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Old   June 9, 2005, 14:14
Default Time Step for DES
Maged Ismail
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Hi all

I am using the Detached Eddy Simulation model for predicting the three-dimensional flow field of a round jet in crossflow. The velocity range is from 15-20 m/s and the domain dimensions are 0.75x0.3x0.3 m. I am using a relatively coarse grid of about 90000 nodes, because the simulation is done on a PC. The unsteady parameters that I have used are:

Time Step Size: 0.0001 No. of Time Steps: 1000 No. of Iterations per Time Step: 20

I have compared the mean results of the DES and results from steady RANS based simulation using standard k-epsilon model against experimental data, and I found that the steady k-epsilon simulation results are slightly in better agreement with the measurements; however both methods highly over-predicted the velocity profiles (with an error of about 250% for the wall-normal velocity).

I just have tow questions:

1. Can I get better results using the same grid size? In other words: Is this grid size appropriate for my problem.

2. What are the appropriate "Time Step Size" and "No. of Iterations per Time Step" that I should use?

Thanks a lot. Maged
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Old   June 27, 2005, 04:43
Default Re: Time Step for DES
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I believe that your grid is very course for DES. Though DES requires less number of grid pts compared to LES, your grid size seems to be very course for a DES simulation. Infact your current grid size might not give good results even with steady state RANS (k-e)models. Also try to play with time steps by doubling and halving your present time step size. See if the results are close to one another. Also you might need to use a longer averaging time (more than 1000 time steps).

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