Mass Matrix of BDM-interpolated Discontinuous FE Spaces

Dear Community,

I am trying to use NGSolve to realize something similar to Section 4.2 of  the paper by Alexander Linke and Christian Merdon , where a mass matrix of the BDM-interpolated discontinuous FE space is added to the bilinear operator of Navier Stokes equations. May I ask if there is any way to directly add to the BilinearForm formula a mass matrix with trial and test bases interpolated/projected into another space, e.g. BDM? Right now I can separately get the mass matrix with bases interpolated by embedding into the target space and back into the original discontinuous FE space, but I do not know how to add this separate mass matrix to the bilinear form to get the operator inverse. Could you provide some insight into this? Thank you for your time!

Hi Wenzheng,

you can do this interpolation easily if you can do it on element-level:

bfm += Interpolate(u, RTSpace) * Interpolate(v, RTSpace) * dx

You can find an example (for MITC plate elements) in the documentation here:
docu.ngsolve.org/latest/i-tutorials/unit...r_Mindlin_plate.html

Best, 
Joachim