Stokes flow with zero mean pressure

Hi Jens,

The stokes system of equations is generated by finding the saddle point of a functional of velocities and pressure. Name that functional J(u,p). For your case you need to add the constraint \int p dx =0 to J. To do so multiply the constraint with a Lagrange multiplier, \lambda, by using and inner product rule. Since the specific functional is a scalar, the Lagrange multiplier is a scalar as well, and it can enter inside the integral. Therefore the functional is transformed to J(u,p) + \int ( \lambda*p) dx. Take the variations of the new functional with respect to the velocity, u, the pressure, p, and the new multiplier, \lambda, and set them to zero, to find the 3x3 system of equations you need to solve. The function space which needs to be used for the multiplier \lambda is defined as R = FunctionSpace(mesh, "R", 0).

Andi