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#1 |
Senior Member
Jiri
Join Date: Mar 2014
Posts: 221
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Hi there,
I simulated a hydrodynamic bearing operation being lubricated by oil. Since it is possible that air may be sucked into the bearing, I included the air onto a surface as boundary condition. The trouble is, that the computation runs only with Air at 25C which has constant density. Applying Air ideal gas leads to divergence due to overflow. Playing with timescale does not work as well as improving mesh quality. Are there any ideas for oil - ideal gas mixture? |
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#2 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,816
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It is normal that when you make the physics more complex that convergence is harder. You have replaced an incompressible fluid with a compressible one, so now you should expect pressure wave effects, compression, adiabatic heating and cooling and all those sort of complex physics.
Without specific details I cannot help, but this simulation sounds highly transient and will require a VERY small time step. Have a look at the CFL numbers for your flow now you are using a compressible fluid, that will guide you to the sort of time step size you need.
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#3 |
Senior Member
Jiri
Join Date: Mar 2014
Posts: 221
Rep Power: 13 ![]() |
Thank you for the reply. Is there any document how to set CFL for multiphase analyses? Must the Courant number be always below 1?
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#4 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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No, CFX is an implicit solver and is not restricted to a CFL or Courant number. I generally recommend you use adaptive time stepping, homing in on 3-5 coeff loops per iteration for transient simulations.
If you want to Courant Number (or CFL for compressible flows) then aim for about 1 to 10 - the actual requirement could be higher or lower than that, but that is a starting point.
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Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. |
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