Help on induced drag in VLM
 

Hello,

I am doing analysis of ocean current turbine using vlm. i computed lift force and Can anyone please let me know how to calculate the induced drag (or any reference).

Thanks
Aneesh

If you where able to calculate lift force then induce drag is pretty much the same. I suppose you calculated the force on each panel by:

Fi=gammai*rho*V*bi (Kutta's formula)

Now, as Fi is a force it has three components in x,y,z respectively, usually in body axes. So:

Fx=sum(Fxi)
Fy=sum(Fyi)
Fz=sum(Fzi)

Then to convert those total components to wind axes simply apply a rotation matrix (Euler angles) which depends on attack and yaw angles solely:

[Di,C,L]'=[rotation matrix]*[Fx,Fy,Fz]'

Di is induced drag, C is lateral force and L is lift. To obtain coefficients divide each of them by q*S.
If you're not happy with my answer, you can check the books by Jack Moran (provides sample VLM code) or the good ol' Katz & Plotkin. You can also check the source code of Tornado, AVL, VLMpc or VorStab, they are all open source VLM codes, and reliable.

That would give you a force perpendicular to the freestream and you wont get drag force but thrust force,.