Flow over a backwards facing step is a classic computational fluid dynamics test
problem which is used extensively for validation of simulation codes. The test
problem essentially consists of studying how a fully developed flow profile
reacts to a sudden expansion in a channel. The expansion will cause a break in
the flow and a recirculation or separation zone will form. To measure and
compare results the resulting length of the recirculation or separation zone is
The stationary incompressible Navier-Stokes equations are applied with
simulation parameters corresponding to a Reynolds number, Re =
389. The inlet velocity is given as uinlet = 4umax
hinlet is the channel height, hstep the expansion step
height, and umax = 1 the maximum velocity. No-slip zero
velocity conditions are applied to all solid walls, and a suitable
outflow condition must also be applied. The reference recirculation
zone length found in the references ,  is estimated to be 7.93
length units (fraction of the step height).
This model is available as an automated tutorial by selecting Model Examples and Tutorials… >
Fluid Dynamics > Flow Over a Backwards Facing Step
from the File menu, and also as the MATLAB simulation m-script example ex_navierstokes4.
Step-by-step tutorial instructions to set up and run this model are linked below.
 P.M. Gresho and R.L. Sani, Incompressible Flow and the Finite Element
Method, Volume 1 & 2, John Wiley & Sons, New York, 2000.
 A. Rose and B. Simpson: Laminar, Constant-Temperature Flow Over a Backward
Facing Step, 1st NAFEMS Workbook of CFD Examples, Glasgow, UK, 2000.