[petsc-users] Interaction between multiple DMDA grid

Hi all, I am trying to modify a single block incompressible solver for flow over open cavity. One way I could do it, is to set part of the domain as flagged and apply velocity field=0 all over it. But it does not seem to be appropriate way (I am wasting alot of grid, just to have a small cavity). So I was wondering, is there any example to interact between two DMDA grids? I am using petsc to solve the poisson equation so if I can just create two set of grid (say DA_grid1 and DA_grid2) and have a interface between them, my problem would be alot more easier. Thanks a lot in advance. Somdeb