boundary integral

Hello everyone,

I have done some NS analysis and now want to do some post-processing using my results, I want to calculate my lift and drag over a certain part of the boundary (i.e its a surface integral). I have my functional defined and I use following line to calculate the drag

     c_d = Integrate(cf=Drag(u_h,p0), mesh=mesh, VOL_or_BND=BND, definedon=mesh.Boundaries("cyl"), order=1)

but i get the error

Anyone knows what this is?

Thanks in advance.

My guess is, that you need to use the trace operator for the HDiv function if you want to evaluate on the boundary, so do u_h.Trace(), but if you provide an example it would be easier.
Best Christopher