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December 5, 2009, 13:47 
CFD Study of a Diffuser

#1 
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Mat S
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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, KE 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 

December 5, 2009, 17:53 

#2 
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Charles
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1 million cells for a model like that sounds quite low. Also, you are flirting with flow separation in an adverse pressure gradient. This is not the natural habitat of the kepsilon turbulence model.


December 5, 2009, 19:47 

#3 
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Mat S
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which would you recommend? komega?,,,wrt to cell count i dont have access to a cluster, the uni one is broken, so i have to keep the cell count down!


December 6, 2009, 02:27 

#4 
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Charles
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Well, you are somewhat resourcelimited then, so probably a good idea to check just how far away from mesh insensitivity you are. As for turbulence models, first of all get hold of Wilcox's book! SST is probably not a bad place to start, but you need to take things step by step here. Going for a low Renumber method is probably advisable.


December 6, 2009, 07:17 

#5 
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Mat S
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ok, ive started by switching my turb model to k omega, and re solving the steady state cases, then look to incorporate the unsteady solver. any thoughts on the time step i proposed?


December 6, 2009, 14:24 

#6 
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Charles
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It doesn't sound unrealistic, but it also helps to a) check your maximum and average CFL numbers, and b) to see how much your solution changes as you refine the time step.


December 6, 2009, 15:04 

#7  
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Mat S
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Quote:
wrt to time step will the solution always be time step dependent since you will always pick up something extra(some tiny negligible disturbance?) with the smaller and small time step? Thanks Mat 

December 6, 2009, 15:10 

#8 
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Charles
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You need to do some reading about CFL numbers before you go any further. As for the second question, do it and see. It is a good way to learn.


December 7, 2009, 06:55 

#9  
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Mat S
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Quote:
wrt to my original questions  for a comparison case can i just use steady state analysis?  no actual force numbers will be calculated! Thanks Mat 

December 7, 2009, 08:45 

#10 
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Charles
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1. Also look under Courant number. 2. Maybe, depends on whether or not there is flow separation.
Generally though, your university lecturer is being paid to answer these questions .... 

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