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Old   September 6, 2010, 02:28
Default Time step too small?
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Derwin Parkin
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Hi guys,

I'm running a transient simulation of a little truck model we have in the wind tunnel. Width Re is around 640,000 for this and I'm using DES.

They have been getting a Cd value of around 0.29. I was always getting around 0.33 but ever since I reduced the size of the time steps I have gotten it to around 0.30, which is great.

However, I noticed that when I reduce the size of the time step to around 0.002 s, the vortex shedding I had observed with time step 0.005 s disappears. So my drag coefficient has improved, but I feel there should be some shedding at the rear, as that's what is happening in the wind tunnel.

Is my time step somehow too small to resolve the shedding somehow? I always thought going as small as possible was the goal.

* It should be noted I made a small geometric change before lowering the time step...it is possible that small change has somehow eradicated the shedding, but I highly doubt it.
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Old   September 6, 2010, 07:39
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Glenn Horrocks
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The smaller timestep will usually mean increased temporal resolution and therefore more shedding, not less. If it results in less shedding then you may be approaching numerical round-off and that is limiting convergence. Have you tried running double precision?
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Old   September 8, 2010, 00:03
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Derwin Parkin
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Thanks, Glen. I'll see what happens when I run it in double precision mode...
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