Have you got any tips on converging high speed flows with CFX?
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In addition to the advice given in the solution 'How to solve supersonic Laval nozzle flows quickly ' (ID 468798), we can also make the following recommendations: In CFX-10, it can help to set the various numerics options under SOLVER CONTROL: * VELOCITY PRESSURE COUPLING/Rhie Chow Option = High Resolution . This reduces wiggle size adjacent to shocks and can also improve stability * ADVECTION SCHEME/Gradient Relaxation = 0.1 (steady state only) . This can improve convergence behaviour. * ADVECTION SCHEME/Blend Factor Relaxation = 0.1 In CFX-11, there is an option to set under SOLVER CONTROL/COMPRESSIBILITY CONTROL the parameters: * Carbuncle Fix = t . This improves 2D stability adjacent to shocks with high aspect ratio meshes. For a reason yet to be understood, it can also improve the path to convergence for steady state calculations. The carbuncle fix implementation essentially prevents the development of the so called 'carbuncle instability' by 1. reducing the order of the advection scheme in the area of the shock and 2. by adding some additional dissipation. For more information on this fix and when it is relevant, please take a look at the attached powerpoints. * High Speed Numerics = t. This enables the numerics options listed above (Carbuncle Fix , as well as SOLVER CONTROL options for CFX-10) Note also that, for high speed flows, a large timestep can sometimes give better convergence behaviour than a small timestep. This is because in a steady state run a small timestep starts to track the true transient, but not perfectly because we aren't converging each time step. But the transient effects it tries to resolve are complex and can be nasty. A large timestep does not try to track these and can therefore be better behaved. Use a small timestep (CFL~1) only if you want to resolve the true transient; else use a timestep based on L/V for the domain. Another general recommendation would be to run double precision. Also sometimes when the flow goes supersonic. setting the expert parameter 'max continuity loops = 2' can help stabilise the run. About the attached ppt files: These show the improvement on convergence (and accuracy) obtained with the implementation of the 'carbuncle fix' in CFX-11. Note that the carbuncle fix option is no longer an expert parameter (as described in carbuncle.ppt), but is enabled under 'SOLVER CONTROL/COMPRESSIBILITY CONTROL' (see more recent ppt high_speed_nozzle_sph.ppt), and is automatically enabled if the option 'High Speed Numerics' is activated. Note also that if you are having convergence problems, and if having the carbuncle fix activated does not make any difference, it might still be because the carbuncle fix parameters need tuning (check in Post if the shock location function actually locates the shock, if not you will need to tune the parameters). **** Entered By: dsclarke @ 07/25/2006 12:24 PM **** |
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