Home

The DS3V Program User's Guide

1. Jonnnnan EFDEN the number density of the flow Jonnnnan in a loop over the number of species L 1 MSP EFSP L the fraction of species L if ITYPS 4 periodic boundary BANG the angle about the x axis such that y tan BANG z of the plane TANG the similar angle of the matching plane l end of the loop over the triangles l end of second loop over the surfaces There is now a program DS3DG EXE that employs menus similar to those in the DS2V and DS8V programs to convert a raw triangle file to this specification 5 The Surface Data Generation Program DS3DG EXE The raw triangle format for a surface consists of a single line for each triangle Each line contains nine floating point numbers that represent the x y and z coordinates of the three vertices The ordering of the vertices must be such that they are in anticlockwise order when viewed from outside the surface This is one of the output options that are available in surface modeling programs such a Rhinoceros www rhino3d com There may be more than one surface in a problem and the raw triangle files must be combined to produce the file DS3TD DAT The specification of this file is NSURF the number of unconnected surfaces Each surface must be either open or closed Every side of a closed surface must connect with just one side of another triangle Every side of an open surface must either connect with just one side of anoth
2. if there is more than one molecule in the sub cell the collision partner is chosen from these The ratio of the mean separation between collision partners to the local mean free path is output by the program and should be small say 0 2 compared with unity over the flowfield The nearest neighbor procedures ensure that the lowest possible values of this ratio are achieved Further reductions can be obtained only by an increase in the total number of simulated molecules A problem with nearest neighbour collisions is that if the interval between collisions happens to be very small a subsequent collision may involve the same collision pair This is unphysical because molecules are always moving away from one another after a collision The program stores the code number of the immediately preceding collision partner for each molecule and this molecule is not allowed to be the next collision partner It has been found that this avoids an error in the heat transfer and shear stress that can be as high as four or five percent The only computational parameter that must be set by the user in Version 2 is the initial number of megabytes to be used for storage All other computational variables are set automatically by the program However there is an optional menu that allows experienced users to increase or decrease the default values of cell size time step etc The traditional specification of a single value of this time step over the whole flowfield i
3. Another strategy to decrease the computational effort for eventually steady flows is to employ a relatively small number of simulated molecules at the start of the calculation and to increase the number when steady flow is nearly attained This option changes the number of real molecules that are represented by each simulated molecule The initial number should not be so low that the criteria for a good DSMC calculation are violated by such large margins that the initial flow is unrepresentative of the real flow In general the increase will not be greater than a factor of about three The option may also be used to reduce the number of simulated molecules if it appears that the eventual number will be beyond the memory capacity of the computer The Number of Molecule Moves in Each Mean Collision Time One of the criteria for a good DSMC calculation is that the time step over which the molecule moves and collisions are uncoupled should be much smaller than the mean collision time This can lead to prohibitive computation times for flows with large variations in the collision time if a fixed time step is used The D3V program employs a variable time step that is a fixed fraction of the local mean collision time Four or five moves in each mean collision time is usually adequate although some systematic changes have been observed to a ratio of nine in some flows The ratio is set in the data and this option provides an opportunity to vary it later in the
4. Moves 5 B in Each Mean Collision Time The Number of Molecule Moves 2 a in Each Mean Cell Transit Time Nearest Neighbor Collisions No Nearest Neighbor Produce a molecule output file Continue the Current Sample This and the following option is applicable only when the flow is assumed to become steady at large times and the specified time at which time averaging is to commence had been reached in the earlier calculation This option continues the existing time average Commence a New Sample Resets the sampling variables so that a new time averaged sample commences from the restart This option is useful when the specified time to steady flow proves to be inadequate In addition it should be applied when the flow has settled down after a change in gas density or a change in the number of 20 simulated molecules The program resets the sample automatically if the fluctuating number of simulated molecule changes by more than eight standard deviations Factor to Change the Density of the Flow Most calculations start from an initially uniform flow and at low Knudsen numbers the computational time to steady flow may be very large It is generally more efficient to establish steady flow at a higher Knudsen number and then increase the density of the flow This option is particularly useful when a number of cases are to be calculated at gradually decreasing Knudsen numbers Factor to Change the Number of Molecules
5. a gradual decline in the statistical scatter as the sample increases but convergence is not the appropriate description of this process ii Additional effects such as non equilibrium chemistry may be included simply by adding to the complexity of the molecular model and the fact that these may change the basic nature of the mathematical models is of no consequence iii Most importantly there are no numerical instabilities While personal computers now readily permit DSMC calculations of two dimensional and axially symmetric flows at overall Knudsen numbers well into the continuum regime three dimensional calculations can only just reach the continuum regime Note that the Reynolds number Re is related to the Knudsen number Kn the Mach number M and the specific heat ratio y by e Z M l 5 22z Kn The total number of simulated molecules is limited and in a three dimensional calculation at normal and low densities each simulated molecule must be regarded as representing an extremely large number of real molecules The macroscopic or continuum flow properties are established through averages over the microscopic or molecular properties and are therefore subject to an unrealistic level of statistical scatter However the number of molecules in a cubic mean free path is inversely proportional to the square of the number density The scatter in a typical DSMC calculation is in fact similar to the real scatter in a gas flo
6. absolute value of the negative data variable 31 Ifit is an exchange reaction Post collision species code of the first molecule Post collision species code of the second molecule The number of molecules is unchanged in an exchange reaction The reaction model accepts data in the form of the continuum rate equations in the conventional Arrhenius form The rate equation data is converted within the program to reactive cross sections using the theory of Bird 1994 Note that this is a quasi equilibrium theory and while it appears to work well for the reactions in hypersonic flows in air large correction factors may be need in other reacting flows For example combustion reactions appear to be to slow and significant non equilibrium effects may be present No of internal d of f that contribute The internal degrees of freedom of the reacting molecules may contribute to the activation energy that is required for the reaction to occur The value of this data item will not affect the reaction rate but if the temperature exponent in the rate equation is negative the curve of reactive cross section against collision energy may not have a physically reasonable shape unless the internal degrees of freedom contribute energy The temperature exponent cannot be more negative than the negative of the sum of this number and 3 2 Activation energy The sum of relative translational energy and the contributing internal energy must exceed this energy f
7. an internal entry file A file generated by DS3V can only be an internal entry file file generated from a single DS2Y boundary can be a boundary or internal entry file A file generated from the whole boundary of a DS2V flow can only be an internal entry file The following parameter is required only for internal entry files from DS2Y The z location m of the start location of a two dimensional entry file or the start angle radians anti clockwise when looking in the positive x axial 0 0000 direction from the positive y axis in degrees of an axially symmetric file Also for DS2 V entry files the distance in metres of the extrusion in the positive z direction of a two dimensional molecule entry line or the fractional fof 360 degrees angular sweep of an axially symmetric entry file This parameter must also be set for boundary entry files For those 1 0000 generated acrooss a DS2V boundary set as the second dimension of the DS3 face Set to unity for a file generated from the DS2 flow plane or for a file generated by DSSY It provides information on the structure of the molecule file and if it was generated by DS2V rather than DS8V it specifies the way in which it is to be converted from a two dimensions to three dimensions Computational ariables Assumed Steady Flow at Large Times Continued Unsteady Flow Employ Standard Options Set Custom Options Either steady or unsteady sampling must be c
8. calculation The program logic sets internal limits on the time step and the actual ratio that appears in the output screen will generally be somewhat higher than the value set in the data The Number of Molecule Moves in a Cell Transit Time If the Knudsen number is high the time step set by the collision rate may be too large This data item sets the maximum time step to a fraction of the time for a typical molecule to transit the cell The recommended value of this parameter is two Nearest Neighbor Collisions This is the default option and should be accepted The these procedures minimize the separation distances between the molecule pairs chosen as potential collision partners No Nearest Neighbor 21 The transient sub cells may have been disabled to speed up a program or to investigate the effect of the sub cells The program should normally be run with transient sub cells and this option would rarely be chosen Produce a molecule output file If this button is checked a molecule output file DSMOF DAT will be produced after the restart The following menu then appears Molecule Output File Generation at a Specified Plane 7 file DSMOF DAT may be generated that comprises the molecules that crass a plane within the flow The direction in which the molecules cross the plane must be specified These flow towards the side of the plane that is On the Same Side as the Origin The plane must not be parallel to and very close
9. in the positive z direction of a two dimensional molecule entry line or the fractional fof 360 degrees angular sweep of an axially symmetric entry file This parameter must also be set for boundary entry files For those 1 0000 generated acrooss a D52 boundary set as the second dimension of the D53 face Set to unity for a file generated from the DS2 flow plane or for a file generated by DSSY End of the loop over the boundaries 38 Boundary at Line of Constant x Coordinate of Boundary m i This must be located at a division boundary Set Initial Flow to Stream or Ret Gas Set Initial Flow to a Vacuum If a separation boundary is set the area beyond larger x this line is set to the secondary stream The total number M of seperate molecule 3 input files DSMIF1 DAT OSMIFM DAT 3 a The maximum number of molecule entry files is four The Number of Molecule File Entry Sources 0 a These are in addition to the file entry boundaries For input files generated by DS2Y there may be multiple enty points for the same set of molecules with set displacements and rotations There are three options with regard to the separation boundary and secondary stream No Separation Boundary There is no separation boundary and no secondary stream Boundary at line of constant x A secondary stream must be set and applies at x coordinates greater than this value Boundary at line of constant y A secondary stream
10. m 0 500000E 20 Fraction of Species Number 1 0 200000 2 0 800000 3 0 0 4 00 5 00 6 0 0 F 0 0 3 00 9 0 0 10 00 11 0 0 12 0 0 x y and z velocity components Temperature and Number Density Fraction of Species A maximum of twelve molecular species may have been defined This sets the flow conditions for the gas that enters across the stream boundaries and which generally fills the flowfield at the start of the calculation It is used for the normalization when the non dimensional option is chosen during the run It should therefore be set as a reference gas even when there are no stream boundaries and the flowfield is initially set as a vacuum The following screen appears in a loop over the six flow boundaries 35 Boundary Conditions at minimum x boundary Plane of Symmetry Stream Boundary Yacuum Interface Min and Max x only and must apply to both Periodic Boundaries Constant Pressure Boundaries m Minimum x only Specular Surface Moving into the Flow Surface Speed ft s 0 0000 Molecule File Entry Boundary Axis This choice is not valid for three dimensional flows Plane of Symmetry All molecules striking the boundary are specularly reflected This is equivalent to setting the boundary as a plane of symmetry Stream Boundary Molecules that are typical of the specified stream enter across the boundary Molecules that strike stream boundaries
11. mandatory for the DS3SD DAT file m Units for Data Entry Eng US Some of the data may be species dependent depend on whether any species have vibrational modes or relate to optional surface reactions The values in the following menu must be consistent with those in the DS3VD EXE file 12 lt Set species dependent information fx Number of species in the gas fl B Maximum number of vibrational zx modes of any species 0 g Maximum number of surface reactions 0 B The program must be able to determine the number of distinct points that comprise the vertices of the triangles The next dialog sets the number such that the points are distinct if they are separated by a distance that is greater than this number Note that a typical dimension can vary by many orders of magnitude and that the coordinates in DS3DT DAT may be single or double precision 4 Set Discrimination Limit Surface 1 comprises 5640 triangles Set a number that is very small in comparison with the smallest distance between the points that define the triangles The program then determines the number of distinct points associated with the triangles assigns numbers to the points and defines each triangle in terms of the point numbers of its vertices The user is then asked to set the type of each triangle There may be thousands of triangles on a surface and it is essential to take advantage of the fact that a given set of pro
12. must be set and applies at y coordinates greater than this value Coordinate of the Boundary The x or y coordinate of the separation boundary if one is set One of the following options must be chosen Set Initial Flow to Stream or Ref Gas The initial zero time state of the gas is set to the stream definition If a separation boundary has been set the gas beyond this boundary is set to the secondary stream Set Initial Flow to a Vacuum There are no molecules in the flow at zero time Note that these options have been carried over from the DS2V program and will be extended appropriately for the three dimensional environment when this is required for a particular application Any queries or problems should be referred to GAB Consulting 39 If there is a secondary stream it is set by the above dialog that is identical to that for the stream and for the specified stream elements The total number M of separate molecule input files DSMIF1 DAT eevee DSMIFM DAT This number includes the boundary input files that have already been set for each boundary and the files that supply the molecules for the file entry sources within the flow Note that a single file can be employed at a number of locations This number cannot exceed four The Number of Molecule File Entry Sources This is a molecule file entry process is distinct from that involving file entry surface triangles If entry triangles have been set this number MUST be set to
13. or constant C1 in eqn 6 53 ooo 1 for a constant Zv or constant C2 in eqn 6 53 soo Characteristic vibrational temperature This is a basic physical property of the gas Vib Collision number Zv or constant C1 in eqn 6 53 1 for a constant Zv or constant C2 in eqn 6 53 If a constant vibrational collision number is to be used the first value is set to that number and the second value to 1 However vibrational relaxation is strongly temperature dependent and iit is preferable to use values correlated by the Millikan White theory The equation numbers refer to the supplementary theory in Bird 1994 End of the loop over the number of molecular species Note that the molecules will be referred to in later data input items by a code number based on the order in which they were specified in this loop Le The first species to be input is regarded as species 1 the second as species 2 and so on The following dialog appears in a loop over the specified number of chemical reactions 30 Chemical Reaction Reaction 1 Pre collision species code of first molecule 0 Pre collision species code of seond molecule 0 H Recombination Exchange Reaction Dissociation oo A Post collision species code of first molecule OF zn i a Post collision species code of second mol 0 ae a Post collision species code of third molecule G r Recombination Post coll species code of recombined mol 0 B Th
14. 0000 Fraction of Species Number 1 0 0000 2 0 0000 3 0 0000 4 0 0000 5 0 0000 6 0 0000 7 0 0000 8 0 0000 9 0 0000 10 0 0000 44 0 0000 12 0 0000 The items in this menu are self explanatory Molecule File Entry Triangle This is a triangle at which molecules enter from the file DSMIF DAT The area defined by the triangles must be consistent with the geometry associated with the generation of the DS2MIF DAT file that has been manually renamed DSMIF DAT The following dialog appears during the run when the flow is being set up 18 Molecule Input File A file DSMIF DAT must exist in the directory from which DS3V is a being run This must be a manually renamed DSMOF DAT file produced by DS2V and the following parameters that were specified for the generation of that file must be re entered here Number of molecules in each record Number of records in the fle 0 pone he surface that is specified for molecule file entry in this set program must be either the extrusion in the z direction of the file generation line in a two dimensional run or a an angular sweep Sweep of the entry line in an dha symmetric run of D52 The z location m of a two dimensional entry line or the angle clockwise when looking in the i positive x direction from the 2 0 plane in ae ofan axially symmetric line Sarre KTE Distance in metres of the extrusion in the panve OAAR Bearers A z directio
15. V calculations A molecule file may also be produce from the molecules crossing a specified plane in a DS3V calculation The molecules from this file may be replayed into a separate DS3V calculation The flowfield grid consists of a background rectangular grid that is uniformly spaced in each of the three directions Every division of this grid is divided into a finer grid of elements typically several hundred elements in each division Solid surfaces must not be thinner than an element The flowfield cells are defined by cell nodes that are initially set to the centers of the divisions and the points at the center of the surface definition triangles The cell associated with a given node is comprised of the elements that are closer to that node than to any other At any stage of the calculation a new set of nodes may be adaped to the flow density and density gradients in order to set new cells with a specified number of simulated molecules in each cell 2 2 Gas Specification and Model The gas may be chosen from a menu that includes ideal air real air with vibration and chemical reactions nitrogen argon and a hard sphere gas Alternatively a custom non reacting or reacting gas may be specified The program employs the physical gas models that have been described and validated in Bird 1994 The gas is a mixture of the VHS models and the cross sections the viscosity temperature index which determines the way in which the cross sect
16. Visual DSMC Program for Three Dimensional Flows The DS3V Program User s Guide Version 2 5 October 2006 G A B Consulting Pty Ltd 144 110 Sussex Street Sydney NSW 2000 Australia Phone Country code 61 City code 2 9279 0073 FAX 61 2 9279 0275 gabird compuserve com http gab com au Table of Contents 1 The DS3V Implementation of the DSMC Method 2 The Scope of the DS3V Program 3 Computer Requirements and Program Installation 4 The Geometry Definition File DS3SD DAT 5 The Surface Data Generation File DS3DG EXE 11 6 Running the DS3V Program 7 The Data Input Dialogs 8 The DS3V Tutorial Demonstration Case 9 The DS3DG Tutorial Demonstration Case 20 26 43 51 1 The DS3V Implementation of the DSMC Method The direct simulation Monte Carlo method is a technique for the computer modelling of a real gas by some thousands or millions of simulated molecules The velocity components and position co ordinates of these molecules are stored in the computer and are modified with time as the molecules are concurrently followed through representative collisions and boundary interactions in simulated physical space This direct simulation of the physical processes is in contrast with conventional computational fluid dynamics which seeks to obtain solutions of the mathematical equations that model the processes The computational task associated with the direct physical simulation becomes feasible when the gas density is su
17. al collision time is then the maximum value of the time step Because of the very small overall time step and the small number of cells for which collisions are calculated at each step the implementation of the traditional indexing routine at each step would involve a prohibitive computational effort This problem has been solved through the introduction of bi directional indexing The traditional index array is comprised of the molecule numbers in order of the cells If the address of the corresponding entry in this array is stored for each molecule the indexing may be kept up to date continuously as molecules move from cell to cell The index array must also be extended with sufficient free space between cells to cope with the fluctuations in the number of molecules in individual cells Some general rearrangement takes place from time to time to allow for particularly large fluctuations or systematic changes in the distribution of molecules In many other procedures the ability to assign different times to molecules leads to a simplification of the program The new procedures are computationally faster than the traditional procedures but because of the data structures necessitated by the continuous indexing the number of simulated molecules that can be employed for a given amount of memory is reduced Because of the automatic setting of the time step it is no longer possible for this to be incorrectly specified The nearest neighbor minimize
18. and two for a recombination or exchange reaction Species code of first reflected molecule Species code of the second reflected molecule Species code of the third reflected molecule Species code of the fourth reflected molecule Species code of the fifth reflected molecule Species code of the sixth reflected molecule There is one reflected molecule for a surface recombination two for an exchange reaction and two or more for a dissociation Reaction energy input to surface 0 0 J Species code of first incident molecule Species code of second incident molecule There is one incident molecule for a dissociation and two for recombination and exchange reactions Species code of first reflected molecule Species code of second reflected molecule Species code of third reflected molecule Species code of fourth reflected molecule Species code of fifth reflected molecule Species code of sixth reflected molecule There is one reflected molecule in a recombination reaction two in an exchange reaction and at least two in a dissociation reaction 33 Reaction energy input to the surface This is the energy from a single event and will be negative when energy is required for the reaction The probabilities of the various surface reactions are set when the surface properties are specified Ideal air This sets oxygen as species 1 and nitrogen as species 2 The vibrational modes are not active and there are no chemical reactio
19. ar Stress 1 N sq m 0 49128 024 Shear Stress 2 N sq m 293856 01 Shear Stress Magnitude N sq m 0 89520 01 Ine Heat Flux w sq m TETAS Pressure ES Refl Heat Flux w sq m ESRB Nisq m 0 2 0 4 Net Heat Flux W sq m E Set Steamine JA est through a ny E E Slice B To 5 T ture K E at x Division 12 10 E Retun counter EMpEraRAE Element 1 IZ E to zero paino E MEE a on Species z A streamline has been set TE this location arc LE Reflections Not Shown H Start 4 psavman 09 Dsav Com K DSMC prog J EX Distributio K E g S30 446m The surface properties at any location are displayed when the triangle number is selected The selected triangle then becomes black in the display Three dimensional streamlines may be drawn through the center of any division in the flow 9 The DS3DG Tutorial Demonstration Case The DS3DG EXE file should be in a directory with the sample triangle file DS3TD DAT The sample triangle file for a sphere of unit radius was generated by the Rhinoceros program and was exported as raw triangle file with the coordinates of the triangles in double precision Two lines were added at the beginning of the file for the number of surfaces and the number of triangles on that surface The DS8DG file may be run to produce a DS3SD DAT file
20. are removed from the flow This boundary conditions maintains a uniform flow in an empty flowfield even for a subsonic or stationary stream Vacuum Interface No molecules enter the flow and all molecules striking the boundary are removed from the flow Periodic Boundaries This applies only to the minimum and maximum x boundaries and must be applied to both Molecules that leave across the minimum x boundary enter across the maximum x boundary and vice versa Constant Pressure Boundaries These boundary conditions require the definition of a secondary stream and an initial flow separation boundary at a specified x location The minimum x boundary is a stream boundary but the number of molecules that enter is 36 made equal to the number that crosses the initial flow separation line in the positive direction The maximum x boundary is a similar boundary for the secondary stream and the number of molecules that enter is equal to the number that cross the flow separation line in the negative direction This boundary is intended for use in calculations involving steady subsonic flow through channels and tubes Specular Surface Moving into the Flow This boundary can be set only at the minimum x boundary and causes a specular piston to move into the flow at a constant speed given by the specified Surface Speed This produces a shock wave ahead of the piston Steady flow sampling should not be employed with this option Molecule File Ent
21. ass Flux This option should only be used when there is separate input for each molecular species The following dialog appears if any surface reactions have been defined Surface Reaction Probability for this Group Probability of surface reaction number 1 ii 2 0 0000 3 0 0000 4 o 0000 5 0 0000 0 0000 7 o 0000 8 0 0000 3 0 0000 10 o 0000 11 0 0000 12 0 0000 13 o 0000 14 o 0000 15 0 0000 16 o oo 17 0 0000 18 0 0000 Probability of surface reaction number Up to 18 surface reactions were defined earlier This is the probability when one of the incident molecules strikes the surface If two are required the other is removed later Solid surface intervals with species dependent properties This choice brings up similar menus to the species independent choice except that the dialog Gas surface interaction for this group is repeated for each molecular species Also it is possible to set the Outgas mass flux This is set separately for each species The outgassed molecules are generated as if they were effusing from the surface at the temperature of the surface Stream entry intervals The surface is a flow entry surface and is commonly used to cut off a region of flow that is undisturbed freestream flow No additional data is required 17 Specified flow entry triangles x velocity component m s y velocity component m s 0 0000 2 velocity component m s omn Temperature K oao Number Density 4cu m 0
22. ated by a plane of constant z The single data item sets the fraction of the bounding parallepiped that is in domain 1 Following this menu the program transfers to Set New or Modified Data About the DSMC method This option brings up an information screen on the DSMC method and a Frequently Asked Questions screen on the method can be called from this The program then returns to the Initial Options menu Adapt the Cells and Continue Continues the current run with cells adapted to the density in the current flowfield sample When selected the user is asked to supply the desired number of simulated molecules in each of the new cells Separate adaptions are made are made for the collision cells and sampling cells The collision cells are adapted first and acceptance of the default eight molecules per collision cell is recommended Set the Collision Cell Size Set the desired average number of molecules per 8 4 collision cell in the flowfield This number affects the quality of the computation The optimum number appears to be about eight but the oroaram will run faster with a laraer number A plot of the new cells appears in a new window when the adaption is complete and the window should be closed for the computation to proceed The sampling cell dialog then appears with a recommended number that depends on the magnitude of the calculation 23 Set the Sampling Cell Size Set the desired average number of mo
23. ave been suppressed in the output The calculation contains the correct average amount of vibrational energy despite the exaggerated scatter in the vibrational temperature at low overall temperatures vi The ratio of the local mean separation of the collision partners to the local mean free path is the key verification parameter for the calculation The largest and therefore the worst value in any cell is output in the flow log There could be some error in the calculation unless this is small compared with unity On the other hand higher values might be tolerated in a small region of the flowfield and the largest value might be an extreme fluctuation in one of the very small cells at the boundary of the surface cell region and the rectangular cells Therefore if the largest value is undesirably large the Coll Sep m f p option for the property in the flow display should be selected The distribution of the ratio can then be studied and a judgement made about the quality of the calculation The ratio can be reduced only by the use of a larger number of simulated molecules in the calculation vii The average number of simulated molecules in the cells may also be selected for the flow property display This is a computational parameter and unlike the flow properties aspects of the cell structure are apparent in the display The smallest numbers will generally be associated with the small cells that are remnants of the rectangular cells that a
24. ctly thermal conducting surface that extends over all triangles in the same ITYPS 0 group if diffuse adiabatic ve or zero TSURF if the triangle is part of an adiabatic surface if perfectly conducting TCAP thermal capacitance of the whole body defined by the surface end if EMISS the surface emissivity end if in a loop K over 1 for ITYPS 1 the number of species MSP for ITYPS 2 IRFM K 0 1 for diffuse CLL model if IRFM K 1 Jann nnnnnnnn EACN K the normal energy accommodation coefficient TMAC K the tangential momentum accommodation coefficient 10 ROTAC K rotational accommodation coefficient Janm if MMVM gt 0 Jonnnnnnnn FRSPEC k fraction specular FRSORB K fraction adsorbed if IT YPS 2 OUTGMF K the outgas mass flux end of loop over 1 or MSP l if MNSR gt 0 Jann nn nnn in a loop K over the number of surface reactions MNSR Jasa PROBSR K the probability of the surface reaction end of loop over MNSR Jonannan the in plane velocity component in direction1 added 1 2 02 Jonnnnan the in plane velocity component in direction 2 added 1 2 02 if ITYPS 3 gas entry triangle Jonnnnan XVEL the velocity component in the x direction YVEL the velocity component in the y direction Jonnnnans ZVEL the velocity component in the z direction EFTMP the flow temperature
25. cules may recombine or dissociate as a result of collisions with solid surfaces The following dialog appears in a loop over the number of molecular species Definition of a Gas Species Definition of Species 1 Reference diameter of the molecule i m at a reference temperature of 0 0000 K Inverse of SS scattering parameter 0 0000 Set to 1 0 for YHS molecular model Viscosity temperature power law 0 0000 Molecular mass 0 0000 kg Number of rotational degrees of freedom of Rotational relaxation Ke Second order polynomial collision number value in the gas temperature Fixed collision number or constant in polynomial 0 0000 Coefficient of temperature in the polynomial 0 0000 Coefficient of temperature squared in polynomial 0 0000 The number of vibrational modes 0 B The physical properties of a number of common gases may be found in the Reference Diameter of the molecular model The variable hard sphere and variable soft sphere molecular models employ the hard sphere VHS or modified hard sphere VSS scattering model but 28 have a diameter that varies with the collision energy The energy for the specified diameter is characteristic of the temperature that is specified after at a reference temperature of Inverse of the VSS scattering parameter This is set to unity if the VHS model is to be used or for VSS molecules to the value appropriate to the Schmidt number of the gas Viscosity temperature pow
26. e code of the third body molecule or roo if negative the code of third body table a m Exchange Reaction Post collision species code of first molecule 0 Pre collision species code of seond molecule H No of internal d of f that contribute Oo od Activation energy 0 0 J 1e10 x the pre exponential parameter 0 0 Temperature exp in the rate equation 0 0 Reaction energy input to gas 0 0 J Pre collision species of the first molecule Pre collision species of the second molecule The code numbers of the two pre collision species The identity of the third body molecule in a recombination is specified later Choose one of Dissociation Recombination and Exchange Reaction If it is a dissociation Post collision species code of the first molecule Post collision species code of the second molecule Post collision species code of the third molecule A dissociation is a reactive collision that converts two molecules to three molecules If it is a recombination Post collision species of the recombined molecule A recombination leads to a single molecule and an unchanged third body molecule The code of the third body molecule or if negative the code of the third body table The first option is allows a reaction to be specified for a particular species as the third body while the second allows all species as third bodies with the relative efficiencies given by the table with code number equal to the
27. e that the surface definition file was produced for Version 1 and does not conform to the specification in Section 4 As noted earlier DS3SD files for Version 1 are accepted by Version 2 if a line containing 1 is added as a new first line The first window to appear provides a check of the geometry that has been defined The graphical representation of the axes bounding rectangle and surfaces can be rotated in three dimensions by mouse movements with the left button depressed Mouse movements with the right button depressed resize the image which can also be moved by the arrow keys POSITION Arrow Keys SIZE Right Mouse Button ORIENTATION Left Mouse Button POSITION Arrow Keys SIZE Right Mouse Button ORIENTATION Left Mouse Button The mouse should be moved around until the geometry of the flow becomes clear The minimum z face is a plane of symmetry and the other faces are stream boundaries The solid surfaces that define the delta wing are coloured triangle by triangle The flow entry surfaces that cut off regions of uniform flow in the corners of 44 the parallelepiped are also shown These are much larger and all appear as blue in this case The surface images are transparent if they are viewed from behind The program is paused while any of the flow establishment windows are displayed and the window must be closed in order for the calculation to continue The next window shows the elements that are intersected by surface
28. ec Diffuse Reflection The classical diffuse reflection model is employed with full accommodation of the translational energy to the temperature that was defined earlier CLL reflection model This model see Reference can provide a realistic model of the reflection processes that occur on extremely clean surfaces It also employs the temperature that was defined earlier and requires the definition of the Normal energy accommodation coefficient and the Tangential momentum accommodation coefficient A major problem is that very little information is available on when this model should be used and the values of these coefficients that should be employed if it is used Rotational accommodation coefficient Irrespective of the model that is used for the gas surface interaction of the translational modes the fraction accommodation of any rotational modes may be set Vibrational accommodation coefficient Similarly for any vibrational modes Fraction of specular reflection 16 A combination of specified fractions of specular and diffuse reflection has often been used to provide an indication of non diffuse effects It is now recommended that the CLL model be used for this purpose Full specular reflection can be used make a surface a plane of symmetry or to make an inviscid flow calculation Fraction adsorbed at surface The specified fraction of molecules that strike the surface are removed from the calculation Outgas M
29. ed to keep pace with the introduction of multi core processors Dual core operation leads to an overall increase in computational speed of about 50 and it is hoped that as the number of cores increase the speed increase per core will be relatively higher A surface triangle can spread over the domain boundary and the parallelization does not have to be taken into account in the DS3SD DAT file 3 Computer Requirements and Program Installation G The program requires Windows 98 or higher Gi A2GHz Pentium processor or higher is recommended Gii 500 MB RAM is required for the program to run efficiently iv The program runs in an XGA 1024 x 768 window Windows 98 required small fonts to be specified in order for the DS2V windows to be sized and positioned correctly With XP it is essential that the DPI setting be set to Normal size 96 DPI This is accessed through Control Panel gt Display gt Settings gt Advanced gt General The program is delivered as the compressed executable DS38VZ EXE This is run to produce the uncompressed executable DS3V EXE of the main program and the program DS3DG EXE for the generation of the surface data file These files should be placed in separate directories It also produces sample data files DS83SD DAT and DS3VD DAT that should be placed in the directory with DS3V EXE and the sample triangle file DS3TD DAT that should be placed in the directory with DS3DG EXE 4 The Geo
30. ehaviour of the total molecule number in this screen iii The log of the computation in the top left hand corner of the screen shows the number of molecule round off substitutions that have been made during the calculation A molecule is removed from the flow and is replaced by randomly selected molecule from the same cell if the computer is unable to fix its position within the flow This problem arises from the six or seven significant digit limitation of 32 bit arithmetic iv The units are quoted when one of the dimensioned output options are chosen The dimensionless output option requires specific definitions for some of the output quantities The flow quantities are expressed as their ratios to selected undisturbed stream quantities The non dimensional positions are the x and y co ordinates divided respectively by either the length or height of the working section The speeds and velocity components are normalized by the speed U of the undisturbed flow The overall temperatures and the separate components of the temperatures are normalized by the overall temperature of the stream The pressure is normalized by the pressure p in the stream The densities and number densities are normalized by the stream density p and number density n A number flux coefficient is N nsU A surface pressure coefficient is p Ppa A4p Uz A shear stress coefficient is t p U oy A heat flux coefficient is AY tpU nae A characte
31. er law The variation in the VHS diameter is such as to produce a coefficient of viscosity that varies with temperature raised to this power 0 5 is a hard sphere gas and 1 0 is effectively a Maxwell gas These should be regarded as limiting values Molecular Mass This is the mass of an individual molecule Number of rotational degrees of freedom A monatomic molecule has no rotational degrees of freedom and a diatomic molecule or a linear triatomic molecule has two Non linear and complex polyatomic molecules have three rotational degrees of freedom Rotational relaxation collision number This sets the rotational relaxation rate It mat be made a constant generally about five or a second order polynomial in the temperature Fixed collision number or the constant in the polynomial Coefficient of temperature in the polynomial Required only if the polynomial option has been chosen Coefficient of temperature squared in the polynomial Required only if the polynomial option has been chosen The number of vibrational modes These do not become significantly excited until the temperature reaches values of the order of the characteristic vibrational temperature For most gases vibration may be neglected if the temperature is less than about 1000K The following dialog appears if there are vibrational modes in a loop over the number of modes 29 Vibrational Data Characteristic vibrational temperature E K Vib collision number Zv
32. er triangle or may terminate on a face of 11 the computational region In the latter case the sides must form a closed line on the six faces the computational region in a loop N over NSURF l NTRI N the number of triangles on surface N The raw triangle file for the surface i e l in a loop M over NTRI N ISEGI 1 M ISEGI 2 M ISEGI 3 M the point numbers of the three vertices the order must be anti clockwise when viewed from the flow outside l l end loop over M end loop over N The first dialog that appears when the DS3DG EXE file is run is an introductory dialog to remind the user that a DS3TD DAT file must be present DS3TD DAT FILE 4 triangle definition file DS3TD DAT must be present The first line must contain the number of separate surfaces NSURF Then in a loop over NSURF The number of triangles NTRI that define the surface Then in a loop over NTRI The three vertices of that triangle 1 y1 21 52 y2 22 43 y3 23 The order of the vertices is such that they are clockwise when viewed from outside the surface Each surface must be either a closed surface without holes or an open surface with triangles that may teminate on one or more sides of the bounding parallelpiped All the data Gncluding that in the DS3TD DAT file must be in either SI or Engineering US units If the latter the values are automatically converted to the SI units that are
33. f the computational variables Flow Composition Retain ensina Eomp Ideal Air Argon Hard Sphere Gas Real Air with Reactions Martian Atmosphere Retain Existing Composition This option is available only when data is being modified and it retains the existing number and definitions of the gas species Custom Composition Allows the detailed specification of the gas species and chemical reactions Modified data files revert to custom composition unless the preceding option is chosen The following dialogs appear under this option Gas Complexity Number of molecular species in the flow H Max no of vibrational modes in any species 0 H The number of chemical reactions 0 H Number of third body tables for recombinations of Number of surface reactions Oo H 27 Number of molecular species in the flow This is the total number that may appear in the stream the secondary stream or in the gas that enters the specified flow entry lines Max no of vibrational modes of any species Vibrational modes need not be set unless the temperatures in the flow are sufficiently high for them to be activated to a significant extent The number of chemical reactions This is the total number that will be specified Number of third body tables for recombinations These set the relative efficiencies of all the molecular species as third bodies in recombination of atoms and molecules Number of surface reactions Mole
34. fficiently low It also becomes necessary under these conditions because the Navier Stokes equations do not provide a valid model for rarefied gases and conventional CFD methods are unable to cope with the large number of independent variables that are involved in applications of the Boltzmann equation to realistic multi dimensional problems The degree of rarefaction of a gas flow is generally expressed through the overall Knudsen number which is the ratio of the mean free path in the undisturbed stream to a typical dimension of the flowfield It is desirable to also define the local Knudsen number as the ratio of the local Knudsen number to the scale length of the gradient of a specified macroscopic or continuum flow property The Navier Stokes equations are valid only when the local Knudsen numbers based on flow speed density temperature and pressure are small in comparison with unity The limit as the Knudsen numbers tend to zero may be identified with the inviscid limit that is modelled by the Euler equations The opposite limit as the Knudsen number tends to infinity is the collisionless or free molecule flow limit in which intermolecular collisions may be neglected The flow regime between free molecule and the limit of validity of the Navier Stokes equations is generally referred to as the transition flow regime A Knudsen number of 0 1 has traditionally been quoted as the boundary between the continuum and transition regimes but the character
35. for a closed surface that describes a nano particle that is subject only to convective fluctuations with a one to one correspondence between real and simulated molecules The temperature history of the nano particle is output in the file PTEMP TXT In plane velocity component in direction 1 m s 0 0000 In plane velocity component in direction 2 m s 0 0000 Direction 1 is normal to the x axis or if the triangle itself is normal to the x axis in the y direction Direction 2 is also in the plane of the surface and is normal to direction 1 Surface Temperature 300 00 K 14 Reflection at a Set Temperature The molecules that are reflected according to the gas surface interaction to be specified later at the temperature that is specied below Zero Heat Transfer Adiabatic Surface The incident heat transfer to the surface is calculated on a progressive basis and the surface temperature is adjusted such that there is net heat transfer to the surface This is commonly referred to as an adiabatic surface and the adiabatic surface temperature appears as one of the options for the surface distribution output Surface Temperature The surface temperature for molecular reflection or the minimum allowable surface temperature if the adiabatic option has been chosen and the surface is insulated zero thermal conductivity However if this is set to zero the surface is assumed to have infinite thermal conductivity such that the
36. gion in the latter case the sides must form a closed line on the six faces the copmputational region in a first loop N over NSURF NPNT N the number of points that define surface N NTRIS N the number of solid surface triangles on surface N NTRIF N the number of flow entry triangles on surface N NTRIC N the number of periodic boundary triangles on surface N ladded 1 2 02 end first loop over N in a second loop N over NSURF l in a loop M over NPNT N XS M YS M ZS M coordinates of point M end of loop over points in a loop M over the triangles sum of NTRIS N NTRIF N and NTRIC N ISEGI 1 M ISEGI 2 M ISEGI 3 M the point numbers of the three vertices the order must be anti clockwise when viewed from the flow outside l ITYPS 0 if the triangle has the same properties as the preceding triangle 1 if the triangle is a solid surface with species independent properties 2 if it is a solid surface with species dependent properties an outgassing surface must be type 2 3 if it is a gas entry boundary with specified properties 4 if it is a stream entry boundary 5 if it is a periodic boundary such a boundary must be in a plane y az if ITYPS 1 solid surface Jonnnnans TSURF if ve surface temperature for diffuse or CLL reflection if ve diffuse or CLL adiabatic with the absolute value as the minimum temperature if 0 0 generates the temperature of a perfe
37. hosen Assumed Steady Flow at Large Times This is chosen when the boundary conditions are such the flow becomes steady at large times and a long time average will be required to produce a large sample The 41 sample is automatically reset whenever the total number of simulated molecules changes by seven or eight standard deviations For most problems this automatically causes a time sample to be made over the eventual steady flow However in some cases the unsteady phase of the flow does not involve much or even any change in the number of molecules The user must then stop the program after the flow has become steady and select the Commence a New Sample option when re starting it A large time averaged sample may then be accumulated Continued Unsteady Flow The flow is not assumed to become steady at any time This option requires a very large number of molecules The flow is time averaged over an interval at the end of each output interval Employ Standard Options The computation is made with the standard or default options that are adequate for most problems Set Custom Options This brings up the following dialog that can be used to modify the standard options Custom Computational Parameters aj Factor to change the standard number of rectangular divisions in direction X 1 0000 y 1 0000 2 1 0000 The number of elements per division standard value is 5 in direction x sH el s ezl s Factor to change the
38. ion changes with the relative velocity are set separately for every molecular species A classical Larsen Borgnakke model is employed for the rotational degrees of freedom while a quantum model is used for the vibrational modes The chemical reaction model calculates reactive cross sections that are consistent with the measured rate constants The classical diffuse reflection model with complete accommodation of the gas to the surface temperature is appropriate to engineering surfaces that have not been exposed for a long period to ultra high vacuum A set temperature distribution may be specified for the surface or it may be specified as an adiabatic surface with zero heat transfer In the latter case the temperature distribution on the surface is one of the output quantities Alternatively the other classical model of specular reflection may be specified for the artificial case of complete slip and no energy transfer at the surface Partial accommodation may be modelled by a diffuse reflection with a specified fraction of specular reflection 2 3 Parallel Operation 8 Starting with version 2 2 the program allows parallel operation on dual core computers and this feature can be tested on a single core CPU The process is largely transparent to the user who needs to set just one additional data item when parallel operation is selected Domain decomposition is employed in a shared memory mode The maximum number of domains will be increas
39. istic dimension of complex flow fields may be specified in many different ways and the use of the overall Knudsen number may be misleading The conservation equations of fluid mechanics are valid for all flow regimes but the Navier Stokes equations depend also on the Chapman Enskog theory for the shear stresses heat fluxes and diffusion velocities as linear functions of the velocity temperature and concentration gradients The Chapman Enskog theory assumes that the velocity distribution is a small perturbation of the equilibrium or Maxwellian distribution In an isentropic flow for which the Euler equations are valid the distribution function conforms everywhere to the Maxwellian The formulation of the Chapman Enskog distribution involves the local Knudsen numbers based on the velocity and temperature gradients It has been found that errors become significant when these local Knudsen numbers exceed 0 1 and the continuum theory is hardly useable when they exceed 0 2 The transport property terms become zero in an isentropic flow and it might be thought that the Euler equation yield correct inviscid flow results at all Knudsen numbers However as the density decreases the collision rate in the gas eventually becomes too low to maintain the isotropy of the pressure tensor A breakdown parameter can then be defined as the ratio of the density scale time following the fluid element to the mean collision rate For a steady flow this parameter can be re
40. lated to the local Knudsen number based on the density scale length Although it was shown many years ago that the Chapman Enskog expansion for the distribution function is not uniformly valid attempts are still being made to extend the range of validity of the Navier Stokes equations to lower densities However the low density effects such as the development of an anisotropic pressure tensor are of a very basic nature and it is unlikely that much progress will be made for other than one dimensional steady flows In addition effects such 4 as thermal and pressure diffusion become more prominent at low densities and these are not generally included in the Navier Stokes formulations It is certain that the necessary extensions in the event that adequate ones can be developed will add greatly to the difficulty of the continuum approach On the other hand once the density becomes sufficiently low for the DSMC solution to be computationally feasible it is a much easier method to apply The main reasons for this are i The calculation is always unsteady with physical time as one of the principal variables in the simulation If the flow becomes steady it is obtained as the large time state of the unsteady flow The method does not require an initial approximation to the flow field and there is no iterative procedure or convergence to the final solution In the case of a time averaged steady flow or an ensemble averaged unsteady flow there will be
41. lecules per 27 4 sampling cell in the flowfield This number sets the resolution of the flowfield output but does not affect the quality of the simulation The recommended value is shown A plot again appears and this window should be closed for the calculation to proceed An information dialog appears when the flow is ready to restart using the new cells Sampling Cell Adaption Complete i xj e j The Flow simulation will now continue In the case of flows that become steady the option will normally be chosen after a substantial sample has been built up However it can be selected any number of times and can be used for flows that remain unsteady IF THE PROGRAM STOPS WITH OR WITHOUT A DIAGNOSTIC WINDOW EXAMINE THE FILE DIAG TXT FOR MESSAGES AND EXPLANATION A number of additional files are generated during the problem set up phase These are mainly for diagnostic purposes Set New or Modified Data This option allows a new calculation to be specified If there is no valid data file on the computer it must be selected before a calculation can be made The following dialog with three choices then appears Create or Modify Data Modify the Existing Data Set New Run with Existing Data Generate Completely New Data Selection of this button asks for a choice of input units 24 SI units are recommended The program is rarely used with engineering units so please report any errors The program
42. metry Definition File DS3SD DAT Version 2 introduces additional options but DS3SD DAT files that were prepared for Version 1 remain valid if an additional line containing the integer 1 is added as the first line The first items in the file are now the version numbers and future programs will be backwardly compatible as far as the data files are concerned This file must be prepared independently of the DS3V program and must take into account the following conventions that relate to the DS3V geometry i The coordinate system is such that with x to the right and y to the top of a sheet z is up from the sheet ii The order of the points that define each triangular element must be anticlockwise when looking from OUTSIDE the surface iii Open surfaces must terminate on the faces of the bounding parallelepiped Faces 1 2 are at XMIN XMAX Faces 3 4 are at YMIN YMAX Faces 5 6 are at ZMIN ZMAX The specification of the data file is as follows comments are preceded by NVERS the n in version n m that creared the data file MVERS the m in version n m that creared the data file only if NVERS gt 1 NSURF the number of unconnected surfaces l each surface must be either open or closed Jona n every side of a closed surface must connect with just one side of another triangle Jonn every side of an open surface must either connect with just one side of another triangle or may terminate on a face of the computational re
43. n of a two dimensional molecule entry line or the clockwise fractional of 360 degrees m sweep of an axially symmettic le BRA zeiss Number of molecules in each record and Number of records in the file These much match the values when the DSMOF was generated in the DS2V program The two real variables set the location and extent of the tw0 dimensional or axially symmetric molecule file in the three dimensional flow Completed xf The surface data file D535D DAT has been generated 19 6 Running the DS8V Program Running DS8V produces the Run Type window that contains six push button style radio buttons and an OK button to move on when the desired button has been selected Run Type New DualDomain Parallel Run About the DSMC Method Adapt Cells and Continue Set New or Modified Data About the DS3 Program Continue the Current Run This is the pre selected button but is valid only when a calculation has been stopped after it had reached the stage at which the flow and surface property displays are displayed Restart files are written every time the print out stage is reached The selection of this option causes the last set of restart files to be read and re starts the run from that flow time Review the Computational Variables and Settings Commence a New Sample Factor to Change the Density of the Flow 1 0000 Factor to Change the Number of Simulated Molecules 1 0000 The Number of Molecule
44. normal to any of the three axes and may be moved along the axis Initially the sampling cell nodes are the centres of the divisions that are entirely within the flow and the centres of the surface triangles The smaller collision cells are generated next and are shown on the following page Note that the whole of the elements that are partly within the flow are coloured in these plots The cells are near uniform in size and the apparent size irregularity is an artefact of the three dimensionality of the cells an that a two dimensional cut intersects only a small cross section of some cells Each cell is assigned a random colour out of a palette of 190 colours and adjacent cells will occasionally have the same colour lt Flowfield Divisions and Elements in Specified Planes 45 iol xj lt Flowfield Cells Some elements are partl ide the surface E 5 xi 46 lt DSMC program DS3 for three dimensional flows Version 2 1 01 z ioj x Pm Progessvelog othe Cure DSMCComputaton Meic Gi __Eng US _ Dimensionless Flow time is MEEKER seconds with MSEE simulated molecules Overal Forces and Moments on Surface Number NES 7325664227 molecular moves 703366643 binary collisions and Forces ate measured in the direction of the axes BERGE c cofections A EE I 2 E 2209 molecule round off substitutions from entering ne The moments ate about the point These are editable fields 7 x JOO m y 0 000 m z 0 000 m A
45. ns It is useful for problems involving air at temperatures less than about a thousand degrees The fractions of the two species are about 0 2 and 0 8 and are set in the stream property dialog Argon Argon is set as a typical real monatomic gas Hard sphere gas This employs a fixed molecular of 4x101 m and a molecular mass 10 kg Real air with reactions This is a five species model with oxygen as species 1 nitrogen as species 2 atomic oxygen as species 3 atomic nitrogen as species 4 and nitric oxide as species 5 The vibrational modes are active and the data is set for 23 chemical reactions The model has been extensively employed for atmospheric re entry studies Martian atmosphere This employs nine molecular species The first five are as outlined for real air and atomic carbon is species 6 carbon monoxide is species 7 carbon dioxide is species 8 and argon is species 9 The model is again intended for atmospheric entry problems and the vibrational modes are active A typical Martian atmospheric composition is 95 6 carbon dioxide and 4 6 molecular nitrogen Argon would have to be added as a trace species but the other species are generated by the fifty nine chemical reactions that are included in the model 34 Properties of Main Stream or Reference Gas x velocity component m s 670 760 y velocity component m s 179 730 z velocity component ms 0 00000 Temperature K 300 000 Number Density cu
46. o reassign the axes The files generated from molecules leaving across one of the DS2V boundaries are necessarily moving in either the x or y directions and when replayed into DS3V The following menu comes up for each input file after the cells have been set File DSMIF1 DAT The file must exist in the directory from which DS3 is being run This must be a manually renamed DSMOF DAT file that was produced by DS2 or DS3V and the following parameters that were specified for the generation of that file must be re entered here Number of molecules in each record 10000 H Number of records in the file 100 THE SET OF MOLECULAR SPECIES MUST BE IDENTICAL IN BOTH PROGRAMS 4 file from the flow plane of a 2D case with DS2V can only be a boundary entry file A file generated from a line in DS2V can only be an internal entry file A file generated by DS3V can only be an internal entry file file generated from a single DS2V boundary can be a boundary or internal entry file 4 file generated from the whole boundary of a DS2Z flow can only be an internal entry file The following parameter is required only for internal entry files from D524 The z location m of the start location of a two dimensional entry file or the start angle radians anti clockwise when looking in the positive x axial 0 0000 direction from the positive y axis in degrees of an axially symmetric file Also for DSZ entry files the distance in metres of the extrusion
47. or a reaction to be possible 1 e10 x the pre exponential factor The reaction rate coefficient is assumed to be in the form kT AT expC E kT where Fa is the activation energy A is the pre exponential factor and n is the temperature exponent The units are such that the rate coefficient k T is in m molecule sec for a binary dissociation or exchange reaction and m molecule sec for a ternary dissociation reaction The input quantity is multiplied by ten orders of magnitude so that it does not become smaller than the minimum number that can be represented by a single precision variable Temperature exp in the rate equation The quantity y in the above equation Reaction energy input to the gas This is positive for an exothermic reaction and negative for an endothermic reaction The following dialog appears in a double loop over the number of third body tables and the number of molecular species 32 Third Body Probability For species 1 intable 0 Relative probability 0 0 Relative probability If this is set to unity the rate coefficient is as set for the reaction Otherwise it is the factor by which the rate coefficient is multiplied The following dialog appears in a loop over the specified number of surface reactions Surface Reaction Reaction 1 Species code of first incident molecule 0 E Species code of second incident molecule 0 There is one incident molecule for a surface dissociation
48. parate to and in addition to the DS3V program Each surface may be set to move in its plane or with some restrictions normal to it The flow input boundaries permit the study of a wide range of problems that involve jets and plumes A secondary stream may be set to occupy part of the initial flowfield and this permits the 7 study of unsteady shock tube type flows and shear flows Alternatively shock waves may be generated by a moving piston type boundary for diffraction studies Special constant pressure boundaries are available for the generation of steady internal flows that are unaffected by the boundary conditions Periodic boundary conditions are also available These features have been carried forward from the DS2V program and may need to be generalized to cover some 3 D flows The upstream region of a three dimensional flow is frequently two dimensional or axially symmetric Starting with version 2 3 it is possible to combine DS2V and DS3V runs through molecule file output input options These procedures automatically allow for the width of the three dimensional flow and differences in time scale It is possible for a number of copies of axially symmetric file molecules to enter a three dimensional flow with various translations and rotations of the molecules For example a study may be made of the impact of multiple control jet plumes on a three dimensional satellite model The molecule files can also connect two DS2V or two DS3
49. peed MEE ms x Comp Tr Temp E y Comp Tr Temp 375 65 K z Comp Tr Temp 34934A Translational Temp MEEA K Rotational Temp 260 29 K Vibrational Temp Color Map rr 2 3 8 Generate file F3 1E TXT Generate file B3V TE BMP H 30 0 Upper Limit Rotational Temp start wi DS3VMAN Micros il Q9 D53 Compaq Yis IE DSMC program D B Distribution of Fl Delete all the flow direction lines S Linear Logarithmic iB Scale Black _ White Eliaan MO 4 39PM The flowfield may be viewed in planes normal to any of the three axes Note that the planes and the properties to be viewed in the plane must be selected by the arrow controls and not through clicking on the buttons The flow properties at any point are selected by left clicking on the location and a right click produces the path line in the plane DSMC program DS3V for three dimensional flows Version 1 2 07 E ioj xj lt Distribution of Surface Properties UR over each Surface Triangle a jol xj Sample Si ize Number Flux Pressure Magnitude of Shear Stress Incident Heat Flux Reflected Heat Flux Net Heat Flux Temperature Number Fraction of Species 1 Triangle Number Generate File S2V 1 ES TxT x coordinate m y coordinate m 2 coordinate m 0 66681E 01 mmn 22668 Sample Size Number Flux sq m s 0 97255E 22 Pressure N sq m 0 36220 She
50. perties generally apply to more than one triangle on a surface and hopefully to all of them The first menu for the setting of the surface properties allows the specification of the number of following Gn numerical order triangles that share the properties of the triangle that is being set 13 Type of next Triangle or Group of Triangles Triangle 1 of s5000 with Vertices at 4997500E 03 0 8000000E 04 0 1570500E 04 S000000E 03 0 8000000E 04 0 000000 50000000E 03 0 000000 0 000000 Solid surface triangle with species mdependent properties cannot be an outgassing surface Solid surface triangle with species dependent properties Stream Entry Triangle Specified Flow Entry Triangle Periodic Boundary Triangle The number of additional following 2 trianales that share these properties H Solid surface intervals with species independent properties This choice brings up a menu to set the properties for this group Properties of Solid Surface Intervals Reflec Zero Heat Transfer Adiabatic Surface This is the minimum temperature in the case of an insulated adiabatic zero heat transfer surface Set it to zero to generate the temperature of an adiabatic perfectly thermal conducting surface Emissivity Set only for an adiabatic surface 0 0000 Thermal Capacitance Assumes a perfectly 0 0000 conducting adiabatic body J deg K z finite thermal capacitance should be set only
51. re not quite cut out when the surface cells are superimposed on the background grid Small cells are also occur adjacent to sharp corners in the surface cell regions while the outermost cells in the corner regions are larger than normal The total sample in the cell is obtained by multiplying the average number of molecules by the total number of samples that is shown in the flow log There is a button marked Pause Program for Detailed Plot for both the flowfield and surface properties 49 Id Properties Us Sample Size Number Density Density u v w Flow Speed Pressure nkT Translat l Temp Rotat l Temp Wibrat l Temp Temperature Mach No Mols Cell Mean Coll Time Mean Free Path mean coll sep m f p ae Sample Size Fraction in Velocity Components U wW Translat Temp Overall 2 Components Rotat l Temp Vibrat l Temp y 12156 25 0 79960E 01 m Sample Size 1742148 Number Density 0 67332E 20 En x Velocity Comp MEME m s y Velocity Comp EE r z Velocity Comp 28 234 m s Flow Speed 507 01 i Translat l Temp EES K Rotational Temp 357 32 K Vibrational Temp EET K Temperature BERET K Density MEAE kocu m Pressure nkT BERRA N s m Mach Number MEMMEN Mean Fiee Path ME mn Molecules Cell 27 649 Mean Collision Time ESSE s m coll sep m f p MEELT BEE Sample Size MEERES Number Faction M x Comp Diff Vel MENSE m s Comp Diir Vel BERRIES ms 2 Comp Diff Vel MEME m s Diffusion S
52. ristic length or area is required for the normalization of the surface forces and moments The input of this parameter is through a pop up menu that appears the first time that non dimensional output option is selected The force coefficients are the force divided by the product of 5 PU 2 and a characteristic area v For gases with active vibrational modes there is an apparent anomaly in that the vibrational temperature is usually output as zero in a low temperature stream This is a result of the quantum model that is employed in the program The minimum vibrational energy of any molecule is the Boltzman constant times the characteristic vibrational temperature The characteristic vibrational temperatures in air are of the order of two or three thousand degrees Kelvin and at temperatures 48 of the order of one or two hundred degrees only about one in a million molecules would be in the first vibrational level This means that there is likely to be no vibrational energy in the number of molecules that are involved in a typical DSMC sample The amount of vibrational energy at the low temperatures is so small that the zero values are of no consequence To balance the zero vibrational temperatures in most samples in a low temperature gas a small sample that happened to include one molecule at the first vibrational level would be assigned an extremely high vibrational temperature These would appear even more anomalous to the casual user and they h
53. ry Boundary There must be a file DSMIFn DAT file where n may be 1 to 4 that is a renamed DSMOF DAT file that was generated by the DS2V program Specify the Molecule File Entry File for this Face a at minimum x boundary The integer M in the molecule input file BE DSMIFM DAT that enters this face must be molecule file that enters a face of the DS3 flowfield must have been generated by DS2V There is a single variable direction x or y if it is generated at a side or two if it is generated from the molecules moving in the z direction The xyz directions are given the numerical codes of 1 2 3 Set the DS3 axis that corresponds to the original x ditection inthe fle Set or 1 203 foxyorz 1 i Use the coordir in the file Spread uniformly along the face Set the DS3 axis that corresponds to the original y direction in the file Set or 1 2 or3 for x y or z 2 Use the coordinates in the file Spread uniformly along the face Set the new direction or 1 2 3 for the z velocity component in the file generated by DS2V 3 B The new directions must be consistent with the right handed three dimensional Cartesian axes in DS3V The first item sets the name of the molecule input file for this boundary The setting of the other items in this menu is more demanding than is the case with the other menus The DS2V flow 37 is necessarily in the x y plane so that it is necessary to make provision t
54. s computationally inefficient particularly for supersonic and hypersonic flows This is because a value that is small in comparison with the mean collision time in the stagnation region is extremely small in comparison with the collision time in the outer regions of the flow This problem is avoided by new procedures that make the time step vary with every molecule as well as with every cell To this end separate time variables are associated with every molecule and every cell The procedures keep all these variables concurrent with the overall time variable The overall time variable is advanced in very small time steps but only a small fraction of the molecules are moved and collisions are calculated in only a small fraction of the cells at any one time step The mean collision time is kept for each cell and collisions in a cell appropriate to a time interval of approximately one fifth the local mean collision time are calculated whenever the cell time variable falls one tenth of the local mean collision time behind the overall flow time 6 Similarly a molecule will generally be moved through a time interval of one fifth the mean collision time of its current cell when its time variable falls one tenth this local mean collision time behind the overall flow time However the logic can often be simplified if there is a molecule velocity dependent restriction that limits the distance that a molecule can move in a single step The step set by the loc
55. sed in the procedures for establishing the collision rate This means that the cell geometry should be chosen to minimise the changes in the macroscopic properties across an individual cell A detailed exposition of the method is available in the reference that will be referred to as Bird 1994 However the DS3V program takes advantage of several important developments in DSMC methodology that have occurred since that time Simple implementations of the DSMC method choose the collision partners from any location in the same cell Later implementations including those in Bird 1994 employed fixed sub cells to reduce the spacing of collision partners The condition relating to the cell size relative to the mean free path then applies to the sub cell size rather than to the cell size The most recent programs including the later versions of the DS2V program employ separate cell systems for the sampling of flow properties and collisions The first molecule for a potential collision is chosen at random from a collision cell If the number of molecules in the cell is less than 30 the nearest molecule is chosen as the potential collision partner For larger numbers a transient rectangular background grid is generated and the molecules are indexed to this transient grid The size of the background grid is such there is approximately one simulated molecule within each grid element The transient sub cell in which it lies is determined from the indexing and
56. standard value for the initial number of simulated molecules 1 0000 Number of molecule moves in a mean collision time for I o 5 accuracy optimized case the standard number is 5 a Number of molecule moves in a mean transit time for the 2 accuracy optimized case the standard number is 2 2 H Factor ta change the standard value of the time interval 1 0000 between successive samplings of the flow properties 1 0000 Factor to change the standard value of the time interval 10000 between the successive outputs of the flow properties For unsteady sampling the fraction of the output interval over which a time sample is taken standard is 0 5 0 50000 Retain the default normal indexing that may involve a time penalty Switch to continuous indexing that requires more memory Retain the default basing of relaxation rates on the gas temperature Base temp dependent relaxation rates on the collision temperature Employ the default nearest neighbor collisions Choose the collision partner from anywhere in the collision cell 42 The data file DS3VD has now been generated The program may be run with this data or it can be recycled for corrections and or alterations The Data Set is Complete Review and or Correct the Data 8 DS38V Tutorial Demonstration Case The DS8VD DAT and DS38SD DAT files that are produced when the DS3VZ EXE file is run are for the supersonic flow past a delta wing Not
57. ten The flow and surface property displays are therefore time averaged over time intervals that are small in comparison with the steady flow time and give a realistic picture of the unsteady flow development process Steady flow time averaging builds up a large sample Ensemble averaging will eventually be added but the unsteady sampling option in the current version essentially continues the unsteady phase indefinitely However the short time averaging may be confined to the specified fraction of the output interval ii The Molecule Number History window plots the number of simulated molecules over a time interval immediately preceding the current flow time There is usually an increase in the number of simulated molecules during the unsteady phase of a flow but when a flow has become steady the fluctuations should be characteristic of the statistical fluctuations These have a standard deviation of approximately the 47 inverse square root of the number of simulated molecules The magnitude of the standard deviation is indicated by the length of the double ended yellow arrow at the center of the window The scatter in a steady two dimensional flow shows the expected characteristics With an assumed steady flow should the number of molecules depart by more than about seven standard deviations from the value at the start of the sampling interval the sample is reset Should a flow become periodic rather than steady it should be clear from the b
58. the errors that may be introduced by the incorrect setting of cell size A poor calculation can be made if the total number of simulated molecule is too small This may cause the mean separation of collision partners to exceed the local free path in significant areas of the flowfield but the output enables this to be readily monitored Reference BIRD G A Molecular Gas Dynamics and the Direct Simulation of Gas Flows Oxford University Press 1994 2 The Scope of the DS3V Program 2 1 Geometry and Boundary Conditions The flowfield limits are specified by the minimum and maximum values of the x y and z co ordinates These six boundaries may be stream boundaries planes of symmetry vacuum boundaries or periodic boundaries This basically rectangular flowfield may be modified by the definition of one or more separate surfaces Each of these must be either a closed surface or an open surface that starts and finishes on a boundary of the basic rectangular parallelepiped Each surface is specified by a set of triangular elements in an externally generated surface definition file As with the DS2V program each of the surfaces may be a combination of solid surface triangles and specified flow input triangles The surface definition triangles also form the sampling elements on solid surfaces This means that many triangular elements are generally required even on a simple flat surface The production of this file generally requires a program se
59. then moves straight to the data input dialogs of Section 5 Modify the Existing Data Set The user may choose to modify an existing DS8VD DAT file Should an end of file be encountered when modifying an existing file the program automatically switches to the new file mode The details of the input menu windows are described in Section 6 New Run with Existing Data There must of course be existing DS8VD DAT and DS3SD DAT files and this starts a new run from zero time About the DSMC method This option brings up an information screen on the DS8V program and a Frequently Asked Questions screen on the program can be called from this The program then returns to the Initial Options menu 7 Data Input Dialogs Initial Magnitude of the Computation Flow Computation Region Minimum x coordinate Maximum x coordinate 25 26 Minimum y coordinate Maximum y coordinate Minimum z coordinate Maximum z coordinate These input variables set the limits on the basic rectangular parallelepiped flowfield The effective shape of the flowfield may be changed through the definition of open surfaces and flow input lines that terminate on these boundaries Approximate fraction of the bounding rectangle that is within the flow The surfaces may be such that only a small fraction if the bounding rectangle is within the flow The setting of this item to the approximate fraction assists with the automatic setting o
60. to a surface or a boundary The plane passes through the three points 1 m 0 0000 y1 m 0 0000 21 m 0 0000 x2 m 0 0000 y2 m 0 0000 22 m 0 0000 3 mn 0 0000 y3 im 0 0000 23m 0 0000 The dialog can be closed only if the points are within the flow and the plane does not pass through the origin Number of molecules in each record 10000 4 Number of records in the file 100 4 Stop the calculation when the file is complete The items to be set in this menu are similar to those for the generation of molecule files in DS2V The program should not be stopped until the file is complete and by checking the box the program may be made to close automatically when the file is complete A DSMOF DAT file that is generated by DS3V may be renamed DSMIF DAT and read into a separate DS3V run It cannot be read into a DS2V run 22 New Dual Domain Parallel Run This option is intended for use on dual core processors It brings up the following sub menu Set the Domain Boundary This option is intended for computers with a dual core CPU The flowfield is divided into two domains for parallel computation The second program starts and stops automatically Parallel computation can be tested on a single core processor The bounding parallepiped is divided by a plane normal to the z axis Set the fraction of the 0 50000 bounding parallelepiped in the first lower z zone The two domains are separ
61. triangles as black and those inside the solid surfaces or beyond the flow entry surfaces as white The elements within the flow are coloured according to the distance from a surface of the division in which they lie The colours show the level of the division relative to the surfaces This is used within the logic of the program to determine whether possible surface interactions need be considered when moving the molecules The main purpose of this window is to allow the user to determine whether the elements have been correctly marked as being within or outside the flow Round off error occasionally causes an element near a surface to be incorrectly marked Should an inside element be marked as outside the problem is obvious because the incorrect marking propagates throughout the region An inside element marked as outside is less obvious because it produces an isolated white element surrounded by coloured elements Note that the plane of the plot can be changed to be normal to any of the three axes The plane may then be moved along this axis by setting the division and the element within that division Should an incorrectly marked element be found the program should be stopped A small change in the data items that set the number of divisions and possibly elements generally fixes the problem Any data and geometry files that lead to persistent problems should be reported The next window shows the initial cells Again this plot may be
62. verage mean coll sep m f p MEMME MEENE flowficld samples since 0 000 m yf 0 0 il 000 initialization at time aae were MSEE molecules Moment about a line parallel to the x axis 0 3356 02 N m iaei 5 Past 0 164E 02 seconds A Moment about a line parallel to the y axis 377E ON C Molecule Number History Moment about a ine parallel to the z axis ERM Deon ove suface oi ee gs ETARE roves preneto pone RR gt er Fon RS oo o ofS Led es Vertically o Hamm vE Pause AE for Detailed a ay a E Pause Program for Detailed Be x 1 fi I m a l 0 2 0 4 3 0 3 2 3 4 3 6 3 8 4 0 Pressure Temperature K This main window is visible throughout the running of the calculation It contains graphics areas that show the surface distributions and the flowfield properties The amount of detail in these is limited and subsidiary screens may be selected by the Pause Program for Detailed Plot buttons The windows are largely self explanatory but some comment and clarification is useful for some of the contents i The Flow time is the time in seconds since the surfaces were instantaneously inserted into the uniform stream The flow time advances in steps equal to the set fraction of the smallest value of the sampled mean collision time in any of the cells The flow properties are sampled at set time intervals With the eventual steady flow option until the flow becomes steady the samples are reset each time the restart files are writ
63. w at a density of about five times normal or sea level density A flow with extremely small physical dimensions may therefore be calculated with a 1 1 correspondence between real and simulated molecules The DSMC method may be modified to take account of the size of the molecules and the fluctuations in the simulation have then been shown to have the same physical characteristics as the real fluctuations This has important philosophical consequences in that fluctuations are neglected in the formulation of the Boltzmann equation so that the DSMC solution has a higher degree of physical realism than a generally hypothetical solution of the Boltzmann equation Fluctuations are included in gas theory only at the level of the Fokker Planck equations and are of course disregarded in the Navier Stokes equations The uncoupling of the molecular motion and collisions over small time steps and the division of the flow field into small cells are the key computational assumptions associated with the DSMC method The time step should be much less than the mean collision time and a typical cell dimension should be much less than the local mean free path The cell 5 dimension should also be small compared with the distance over which there is a significant change in the flow properties The DSMC method uses the cell system only for the sampling of the macroscopic properties and for the selection of possible collision partners although the sampled density is u
64. whole surface comes to a uniform temperature with no net heat transsfer to the surface as a whole Emissivity This need be set only for adiabatic surfaces so that thermal radiation is taken into account in calculating the temperature for zero heat transfer Thermal Capacitance This should be set only for the infinite conductivity case and is the heat reqired to raise the temperature of the body defined by the surface surface through one degree Note that this option should be used only in the context of nanoparticles and a file of the nanoparticle temperature history is produced In Plane Velocity in diretion 1 The surface moves in its own plane in a direction normal to the x axis or if the triangle is normal to the x axis in the y direction This has effect only on the molecules are reflected diffusely In Plane Velocity in diretion 2 The surface moves in its own plane in a direction normal to direction 1 This has effect only on the molecules are reflected diffusely 15 Gas Surface Interaction for this Group for whole gas with full accommodation to the surface temperature CLL reflection model with normal energy accommodation coefficient 0 0000 and tangential momentun 0 0000 accommodation coefficient Rotational accommodation coeff 1 0000 Vibrational accommodation coeff 1 0000 Fraction of specular reflection 0 0000 Fraction adsorbed at surface 0 0000 Outgas mass flux 0 0000 kg sq m s
65. zero Although there can be only one DSMIF DAT file the molecules from this can be translated and rotated to multiple locations The following menu is repeated for instance of molecule entry Molecule Entry Location and Direction Copy Number 1 of File DSMIF DAT IT Enter the molecules with a rotation of the axes Set the point relative to the file axes about which the rotations occur x im 0 0000 y fm 0 0000 z m 0 0000 Set the anti clockwise looking towards the origin from the positive side rotations about the three file molecule axes in the right handed system x right y up z out from paper x deg 0 0000 y deal 0 0000 2 deg 0 0000 Enter the molecules with a translation of the molecules through mJ E yim 0 0000 2 m 0 0000 As with the file entry triangles the following menu comes up during the early stages of the run Molecule Input File Fie DSMIF1 DAT The file must exist in the directory from which DS3 is being run This must be a manually renamed DSMOF DAT file that was produced by DS2 or DS3V and the following parameters that were specified for the generation of that file must be re entered here Number of molecules in each record 10000 E Number of records in the file 100 THE SET OF MOLECULAR SPECIES MUST BE IDENTICAL IN BOTH PROGRAMS 4 file from the flow plane of a 2D case with DS2 can only be a boundary entry file A file generated from a line in DS2 can only be

The DS3V Program User's Guide

Contents

Download Pdf Manuals

Related Search

Related Contents