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-   -   lift and drag ceofficient problem (https://www.cfd-online.com/Forums/cfx/91510-lift-drag-ceofficient-problem.html)

 icemaniac178 August 12, 2011 22:49

lift and drag ceofficient problem

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.

 Hutaru August 13, 2011 04:55

urgently need....How To Calculate Friction Drag in CFX-13

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:p

 mauricio August 14, 2011 16:04

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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.)

 icemaniac178 August 15, 2011 00:56

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?

 mauricio August 15, 2011 18:48

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Quote:
 Originally Posted by icemaniac178 (Post 320068) 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.

 icemaniac178 August 16, 2011 23:37

thanx mauricio,
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

 ghorrocks August 17, 2011 18:40

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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