Problems with SUPG body force term

FEM question

Dear friends,

I have been getting problems with the SUPG term of the body forces (BF) in a program I'm implementing. I have a SUPG/PSPG finite element incompressible Navier-Stokes program where every time I "turn on" the evaluation of the SUPG term of the body force the program blows up. This problem doesn't happen when I use only the Galerkin term and the result seems to be ok. I know that the SUPG term of the forcing terms introduces more non-linearity in the problem, so, my body forces evaluation is before the predictor-multicorrector loop and are linearized with the time lagged velocity field. So, what could be wrong? My SUPG BF vector is defined by:

f_SUPG = int[ tau_SUPG * (u grad w) * f]

int = integral operator grad = gradient operator tau_SUPG = stabilization parameter u = velocity vector (lagged in time for linearization purposes) w = weighting function f = force vector

being f and w defined by:

f = sum(N_a*f_a) and w = sum(N_b*c_b)

would reach:

f_SUPG = tau_SUPG*{int[u*grad*N_a*N_b]*f}*c_b

could anybody give me a hand to solve it?

thanks

FEM