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

ndabir · January 28, 2010, 18:48

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?
for more information I should say my blade is a well-known c3x blade and it's setting angle is 59 degrees.

Thanks for your answers

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.

January 28, 2010, 18:48	high amount of drag coefficient. about 15????Is it okay?	#1
ndabir Senior Member navid Join Date: Jan 2010 Posts: 110 Rep Power: 16	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/(0.5densityvelocity^2area) I used the inlet conditions for density and velocity and I also used chordspan 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? for more information I should say my blade is a well-known c3x blade and it's setting angle is 59 degrees. Thanks for your answers

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January 30, 2010, 06:41		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	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.

February 1, 2010, 05:29		#3
ndabir Senior Member navid Join Date: Jan 2010 Posts: 110 Rep Power: 16	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."

February 1, 2010, 18:10		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	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.