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July 20, 2011, 09:04 
convergence with simple foam and different fvSchemes

#1 
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Hi,
I am trying to simulate flow through a valve, with one inlet and one outlet. I have to calculate the pressure drop across the valve for different flowrates, and compare it with experimental values. The flow is turbulent. I am using kepsilon model and simple foam solver for incompressible fluid. i run the calculation in two parts, which are as follows: 1. in the first part, the fvSchemes directory is the following, which is run for the first 700 steps. ddtSchemes { default steadyState; } gradSchemes { default Gauss linear; grad(p) Gauss linear; grad(U) Gauss linear; } divSchemes { default none; div(phi,U) Gauss upwind; div(phi,k) Gauss upwind; div(phi,epsilon) Gauss upwind; div(phi,R) Gauss upwind; div(R) Gauss linear; div(phi,nuTilda) Gauss upwind; div((nuEff*dev(grad(U).T()))) Gauss linear; } laplacianSchemes { default none; laplacian(nuEff,U) Gauss linear corrected; laplacian((1A(U)),p) Gauss linear corrected; laplacian(DkEff,k) Gauss linear corrected; laplacian(DepsilonEff,epsilon) Gauss linear corrected; laplacian(DREff,R) Gauss linear corrected; laplacian(DnuTildaEff,nuTilda) Gauss linear corrected; laplacian(1,p) Gauss linear corrected; } interpolationSchemes { default linear; interpolate(U) linear; } snGradSchemes { default corrected; } fluxRequired { default no; p; } In the second part of the calculation, I switch from Gauss upwind to Gauss linear limitedV 0.3 for div (phi, U) case, with everything else being the same, and run it from 700 to 1200 time steps. The problem is that for lower flowrates (of the order of 75100 L/min), both the schemes gives good results. However, as I increase the flow rate (250350 L/min), for the Gauss upwind, the convergence of the residuals is quite good, but when I switch it to linear limited, the solution blows up completely. Can somebody tell me why it is happening and what is the solution? I have attached the fvSolution and checkMesh with this thread.
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July 20, 2011, 09:44 

#2 
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maddalena
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Hello,
first of all, I suggest you to check where the solution diverges. Try to look at your pressure and turbulence fields a little bit before than everything blows away. Maybe check local residual as well. I have the feeling that something is going wrong on your high skew cells. The reason your first setup reach convergence is that you select more diffusive schemes in comparison of the second setup. Whit a diffusive scheme, convergence is easier but the results may be less accurate. In my opinion, a good point where to start for you is this thread: http://www.cfdonline.com/Forums/ope...imesteps.html There is for sure something more / something better on the forum, but I started this discussion and found it really really useful. Note that it is based on a simpleFoam case, kepsilon model... mad 

July 20, 2011, 10:21 

#3  
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Quote:
Thanks for the answer. My solution blows up in the first 3040 steps of the second part. I post here the last steps of the calculation Quote:
I will try to run the simulation again with a change of mesh to reduce the number of skew faces and post the results here. Is it ok to start the simulation straight with the higher order scheme that gauss upwind? Can you throw some light on how the solution varies, and what should be the best way to approach a good solution?
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July 20, 2011, 10:26 

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mad 

July 20, 2011, 11:01 

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Ok . i was a getting a little impatient with my results not turning out nicely.
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July 20, 2011, 11:09 

#6 
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July 20, 2011, 11:13 

#7 
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Felix L.
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Just a thought here:
Is your flow separating somewhere inside your domain? Maybe at those higher mass flow rates where your case is diverging. It's possible that the higher resolution schemes capture a flow separation near the outflow area of your domain which might cause partial inflow there and thus the divergence. Stop your simulation say 20 steps after you changed your numerical schemes and have a look at your flowfield to get an idea. Greetings, Felix. 

July 21, 2011, 02:32 

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