%***********************************************************************
%  This code is written by Chiu-Yen Kao
%  Copyright is reserved  !!


function phi = enforceBC(phi)
global nx ny nxs nys nxe nye 

      for i = nxs : -1 : 1
         for j = nys : nye
            phi(i,j) =  0;
         end
      end
      for i = nxe : nx
         for j = nys : nye
            phi(i,j) = 0;
         end
      end
      for j = nys : -1 : 1
         for i = 1 : nx
            phi(i,j) = 0;
         end
      end
      for j = nye : ny
         for i = 1 : nx
            phi(i,j) = 0;
         end
      end