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A shock appears after several periods of the transient simulation 

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November 1, 2014, 21:40 
A shock appears after several periods of the transient simulation

#1 
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Cai Tao
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Hi，I came across a problem when I use the kEplison model to do the transient simulation of flow around a square cylinder.
A shock appears after several periods of the simulation, the shock vibrates every timestep, and the amplitude remains unchanged. the attachments shows the phenomenon. point E is a monitoring point of the pressure in the wake area of the cylinder. the period is about 0.01s, and the phenomenon appears near the 27th periods. 1.png 2.png 

November 3, 2014, 09:16 

#2 
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Philipp
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Hi,
We need more information. 1) What solver do you use? 2) What do you mean by "periods of the simulation"? 3) Is it possible that you just see a very poorly resolved vortex street? 4) some time steps at the end of your log file
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November 4, 2014, 05:13 

#3 
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Cai Tao
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Hi，RodriguezFatz
thank you very much. the informations: （1）the file of fvsolution // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // solvers { p { solver GAMG; tolerance 1e06; relTol 0.1; smoother GaussSeidel; nPreSweeps 0; nPostSweeps 2; cacheAgglomeration on; agglomerator faceAreaPair; nCellsInCoarsestLevel 10; mergeLevels 1; } pFinal { $p; tolerance 1e07; relTol 0; } "(UkepsilonRnuTilda)" { solver smoothSolver; smoother GaussSeidel; tolerance 1e07; relTol 0; } } PISO { nCorrectors 5; nNonOrthogonalCorrectors 0; pRefCell 0; pRefValue 0; } and the file of fvschemes // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // ddtSchemes { default Euler; } gradSchemes { default Gauss linear; } divSchemes { default none; div(phi,U) Gauss limitedLinearV 1; div(phi,k) Gauss limitedLinear 1; div(phi,epsilon) Gauss limitedLinear 1; div(phi,R) Gauss limitedLinear 1; div(R) Gauss linear; div(phi,nuTilda) Gauss limitedLinear 1; div((nuEff*dev(T(grad(U))))) Gauss linear; } laplacianSchemes { default Gauss linear corrected; } interpolationSchemes { default linear; } snGradSchemes { default corrected; } fluxRequired { default no; p ; } (2)"periods of the simulation" means ：At first of the simulation, the simulation is correct（there isn't any shock）， while after a period of time，the shock begins to appear. (3)Actually, the transient simulation is based on the result of steady simulation, and I use the software CFX simulated the case, the result is right. (4)some information of the file "log": Time = 0.04264 Courant Number mean: 0.122131 max: 1.2103 smoothSolver: Solving for Ux, Initial residual = 4.04218e05, Final residual = 7.8952e08, No Iterations 6 smoothSolver: Solving for Uy, Initial residual = 0.0019072, Final residual = 6.09567e08, No Iterations 12 smoothSolver: Solving for Uz, Initial residual = 0.000173136, Final residual = 8.44166e08, No Iterations 8 GAMG: Solving for p, Initial residual = 0.000218439, Final residual = 1.89013e05, No Iterations 8 time step continuity errors : sum local = 4.78031e10, global = 6.72354e11, cumulative = 4.2685e08 GAMG: Solving for p, Initial residual = 8.85436e05, Final residual = 7.51789e06, No Iterations 7 time step continuity errors : sum local = 1.90134e10, global = 2.34216e11, cumulative = 4.27084e08 GAMG: Solving for p, Initial residual = 1.66438e05, Final residual = 1.5961e06, No Iterations 14 time step continuity errors : sum local = 4.0367e11, global = 5.13811e12, cumulative = 4.27135e08 GAMG: Solving for p, Initial residual = 3.36047e06, Final residual = 9.43844e07, No Iterations 8 time step continuity errors : sum local = 2.38714e11, global = 3.1315e12, cumulative = 4.27167e08 GAMG: Solving for p, Initial residual = 1.55151e06, Final residual = 9.86122e08, No Iterations 32 time step continuity errors : sum local = 2.49409e12, global = 3.44097e13, cumulative = 4.2717e08 smoothSolver: Solving for epsilon, Initial residual = 1.33639e05, Final residual = 3.68449e08, No Iterations 5 smoothSolver: Solving for k, Initial residual = 4.62431e05, Final residual = 9.74823e08, No Iterations 6 ExecutionTime = 7549.61 s ClockTime = 8494 s Time = 0.04268 Courant Number mean: 0.12213 max: 1.21029 smoothSolver: Solving for Ux, Initial residual = 4.03712e05, Final residual = 7.88954e08, No Iterations 6 smoothSolver: Solving for Uy, Initial residual = 0.001907, Final residual = 6.09236e08, No Iterations 12 smoothSolver: Solving for Uz, Initial residual = 0.000173084, Final residual = 8.4416e08, No Iterations 8 GAMG: Solving for p, Initial residual = 0.000218598, Final residual = 1.88857e05, No Iterations 8 time step continuity errors : sum local = 4.77668e10, global = 6.71387e11, cumulative = 4.26499e08 GAMG: Solving for p, Initial residual = 8.87166e05, Final residual = 7.51532e06, No Iterations 7 time step continuity errors : sum local = 1.9008e10, global = 2.34088e11, cumulative = 4.26733e08 GAMG: Solving for p, Initial residual = 1.66435e05, Final residual = 1.59465e06, No Iterations 14 time step continuity errors : sum local = 4.03328e11, global = 5.13204e12, cumulative = 4.26784e08 GAMG: Solving for p, Initial residual = 3.35917e06, Final residual = 9.41966e07, No Iterations 8 time step continuity errors : sum local = 2.38254e11, global = 3.12867e12, cumulative = 4.26815e08 

November 4, 2014, 06:52 

#4 
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Philipp
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Hi again,
1) I still don't get what you mean by "period". Do you see periodic vortices in the wake of the body and the period T = 0.01s? 2) In the log file you show information of time = 0.04264. In your upper picture the whole pressuremess starts at time = 0.27s. Can you show the log output of a time step when  what you call a "shock"  is already there? 3) Your max. Co. > 1. Does this all happen if you use a smaller time step? Try to halve the time step! 4) What about changing to safer numeric schemes? Such as upwind and uncorrected laplacian?
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November 5, 2014, 07:27 

#5 
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Cai Tao
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Hi，RodriguezFatz
1) the period T = 0.01s is base on the st（Strouhal number）of Flow around a cylinder which value is 0.2. The diameter of my model is 0.2m, and the U∞=10m/s. So the f=0.01hz. 2)I show information of time = 0.04264 is another case, but it also have the same phenomenon at that time step. The good news is that I change the mesh, the y+ is increased form 30 to 49. i didn't change any other sets, and the shock phenomenon disappeared. i think the Co may be the cause of the shock phenomenon. 

November 6, 2014, 06:19 

#6 
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Philipp
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You say the log output is during what you call "shock". But the simulation is converged... this doesn't look like a COproblem. For COproblems the simulation diverges and explodes.
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November 8, 2014, 08:38 

#7 
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Cai Tao
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Thank you again.
I just change the mesh(do not change any other settings), and then the phenomenon disappears. I still look for the reason that lead to the phenomenon. 

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keplison, shock, transient simulation 
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