CFD Online URL
[Sponsors]
Home > Forums > CFX

physical timescale

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

Like Tree1Likes
  • 1 Post By michael_owen

Reply
 
LinkBack Thread Tools Display Modes
Old   October 6, 2010, 10:23
Default physical timescale
  #1
Senior Member
 
Join Date: Jul 2010
Posts: 142
Rep Power: 6
keeper is on a distinguished road
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
keeper is offline   Reply With Quote

Old   October 6, 2010, 16:02
Default
  #2
Senior Member
 
Michael P. Owen
Join Date: Mar 2009
Posts: 195
Rep Power: 7
michael_owen is on a distinguished road
A.NSYS C.FX-Solver M.odeling G.uide | A.dvice o.n F.low M.odeling | T.imestep S.election |


T.imestep S.election
P.revU.p / H.omeT.ransient T.imestep C.ontrol
N.ext
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
hassan1201 likes this.
michael_owen is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Superlinear speedup in OpenFOAM 13 msrinath80 OpenFOAM Running, Solving & CFD 17 August 22, 2009 04:59
Solid Timescale Control Parthipan CFX 2 August 24, 2007 11:07
Advice on Physical Timescale Oli CFX 0 January 16, 2007 11:41
Controling Physical Timescale petr CFX 1 January 5, 2007 19:57
SST in 2D? Jesper Sørensen CFX 16 December 23, 2006 09:40


All times are GMT -4. The time now is 12:42.