# this is a code snippet that will force the average velocity in the region of the upper and 
# lower boundaries to approach 0, in both x and y. This is the so-called 'Law of the Wall'

# place zero velocity cells on the upper and lower boundary
for j in range(0,lenx):
    if cell[1,j-1]&2:
        celln[0,j] += 16    #upper boundary is even
    if cell[1,j]&4:
        celln[0,j] += 32    #upper boundary is even
# lower boundary is odd
    if cell[leny-1,j]&32:
        celln[leny,j] += 4
    if cell[leny-1,j+1]&16:
        celln[leny,j] += 2