The flow over a backward facing step is also a popular test problem for developing flow simulation codes. It consists of a fluid flowing in a straight channel which abruptly widens on one side. Numerical results obtained using a wide range of methods can be found in Gartling (1990). When the fluid flows downstream it produces a recirculation zone on the lower channel wall, and for sufficiently high Reynolds it also produces a recirculation zone farther downstream on the upper wall. The boundary conditions and problem domain are present in Figs. below. Note that the Fig. shows only the part of the computational domain that contains all the essential features.