[blobuin]

Hello!
I have a question, I could not find an answer to in this forum or googling. As I am rather a beginner, I cannot wrap my head around it. It might be very easy to the experts.

I have an object placed in an atmospheric boundary layer (ABL) for which I would like to calculate the lift and drag coefficients using openFoam. The ABL is defined by a reference velocity Uref at a height Zref. Zref is significantly higher up than the object. The object is located at a height where there is a high velocity gradient in the ABL.

The forceCoeffs function in openFoam has a magUInf parameter specifying the free stream velocity. However in the described situation it is unclear to me which magUInf I should set:

Possibilities for selecting the free stream velocity magUInf:
(1) magUInf = u(z_min)
(2) magUInf = u((z_min+z_max)/2) =u(z_center)
(3) magUInf = u(z_inc) //at incident height
(4) magUInf = u(z_max)
(5) magUInf = Uref
(6) magUInf = something else

It would be great to get an answer here!
Thank you all!

Uref_Cd_Cl.png