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timesteps reqd for the convergence of unsteady flo

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Old   July 27, 2005, 11:42
Default timesteps reqd for the convergence of unsteady flo
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satyam
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hi im simulating the flow past a barge(a displacment vessel) using vof,realizable k-epsilon,unsteady model.iam using maximum iterations per time step as 20 and the time step size as 0.001.can any one suggest me for how many time steps are reqd to acheive the convergence. thanks in advance

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Old   July 28, 2005, 07:33
Default Re: timesteps reqd for the convergence of unsteady
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Swarup
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This is completely problem dependent for any model. Swarup.
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Old   July 28, 2005, 07:39
Default Re: timesteps reqd for the convergence of unsteady
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satyam
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how can i calculate the min time required
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Old   July 28, 2005, 07:51
Default Re: timesteps reqd for the convergence of unsteady
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dirk
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What do you think should the converged solution in your unsteady simulation be ? What do you want to compute ?
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Old   July 28, 2005, 07:56
Default Re: timesteps reqd for the convergence of unsteady
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satyam
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i want to compute the total force,convergence of the coefficient of drag
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Old   July 28, 2005, 09:30
Default Re: timesteps reqd for the convergence of unsteady
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dirk
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Then you simply have to monitor the development of drag and hope that it reaches a steady state or oscillates around a steady mean value.

Dirk
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Old   July 28, 2005, 09:37
Default Re: timesteps reqd for the convergence of unsteady
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Swarup
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then you should run simulation until Cd tapers off. do you have a target for total flow time? if so, then you should see that before your flow time is over, you have achieved convergence. further, if you are assuming incompressibility, then you should try to obtain convergence as early as possible. you have time step size deltat and number of iterations per time step to choose from. one more issue is whether your problem is inherently unsteady and has no steady state solution (like free convection)or you are finally interested in obtaining a steady state solution. Swarup
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