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Old   June 20, 2001, 03:04
Default Turbulent viscosity
Christian Holm
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I have made a simulation upon a rectangular box. Inlet in one end, and outlet in the other end. Inlet vel. is approx. 20 m/s. There are no temperature differences. The problem is that I get a too high turbulent viscosity (the turbulent/laminar viscosity ratio is limited to approx. 10^5). I first considered that it could be due to a too fine grid combined with a "coarse" turbulent model (k-e model), so I tried with a coarser grid. No effect. I have also tried other models (RS, LES) but it did not do the trick (well, the LES did with a good initial guess).

Q: Is it true that there are problems with combining a fine grid with a "coarse" turbulent model? Q: Do anyone have suggestions as to solve the high turbulent viscosity problem?

I would appreciate comments to the two questions.

Cheers Christian

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Old   June 20, 2001, 08:36
Default Re: Turbulent viscosity
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In fact turbulent/laminar viscosity ratio=10^5 is typical value. Don't worry. For instance, in my numerical simulations of supersonic turbulent jets flowing into atmosphere I got maximum dynamic turbulent viscosity approx 0.5 Pa*sec (molecular vis is about 1e-5 Pa*sec). I used Menter's k-w model and got good agreements with experiments.

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Old   June 21, 2001, 05:09
Default Re: Turbulent viscosity
Christian Holm
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I would think that the more turbulent the flow is, the higher the ratio. And I guess that supersonic can be considered very turbulent, and a high ratio could be expected. My case on the other hand is not that turbulent, and I am just surprised that the ratio is that high, and I am afraid that it might contaminate the solution. I can see that the convergence is more stable when I lower the inlet vel. Do you have any suggestions what to do about a potentially too high viscosity ratio ?

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Old   June 21, 2001, 13:38
Default Re: Turbulent viscosity
John C. Chien
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(1). Take the flow over a flat plate as a test case. (2). Run some cases of flat plate flow, refine the mesh, and check the results against the test data.
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Old   June 23, 2001, 22:04
Default Re: Turbulent viscosity
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Try to check the mesh quality, by means of the distributions and values of the y-plus and y-star.

Good luck,
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