# forceCoeffs - refL and refA

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January 19, 2014, 16:41
forceCoeffs - refL and refA
#1
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Tobias Holzmann
Join Date: Oct 2010
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Hi all,

just a simple question.
The refLength and refArea in my case is that one I showed in my pictures.
Is that correct?

meshLength_x = mesh length normal to the visible plane (2D)
meshLength_x = surface length normal to the visible plane (3D)

PS: Does it matter if i have the dragDirection set (0 1 0) or (0 -1 0) - should only be a sign topic or?
PPS: pitchAxis - what does that mean? I set it to (0 0 0).

Regards Tobi
Attached Images
 0degree.jpg (14.0 KB, 24 views) 15degree.jpg (14.9 KB, 18 views)

 January 19, 2014, 17:01 #2 New Member   Gabriel Boucher Join Date: Jul 2013 Posts: 23 Rep Power: 5 For the drag direction, the only difference is the sign of the force output. It doesn't really matter.

January 19, 2014, 17:08
#3
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Tobias Holzmann
Join Date: Oct 2010
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Quote:
 Originally Posted by Alfalfa For the drag direction, the only difference is the sign of the force output. It doesn't really matter.
Thanks

I think the calculation of ref length and area are okay because you did not correct me

 January 19, 2014, 17:43 #4 New Member   Gabriel Boucher Join Date: Jul 2013 Posts: 23 Rep Power: 5 I don't know about the ref length. From Fundamentals of Fluid Mechanics, "A (characteristic area) is taken to be the frontal area - the projected area seen by a person looking toward the object from a direction parallel to the upstream velocity." So your area is good. For the length, I am not sure. I know that for airfoil you usually take the length of the chord. For a flat plate, I you should use the length of the plate in the flow direction, but I might be wrong. For the pitchAxis, I am not sure but I think it is used for the moment calculations.

 January 19, 2014, 22:17 #5 Member   Eric Robertson Join Date: Jul 2012 Posts: 95 Rep Power: 6 The length of your object (from nose to tail) is the reference length regardless of its angle of attack with the flow. This is true unless you have a special definition for this. The reference area is the surface area. If this is an airfoil, then this quantity is simply the reference length times the cell "thickness" (for 2D), which gives you the area of the top surface of the airfoil. Your pitch axis should correspond to the "aerodynamic center" of your airfoil. For your object, which appears to be perfectly symmetrical, the pitch axis should be set as the point at the center of your object. This is only crucial if you care about finding Cm.

 January 20, 2014, 04:35 #6 Senior Member     Tobias Holzmann Join Date: Oct 2010 Location: Leoben (Austria) Posts: 1,395 Blog Entries: 5 Rep Power: 24 Thanks for your answers. Then I did the right things Regards Tobi

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