Fixing values in a poisson eqn

[Ben Tay]

Hi,

I am trying to solve a pressure poisson eqn resulting from the NS eqn. 
How can I fixed the pressure at some nodes while solving the poisson eqn?

I tried to make the coefficient of the diagonal location of the node 
very big e.g. 1e100. I also use

call KSPSetInitialGuessNonzero(ksp,PETSC_TRUE,ierr)

and insert the original values as initial solution:

do i=1,blk(1)%size_x
           
            do j=1,blk(1)%size_y

                call VecSetValue(xx,k,blk(1)%p(i,j),INSERT_VALUES,ierr)

                k=k+1

            end do

        end do

 in my code. However, after solving the eqn, the value I got is 1e-16 or 
0. So what have I done wrong?

Thanks