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MANUAL DE SHYNE (GiD+)

1. a Hull l X Starboard Flotation Ex 2 22 Assign Entities Draw Unassign E Lose DO Da D Figure 44 The next operation to be performed is to introduce the problem data This is done with the Problem Data submenu within the Data menu We must fill in the information required in the menu boxes Problem Data Length 121 92 Beam 16 255 Draft 6 501 Speed kts 12 Accept data Close Figure 45 9 Another very important step is to define the direction of the normal vectors of each surface check these directions we select Draw normals gt Surface within the Utilities menu The normal limes of the surface will be drawn and thus we will be able to check their orientation They should point to the inside of the hull for the hull surfaces and they have to point down for the free surface If any of the lines do not fulfil this condition normally it is a random condition we have to swap its direction To swap it the option SeepSeme situated in the menu at the right hand side of the screen must be used PES cea nee 23 ETISN CFD tutorial ew filma Leo POO M GlYors e ila 4 au T ener mouse wheel momi deser pam Figure 46 With this option activated we will change the
2. DEN ERI E A E birra E i A UU O UU UU er er UU UU UU UU UU UU E E 0 SD IE E E ya O O I A DI DOD Ra na E TATTO mM EIS o nae A N O O CR I n N 0 A A gg E A E 0 pa S AAA I T 1 AAA m Em Do ee pt TEN AAA me ee H SY A SH oO Se ttt SF a E dD N
3. Propnocnzi ui Da MVolumas F Sudaces F Cut fs alphabesc oder E aS GurloceSet x j 5 EuraceSat 2 5 corel Pick LEFTMOUSE to desplace view ESC to quil Fick LEFTIMOLISE to desplace view ESC to quit Figure 124 Once the calculation is finished we have to get into the postprocess context of GiD This can be done simply when requested after the calculations or with the F es Postprocess command We have to set certain values for a correct interpretation of the results View Style and View Results menus have to be active First we have to select the type of view Within view menu we select Contour Fill With ETS N_CFD we have the possibility to visualize different results in GiD Speed field can be visualized in both the hull and the free surface Both pressure coefficients on the hull and the wave pattern on the free surface can also be visualized In order to select one of the visualizations we will use the View Results menu Visualization of the Pressure Coefficient 57 ETISN CFD tutorial Pressure Coefficient Cp This coefficient is a non dimensional way of representing the pressure 2 Cp 1 In order to visualize the pressure coefficient function on the hull we have to select it on the View Results menu figure 125 left hand side After selecting it we click The pressure coefficient distribution appears only on the hull since it makes no sense to draw this pres
4. olumes Signal Distance Draw normals a Repair Pe Collapse Ld ta P d LIncollapse E Figure 13 We encircle the area containing the elements that we want to identify In this case we will choose a small zone that includes part of the original line and the duplicated one The following box should appear Figure 11 List Entities HMIUBBLINE 52 Layer HigherEnt 1 Cond o Mo meshing information End Pret 8 Mum Initial Pnt 14 Points Enots Aura Points 38 Degree 1 Mon rational 1 251776 0 000000 1 050000 1 254888 n 003423 1 050000 Aarena er AA o IR non ono Use and Shift Tab Figure 12 Where Num identifies the line We have to also look at HigherEnt which reports the number of patches that lie on this line In this case there is only one We can therefore deduce that line 52 is the one bounding the stern panel If we take the next line copy 53 using Next we see that no patch lies on it Lets create the line joining line 53 with the lower one To do this the best method is to convert both lines into a polyline by pressing the polyline button E We select with the mouse the lower line and with the command ne we introduce the number corresponding to the upper one 53 If a message appears saying not correct polyline the probable cause is that when doing the copy both lines were not in contact at their ends This happen
5. be 5 53 ETISN CFD tutorial Where V and g are dimensionally coherent The wavelength for this case study is 10 86 m Since our free surface is 48m long we see that we can house 48 10 86 4 42 complete waves within our domain Taking into account that the representation of a wave requires approximately 15 panels we see that we will need at least 67 panels in the lengthwise direction For the transverse direction we choose 30 panels so that the panels have an aspect ratio close to 1 The free surface will have a layout similar to figure 115 SU OR O IR E E E ID A ENI DI I e ge III I ATI TTI PS i I O SO EE SD O SO UO I O E OS I MH TA a P T i i I E SI I 8 E II I3 TE 88 a a lt USD SE DS O E O A I O I DA DS a pe a aS A DE E O DI E OD E Ag PO gy get A I pg ME ag oS E y I I E I n I SA pg O UU O O Re N pg N N Es
6. O E E DN E HH SEA IS SI HA VS AO SS I DI EEE aa EEE EEE EEE EEE EEE EEE A E ES I SR I ES a a AAA A PAE Figure 115 There are no strict rules for the hull mesh and the appendages We will mesh with a density sufficient to be sensitive to the form and the details We will try to ensure that the forward third has a denser grid than the rest We will also pay greater attention to the definition of the leading edges of the different appendages We use the command Meshing Structured Concentrate elements to get this We must select the edges of the side where we want a greater density We select a weight value that has to be fine tuned by trial and error In figure 22 we see the keel after selecting the top and bottom edges as the direction for the longitudinal variation of the panel density Next a box will appear to set the weight for the mesh in that direction Concentrate elements start Weight End Weight 0 0 04 Ok Cancel Figure
7. 2 fe Yow va Merwg X 8 e 2 8 2 D m E ANNO m 2 ki 4 Pa Erter sere othe paja EI eoc OF lowing E EL IN Hemos ae aote ape Se RAR uv Figure 9 We are going to apply the previously described method to this surface and more precisely to the two left segments Because it is used by a neighbouring panel we have to duplicate the upper segment For this we use the Copy command Qus Metro Case ti lc Efes FI pg GRFC 21 6 71 Entities type NT NES Trensiermation First pont x 00 Hum oo w ua Fs lo Pra ho Second point Ek y Second paint Hum x Mens fo ij F Dupi j plicato antimo Do extrude No a Do i r F A r 7 hs x X 2 Multiple copian h gt a gt en Presa Finish whan Saler Cancel n gt in i Figure 10 Once the line has been chosen in red press Finish Since the two lines are superimposed we will not notice any change Before joining it the new line has to be identified by the function List ETISN CFD tutorial DITE Data Meshing Calculate Help Ctrl u e Lawers trl l is Ereferences Ctrl Calculator Graphical Status Mass Copy C trl c Move Ctrl Renumber Surfaces
8. 4 0 25 View No Result factor Results j Component A Figure 52 b 4 First we have to select the type of view Within view menu we activate Contour Fill Analysis TIME ANALYSIS A Steps o F a pl 0 25 View No Result faci Display Vectors Contour Fill Results Contour Lines Component Sh Min Mex Apply Close Figure 53 Within the Results box pressing x we can see all the variables that can be visualised View Results Analysis TIME ANALYSIS A Steps ps 4 b 0 25 View Contour Fill factor Results Presure Caef Component Presure Wave Pattern LL Figure 54 Visualisation of Pressure Coefficient Pressure Coefficient Cp This coefficient is a non dimensional way of representing the pressure on the hull V 2 Cp 1 E V 00 26 ETISN CFD tutorial In order to visualise the pressure coefficient function on the hull we have to select it in the previous menu After selecting it we press Apply Pressure coefficient distribution appears only on the hull since it makes no sense to draw this pressure coefficient on the free surface where the pressure is atmospheric For a better visualisation we will deactivate the layer that contains the free surface We use the Select amp Display Style menu 5 SurfaceSet 1 corresponds to the hull mesh and S SurfaceSet 2 to the free surface mesh By deactivati
9. Figure 93 44 ETISN CFD tutorial LONGITUDINAL CUT AT 1 5508B Figure 94 We have put these graphs in a non dimensional format in the same way that we did for the S60 example We refer to the explanation of that example to fully understand these longitudinal cut graphs 45 ETISN CFD tutorial Tutorial Example 3 Class Yacht This example is aimed at illustrating an alternative form to proceed with the hull mesh generation In previous examples the hull is defined by lines which bound the surface patches forming the hull in the grid generator GiD An alternative method would be to have the case study s geometry as surface patches directly in IGES format This is the possibility we are now going to study in more detail We will use an America s Cup type hull This geometry is defined by means of an IGES file This file has been generated with a commercial program for ship design It was later exported as NURBS surfaces in a 3D IGES format Figure 95 Preprocess Once we have the geometric form in a GID compatible format we can start with the preprocess phase following the same steps as in the previous cases First we import the IGES file to GiD using the commands Files mport ges 10 0 105 46 ETISN CFD tutorial R B Ld eiee 5 __ Figure 96 The first thing to do will be to eliminate the port section of the
10. POUT CNIC A 1 Coordinator Antonio Souto Iglesias ETSIN UPM Model Basin Research Group CEHINAV ETISN CFD tutorial Contents INTRODUC TON conri 3 USING TMS Ma labate 3 FOUNDATION pu 4 TMS Dane Melodia riada 4 nn o 5 EXAMPLE 1 LINER WITHOUT BULBOUS BOW 560 5 PresproGessiN 5 MESHING WEN GID dta E 6 Process phases ETSIN Criada dudas 22 POSI OCA 25 Visualisation of Pressure 1 1 26 Visualisation of the wave pattern mmn 27 Velocities field visualisation 2 28 Sa 30 TUTORIAL EXAMPLE 2 FISHING VESSEL WITH BULBOUS 32 PIGDI OCGSS Lope E 32 Grid generation WIE Ib a 32 Calculation Process EV SIN CEDE a o a n 38 A amica aa 39 Pressure Coefficients Visualization 41 Wave pattern visualization 41 VelocitiviFiela VIisgalizatliona ilaele 42 Longitudinal icu 43 TUTORIAL EXAMPLE 3 IACC CLASS 46 o 46 Calculation process ETSIN na 56 POSEDFOGESS e 57 Visua
11. 116 Before proceeding with the hull meshing forward and stern patches must be modified since they are three sided surfaces We have chosen the division points shown in figure 117 54 ETISN CFD tutorial Figure 117 Next we show the different elements of the hull with the proposed mesh pu animalito mE SS ee eee 5 RR 118 if EE simona nti HUH 114545559 i m d ATI a me 31 1 3 OTT ase Ta Figure 119 Example file IACC_3 gid 55 ETISN CFD tutorial PTT ft E LELEEEEEET TE ELT a o Figure 120 Once we have a mesh we have to make some verifications prior to the calculation First we have to ensure that the normal vector s orientation points towards the inside of the hull We must also verify that the free surface s normal vector points downwards For the visualization of the normal vectors the command Utilities Draw Normals Surfaces is used In order to change the orientation we will right click and then Contextual Swap Some Figure 121 Calculation process ETSIN_CFD The last step before calculating is to define inside GID context the type of problem we are solving
12. HULL 2 HULL 3 HULL 4 Figure 137 In these first representations we can see that the zones with greater gradients are in the forward third of the hull We zoom in on these regions to further analyze them and obtain the corresponding conclusions 66 ETISN CFD tutorial 3 HULL 1 i A HULL 3 HULL 4 Figure 138 The first visual impression is used to notice the bad hydrodynamic properties of the bulbous bow of design 1 Considerable gradients appear for the speed field leading to higher deformations on the free surface and therefore greater wave resistance Hulls 2 and 4 seem to have little to present in this phase when comparing them with hulls 1 and 3 due to the obvious shape difference between them For a more precise analysis it is essential to use the free surface longitudinal cuts corresponding to these designs In order to better compare the different cases we use one graph for the four corresponding longitudinal cuts of the wave systems The usual thing is to represent these cuts at several specific distances from the centre line These distances are 0 5665 B 1 0587 B and 1 5508 B as has already been mentioned 67 ETISN CFD tutorial 11 Kts 0 5665B Hull 1 Hull 2 Hull 3 Hull 4 Figure 139 The first cut the one closest to the hull is the one that provides the best information for the amplitude of the
13. Rojas L Diez a os de I D en el Canal de la ETSIN XXXVI Sesiones T cnicas de Ingenier a Naval Nacional Cartagena Noviembre 1999 Asociaci n de Ingenieros Navales y Oce nicos de Espa a 8 P rez Rojas L Zamora R Souto A Abad R La contribuci n hidrodin mica del Canal de la ETSIN al Proyecto Copa Am rica Espa ol XXXVIII Sesiones T cnicas de Ingenier a Naval Barcelona Noviembre 2000 9 Souto A Nuevas herrramientas de dise o de formas de buques basadas en c digos de flujo potencial Tesis Doctoral Departamento de Arquitectura y Construcci n Navales 5 1 Navales U P Madrid 2001 70
14. both the hull and the free surface We get this with the Se ect amp Display Style window activating and deactivating 5 Surfaceset 1 and S Surface 2 corresponding to the hull and free surface If we select S Surface 1 and press Off only the free surface will remain visible We must not forget to click the 4pp y button after any change to update all the colour scales 42 ETISN CFD tutorial EN Figure 90 2OO0G amp GINo IR A sis MEER S gt gt IA X we Cog Contour Fill of Velocities velocities Figure 91 Longitudinal Cuts The results files kept in the folder resu ts within the folder casename gid where the mesh used for the calculation was saved This folder also contains the ascii files long cut 1 5508B dat long cut 1 0587B dat and long cut 0 5665B dat Inside these files are the curves corresponding to the longitudinal sections of the free surface at 0 5665B 1 0587B and 1 5508B With these graphs we can visualise the height of the generated waves and their other characteristics This is one of the most useful tools when optimising the ship s hull as will be discussed later They other characteristics can easily be imported from Excel for instance They are represented in the following figures B ship s beam 43 ETISN CFD tutorial LONGITUDINAL CUT AT 0 5665B Figure 92 LONGITUDINAL CUT AT 1 0587B
15. dat and long cut 0 5665B dat Inside these files are the curves corresponding to the longitudinal sections of the free surface at 0 5665B 1 0587B and 1 5508B With these graphs we can visualise the height of the generated waves and their other characteristics This is one of the most useful tools when optimising the ship s hull as we will discuss later They can easily be imported to Excel for instance They are represented in the following figures 14 B ship s beam 61 ETISN CFD tutorial 0 006 0 004 0 002 0 002 0 004 0 006 LONGITUDINAL CUT AT 0 5665B Figure 132 0 005 0 004 0 003 0 002 0 001 0 001 0 002 0 003 0 004 0 005 0 006 LONGITUDINAL CUT AT 1 0587B Figure 133 62 ETISN CFD tutorial LONGITUDINAL CUT AT 1 5508B 0 005 0 004 0 003 0 002 0 001 0 5 1 1 5 2 25 3 3 5 0 001 0 002 0 003 0 004 0 005 0 006 Figure 134 These are non dimensional graphs Along the X axis the free surface X component is represented It is in its non dimensional form with LBP as the reference length The orientation is the common one Therefore 0 corresponds to the ship s bow and x 1 corresponds to the stern Along the vertical axis we have the free surface height in non dimensional form taking again LBP as the reference length So if we want to know the
16. file called casename flavia res containing the data for the visualization of the results Directory eyusers clopez ETSIN CFD pesquero bulbo MOTOP bulbol gid 4 z i 7 resultados MOTOP_bulbol flavia bon E bulbo1 msh bulbo1 1 dat E bulbol flavia res bulbol1 out bulbal cnd bulbol geo bulbol prb E bulbol dat bulbol lin bulbol uni c MEN MCN MM a i File name bulbol flavia res Open Files of type Cancel Figure 83 Once opened we can visualize the different results offered by the code 6214 A e A jew No Result factor Results j Component A Apply Close Select amp Display Style Volumes M Surfaces M Cuts alphabetic order S SurfaceSet 1 a l 5 SurfaceSet 2 rri ed 6 Rename a O nr Delete 3 3 Style Body Render Smooth O Culling None Conditions None To back Send to Close IS FED E es t sd Figure 84 To start we select Contour Fill the window indicated in the following figure View Results Analysis TIME ANALYSIS i o zl 0 25 View No Result taq Display Vectors Lie Contour Fill Results wen Contour Lines Component amp h Min Figure 8
17. hull including the rudder the keel and the bulb The following step stems from the necessity of having no surfaces that get into the hull or fit the definition of dry submerged areas Due to this we need to trim the hull in its intersection with the keel and the rudder and trim both rudder and keel in their intersection with the hull This operation is performed with the option Geometry Create ntersection Multiples Surfaces Once we have selected the surfaces to trim we should press Esc This operation performed on the surfaces of the keel and the bulb should give rise to figure 96 We see that after creating the intersections there are some new inner surfaces that we have to eliminate because they are not going to form part of our final geometric shape The previous example of the bulb and the keel would finally end up as shown in figure 99 Figure 97 Figure 98 Example file IACC 1 gid 47 ETISN CFD tutorial Figure 99 The surface trimming operation has to be performed between the rudder and the hull as well An important recommendation is to put each element in a specific layer in order to work with them more efficiently A good proposal is the one shown in figure 100 On Off Freeze Un Name B E Bulb Rudder Free_Surface Layer To use y ee_Surtas a Sel New Delete Rename al To back Send Tol _ Clos Figure 100 Another important step in the preprocess is t
18. in order to improve the smoothness of that region Another interesting aspect is that patches that are flat or almost flat like the ones on the keel and some of the sternpost can be meshed with a lower density than those of the hull 36 So SO SS ETISN CFD tutorial This is one of the many possible meshes to Example file MOTOP gid Figure 77 make the calculations that we describe next 37 ETISN CFD tutorial Calculation Process ETSIN_CFD The first thing to do as previously discussed is to define the kind of problem Problem Type within the Data menu In our case we will select 7S N CFD The following operation will be to assign conditions to the different elements of the mesh basically hull and free surface Within the Data menu the Conditions submenu is opened and the Hull Condition is assigned to the hull condition Hu if we show conditions with the Draw command The Starboard Flotation condition is assigned to the free surface condition Starboard Flotation if we show conditions with the Draw command Starboard Flotation aj Value 0 0 Assign Entities Drew Unassign Ch This Starboard Flotation Colors All conditions xclude local axes Field s value _ Only local axes Field s color Include local axes Figure 78 X rboard Flotation C a di E c A 1127 72M Figure 79 Another form to fill within the Da
19. is so important to keep the scales consistent for the different case studies for the same graph types We have selected a scale ranging between 2 and 1 for the pressure coefficient This can lead to the non representation of some small regions for instance the rudder s lowest edge This is the part of the ship where the gradients are greatest If we do not set this limit the entire colour range will be concentrated on the rudder and the rest of the hull will appear monochromatic For the range definition we use the icon to define the maximum and the icon to define the minimum For 4 range selection to reset the range when switching visualization we must use this icon 78 Wave pattern visualization Another feasible visualization is the wave pattern on the free surface Again in the View Results menu we select the visualization that we want with this function E this case the wave pattern To only visualize the free surface we enter into the Se ect amp Display Style menu In this menu when deactivating S Surface Seti only the free surface remains visible Do not forget to click App y after every change We must also set maximum and minimum visualization values for the incoming comparisons Figure 127 Velocity field visualization The last visualization option is the speed field This magnitude gives a lot of information both for the hull and for the free surface The flow is potential and therefore t
20. line with the centre line skipping the transom 33 ETISN CFD tutorial For the creation of the line that skips the transom we will work in the layer where we have previously copied the waterline In this layer we will create a line that joins the water line to the centre line Figure 68 Figure 69 The following step will be to split both lines and to join them at the intersection point To do this we use the divide command as already explained We will also eliminate the unnecessary transom line Figure 70 In order to obtain a smooth junction with the centre line we will use the command Geometry gt Create gt Arc tangents This command asks for the radius of the connecting arc We will continue trying until obtaining a smooth join In this case a value of 2 was used Utilities Data Meshing Calculate Help Arc tangents NURBS line NURBS surface Volume Contact Intersection Object Volume boolean gt 4 4 4 gt 4 Figure 71 34 ETISN CFD tutorial The final result has to be similar to what is shown in the following figure Figure 72 In order to define the panel number we will calculate the reference wavelength 1 2xv g With this value we can calculate the number of panels in the longitudinal direction In order to decide the adequate number of panels in the transverse direction we will have to take into account the RAM memory of our computer We set the pan
21. mesh in order to appreciate the details of some of the complicated regions A LI Sata ier ew a aay Sat Se BEE EE EET SSSSSSSS I CEELEEREEA P TALI POSI FU COAT AAA roi Wa EEE Ag I PELS SASHA o ECT HE ERAI ICH Too tte Poe Er HERA 6 6 4 SA RI 2 EPA NS DONOSO OOO OSORNO Epa e PFA IPSS SSS SSS SS E EY ST SY SND ma Figure 74 gt ls NS 77 4 T o CIT XA ANAL dal f N gt A ed m Pl o 7 n A LAN md I E ai QY ATTI LT Oo EE AS AAA Co tt AL d 1 TS cd MS A i AA T ml md T _ NA A e Y SGL ET A AEE ER EE EE Wen AAA AAA 7 2H NO To SAY AAA LT AAA YAA NAAA GA 1 E 2 2 77 AAA ALA um Sl A _j NOOO th di Figure 75 In Figures 74 and 75 we observe the intersection of the keel with the hull 2 die L _ aa WV l Jil Y a i MNA ie Il 4 4 Figure 76 In the stern region special attention must be given to the sternpost The situation is similar to the bulbous bow with neighbouring patches bounded by different length edges We proceed in the same way as in the bulbous bow In this case to refine the mesh we have sharpened the sternpost s upper patch
22. the calculation power of our equipment As a final consideration it is important to point out that in order to not exceed the memory limit in the calculations we should reduce the number of panels on the hull We can vary the size of the free surface and even experiment with the mesh density in the transverse direction of the free surface But we cannot reduce the 15 panels per wavelength for the mesh in the direction of the free surface For 560 case we have used 170X23 panels in the free surface It looks like this Figure 39 Figure 39 Special Mesh in the stern region gt Example file S60_4 gid 20 ETISN CFD tutorial As we have already commented ETSIN CFD calculates the potential flow around symmetrical hulls and since this is a potential calculation there is a difference between the calculation and the experimental results in the stern region This is because in these zones viscous effects generally dominate at low speeds and they are not considered here This fact means that the results diverge strongly in the stern region for transom stern ships For dry transom stern ships a specific module of ETSIN CFD exists in which a different mesh type is used for the free surface and specific conditions are applied for the stern region In our case we are dealing with ships without transom stern or with a wet transom stern The latter type and some others with thicker lines in the stern region can induce bad results unles
23. 110 In this case we have to separate the forward region of the bulb in order to obtain a better definition These types of decisions are usually better made after designing a preliminary mesh and trying out different possibilities and then checking whether the grid is satisfactory 51 ETISN CFD tutorial p A pro Figure 110 After some tests the most reasonable option seems to be to divide the bulb in two halves upper and lower We take a horizontal plane passing through the trailing edge at the bulb stern as the section plane We write down the coordinates of the points that define the straight line of the trailing edge and then from this segment we define the edges of the patch that will be used for the section Figure 111 We will start by dividing the bulb surface with this section plane Now our experience says that it is usually necessary to create zones in the bow and stern with smaller surface patches The final form should be similar to figure 112 C me 7 Figure 112 The forward region is formed by 3 sided patches We have to split some of the edge segments to get 4 sided patches as shown in figure 113 52 ETISN CFD tutorial Figure 113 The following step will be to study the viability of the stern patches of the b
24. 5 40 ETISN CFD tutorial Pressure Coefficients Visualization We select the corresponding option in the View Results window and deactivate the free surface layer in the Se ect amp Display Style window Volumes Suraces F Cuts An UT TMEANALYSIS ANALYSIS alphabetic order mp pu 4 S 2 View i Contour Fill amp E Panone factor O z Dette Results Presure Coef 0 Sye Component Render Smooth E Culling None Conditons None To bacs Send to Close Figure 86 The pressure coefficients graph should look like this ROO G KGIYO ela A ta de E er Presse FL 0 85754 0 71507 Bi 0 57261 7 043015 9 28788 0 14522 000584 84397 gt D TH h 042487 x Contour Fill af Presure Coel Figure 87 Wave pattern visualization We follow the same steps by choosing Wave pattern deactivating the hull layer and activating the free surface layer 41 ETISN CFD tutorial 4 lw d Co O gt IS LED Ez es Gn m E a Figure 88 Velocity Field Visualization We choose Velocities in Results box and Velocities in the Component combo box With the hull and free surface we get the following graph 6214 w Se x S LEE es 4 Figure 89 As with the S60 we separate the visualization to see
25. BS line to ease its handling Moreover each polyline can only be used to bound a single patch which produces several small shortcomings We use the Convert to NURBS command to perform this conversion Utilities Data Meshing Calculate Help i Miew geometry lt D Create Delete Move Paint cc Explode polyline Divide Join lines end points Swap arc Edit polyline Edit NURBS line Edit NURBS surface Hole NURBS surface Edit SurfMesh Convert to NURBS gt Simplify NURBS Surface ETISN CFD tutorial Figure 6 Error Il A few panels have not been created Causes it might be that the surface is defined by 5 sides instead of 4 As in error there are incomplete lines A gap between 2 lines preventing the surface from being created could also exist Solution using a polyline and a NURBS Line we create a single line from the 2 segments as in error 1 If the problem is due to the existence of a gap we have to join the line ends Since there are 4 corners for each panel and we do not know which one is the bad one we have to test them one by one In the example we are studying the problem is due to the existence of divided lines We can also mention the fact that even after joining the lines or creating the polyline and transforming it into NURBS we should finally be able to create the surface but this is not always the case A few nodes can be misplaced and thus it is never superf
26. Since this is a generic case of symmetrical flow without transom without drift and list we will indicate Data problem Type ETSIN CFD Data Meshing Calculate Help ansyshb ETSIN_CFO Problem type War Interval Examples Local axes tdyn4ll Transform Internet Retrieve Others Figure 122 The following operation will be to assign conditions to the different elements of the mesh basically hull and free surface Within the Data menu the Conditions submenu is opened and the Hull Condition is assigned to the hull and appendages condition 1 if we show conditions with the Draw 56 ETISN CFD tutorial command The Free Surface condition is assigned to the free surface condition 2 if we show conditions with the Draw command Another form that must be filled in within the Data menu will be the one corresponding to the principal dimensions of the problem Problem Data figure 123 Accept data Close Figure 123 We will regenerate the mesh Generate option in the Meshing menu so that the new mesh reflects the changes done so far In this case we have a 3721 panel mesh We are now ready to launch the calculation process Within the Ca culate menu we open the Calculate Window submenu and press Start to begin the calculation Postprocess 1 Gio A 5 di
27. at makes the panel as quadrangular as possible This way the mesh will have a more uniform look In the following figure the segment where the line was divided is indicated in red Figure 18 Doing the same thing with the stern one we finish the process of covering the hull with surfaces 12 ETISN CFD tutorial Figure 19 Let us now proceed with the meshing of the hull In the Meshing menu go to Structured gt Surface then select the whole hull Calculate Help Quadratic elements Assign unstruct sizes Structured Lines v Mesh criteria Surfaces Elementtype Volumes Boundaries Concentrate elements A Reset mesh data Sy Cancel mesh aN Generate Ctrl g Mesh view Ctri m Mesh quality Edit mesh Figure 20 Press ESC and a window should appear to request a number of divisions for the surface edges We will try first with 5 We press OK and then select the line for these 5 divisions For example we select the edge of any surface and see what happens Here we have selected one of the waterlines in the central section of the hull Since the mesh has to be continuous we can see the horizontal lines of the surface get automatically highlighted A surface will have the same number of cells in the horizontal vertical direction as its neighbour Although this is the usual way of creating the mesh it has to be modified in some instances as will be seen
28. ation is linear we can combine its particular solutions to obtain a new one that is still a valid solution The elementary solutions are sources located on the boundaries discretized in panels whose intensities are adjusted in order to have the flow verify the boundary conditions impenetrability on the hull and the free surface being a streamline under atmospheric pressure The potential speed field produced by a flat panel on which a uniform distribution of sources has been placed is known Thus if one considers any panel and the knowledge of the intensity of the sources the speed induced at any point in space can be directly calculated In the developed method both hull and free surface of the water near the boat are represented as two surfaces formed by panels One assumes that for each panel there is a uniform distribution of sources whose intensities are unknown Moreover a control point is located on each panel s centroid The control points that lay on the hull have to fulfil one rule the speed vector has to be tangent to the panel The speed vectors located on the free surface of the water must fulfil two conditions The cinematic condition that indicates that the speed of the liquid must be tangent to the free Surface The dynamic condition that requires the pressure to be equal to the atmospheric pressure We can combine these two equations in a single equation The problem consists in figuring out the value of the sources on
29. cal laws that govern the behaviour of fluids water inside a glass as well as gases at supersonic speeds can be summarized in relationships between the variation of fluid speed and the causes that create this variation Among these causes are the pressure differences the viscosity between particles and finally the action of gravity In the problems of naval hydrodynamics that this software can deal with viscosity has a residual importance and will not be considered Wave generation is basically a non viscous phenomenon albeit very difficult to solve Since the problem is perfectly well known one could think it is easy to solve but this is far from being true Except in very simple cases those without free surface an analytical solution does not exist and it is necessary to resort to approximate numerical methods The tricky part is that an infinite number of numerical methods exist A great family is formed by the ones that distribute the control points across the whole volume of the fluid and later impose on these points discrete forms of the differential equations that govern the phenomenon The method used in this report belongs to the other great family characterised by having control points on the boundary surfaces of the domain Particular solutions are used and are combined to obtain a global flow that fulfils the boundary conditions of the problem governed by the Laplace equation which refers to the velocity potential As the Laplace equ
30. e ETISN CFD tutorial We use Rhino as the CAD software but any other package capable of exporting IGES files that will be used by is just as good In order to illustrate the handling we have used a Series 60 model available the examples 560 0 3DM The reason for using a CAD software before generating the mesh is to define the hull with enough lines for GID to be able to generate surfaces from them These surfaces will define the submerged part of the hull and once processed will be the basis for the mesh generated with GID Meshing with GiD Using the first step is to import the IGES file of the form GES read and to save it save as as a GID file Project UNNAMED MES View Geometry Utilities Data Meshing Calculate Help New Ctrl x Ctri n qs 2144 Da ua o Save Ctrl s Save as Ctrl x Ctrl s Read Batch file DXF Read Ctrl d Insert nenme the Paraenlid read Figure 2 In order to avoid problems that could occur while using GID it is safer to copy all the imported lines to an auxiliary layer The use of several layers simplifies the meshing work and some cases you may have to erase a few empty layers created when reading the IGES file With the imported lines we can generate surfaces using the command Geometry Create NURBS Surface Automatic We want them to be exactly defined by 4 lines Ge Des Meshing Calculate Help View geom
31. e is LBP aft of the stern Therefore the x length of the free surface mesh is approximately 2 25 times LBP In the 17 ETISN CFD tutorial transverse direction the line should be around 1 away from the centre line Remember that all this data is a general estimation and only experience with the peculiarities of each case can suggest appropriate dimensions We will now create a line NI defined with the command line from the point 0 0 0 that passes through the vertexes of the free surface which we have calculated with the previous measures This way we are defining the starboard part of the free surface line In this particular case the vertexes are 0 0 0 121 0 0 121 90 0 151 90 0 151 0 0 50 0 0 po Figure 32 With the command Geometry gt Edit gt Divide gt Near point we split the horizontal line that goes inside the forward waterline end Eps Utilities Data Meshing Calculate Help 0 21 21 View geometry Create Delete Edit Move Point Explode polyline d Num Divisions Join lines end points Swap arc Edit polyline Edit NURBS line Parameter Polylines Surfaces gt Edit NURBS surface Hole NURBS surface Edit SurfMesh Convertto NURBS gt Simplify NURBS Figure 33 Once we have selected the line that we want to divide red coloured in figure 37 we then indicate the division point by right clicking the mouse the F gure 34 context men
32. e to be discarded in our calculations This includes the dry part of the hull and the auxiliary free surface patch that we created in the previous step to define the water line Also we have to verify that the lines defining the free surface patch are in the layer corresponding to the free surface After this process the layout should be similar to the one of figure 103 Figure 103 We will reconstruct the centerline of the free surface patch so that it contains the water line we have just generated It might eventually be necessary to copy this line into the free surface layer as it could have ended up in the hull layer after this process In order to avoid stability problems in the stern region calculations the connection between the waterline and the hull must be as smooth as possible We do this by drawing a straight line from the stern part of the waterline to the centerline Later on we smooth the junction with an arc using the command Geometry Create Arc Tangents radio 5 A possible solution is the one shown in figure 104 49 ETISN CFD tutorial Figure 104 With the obtained waterline we trim the centerline to form the inner edge of the free surface patch With this line and the other three segments we can define our final surface patch Figure 105 In order to be able to mesh a surface patch the patch has to be a four sided one If the patch is bounded by three lines we have to split one of the lines to get the four
33. each one of the panels so that the conditions are fulfilled for all the control points As opposed to the methods of discrete volumes finite elements and finite differences the panel method has the great advantage of allowing us to obtain more accurate results for the speed field on the hull surface using a much smaller number of control points This allows us to do a quick and relatively precise calculation of the free surface deformation The optimisation of forward resistance due to wave generation requires the knowledge of the pressure distribution along the hull and the deformation of the free surface This is why the panel method is a good choice in engineering ETISN CFD tutorial Technical Notes It is very important for a correct operation of the program thatthe decimal separator symbol is assigned to the point and not to the comma The default option in Windows 15 the comma In order to change this option it is necessary to modify the settings in the regional configuration menu of the Control Panel Example 1 Liner without bulbous bow S60 In this and the following two chapters 3 examples are presented whose guided execution will let us explore the system s possibilities The first example deals with a ship without bulbous bow The second consists in creating the mesh of a ship s bulbous bow with the corresponding particularities due to the existence of the bulbous bow The third example corresponds to a racing yacht wi
34. el number as to not surpass the RAM capabilities In the hull mesh the bulbous bow region has to be carefully considered In this region a dense and uniform grid has to be created like the rest of the forward region of the hull 3 co ome oe Td e A 5 5 Ca Ww O A TA 4 NATIA CLA CN Y E Lan a I IATA TS EEE T SA lt Figure 73 Considering the bulbous bow an especially interesting area to mesh is the one inside the circle in figure 73 In this region the boundary of the two neighbouring panels is not the same Each one is bounded by different lines The problem is that the upper patch of the bulbous bow does not contain the same number of divisions in the vertical direction because it is shorter than the one to the left regarding the vertical direction The problem arises when selecting the row of upper patches to mesh A solution to avoid these situations is to consider these lines during the previous modelling with the CAD program We can select the section that starts the bulbous bow as the one corresponding to the forward point of the waterline Since this is not always feasible we explain the solution used in this example Through trial and error we mesh the bulb patch until there is sufficient continuity with the neighbouring patches We are referring to the number of divisions per unit length 35 ETISN CFD tutorial We will now show different views of the
35. etry bela P 1 83 Point Delete Line Arc Arc tangents HURBS line Polline Edit M olurrie Contact ed Figure 3 We notice that all 4 sided surfaces have not been created Occasionally the line system is modified during the importing process of the IGES file For instance once imported a single segment could be split into two Thus making it impossible to create surfaces from such lines To solve this problem you can use Geometry Edit J oin Lines end Points and select the 2 lines to be joined rebuild a line we create a polyline from the fragments geometry gt Create Polyline Possible errors in the surface creation phase t Exemple file 560 0 6016 2 Example file S60_1 gid ETISN CFD tutorial CENIZAS AMOO E Pe Pick LEFTIIOUEE in rotate ESC to quit Pa A _ LL Figure 4 Error 3 sided surface has been created the stem region Cause The surface that was created has 4 sides but because one is very short we can hardly see it This occurs when we are not careful enough in the previous step Solution Since this surface is not valid we remove it Figure 5 From the two lines that form the side with the small segment we create a polyline with Create Polyline z Once created it will constitute a single entity that has to be converted into a NUR
36. faces defined by red lines Lines that bound several surfaces should not be divided as they form the boundary between them We use Divide gt Near Point function to divide the line ALII queant Meshing Calculste Help Pl 8 Kraate Li Foe Delete E _ bone Point Esplode polline LO Hum Divisions o Pa n alcun Brus end pairis Miser port 7 1 Swap arc Parameter 5 Edit polline Pobdinez I Edit MUFRES line Surfaces a sura ia E T 75 Hole MUROS surface n prec m Edit Suritde sh Qn a Ta m e Sa a Pal e Corner to MURES _ i im mn be a simplify NURDS T s T E di y ae Fa Pal Y a la Sur rd E Figure 16 To define the chosen point for the division the easiest thing to do is to select the line that we want to divide right click the mouse and in the contextual menu choose Point in Line Thus if we do not indicate the zone of division very accurately the program will choose the point on the line closest to the centre of the mouse pointer Zoom gt es 4 Rotate 5 oimn ine i Point in surface d pai i Tangentin line Redraw i i Z Render gt dala in surface Label ions Layer gt scape Copy paste Quit Figure 17 We should choose the point th
37. fications in hull shapes bulbous bow designs appendages We get knowledge of the quality of the alternatives by comparing all of them To precisely define the required power in order to project the engine system the best thing to do now is to do a towing test in a model basin of the selected alternative With this experiment the speed resistance curve is obtained and used afterwards to select the engine and design a suitable propeller 69 ETISN CFD tutorial References 1 Bruzzone D Numerical Evaluation of the Steady Free Surface Waves Proceedings of CFD Workshop Tokyo 94 Vol 1 Pag 126 134 2 Centro Internacional para m todos num ricos en Ingenieria CIMNE GiD Manual de Utilizaci n 3 Dawson C W A Practical Computer Method for Solving Ship Wave Problems Proceedings of Second International Conference on Numerical Ship Hydrodynamics Berkeley pp 30 38 1977 4 Garc a Espinosa J P rez Rojas L Valle Cabezas y Chac n Alonso J R El Proyecto BAJ EL Una herramienta de diseno hidrodinamico de buques de pasaje XXXIV Sesiones T cnicas de Ingenieria Naval Barcelona 1998 5 McNeel R amp Associates Rhinoceros Nurbs modeling for Windows User Manual 1993 2001 6 P rez Rojas L Souto Gonzalez L M Los CFD Computational Fluid Dynamics en el diseno de buques Aportaciones de la E T S I N U P M IV Simposio Mar timo Internacional 13 15 de junio 2001 La Habana Cuba 7 P rez
38. figures 44 and 45 the mesh was made for a container vessel 200 meters long sailing at 9 kts The transom stern is wet This area was smoothed in the free surface panelization In order to do this the connection of the waterline was made tangential to the centre line Process phase ETSIN_CFD Once the hull has been meshed we must proceed with the proper flow calculations We first have to define the kind of problem that we want to solve In our case the Problem Type will be the one that we have called E7S N_CFD In the Data menu we choose Problem Type Within this menu ETSIN CFD should appear as an option We select this type of problem and introduce the required data for the calculation For more details on the calculation basis see 9 Within the Conditions section of the Data menu we must indicate which part is the hull and which part is the free surface so that they are separated during the calculation process To obtain this the first thing we must do is to deactivate one of the layers For example we deactivate the free surface layer and we work with the hull layer We choose the condition u default in the Conditions window We leave Va ue as 0 0 and press the Assign button highlighting the entire hull This same operation has to be done with the free surface that we will activate after deactivating the hull layer The Starboard flotation condition has to be assigned to the free surface 22 ETISN CFD tutorial
39. generated waves on the free surface Within it the bow wave and the forward third of the ship are the regions with the most precise definition for our CFD If we focus on this region we can see that the wave with the smallest amplitude is the one generated by hull 3 followed by hull 1 and then with quite a big difference the worst ones are hulls 2 and 4 This makes the choice of a bulbous bow a good one and we now have to choose between hulls 1 and 3 For a more educated choice between hulls 1 and 3 we study the next wave cuts and verify whether hull 3 is still the one with the smallest wave amplitude 11 Kts 1 0375B Figure 140 68 ETISN CFD tutorial 11 NUDOS 1 5665B Figure 141 We can now verify that hull 3 presents the lowest amplitude waves in the forward region of the cut This analysis allows us to state that the best choice regarding wave generation and therefore wave resistance is hull 3 This example has demonstrated how to discard among different alternatives those that will produce a higher wave resistance Therefore taking this process to its limits we are able to evaluate the best design among those of a series of close alternatives To perform this comparison we visualise on the same graph and for a certain speed the wave cuts corresponding to each case This methodology provides us with a good tool when devising modi
40. he speed on the hull surface is not the ship s speed itself To correctly interpret this output data we must consider the ship to be still and that the water advances towards it at the ship s forward speed 59 ETISN CFD tutorial Again we follow the same steps and visualize Ve ocities There is also the possibility of choosing any speed component as well as the module TIME ANALYSIS Analysis 0 25 View Contour Fill factor Results velocities j Component velocities 128 129 Since all the colour range is concentrated on the rudder s tip figure 129 we must manually set the range limits to make our visualization useful These limits must be set for every visualization option For instance since the smallest velocity regions correspond to the appendages we must show the velocity gradient on the hull without the appendages and set a greater value for the minimum 60 ETISN CFD tutorial elocities 5 4 8333 Figure 130 In figure 130 we can see that the forward part of the bulb and the rudder s leading edge do not appear because their velocity range is out of the selected range Figure 131 Longitudinal cuts The results files kept in folder resu ts within the folder casename gid where the mesh used for the calculation was saved This folder also contains the ascii files long cut 1 5508B dat long cut 1 0587B
41. hin the folder casename gid where the mesh used for the calculation was saved This folder also contains the ascii files long cut 1 5508B dat long_cut_1 0587B dat and long_cut_0 5665B dat In these files there are the curves corresponding to longitudinal sections of the free surface at 0 5665B 1 0587B and 1 5508B With these graphs we can visualise the height of the generated waves and other characteristics This will be one of the most useful tools when optimising the ship s hull as will be discussed later They can easily be imported from Excel for instance For the S60 example used in the tutorial the graphs are shown in the following figures LONGITUDINAL CUT AT 0 5665B Figure 62 B ship s beam 30 ETISN CFD tutorial LONGITUDINAL CUT AT 1 0587B Figure 63 LONGITUDINAL CUT AT 1 5508B Figure 64 These are non dimensional graphs Along the X axis the free surface X component is represented It is in a non dimensional form with LBP as the reference length The orientation is the common one Therefore x 0 corresponds to the ship s bow and x 1 corresponds to the stern Along the vertical axis we have the free surface elevation in non dimensional form taking again LBP as the reference length So if we want to know the proper value of the height of some of the waves we must multiply its graphed value by LBP 31 ETISN CFD tuto
42. later is 4 0 4 m RN QW A 3 b 7 ea a et i Y TN 755 NI gt SX 65 Va i gt A OM o N 3 y A un Ba JT gt SI Figure 21 13 ETISN CFD tutorial Let us press ESC and Cancel the operation We have now assigned to these surfaces the proper number of cells in the desired direction We can foresee that with the others the same thing will have to be done First we press Generate within the Meshing menu In the window that appears there is no need to change anything since the size should be assigned automatically until the surface is complete Figure 22 We already have our first mesh but the definition is not good enough In this mesh it is possible to see how in the central area the five panels previously assigned appear Repeating the previous operation with all the panels we are able to refine the mesh For example we select again the whole hull with Structured gt Surface in the Meshing menu Pressing ESC we introduce 7 as the number of segments per line then Ok again and finally highlighting all the horizontal lines of the hull to mean that for each horizontal line there must be 7 panels in the mesh We then press ESC Cancel Generate and verify the results Figure 23 We can see that for the smallest panels stern no seria bow the concentration of panels is greater as expected Once this mechanism is properly understood we can refi
43. line of this patch is a polyline and therefore the panel is bounded by four entities Another complicated patch is the one right at the beginning of the sternpost the one that links the keel with the beginning of the sternpost Figure 66 This patch falls under the same category as the one bounded by three lines We saw that the solution was to split one of them in two and to create the surface with those four lines Figure 67 Other problematic patches are those at the end of the keel Some of them are defined by three lines instead of the four necessary Again this is solved by dividing one of the lines and generating those patches from four lines The transom stern region in the free surface panelization has already been commented on in the previous example As a reminder we said that the solution for the cases of wet transom was to model a free surface that does not consider the zone of the transom stern What has to be done in this case is to extend the waterline in order to smoothly join it to the centre line In order to create the free surface patch some of the ideas taken into account in the previous section were used A possible free surface would be formed by the waterline and the segments between the points LBP 0 0 and 5LBP 4 0 0 5LBP 4 0 0 and 5LBP 4 LBP 0 5LBP 4 LBP 0 and LBP LBP 0 LBP LBP 0 The last segment will be made between LBP 0 0 and the end of the segment we created to join the water
44. lization of Pressure Coefficient 57 Wave pattern visualization 59 Velocity cai 59 Longitdcdinal CUS 61 HULL OPTIMIZATION 5 2 0422 0022 63 ETISN CFD tutorial H ull desighi alternatives iaa 64 Speed field graphs on the 5 2 02 mH 66 REFERENCES cali 70 ETISN CFD tutorial Introduction ETSIN CFD is a computer code aimed at the optimisation of ship hull forms based on the calculations of potential flows with free surface 3 9 Nowadays the numerical simulations carried out on computers have become an irreplaceable tool for the optimisation of forms The commonly called CFD that stands for Computational Fluid Dynamics are used in the industry to predict the behaviour of fluids without having to resort to expensive scale models that simulate a process These tools allow us to test a greater number of models in a shorter time and with reduced costs compared with traditional tests Thus it is logical to use them in the first stages of the design in order to predict concept failures as soon as possible 6 One of the main applications for the naval industry is the optimization of ship forms and the calculation of forward resistance Although the accuracy of the calculation of resistance is quite low the pote
45. luous to check the connections for these points through the Join Lines end Points function Utilities Data Meshing Calculate Help s Miewgeomety Koo lt a Create Li Delete Li 006 Explode polyline Divide Join lines end points Swap arc Edit polyline E dit NURBS line E dit NIUFRES surface Hole NUFRES surface Edit Surfd4esh Conver to NURES Ld Simplify MURES k Figure 7 After fixing these errors and creating all the missing 4 sided surfaces the model should look like what is shown in figure 8 Figure 8 gt Example file S60 2 gid ETISN CFD tutorial There are still 3 surfaces to be created one for the stem region and 2 for the stern region The surface at the stern end and the one at the stem end have only 3 sides The third patch from the left of the stern region is defined by 5 sides Error Ill in some areas 5 sided patches the most natural Causes in a few areas especially for the stern it is not always easy to only obtain four sided surfaces Solution by joining two lines we can define these surfaces with only four sides lines This will result in a new problem for the adjacent panel that will not be able to use this new line as an edge To solve this we have to duplicate the original line in order for the nearby panel to use it We will now analyse in detail the 560 example where its stern presents an area with such a problem Pee
46. mesh makes calculation times higher and memory restrictions apply If we exceed the capacity of the ram memory on our computer it will be impossible to make the calculation A rough estimation of the necessary ram memory needed for the calculations can be obtained with the formula RAM 1 2 10 N MB With being the number of unknowns n9 of panels of half hull and half free surface panelization As an example computers with 256 Mb RAM can solve 5000 panel problems With these principles in mind we can efficiently refine the mesh First we are going to improve the mesh in the aft part With the Structured gt Surface function we select the surfaces in that zone and change the number of panels in the horizontal and vertical directions until an acceptable mesh is 15 ETISN CFD tutorial obtained One of them could be the following 5 in the X direction bow stern and 4 in the Z direction TPL TAT LIL Uy My WT A AU LS b b b b b Figure 26 Figure 27 Contextual Rotate Redraw Render Copy paste Zoom Pan create another layer in which we will draw the free surface Once the layer is created and activated Once the hull mesh is completed we go on to create the free surface one To do this we will for use View gt Layer gt To use gt name of the layer we the waterline to the free surface layer panels for the stern and central zones Once the density of panels in the b
47. n the hull and on the free surface The flow is potential and so the speed on the hull surface is not the ship s speed itself To correctly interpret this output data it is necessary to consider the ship still and that the water advances towards it at the ship s forward speed Again we follow the same steps and visualise Ve ocities There is also the possibility of choosing any Velocity component as well as the module Analysis TIME ANALYSIS Steps E 4 b y 0 25 View Contour Fill factor Results Velocities Component gt Velocities Y Velocities 2 Velocities lala Apply Figure 58 28 ETISN CFD tutorial POvelsgionoe s ae a R A NOE E Contour Fill of Velocities velocities Figure 59 This representation of the free surface together with the hull is not the best one GID distributes the colour range automatically and this causes because the data value limits are always on the hull the free surface gradients to be ill defined It is better to deactivate the hull layer when studying the free surface information 2 NO E 3 LEE es 6 Contour Fill of Velocities velacities Figure 60 29 ETISN CFD tutorial 2OGS amp GIQe Sa DES IN x 7 SIRO Bi a Figure 61 Longitudinal cuts The result files kept in folder resu ts wit
48. ne the mesh to a convenient level and start varying the number of rows of cells in the vertical direction until we consider it sufficient Again we select the whole hull and assign for example the value of 9 panels per line In this case vertical lines have to be selected by highlighting one the contiguous ones are highlighted automatically The new mesh should look better 14 ETISN CFD tutorial Figure 24 In this initial mesh in comparison with the upper part of the hull the bottom part is less defined as far as number of cells in the vertical direction is concerned In order to homogenize the mesh it Is necessary to change the number of cells in the vertical direction of the lower vertical lines We increase it from 9 to 14 and regenerate the mesh Figure 25 The mesh now looks more homogenous and we are getting closer to a decent result We should keep in mind that our mesh must meet certain requirements It has to have a high density zone close to the bow since it is in this zone where the potential flow is best calculated from the non viscous hypothesis For this reason we will pay special attention to this area When we have a bulbous bow we have to be more careful since acceptable results highly depend on a good mesh In the intermediate and stern areas the convergence of the calculated potential flow with the real one is usually poorer there is therefore no need for the mesh to be so dense Having a very dense
49. ng the free surface one and pressing Apply in the View Results menu only the hull will remain visible Select amp Display Style Volumes M Surfaces M Cuts alphabetic order S SurfaceSetl 5 SurfaceSet 2 ed 6 r Rename O Delete Style Body Render Smooth 2 amp 5 Culling None Conditions None To back Send to Close Figure 55 9190 6 14 wd Co bh st A D Presure Coef 0 46083 0 39312 0 32541 0 2577 0 18999 MS ISE e 08 gt ES Be 0 12228 0 054565 0 013146 0 080857 0 14856 Contour Fill of Presure Coef Figure 56 Visualisation of the wave pattern Another feasible visualisation is the wave pattern on the free surface Again in the View Results menu with this function we select the visualisation that we want this case the wave pattern To visualise only the free surface we enter it in the Se ect amp Display Style menu In this menu deactivating 5 SurfaceSet1 means only the free surface will remain visible Do not forget to click App y after every change 27 ETISN CFD tutorial P OCIS So Y lw Cog hp gt HE Contour Fill of Wave Pattern Figure 57 Velocities field visualisation The last visualisation option is the velocity field This magnitude is full of physical meaning both o
50. normal direction of all the selected surfaces The final distribution should now be as it appears in Figure 47 Figure 47 The last thing to do is to regenerate the mesh so that this data and the changes made can be reflected in the final calculation mesh The Generate option from the Meshing menu asks for the size of the generated panels but it is automatic and the value we set is disregarded The mesh will then be rebuilt with the correct orientation of the normal vectors Now the mesh is ready for the process phase In order to start the calculation we must enter the Ca culate menu Inside we have several options We can directly press Ca culate This will start the calculation and give us a message when finished The other option is to open the window Ca culate Windows Within this window we will be able to launch the calculation with the Start button This is the recommended procedure and the one we will follow here Project Starttime UID 4 Output view Kill Start remote Update Close Figure 48 24 ETISN CFD tutorial After starting the calculations the screen will change and the name of the current case will appear Project Starttime UID float casco Thu Jan 24 15 33 50 181 4 Output view Kill Stert remote Update Close Figure 49 We have already commented on the memory necessities for the calculations These calculations also require reso
51. ntial flow methods are the most common in improving ship forms The method we have conceived is of this type Numerical results are very interesting because one can visualize the fluid s behaviour at each point of the hull whereas towing tests only provide overall results such as the resistance or the trim angle For this reason it is much easier to devise modifications aimed at reducing the resistance without relying on the investigator s intuition and experience Nevertheless towing tests tanks are still necessary With the present state of the art techniques it is not possible to compute the total resistance within the precision levels required by the Shipyards and some important phenomena for example the interaction between the propeller and the hull do not yet have a satisfactory model However numerical tests allow for the reduction of the number of scale tests by only needing the construction of one or two models With these definite tests it is possible to determine the propeller and the propulsive plant The towing tank of the Naval Architecture Department of Universidad Polit cnica of Madrid has been investing concerted efforts in the development of software for numerous fields such as manoeuvrability seakeeping and of course forward resistance optimisation 8 The works in the field of forward resistance began in the frame of the Spanish Challenge for the 1992 America s Cup Since this event has been described on numero
52. o trim the hull with at the free surface First we will define the free surface patch that we are going to use Following the parameters defined in the previous tutorials we will draw the edges of the free surface We must take into account that the forward end of the free surface will be 1 4 LBP forward of the hull The lateral side of the free surface patch will be 3 4 LBP away from the centerline and the back side of will be at LBP from the stern We will have defined a quadrilateral like the one in figure 101 Figure 101 Since we have not defined the flotation line we will create it from the intersection of the free surface and the hull First we will create the free surface patch from the quadrilateral defined in the previous step Later on we must intersect the free surface quadrilateral with the hull using the command sequence Geometry Create ntersection Surface Surfaces and selecting the hull and the quadrilateral This will produce the main water line The problem arises when dealing with surfaces that are not so well defined mainly the hull In this case we can create a copy of both surfaces another layer and extend to portside the free surface centerline so that the free surface encircles the hull Once the new line has been defined we can put the new elements back in the free surface layer as can be seen figure 102 48 ETISN CFD tutorial Figure 102 Again we have to eliminate all those surfaces that ar
53. ow zone is properly We now repeat the same optimisation operation for the bow zone and reduce the concentration of increased we should have a mesh ready for computations 16 Figure 28 Example file S60_3 gid ETISN CFD tutorial In order to copy the waterline we must have the two layers in on state and the layer of the free surface activated 7o use The next step is to copy the waterline to the new layer using the copy command Within this command context select lines as the entity type We then click se ect and select the lines that form the waterline of our hull within the drawing We do this in order to copy the waterline to the layer in use the free surface one Copy Entities type Lines Transformation Translation First point Pick 2 0 0 Second point Num x 0 0 m i re Duplicate entities Le Do extrude No Multiple copies fi Select Cancel Figure 29 If we now deactivate the layer where the hull drawing is only the waterline should appear on the screen 331215 7 7 NOOR DIBA le amp Figure 30 The following step is to create the limits of the free surface the line we have just created will form part of this limit or contour It should look something like Figure 31 Com tik A a ge M D gt B IA SNOOP RM Figure 31 Usually the fore line is LBP forward of the bow and the aft lin
54. proper value of the height of some of the generated waves we must multiply its graph value by LBP Hull Optimization Examples In this chapter a series of examples will be presented dealing with how to optimize hulls through the use of the ETSIN_CFD code Optimization is achieved by comparing alternatives hull forms Meshes are prepared for the different ship forms changing bulbs stems and other elements that play an important role in wave generation The calculation case is performed for each alternative at the ship design speed With the obtained data it is possible to decide which of these designs is best In our search for the optimum design we will have to make a series of assumptions Since this method calculates the potential flow and not the viscous one the optimal design is the one that generates the smallest amplitude waves The smallest amplitude is related to the smallest wave resistance and hence to the smallest forward resistance When the amplitudes of the waves are similar attention has to be paid to the pressure coefficient distribution and speed fields the hull and free surface gradients in these fields are greater in 63 ETISN CFD tutorial any particular design this means that this choice is worse regarding wave generation simply because these gradients are a sign of higher waves Since the calculations do not consider the viscous effects results will be more accurate in the forward third of the h
55. rial Tutorial Example 2 Fishing Vessel with bulbous bow In order to illustrate the calculation process of a ship with bulbous bow we are going to use an optimised fishing boat This optimisation process is described in one of the following sections We have already explained the first example commenting on almost every detail We will now use that example to concentrate on the new features that a case with bulbous bow implies Preprocess Again we need a CAD definition usually in IGES or DXF format describing the starboard side of the submerged part of the hull Grid generation with GiD Once we have imported the IGES file we must save it in casename gid We are now ready to start the grid generation process There is no difference between the phase of preparation of the hull panels and the S60 example The same errors usually appear and they are overcome in an analogous way We will just comment on the specific problems that arise from the existence of a bulbous bow Stern region Figure 65 Several surface patches can be problematic The second patch from the left in figure 65 seems to be bounded by five sides This is not really the case since the solution was to duplicate the lower line shared with the lower panel This duplicated line together with the following segment was Example file MOTOP igs 32 ETISN CFD tutorial made into a polyline to bind the patch that we wanted to form Thus the lower edge
56. s a particular mesh is used This is due to the huge curvature of the panel lines in that region which is needed to accurately follow the transom waterline In order to avoid divergences we will have to mesh this region of the free surface in a special way The criterion is to get a smooth union between the waterline and the centre line a little aft of the stern Some examples of these cases are shown in the following figures HH en RUE TT IHR EE Ag eo 7 7 111 1771 O QR Ne a TAN MI Figure 40 MA ATA O DE Figure 41 The mesh of figures 42 and 43 corresponds to a bulk carrier 185 m long sailing at 9 kts Due to such a big length and the low speed of calculation the mesh had to be very dense We had to shorten the free surface at the stern so as not to exceed the computer s calculation limit As we can see the stern zone has been smoothed at the free surface in order to avoid the divergences we have just commented on 21 ETISN CFD tutorial O 1 CC 1 a Ad EEC CIA A CIN UN UA MA dar VO A ug NOTI Figure 42 5 Figure 43 In
57. s often and is solved with the Join Lines 10 ETISN CFD tutorial end Point command as explained above For this reason you should be especially careful when selecting the lines to join The lower one can be selected with the mouse but the upper one has to be selected with the command line with its corresponding number 53 If the previously mentioned window appears it means that the only problem was that they were separated Therefore we must press yes and make the conversion to polyline AL shortest distance is between point 7 14 and point 39 0 000000 Join these points Figure 13 Once we have the polyline we can create the surface Again problems can appear with the lines whose endings are not in contact This is solved once again with the oin Lines end Point command Figure 14 Error Panels are limited by three lines instead of the four needed Cause Due to an especially complicated geometry bow stern etc some zones are difficult to define with four lines Solution We must divide one of the lines to solve the problem This way we are able to create four sided panels It does not matter if the line to be divided 15 not curved or has something similar to a vertex In our example this happens for the aft panel and in one of the stems Let us study with detail the fore one Figure 15 11 ETISN CFD tutorial The line that we have to divide is the only one not bounding any sur
58. sure coefficient on the free surface where the pressure is atmospheric For a better visualization we can deactivate the layer that contains the free surface In order to do this we use the Se ect amp Display Style menu figure 125 right hand side S Surface Set J corresponds to the hull mesh and S Surface Set 2 to the free surface mesh By deactivating the free surface layer with the icon 9 and by pressing Apply in the View Results menu only the hull will remain visible on the hull Select amp Display Style x Volumes Surfaces Cuts alphabetic order S SurfaceSet 1 S SurfaceSet 2 mr e 6 Rename Analysis TIME ANALYSIS Steps i 1 KE Delete IE O 0 Delete View Contour Fill Style Body factor Render Smooth E Results Presure Coef Culling None Component Presure Coef Conditions None Apply Close To back Send to Close Figure 125 Presure Coef 1 0 66666 0 33333 9 954e 06 D 33335 0 66668 1 1 3334 1 6667 2 Figure 126 58 ETISN CFD tutorial GID automatically defines the scale limits for the graphic representation of all the results We recommend that you set them manually and keep them for the incoming comparisons 75 CFD is basically a comparative tool The most reliable information is obtained from comparing the results of different ship hull forms This is why it
59. ta menu will be the one corresponding to the principal dimensions of the problem Problem Data 38 ETISN CFD tutorial Problem Data Length m 36 6 Beam m 8 Draft 3 7 Speed kts 11 Accept data Close Figure 80 We now draw the normal vectors of the patches and swap with the SwapSome command those that are not oriented towards the inside part of the hull and set the normal vector of the free surface oriented downwards Figure 81 We will regenerate the mesh Generate within the Meshing menu so that the new mesh reflects the changes done so far We are now ready to launch the calculation process Within the Ca cu ate menu we unfold the Calculate Window submenu and press Start to begin the calculation Project Starttime UID MOTOP_bulbol Wed Feb 13 15 06 33 184 4 b Output view Kill Start remote Update Close Figure 82 Once the calculation is finished we select Postprocess to start the postprocessing phase Postprocess We can start the postprocess phase directly from the final screen of calculation or with the Postprocess Icon 39 ETISN CFD tutorial If we do it this way we will have to open the file where the results are automatically saved after the calculation This file will be in the directory casename gid where casename is the name that we gave at the beginning of the case study Within this folder we will find a
60. th its corresponding difficulties due to the existence of several appendages In the first two cases the geometric definition is based on a set of lines the third case we directly import a definition of the hull through surface entities Pre processing The calculations we are going to do are based upon the geometric form of the ship we want to study These forms are defined in IGES file which contains CAD definition of the hull to mesh With this definition we will try to obtain a set of lines in three dimensions that will let us generate by means of surfaces a computational representation of the submerged part of the hull In order to make the preprocess easier the geometric representation has to meet with a few requirements Perspective p iN Figure 1 shape shown in figure 3 is ready to start the pre process lines defining this hull are all connected at the different vertex nodes of the draft After the preprocess the shapes have to become surfaces that cover the entire hull Keeping this mind it 15 advisable that the initial drawing be the sort of mesh that facilitates the later performance of grid generation Thus we will try to ensure that this geometric design formed by lines will be the base for those surfaces Therefore we will use as a basis some frames and longitudinal lines that define the future surface patches trying to set them as square as possibl
61. th side as has repeatedly been shown If the surface patch is bounded by 5 lines or more the only option will be to split the patch Therefore we will have 2 four sided surfaces We encounter this type of problem both for the hull and for the bulb The hull has been sectioned by the keel the rudder and the free surface patch It is bounded by 6 lines Each line is limited by the points that we show in figure 106 Figure 106 We are now going to split the hull in order to define it entirely with four sided patches To divide the surface patches we will use the command Geometry Edit Divide Surfaces Near Point This operation will offer us two directions in order to split the surface We will select the appropriate direction in accordance with the criteria we have chosen for the whole splitting operation 14 Example file IACC_2 gid 50 ETISN CFD tutorial Q Choose NURBS sense VSense Cancel Figure 107 In the case shown in figure 107 we indicate the forward point on the bow and choose the direction U Figure 108 In figure 108 we see how the forward surface is bounded by 3 lines The aft surface is bounded by 6 lines If we divide the lines by the points shown in figure 108 we obtain a patch distribution with which we can proceed with the meshing operation as can be seen in the next figure Figure 109 A similar process has to be undertaken with the complicated geometric shape of the bulb figure
62. them one by one The ones that are separated have to be joined with the ines end point command In the end we should have four lines that form a closed perimeter one of which contains the starboard part of the waterline With these lines we create a four sided surface like we did for the hull 19 ETISN CFD tutorial Figure 38 The mesh criteria is stricter here than for the hull mainly regarding the minimum number of panels Number of panels in the longitudinal direction To calculate this number we to know the ship s speed V for this case The characteristic wave length for this speed is obtained with the formula 22 n V g The minimum number of panels for each complete wave is around 15 Since we know the total length of the free surface we can calculate the minimum number of necessary panels in the X direction In this case the length of the free surface is 272 m If we suppose that the speed is 12 kn 6 1728 m s this produces 24m long waves Since we need 15 panels for every 24 meters we need about 170 panels in the X direction Number of panels in the transverse direction To adjust the number in the Y direction we have to get an aspect ratio of the panels that is as close to 1 as possible This is not always viable since it could lead to a too great a number of panels for our equipment s RAM memory Therefore this aspect ratio is considered as ideal and the real limit will always be imposed by
63. u should appear Contextual Zoom Rotate gt Point in line Pan b Pointin surface Redraw Tangent in line Bender Normal in surface S Options Layer Escape Join C a Copy paste Quit Figure 34 18 ETISN CFD tutorial Select oin C a is with the mouse and then select a zone close to the intersection of both lines Once the division is made we only have to eliminate the line piece that is outside our free surface BN Ie BM Figure 35 With the erase command activated Figure 37 we select the one that we want to erase and we press Esc The same operation in the bow zone is repeated and we verify that both lines are joined in the division vertex with the o n ines end point command used previously The appearance at this point should be something like Figure 36 The last operation before creating the free surface is to convert all these line edges into one single polyline From this polyline a four sided surface will be created in which one of the sides will be the waterline polyline In order to create this polyline we use the icon amp and with the mouse drawa box that includes most of the lines we want to convert The rest of them can then be selected with the mouse Figure 37 After these operations a new green polyline should appear If this is not the case the most probable reason is that some of the lines are not properly joined and we should check
64. ulb We must try meshing this region and if the mesh quality is not good enough we will have to create new lines or rebuild the surface patches During the visualization of the geometric shapes it sometimes seems that some of the edges of the shapes do no exist The reason is that these edges also bound other patches in a deactivated layer This means that during the meshing process both lines will have the same number of divisions To avoid this problem and to have the freedom to choose the mesh density in every surface we must ungroup both surfaces This operation is performed with the command Utilities Uncollapse Surfaces indicating the surface in this case the keel as shown in figure 114 In this way we create a new edge line and both surfaces have different patches To visualize this border we put the surface patch in this case the keel in the right layer right clicking Layer Send Surfaces and selecting the keel patch Figure 114 Let us proceed with the meshing of all the entities Again through trial and error we try to obtain a smooth mesh on the hull and a density on the free surface that accomplishes the requirements explained in the previous chapters The density of panels on the free surface is mainly defined by the speed and length of the free surface patch This case study will be tested at a speed of 8 knots In the first estimation it will generate waves of a wavelength determined by the following equation 2
65. ull Viscous effects are more important from the forward third of the hull onwards and dominate in the stern region especially at low speeds Therefore this tool must be focused on the optimization of the forward part of the hull with special attention paid to the bulbous bow if present In order to illustrate this optimization method we are going to present the case of four fishing boat designs that were optimised and then decide which among them would be the one with the smallest wave resistance Two of them are with bulb and two without Hull design alternatives Hull 1 Hull2 C U Figure 135 A hull and free surface mesh were prepared for each design proposition 64 ETISN CFD tutorial 1 ME UN Ru PN LULA A y XY E n D nt PROS m ary DERE SE S WX VS FIVE RAI any asl SR hg yk PRA id M US Y MENA 4 ta val i Y SORA WM iN A M NS ee an di D e ns Tuy A A MH FE E y VERON nd JAN Mi M na Neat 136 The following step is to launch the calculation processes and once concluded we will obtain the INa the GID postprocess and analyze the graphs of the longitudi In thi ION WI maps of speed distribut cuts 65 ETISN CFD tutorial Speed field graphs on the hulls HULL 1
66. urces from the system and we recommend running them on a dedicated computer so that it does not slow down any other work Once the calculations have been performed a warning window will appear on the screen offering us the possibility of directly accessing the postprocess phase which we should accept Process info x Process 860 5 started at Thu Jan 24 19 57 11 has finished OK Postprocess Figure 50 Postprocess Once the calculation is finished we can get into the postprocess context of GID This can be done when requested after the calculations or directly through the Files gt Postprocess command We have to set certain values for a correct interpretation of the results View Style and View Results menus have to be active For more details consult manual 2 View Uter Docu Viewsesuti Options Windows Tu mE W Volumes M Surtaces E Cuts alphabetic order I SSertaceSet sl SSurtaceSet 7 ne Bine e sye mes Bes uum ZOO RAMAS Figure 51 25 ETISN CFD tutorial With E7S N CFD we have the possibility of visualising different results The speed field can be visualised on both the hull and the free surface Pressure Coefficients can also be visualised on the hull and the wave pattern on the free surface In order to select one of the visualisations we will use the View Results menu View Results xj Analysis TIME ANALYSIS Sco ss
67. us occasions as the formula 1 of sailing the basin group had to commit itself completely to the world of the computer simulations 9 in order to meet expectations The codes designed at that time and improved later have been put under numerous processes of validation comparing our results with those obtained from commercial programs of recognized prestige and with the available experimental data 4 At the moment new programs are still being developed with the purpose of analyzing more intricate processes like the turbulent flows with formation of vortices and many efforts are dedicated to the use of the software that has already been validated with practical aims Using this Manual The analysis of a problem using this code consists of a three stage process First the preprocess that consists in the treatment of geometric forms and grid Second the calculation itself and third the postprocess which is the visualization and interpretation of the results In agreement with this we have divided the manual into several different parts The first one is Summary of the numerical and mathematical foundations of the calculation module In the second part the setup process of the code is described The third part consists of a tutorial Three practical examples the three necessary steps that we have described previously preprocess calculation postprocess are explained ETISN CFD tutorial Foundations The panel method The physi

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