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cfd seeker June 15, 2011 05:25

CFD of general missile body
hi all, I am new fluent user and i m doing CFD analysis of a general stepped missile body(diameter is different at different locations) in fluent. My aim is to find the CL,CD,CM at M=2 and AOA ranging from 2 to 10 degrees. I am using coupled implicit solver with steady state. I have made the mesh in Gambit and it has 2.6 million cells. Currently i am doing a run at 2 deg AOA and it took 14,000 iterations to converge(not fully converged,still the cofficients has some fluctations). My mesh is very large so my computer is taking 1 min/iteration. I have kept the courant no to 1 because solution diverges as i increase it beyond 1. So my solution is converging very slowly and taking a lot of time. I have heard that there are some MULTIGRID settings which aid in fastening convergence but i am totally illetrate in their knowledge. Can somebody please help me in these MULTIGRID settings or suggest something else which helps me in faster convergence?????
Any suggestions will be welcomed as my solution is taking alot of time and i have limited time to perform analysis at other AOA's.
Thanx in advance

skullCFD June 15, 2011 17:06

I have a similar (actually, worse) problem. My grid is 6 million cells. Each iteration is taking 2 min on a quad-core, 64-bit xeon machine and a total of about 30000 ite to converge. I wonder what flow model you are using. One thing I did not understand is why you have a Courant number in a steady simulation?

cfd seeker June 16, 2011 01:42

helo, i am using Coupled Implicit solver which uses Courant number even in steady flow and mate do u know anything about Multigrid settings which helps in faster convergence??? if you know anything about them then any ideas from your side will be welcomed.

madhuvc June 16, 2011 13:06

if this what you guys are talking about....

skullCFD June 17, 2011 19:19

Hi cfd seeker and madhuvc, All I know about multigrid methods is from what I read from Fluent manual few years ago. It says the solution progresses from zones with fine grid to zones with coarse grid. So this can be faster compared to solving the equations on the entire grid in a unique way. So you may not see an improvement in convergence time if your entire grid is mostly of the same size. I tried it one time and it did not speed up convergence for my case. Next time it could not obtain a converged solution. So, I think its efficiency and ability to obtain a converged solution depends on the problem i.e., can the problem be solved by such a progression of solution from finer grid to coarser grid, for example, what if you have coarser grids in a problem in the area where the physics is becoming important as the solution progresses but the multigrid solver tries to solve that important zone after solving some physically unimportant zone where the mesh is finer. But I am sure some people might have had success stories with this method. I request anyone who is more knowledgeable about this method in Fluent to kindly correct my understanding if I am wrong. Thanks.

cfd seeker June 24, 2011 02:26

hello skullCFD thanx for taking out time, mate that's exactly my understading but as for as theory is concerned this is OK. But i want to know what parameters needs to be modified in that Multigrid panel which helps me. For my your post i can infer that u are not pretty much clear in this regard but you are talking about 1 or 2 expriences, kindly let me know about them.

skullCFD June 24, 2011 16:59

Hi, The meaning of each of those columns in the panel are given in: I only changed the 'Max Coarse Levels' when I worked with it previously. Higher number for 'Max Coarse Levels' requires more memory but speeds up convergence. As I experienced before, it can diverge as well. If so, it might work if we slightly reduce this number. BTW, my case is still running :D

cfd seeker June 28, 2011 02:54

thanx skullCFD for replying, i will study this topic in detail and one more thing, i have posted a topic named "Turbulent Boundary Conditions" few days back but i didnt get any reply on that. Kindly take out some time and reply in that thread so that i can get some knowledge regarding that topic.
thanx in advance

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