Large Eddy Simulation with too high Mach number
Hello,
I would like to calculate a Large Eddy Simulation to the investigation of an aero acoustics problem in high speed compressible flow. The LES calculation was initialized from a converged transient SST result. The geometry is quite complicated, I am fully aware that the mesh should be more finer but I have very short time. I applied tetra mesh, its quality is about 0.30.5 . The mesh is fine in the narrow cross sections but it’s coarse in the zones which is not investigated. The time step is 1e6 s. Based on the convergence history the residuals is OK, but the Mach number is unrealistic: COEFFICIENT LOOP ITERATION = 15 CPU SECONDS = 2.018E+05   Equation  Rate  RMS Res  Max Res  Linear Solution +++++  UMom  0.58  2.5E05  5.9E03  7.5E04 OK  VMom  0.80  2.8E05  1.3E02  8.2E04 OK  WMom  0.83  5.8E05  1.6E02  1.2E03 OK  PMass  0.89  9.2E06  3.8E03  5.0 5.1E04 OK +++++  HEnergy  0.84  3.1E05  1.2E02  5.8 3.5E08 OK +++++ +  Notice: The maximum Mach number is 4.458E+01. + I thought that this numerical error was caused by the mesh but based on the result the mesh quality is OK in the zone of the high Mach number. The error can be detected in various locations but it always is in one node. (not in a large area). This notice can be seen after every time step then the solver stops with „overflow”. Could anybody help me to solve this problem? Thanks in advance Roland 
Guess there is something basic wrong. Check your setup.
BTW: The resolveable vortices size depend on your cell size. IMHO you have to be fine anywhere or us DES/SAS 
Use the post processor to find where the area of rapid flow is, and whether it is a problem.

Quote:
By the way, In my opinion the convergence history is qiute unusual. The notice (high Much number) was present for 470 timesteps, but the convergence was stabil. Finally in one iteration the calculation has stoped. TIME STEP = 474 SIMULATION TIME = 4.7400E05 CPU SECONDS = 6.306E+05  COEFFICIENT LOOP ITERATION = 1 CPU SECONDS = 6.306E+05   Equation  Rate  RMS Res  Max Res  Linear Solution  ++++++  UMom 10.36  2.7E03  1.7E+00  8.5E02 OK  VMom  4.02  9.0E04  5.8E01  4.7E02 OK  WMom  1.16  3.8E04  1.7E01  7.4E02 OK  PMass  7.51  5.0E05  1.2E02  5.8 3.3E04 OK ++++++  HEnergy  0.59  1.2E03  6.9E01  6.8 1.1E07 OK ++++++  COEFFICIENT LOOP ITERATION = 2 CPU SECONDS = 6.307E+05   Equation  Rate  RMS Res  Max Res  Linear Solution  ++++++  UMom  1.45  3.8E03  2.5E+00  7.2E03 OK  VMom  1.14  1.0E03  6.7E01  1.4E02 OK  WMom  3.84  1.5E03  9.6E01  1.7E02 OK  PMass  0.80  4.0E05  1.1E02  5.8 2.3E04 OK ++++++  HEnergy  1.00  1.2E03  9.2E01  6.8 1.2E07 OK ++++++  COEFFICIENT LOOP ITERATION = 3 CPU SECONDS = 6.308E+05   Equation  Rate  RMS Res  Max Res  Linear Solution  ++++++  UMom  0.00  8.3E17  4.9E14  9.8E+11 *   VMom  0.00  3.8E17  2.3E14  3.0E+12 *   WMom  0.00  1.1E16  7.4E14  8.8E+11 *   PMass  0.00  0.0E+00  3.6E28  10.5 6.0E+08 F  +++++ ...how can it happened? What can cause this sudden "shock" in my calculation? Thanks Roland 
No, not an unusual convergence history at all. Just a simple divergence. You need to stabilise it numerically.
Try: * Double precision number * Tighter convergence * Improving mesh quality * Smaller timesteps * lower order discretisation (not a good idea with LES but at least it will probably converge) * Read the documentation on "obtaining convergence" 
All times are GMT 4. The time now is 22:57. 