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-   -   high amount of drag coefficient. about 15????Is it okay? (http://www.cfd-online.com/Forums/cfx/72219-high-amount-drag-coefficient-about-15-okay.html)

 ndabir January 28, 2010 18:48

high amount of drag coefficient. about 15????Is it okay?

Dear all

I am simulating a stator vane in cfx. I want to get drag coefficient for the blade so I used the force given by cfx-post and used the usual formula: force/(0.5*density*velocity^2*area)
I used the inlet conditions for density and velocity and I also used chord*span for the area. Have I used the right variables?
by doing this I obtained the drag coefficient of about 15. Isn't it too large? Shouldn't be always lower than unity?
Please tell me what is my problem? Is the area I am using ok?

 ghorrocks January 30, 2010 06:41

If you are comparing to literature values then they should define how they calculate it. If you cannot find a sensible existing definition then I would define density based on the average density in the blade region and the velocity based on the volume flow rate divided by the flow area at mid span.

The velocity and density at the inlet can be different to that at the blade by a factor of 2 or more. I reckon that's where your error is coming from.

 ndabir February 1, 2010 05:29

I found where is my problem. I asked from a professor about it and he said: "The ratio of lift/drag is more important than their exact values. so only the forces are important and you shouldn't be anxious of defining a proper density or velocity and also the surface area. the reasonable lift/drag ratio should be between 10-20."

 ghorrocks February 1, 2010 18:10

If all you want is the L/D ratio then the only thing you need to define is Re number (or maybe Ma number if compressible).

So for your case, if all you need is a L/D ratio you can simply get the forces in the two directions and take the ratio. Then the fluid properties is not important. But you still need to make sure you are running at the right Re and Ma numbers.

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