Lift Coefficient Problems

hannah72206 · June 27, 2012, 15:45

I'm running Fluent on a simple UAV design and I'm trying to get a lift coefficient curve. I have a set of design points that run from an angle of attack of 0 to 25 degrees. I get a max lift coefficient around 16 degrees. It goes down at 18 degrees. Then goes back up when it gets to 20 degrees! What is going on?

I've tried both steady state and transient with no real difference.

cfd seeker · June 30, 2012, 03:36

Running steady state solver is out of question at such a high angle of attack because there is lot of flow separation at these angle of attacks but the problem you are telling looks quite strange. Give it a try by changing your time step and then let me know what happens.
Regards

hannah72206 · July 3, 2012, 12:02

It turned out to be as simple as a mesh problem in that the mesh wasn't fine enough around the UAV. The wind tunnel was so large that even when the cells looked small, it wasn't enough. I have fixed it now, and I'm getting a beautiful coefficient curve even with steady state. (I'm not going too far past the stall angle) Thank you!

June 27, 2012, 15:45	Lift Coefficient Problems	#1
hannah72206 New Member Hannah Mac Join Date: Jun 2012 Posts: 15 Rep Power: 13	I'm running Fluent on a simple UAV design and I'm trying to get a lift coefficient curve. I have a set of design points that run from an angle of attack of 0 to 25 degrees. I get a max lift coefficient around 16 degrees. It goes down at 18 degrees. Then goes back up when it gets to 20 degrees! What is going on? I've tried both steady state and transient with no real difference.

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June 30, 2012, 03:36		#2
cfd seeker Senior Member Join Date: Mar 2011 Location: Germany Posts: 552 Rep Power: 20	Running steady state solver is out of question at such a high angle of attack because there is lot of flow separation at these angle of attacks but the problem you are telling looks quite strange. Give it a try by changing your time step and then let me know what happens. Regards

July 3, 2012, 12:02		#3
hannah72206 New Member Hannah Mac Join Date: Jun 2012 Posts: 15 Rep Power: 13	It turned out to be as simple as a mesh problem in that the mesh wasn't fine enough around the UAV. The wind tunnel was so large that even when the cells looked small, it wasn't enough. I have fixed it now, and I'm getting a beautiful coefficient curve even with steady state. (I'm not going too far past the stall angle) Thank you!