# DNS of sphere at Re=3700 blows up

 Register Blogs Members List Search Today's Posts Mark Forums Read

 June 15, 2022, 13:19 DNS of sphere at Re=3700 blows up #1 New Member   Nikolaos Beratlis Join Date: Jan 2010 Posts: 15 Rep Power: 15 I am trying to run DNS of a sphere at Re=3700. I will be comparing the results of OpenFOAM against other codes and results in the literature. I created an O grid for the mesh around a sphere in a cylindrical domain. The inlet is 5D from the sphere, the outlet is 20D and the radius of the cylindrical domain is 10D. Here are some photos of the mesh near the sphere. Around the sphere the grid is slightly stretched in the radial direction to cluster points near the surface of the sphere. The resolution is kept fine in a region behind the sphere up to 5D to resolve the near wake. This photo showing a cross section cut showing the grid around the sphere and the wake Another photo showing a cross section cut in the wake with crinkle clip The simulation blows up after 20 iterations. I ran the simulation with both pisoFOAM and simpleFOAM without any success. Simulationtype is laminar. Here is the output of the pisoFOAM run. Code: ```Time = 0.036 Courant Number mean: 0.0422403 max: 1.36537 smoothSolver: Solving for Ux, Initial residual = 0.0587041, Final residual = 8.42802e-06, No Iterations 5 smoothSolver: Solving for Uy, Initial residual = 0.0587602, Final residual = 7.79945e-06, No Iterations 5 smoothSolver: Solving for Uz, Initial residual = 0.00212533, Final residual = 2.55993e-06, No Iterations 3 GAMG: Solving for p, Initial residual = 0.99465, Final residual = 0.0430159, No Iterations 2 time step continuity errors : sum local = 1.02157e-06, global = -6.00863e-12, cumulative = -3.02265e-08 GAMG: Solving for p, Initial residual = 0.979142, Final residual = 8.77358e-07, No Iterations 21 time step continuity errors : sum local = 2.42567e-11, global = -3.09213e-12, cumulative = -3.02296e-08 ExecutionTime = 401.43 s ClockTime = 403 s forceCoeffs forces write: Cm = -0.0331513 Cd = 0.697653 Cl = 0.0638549 Cl(f) = -0.00122385 Cl(r) = 0.0650787 Time = 0.038 Courant Number mean: 0.0422442 max: 2.85164 smoothSolver: Solving for Ux, Initial residual = 0.0998439, Final residual = 8.97566e+216, No Iterations 1000 smoothSolver: Solving for Uy, Initial residual = 0.100005, Final residual = 2.91442e+216, No Iterations 1000 smoothSolver: Solving for Uz, Initial residual = 0.00432983, Final residual = 2.49464e+215, No Iterations 1000``` The simulation blows up suddenly from one time step to the next. The time step is 2e-3. I ran with smaller time step of 2e-4 and I still get the same problem. Here is the fvScheme file Code: ```ddtSchemes { default backward; } gradSchemes { default Gauss linear; } divSchemes { default none; div(phi,U) Gauss linear; div(phi,k) Gauss limitedLinear 1; div(phi,s) bounded Gauss limitedLinear 1; div((nuEff*dev2(T(grad(U))))) Gauss linear; } laplacianSchemes { default Gauss linear corrected; } interpolationSchemes { default linear; } snGradSchemes { default corrected; }``` and the fvSolutions Code: ```solvers { p { solver GAMG; tolerance 1e-06; relTol 0.1; smoother GaussSeidel; } pFinal { \$p; smoother DICGaussSeidel; tolerance 1e-06; relTol 0; } "(U|k|B|nuTilda|s)" { solver smoothSolver; smoother GaussSeidel; tolerance 1e-05; relTol 0; } } PISO { nCorrectors 2; nNonOrthogonalCorrectors 0; }``` and the controDict Code: ```application pisoFoam; startFrom startTime; startTime 0; stopAt endTime; endTime 300; deltaT 2e-03; adjustTimeStep yes; maxCo 1.0; writeControl adjustableRunTime; writeInterval 10; purgeWrite 0; writeFormat binary; writePrecision 6; writeCompression off; timeFormat general; timePrecision 6; runTimeModifiable true;``` Any help with why it is blowing up and what to try next?

June 18, 2022, 05:40
#2
Senior Member

Josh Williams
Join Date: Feb 2021
Location: Scotland
Posts: 107
Rep Power: 4
Quote:
 blows up after 20 iterations
This would suggest to me that the flow begins to develop (like flow separation behind the sphere). And then it is not being dissipated. I guess maybe you need a longer domain or finer mesh to promote dissipation of the turbulence by molecular viscosity. To see if the lack of viscous dissipation is an issue, you may try use an LES model (just as a test to check this is the issue).

Alternatively, it may be a numerical issue in your solution or boundary conditions. It seems your tolerances are quite low. We typically go for like 1e-10 for pressure and 1e-8 for velocity. Also why using two different solvers for pressure (GAMG and DICGaussSiedel)? I am unsure what pressure condition you use at the outlet but if there is still recirculation at the outlet you may try a stabilising BC (totalPressure for p, inletOutlet for U).

 Tags blow up, dns, laminar, sphere, unstable