|
[Sponsors] |
June 29, 2006, 07:15 |
FSI with MFX
|
#1 |
Guest
Posts: n/a
|
HI All
I am trying to simulate a coupled flow and structure problem with ANSYS+CFX (two way) coupling using MFX feature of ANSYS10.0 So far I am not much satisfied with it due to the following reasons: -- It requires ANSYS classic knowledge and learing ANSYS apdl scripts needs a lot of time. In short it is not user friendly. -- Secondly, convergence of Stagger iterations is really poor. I tried everything so far, i.e. varying time steps, under-relaxation etc. but there is no much success. In fact Stagger-iterations do not converge at all in my case. Therefore I would like ask the following: Is any user successfully using FSI feature of ANSYS10.0 for two-way coupling(Ofcourse not the people/developers from ANSYS)? Can anybody share with me his/her expereince on ANSYS-CFX coupling with MFX e.g. convergence robustness, limitations, drawbacks etc? Another question: which is the best commercial or freeware code for a FSI simulation. many thanks in advance VK |
|
June 29, 2006, 20:04 |
Re: FSI with MFX
|
#2 |
Guest
Posts: n/a
|
Hi Kumar,
You are at the moment using the most user friendly software.It gives you reasonable results for FSI. Whats your problem with convergence.Post it. Thanks Regards Rajit |
|
June 30, 2006, 04:10 |
Re: FSI with MFX
|
#3 |
Guest
Posts: n/a
|
Yes ofcourse CFX is most user friendly CFD code but not ANSYS classic with apdl.
My problem is convergence is that the Stagger iterations do not converge at all. Either I get negative volume message in CFX or the following messages from the two codes (If somebody wants to look into my case, I can send him the input files): CFX=> +--------------------------------------------------------------------+ | An error has occurred in cfx5solve: | | | | The CFX-5 solver exited with return code 1. No results file has | | been created. | +--------------------------------------------------------------------+ End of solution stage. +--------------------------------------------------------------------+ | The following transient and backup files written by the CFX-5 | | solver have been saved in the directory | | D:\Daten\KV\Projects\FEC\FSI\PIPE_LANG_CADFEM\CFX\ run_004: | | | | 0_full.trn | +--------------------------------------------------------------------+ +--------------------------------------------------------------------+ | The following user files have been saved in the directory | | D:\Daten\KV\Projects\FEC\FSI\PIPE_LANG_CADFEM\CFX\ run_004: | | | | mon | +--------------------------------------------------------------------+ ANSYS=>>>>>>>>>>>> *** ERROR *** CP = 2271.578 TIME= 23:11:23 One or more elements have become highly distorted. Excessive distortion of elements is usually a symptom indicating the need for corrective action elsewhere. Try incrementing the load more slowly (increase the number of substeps or decrease the time step size). You may need to improve your mesh to obtain elements with better aspect ratios. Also consider the behavior of materials, contact pairs, and/or constraint equations. If this message appears in the first iteration of first substep, be sure to perform element shape checking. |
|
June 30, 2006, 04:20 |
Re: FSI with MFX
|
#4 |
Guest
Posts: n/a
|
Yes, ANSYS is good solution. And related to APDL languge in ANSYS try to compare it to programing language of other codes, for example - MSC PCL (in Patran). Feel difference... If someone have enough experience in programming then APDL will be no problem.
|
|
June 30, 2006, 04:36 |
Re: FSI with MFX
|
#5 |
Guest
Posts: n/a
|
I AM really concern about this!but i am a pre-learner,
maybe we can communicate by e-mail. |
|
June 30, 2006, 08:03 |
Re: FSI with MFX
|
#6 |
Guest
Posts: n/a
|
Yes APDL is as simple as Fortran77 but for a beginer the problem is to know "what is what" and "who is who".
Anyway thanks for the response and let us not get ourselves involved in this discussion on simplicity of ANSYS-APDL. It would be great if someone can shed more light on convergense issues related to coupled simulations. In my case I even tried to increase the number of stagger-iterations per time-step to 100 and made the numerical time step much smaller than the physical time scales. Still I see no success. That is why I was wondering if CFX users are currently intensively expoilting this feature of the code. People who are interested and currently involved in FSI computations, I welcome all of you to have open discussion on this topic so that we all can benefit from this and remember the objective of these discussions is not to criticize any code or feature. Can I ask you guys to post one or two working example (with input files) of FSI, ofcourse not the typical ANSYS examples of FSI. many thanks |
|
June 30, 2006, 09:27 |
Re: FSI with MFX
|
#7 |
Guest
Posts: n/a
|
If elements are becoming distorted, then usually it's because they are moving too far in one iteration, because they are not good quality to begin with or the mesh stiffness is not appropriate. The first thing to do it to make sure you have a good quality mesh to begin with and that the fluids-only solution runs OK. Looks at the forces in the fluids-only solution and check they are reasonable. Next, make sure you have set up the mesh stiffness parameter to an appropriate value (see tutorial 20). It should not be a constant value. Next, consider the forces applied to the FSI boundary. How far do you expect it to move in one timestep? The distance it moves should be small relative to the geometry. The CFX Reference Pressure is not included in the forces transimitted to ANSYS by default. In general you'll need a converged fluids-only solution as the initial guess so you can avoid force spikes during start-up. hope this helps! Mike
|
|
June 30, 2006, 10:06 |
Re: FSI with MFX
|
#8 |
Guest
Posts: n/a
|
Try to see 2-way coupling between Abaqus & FlowVision (http://www.capvidia.com/index.php?id=22)
|
|
June 30, 2006, 10:34 |
Re: FSI with MFX
|
#9 |
Guest
Posts: n/a
|
Hi Mike
Thanks for a detailed reply. As far my fluid mesh is concerned, the quality of the mesh is quite good. It is a hexa mesh with a minimum angle of 60 deg between the faces of cells, i.e. skewness is well controlled. I start first with a steady state computation whose results, I use as an initial guess for the FSI simulation. From the steady solution, the force acting on the wall and pressure field in the domain look ok. Mesh stiffness parameter is not constant, it is defined as: stiffness = 1 / Volume of control volume The movement of the boundaries in my case is mainly governed by a force acting on the structure side, I am interested to see how fluid affects this movement of the structure. From a transient analysis, without CFD, I observed that the maximum displacement (in one cycle) in the structure is around 2-3% of its dimension and I had 64 time steps in one cycle. Perhaps this movement is too much for the solver. Reference pressure: I also expected that reference pressure should not be transmitted to ANSYS by defualt. Since I have an incompressible case, I am setting P_ref = 0 Pa. From your remarks it seems all my settings are ok. Still, I attach my ANSYS input file with this post so that one can see if the settings are ok for the ANSYS solver. *******************ANSYS INPUT FILE*********************** fini /clear /COM,ANSYS RELEASE 10.0 UP20050718 15:23:39 05/09/2006 !/input,menust,tmp,'',,,,,,,,,,,,,,,,1 WPSTYLE,,,,,,,,0 /PREP7 /TITLE Coupled FVM/FEM modelling !**************************** !*** Craete keypoints******** !**************************** fsi_flag = 1 f_flag = 1 pi=4.*atan(1.) r_inner=10*1.0e-3 r_outer=10.5*1.0e-3 rad=0.5*(r_inner+r_outer) Length=400.0*1.0e-3 area_t=pi*(r_outer**2-r_inner**2) dens_sol=4510.0 E_st=1.027e11 dens_liq=1000.0 Pratio=0.34 den_tot=(area_t*dens_sol+pi*r_inner**2*dens_liq)/area_t !########################## !##Geometry creation####### !########################## K,1,0,0,-Length/2, K,2,0,0,Length/2, !********************** !*** Create axis line !********************** L,1,2 !***************************** !*** Create arcs and surfaces* !***************************** circle,1,r_inner circle,2,r_inner ADRAG,6,7,8,9, , , 1 !################################# !*****Geometry Finished*********** !################################# !!!!!!!!!!!!!!!!!!!! !******************************* ! Material Properties ********** !******************************* ET,1,SHELL93 !**Element type MP,EX,1,E_st !**Youngs Modulus MP,dens,1,den_tot ! density MP,NUXY,1,Pratio!Poisson Ratio R,1,dth ! Real constant !* !****************************** !*Generate mesh !***************************** LSEL,s, , ,2, 9, ! select lines LESIZE,ALL, , ,5, ! resolve above selected lines lsel,none lsel,s,,,11,12 lsel,a,,,14,16,2 lsel,a,,,1, lesize,all,,,21, MSHK,1 ! use mapped mesher MSHA,0,2D ! use only quads AMESH,ALL ! mesher executed finish !********************* !!Transient Analysis !******************** fcn=400.0 omega=2*pi*fcn per_res=64 delt=1/(per_res*fcn) n_per=16 f_max=1.5 nsteps=n_per*per_res !********************* !!FSI Analysis !******************** /PREP7 ALLS DDELE,ALL *if, fsi_flag, eq, 1, then mp,dens,2,dens_sol mp,dens,1,dens_sol totim=delt*nsteps nsel,s,loc,z,Length/2 nsel,a,loc,z,-Length/2 d,all,ux,,,,,uy,uz ! D,ALL, ALL nsel,all alls fini /SOLU ANTYPE,4 !Trasiente Analyse EQSL,SPAR,,,,, 1.00000000 NLGEOM, 1 TRNOPT,FULL NSUBST,1,10,1 resc,,none nsub,1,10,1 nsel,none ! Apply Boundary condition alls esln cm,FSIxx,elem SF,all,FSIN,1 nsel,none nsel,s,loc,z,-0.001,+0.001 *get,nnum,node,0,count *dim, force,table,nsteps,1 ! TABLE fr Kraftverlauf definieren *do,tt,0,nsteps,1 force(tt,0)=tt*delt force(tt,1)=f_max*sin(omega*tt*delt)/nnum *enddo *vplot,force(1,0),force(1,1) F,all,FY,%force% kbc,0 autots,on alls DELTIM,delt,delt,delt OUTRES,BASI,5 TIME,totim !*** * MFS/MFX Aktivation MFAN,ON,MFAX MFCL,MFPS ! -> Simultane Berechnung 2-Weg Kopplung Struktur <-> Flssigkeit !MFPSIMUL,group1,ANSYS,CFX, !MFSORDER,group1, ! -> Sequenzielle Berechnung 1-Weg Kopplung Struktur -> Flssigkeit MFTIME, totim MFDT, delt, delt, delt,OFF MFIT, 101, 1, 15 MFRELAX,DISP, 0.40000000 ,RELX MFRELAX,FORC, 0.40000000 ,RELX MFRELAX,TEMP, 0.40000000 ,RELX MFRELAX,HFLU, 0.40000000 ,RELX MFRELAX,HGEN, 0.40000000 ,RELX MFRELAX,VELO, 0.40000000 ,RELX MFRELAX,ALL , 0.40000000 ,RELX MFRSTART, 0.00000000 MFPS, group1, ANSYS MFPSIMUL, group2, CFX MFSORDER, group1, group2 MFLCOMM, SURF,ANSYS,1,DISP,CFX,PIPE,Mesh Displacement,NONC MFLCOMM, SURF,CFX,PIPE,Total Force Density,ANSYS,1,FORC,NONC MFLI, alls finish *endif !ENDIF FSI_FLAG ************ANSYS INPUT FILE ENDING************************ thanks for your help. VK |
|
July 21, 2009, 05:22 |
|
#10 |
Member
SMN
Join Date: Jun 2009
Location: CANADA
Posts: 71
Rep Power: 17 |
hi V. Kumar:
now in 2009 i have the same problem in fsi modeling...has your problem been solved???would you please help me now? regards |
|
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
FSI - verification force mapping with mfx | Vinzent | CFX | 15 | September 18, 2011 19:00 |
MFX FSI for linear case. Help needed. | peiji1984 | CFX | 4 | February 22, 2011 13:11 |
how to extend FSI 2D codes to 3D, need advises | abouziar | Main CFD Forum | 1 | May 30, 2008 05:08 |
MFX - for FSI between Ansys Multiphysics and CFX? | Zerocles | CFX | 1 | June 5, 2006 09:23 |
MFX , FSI, tutorial? example? | Tom | CFX | 1 | May 17, 2006 10:58 |