lift and drag ceofficient problem

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 August 12, 2011, 22:49 lift and drag ceofficient problem #1 Member   iswadi Join Date: Feb 2010 Posts: 44 Rep Power: 8 i have done my analysis on my wing. it was run at 2 different conditions. 1st condition was set at 15m/s velocity speed. 2nd condition was set at 10 m/s velocity speed. the main value that i am trying to get was the lift and drag coefficient (CL and CD).i have adopted almost the same step as the CFX airfoil tutorial provided in the Ansys customer portal. below was the formula that i adopted from the tutorial AOA = 10[deg] Denom = 0.5*massFlowAve(Density)@Inlet*Uinf^2*9.045[m]*0.001[m] Drag = cos(AOA)*Fx+sin(AOA)*Fy Fx = force_x()@Airfoil Fy = force_y()@Airfoil Lift = cos(AOA)*Fy-sin(AOA)*Fx Uinf = 10[m s^-1] Ux = Uinf*cos(AOA) Uy = Uinf*sin(AOA) cD = Drag/Denom cL = Lift/Denom the problem that i`m facing now is the final results for lift and drag coefficient values for both condition (15m/s and 10m/s) are same!! i knew it was incorrect. i`m completely stuck right now. i dont know where was my mistake. can anybody give me some idea where i should start in tracing my mistake. FYI, my analysis was involving a FSI/ multifield analysis.

 August 13, 2011, 04:55 urgently need....How To Calculate Friction Drag in CFX-13 #2 New Member   Ronald Hutauruk Join Date: Dec 2010 Posts: 4 Rep Power: 7 Dear Friends, do you have some tutorials how to calculate friction drag in ansys CFX-13. I need information for calculating viscous drag including pressure drag and friction drag in catamaran (demi hull) thx hutaru Indonesia

 August 14, 2011, 16:04 - #3 Member   Mauricio Labarca Join Date: Aug 2009 Posts: 35 Rep Power: 9 Hello ice, I'm not sure why you are getting those results but have you tried comparing the values of the data you're getting from your flow field in both cases? that'd be massFlowAve(Density)@Inlet, force_x()@Airfoil force_y()@Airfoil. I'm guessing you already compared all the other values ( Denom, Lift, Drag, etc.) hutaru:Can't help you with that, but open a new thread and maybe someone else will.

 August 15, 2011, 00:56 #4 Member   iswadi Join Date: Feb 2010 Posts: 44 Rep Power: 8 dear mauricio, i have made comparison of force_x()@Airfoil and force_y()@Airfoil at 15m/s and 10m/s. indeed, there were huge different between them. the denom was constant. however i have made a manual calculation but still maintaining and used the value of force_x()@Airfoil and force_y()@Airfoil from computational. i was surprising to see that the formula below giving me a quite different value in manual results compare to computational results!! Drag = cos(AOA)*Fx+sin(AOA)*Fy Lift = cos(AOA)*Fy-sin(AOA)*Fx one more thing. i have gone through some of the posts in this forum. some of them were mentioned about Force_y () @Airfoil is equal to LIft ? Force_x () @Airfoil is equal to Drag ? is it true? do anybody have experienced in this situation? do anybody have their own formula in determining the LIFT and DRAG? Please share with me... Last edited by icemaniac178; August 15, 2011 at 04:05.

August 15, 2011, 18:48
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Mauricio Labarca
Join Date: Aug 2009
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Quote:
 Originally Posted by icemaniac178 i was surprising to see that the formula below giving me a quite different value in manual results compare to computational results!! Drag = cos(AOA)*Fx+sin(AOA)*Fy Lift = cos(AOA)*Fy-sin(AOA)*Fx
Hello ice, i've checked the documentation and it turns out cos, sin CEL function get the input arguments in radians, did you convert the value in AOA before passing it to the drag and lift functions?

about the forces. As far as i know The force command operates on a boundary condition and gets the resultant force of all the nodes you have in the surface that makes your BC. My guess is that You'd have check the theory on how those Fx and Fy are supposed to be applied and obtained, one thing are the forces applied in one point of the airfoil, and another a balance obtained over the whole surface.

 August 16, 2011, 23:37 #6 Member   iswadi Join Date: Feb 2010 Posts: 44 Rep Power: 8 thanx mauricio, i will check about the angle in radian unit. by the way, i have checked the theory about lift and drag coefficient. it really make sense where the theory says that the lift coefficient is almost constant when Re increase more than 10,000. Only Clmax is really affected due to Re changes. Drag coefficient also affected but only at minimum magnitude. Cd changes is more clearly when the Re <10,000. Anybody who have experienced on this matter is highly appreciated to share their knowledge. i believe this would be benefits to all

August 17, 2011, 18:40
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Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,638
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Quote:
 did you convert the value in AOA before passing it to the drag and lift functions?
There should be no need to convert units. When you define the variable you give its units and the conversions are done automatically, so 10 [degree] is internally converted to radians.

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