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November 28, 2005, 04:19 |
steady vs unsteady?
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#1 |
Guest
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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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November 28, 2005, 05:33 |
Re: steady vs unsteady?
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#2 |
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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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November 28, 2005, 06:23 |
Re: steady vs unsteady?
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#3 |
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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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November 28, 2005, 16:42 |
Re: steady vs unsteady?
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#4 |
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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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November 28, 2005, 16:59 |
Re: steady vs unsteady?
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#5 |
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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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November 29, 2005, 02:18 |
Re: steady vs unsteady?
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#6 |
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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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November 29, 2005, 14:51 |
Re: steady vs unsteady?
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#7 |
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Use a pressure boundary condition instead of outlet. See if you get some better results
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