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Old   November 28, 2005, 05:19
Default steady vs unsteady?
  #1
Fer
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Hi all, I'm studying the airflow around a vehicle (in 2D and 3D) for different inlet-velocity cases. Now, I'm working with the 2D case. The problem is that I'm not able to reach convergence. I'm using simple turbulence models (k-e standard, RNG and Realizable) and I only reach convergence if I use a 1st order scheme + low vel-inlet. I'm sure that the grid is ok (high quality, I believe).

The fact is that I've only used steady simulations. Can anybody tell me if it could be an unsteady case?

Fer

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Old   November 28, 2005, 06:33
Default Re: steady vs unsteady?
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bharat
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Hi Fer, What range of reynolds number u r using. If its high change to compressible solver.

hope this helps bharat
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Old   November 28, 2005, 07:23
Default Re: steady vs unsteady?
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Fer
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I think Reynolds isn't too high; well I'm trying with velocities in the range between 10 and 50 m/s. Well, the fact is that in some areas of the car the flux velocity is about 1.7-2*vel_inlet (easy to calculate aproximately using, for example, Bernoulli's equation). Could those zones motive that problems with convergence?

Fer

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Old   November 28, 2005, 17:42
Default Re: steady vs unsteady?
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Anindya
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what are your other boundary conditions? For your 2D case you should get convergence with higher order schemes instead of just 1st order. Something is not right in your simulation.
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Old   November 28, 2005, 17:59
Default Re: steady vs unsteady?
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Freeman
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Hi Fer. I'm having similar problems so I made a Mass-Weighted Average plot of the outlet velocity and it showed me a fluctuation instead of getting stable as one could expect if it would converge, so I am starting to think that I'm in an unsteady case.

But when I run an unsteady simulation, after 1500 iterations start to fluctuate periodically, and also the Mass-Weighted average of the outlet velocity: could this be a convergence criterion? All unsteady problems shows these periodic fluctuations? And those fluctuations means that I have arrived to the convergence?

ThAnxs to all!
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Old   November 29, 2005, 03:18
Default Re: steady vs unsteady?
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Fer
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Well, my boundary conditions are: - Static Wall for the car surface and for the ceiling. - Velocity-intlet at the inlet edge. - Outflow at the outlet edge. - Translational wall at the floor (moving at the same vel as the vel-inlet).

Perhaps there are problems using the outflow, but I don't know. In that case, tell me what's the most convenient to use in that edge, please.

Thanks

Fer

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Old   November 29, 2005, 15:51
Default Re: steady vs unsteady?
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Anindya
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Use a pressure boundary condition instead of outlet. See if you get some better results

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