Time step too small?
 

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.

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?

Thanks, Glen. I'll see what happens when I run it in double precision mode...