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-   -   Correct lift but wrong pressure drag - possible? (http://www.cfd-online.com/Forums/main/13868-correct-lift-but-wrong-pressure-drag-possible.html)

zx July 27, 2007 00:39

Correct lift but wrong pressure drag - possible?
 
I'm simulating the flow past a cylinder using the immersed boundary mtd. I tried to calculate the lift and drag variation at Re=185 and 40

Comparing with my old structured grid and other papers, the lift is similar. Viscous lift is small so it's mainly pressure lift. But the drag is smaller than the correct answer, about 0.5-0.7 of it. On comparison, the viscous drag is about the same, but the pressure drag is much less than the correct answer.

Is this possible? Correct pressure lift but wrong pressure drag... or is it more likely that my post processing is wrong somewhere. Just would like to know which direction I should be heading...

Thanks...

zx July 27, 2007 02:13

Re: Correct lift but wrong pressure drag - possibl
 
I've also compare the pressure coefficient. The paper I had had a graph of the Cp around the cylinder at Re=40.

The x axis is from 0 to 180deg. Since cylinder flow at Re=40 is a symmetrical one, can I assume that the graph from -180 to 0deg will be a mirror image of the graph from 0 to 180deg?

My graph from -180 to 180deg has the part from around 30 to 180deg similar to the paper's graph. However, from -180 to 30deg, my graph is not smooth and there's some zig-zag. Does it mean that my evaluation of pressure is wrong somewhere?

Lionel S. July 27, 2007 09:10

Re: Correct lift but wrong pressure drag - possibl
 
"Correct pressure lift but wrong pressure drag"

I'm sorry I didn't read all your message, but generally drag is more difficult to compute because this involves second derivatives (the shear stress of the fluid). I think that it's a very common problem.

I would advice using a finer mesh in the boundary layer to get good results.

ag July 27, 2007 09:56

Re: Correct lift but wrong pressure drag - possibl
 
Since lift and drag are integrated quantities it is possible to compute one correctly and screw the other up using the correct pressure field. My first thought would be that you have an error in your drag computation. Can you verify that the pressure field is correct?

zx July 28, 2007 00:38

Re: Correct lift but wrong pressure drag - possibl
 
Ya thanks. On comparing the pressure coefficient, I figured that the pressure calculation's wrong somewhere (has to do with angle). I've now corrected it and it's working fine.


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