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Old   September 4, 2002, 18:03
Default Time Stepping
  #1
Chan K I
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Hi,

I am a running an internal flow time-marching simulation using Jameson's Runge-Kutta scheme.

At different CFL numbers, the converged solution is different. Any ideas what could be the problem?

Thanks very much Chan K I
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Old   September 5, 2002, 02:05
Default Re: Time Stepping
  #2
Tom
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You are sure that the implimentation of the Jameson scheme is correct? Perhaps you can test your code with a well defined (laminar) problem.

Tom
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Old   September 5, 2002, 11:22
Default Re: Time Stepping
  #3
Chan K I
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Hi Tom,

Thanks for your reply.

The scheme has been well-assessed beforehand and is being implemented on an internal flow case.

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Old   September 6, 2002, 16:07
Default Re: Time Stepping
  #4
Pao
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Are you running the transient solution? or steady state solution?
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Old   September 6, 2002, 17:30
Default Re: Time Stepping
  #5
Chan K I
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Hi Pao,

I am running a time-marching solution to a steady-state.

Noticed that with same initial conditions, different time steps give different steady-state solutions.

Thanks very much
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Old   September 6, 2002, 17:36
Default Re: Time Stepping
  #6
Pao
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But after a long iteration, the steady state solution is independent from the time step. This is my experience.
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Old   September 7, 2002, 04:50
Default Re: Time Stepping
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versi
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You may encouter MULTIPLE solutions for an encolsed region, if Re is high enough. This is physically possible. For example, Spherical Coutte flow may have 0-vortex, 1-vortex and 3-vortex flows at the same Re number. Please specify your flow problem in detail.
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Old   September 7, 2002, 09:51
Default Re: Time Stepping
  #8
Jim Park
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As early as 1972, Pat Roache published a study showing multiple (steady state) numerical solutions for the same physical problem as he changed the time step.

Try Roache, Patrick J., "On Artificial Viscosity," Journal of Computational Physics, Vol 10, No. 2, October, 1972.
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Old   September 8, 2002, 14:43
Default Re: Time Stepping
  #9
Chan K I
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Hi guys,

Thanks so much for your comments!

Pao: The small time step calculation has been run for a reeally long time and the observed trend for mass flow (this is an internal flow problem) shows it converges to a smaller value than for the large time step case.

Versi: This is a time-marching solution to the compressible Euler equations. Hence Re can be taken to be infinity.

Jim: Am going to browse through the paper.

Any more ideas out there?

Cheers

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Old   September 9, 2002, 10:44
Default Re: Time Stepping
  #10
Pao
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How about the external flow calculation? Did you see time step affect the solution? Or this only happen for internal flow problem?
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Old   September 10, 2002, 10:05
Default Re: Time Stepping
  #11
Chan K I
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The code was developed for internal flow cases. Previous internal flow simulations did not yield the problem. Only encountered it recently.

Cheers
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