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-   -   Bluff body used for diffuser testing (http://www.cfd-online.com/Forums/cd-adapco/70775-bluff-body-used-diffuser-testing.html)

lemat1 December 5, 2009 10:11

Bluff body used for diffuser testing
 
Hi all, i am currently conducting a comparative study of underbody performance if a bluff body with a diffuser. The model is fixed with a moving ground at 40m/s. i am assessing drag and downforce through changes in ramp angle, and yaw condition using a constant ride height. I am pretty sure that vortices should be shed from the body, and that these will affect the drag on the body yet i am unable to detect any in my results. I am using an implicit unsteady flow condition, K-E turb model and RANS solver. My Re number is 1.4x10^7, giving an estimated (from research) strouhal number of 0.28


my questions are;

1 - should i worry about vortices? im am comparing results against themselves so if i ignore this on all is that valid?(i.e. a constant effect) - i do not want exact downforce figures just which is most efficient (lift/drag) - could even switch to steady state?.-----i suspect this is wrong

2 - my time step is set 0.002 seconds which corresponds to 50 steps per vortex shed, at f=9.7Hz i.e. delta t = 1/(50*f) - is this enough/too fine?

3 - how many internal iterations per time step? in a steady state i see convergence at about 250 steps, so is this a good starting point?

4 - the model is 1.1m*0.7 m, so consequently the tunnel huge to maintain 5% blockage, is it possible i am not seeing vortices due to mesh density? im at nearly 1 million cells - using the surface wrapper and polyhedral mesh. refinement around the bluff body! - is this ok?

Thanks all

Regards

Mat


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