# finally, we need to impose the flow condition to make the gas move
# an example would be a shear flow, established by having the upper boundary
# giving a 'kick' in a prefered direction, such as:

for j in range(0,lenx):
    if cell[1,j-1]&2:       # anything coming in from the left, goes right, comes in from right, reflected back right
        celln[0,j] += 32    #upper boundary is even
    if cell[1,j]&4:
        celln[0,j] += 32    #upper boundary is even

# note that a shear boundary is much easier than flow across the boundary since mass needs to be conserved
# in the interor of the problem