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How to explain the lift force in SU2's source code?

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Old   October 10, 2022, 23:27
Question How to explain the lift force in SU2's source code?
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Jay Wee
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I know the drag always aligns with the freestream direction, so it is quite easy to compute drag by:
drag = <total force vector> * <unit vector of freestream direction>


However, the lift force's direction is not obvious. I think the lift formula should be
<lift vector> = <total force vector> - drag * <unit vector of freestream direction>
then the magnitude of lift could be got by

lift = sqrt(<lift vector> * <lift vector>), which is a complex formula.


But in the SU2's code, the lift formula is very simple: (click here to check source code: https://github.com/su2code/SU2/blob/...Base.inl#L1979)

InvCoeff.CL[iMarker] = -ForceInviscid[0] * sin(Alpha) + ForceInviscid[2] * cos(Alpha);


It looks strange to me, b/c lift has no dependence on Beta. Anyone could help to explain or post some reference link? Thanks.

Last edited by jaywee; October 11, 2022 at 08:17.
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Old   October 22, 2022, 12:59
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Ercan Umut
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As a graduate student I am really not sure about this but I guess they are taking normal and axial force's coefficients and using them on the lift and drag calculations. When I first saw the eqns. in code I tought they reminded me of something and found them on the Anderson's Fundamentals of Aerodynamics book. I attached 2 pictures as reference. In my opinion, they are not as easy as they look, at first glance at least you get to used to them in time.
Attached Images
File Type: jpeg anderson2.jpeg (57.3 KB, 49 views)
File Type: jpeg anderson.jpeg (116.6 KB, 44 views)

Last edited by CleverBoy; October 22, 2022 at 16:02.
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Old   October 23, 2022, 20:38
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Pedro Gomes
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The result of <force vector> - drag * <drag direction> is not the lift vector, but rather lift + sideforce
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