Have a look at my answer here: https://www.researchgate.net/post/How_to_modify_predefined_model_equations_in_COMSOL

Thank you for your answer Rena, but I cannot see the weak form of the equation!

let's say you have the momentum equation and the constraint

d_t v + div T - f = 0

div v = 0

where v is a vector quantity (velocity), d_t is the time-derivative, T is a flux (second order tensor, i.e. stress tensor) and f is a vector source term (a force),

the weak form (lagrangian) for this system of PDE is

W1 = d_t v . u - grad u : T - f . v

W2 = q div v

in comsol "language" you need to put these equations in two physics. One has three unknowns (the three velocity components) and the other has one unknown (that is the pressure p). It would be better to have for the velocity a Lagrange-2 discretization and for the pressure a Lagrange-1 discretization (I mean polynomial order). Then you have four contributions

test(u)*u-(test(ux)*Txx+test(uy)*Txy+test(uz)*Txz)-test(u)*fx

test(v)*v-(test(vx)*Tyx+test(vy)*Tyy+test(vz)*Tyz)-test(v)*fy

test(w)*w-(test(wx)*Tzx+test(wy)*Tzy+test(wz)*Tzz)-test(w)*fz

test(p)*(ux+vy+wz)

to be inserted in the boxes "weak" under the weak form PDE.

Best

F