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 keeper October 6, 2010 09:23

physical timescale

hi everyone...
I have one simple question. Please can anyone tell me what significance does timescale control have??? I mean in solution in which equations is this used. I know it is trivial question but I really don't know..

Thank you very much for all your answers

 michael_owen October 6, 2010 15:02

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Steady State Time Scale Control

For steady-state problems, the CFX-Solver applies a false timestep as a means of under-relaxing the equations as they iterate towards the final solution. Because the solver formulation is robust and fully implicit, a relatively large time scale can typically be selected, so that the convergence to steady-state is as fast as possible.
A steady-state calculation will typically require between 50 and 100 outer loop iterations to achieve convergence. If you expect that the actual flow being simulated would take a long time to reach a steady-state condition, given the initial conditions you have specified, then a greater number of outer loop iterations may be required. In this case, you can usually reduce the number of iterations required by setting initial conditions that more closely resemble the steady state flow.
Although a relatively large time scale can be used, if the time scale is too large the resulting convergence behavior will be “bouncy.” If this is observed, then the first thing you should try is to reduce the time scale, say, by a factor of four. If there is no noticeable improvement, then the convergence problem may be caused by another factor. If the time scale is too small, then the convergence will be very slow.
In addition to the advice in the following sections, you will probably require a small physical time scale for the following situations:
• Poor mesh quality
• Transonic flow
• Large regions of separated flow
• Openings with simultaneous inflow and outflow
• Free Surface flows: In these cases it is often sufficient to use a smaller timestep only for the Volume Fraction equations
• Multiphase flows

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