Backward Facing Step Flow
Hello to everyone
I've written a code to simulate laminar backward facing step flow. The main features of the method I have developed are as follows:  incompressible flow  finite volume  Simple algorithm on Staggered grids  1st order Upwinding for convection terms  central differencing for diffusion terms  Iterative solution to obtain steady solution (no time marching)  using line TDMA for the solution of algebraic equations of xy momentum and pressure correction at lower Reynolds numbers the code works satisfactorily but at higher Reynolds numbers (e.g. 600, 800) the L2 norm of mass unbalance decreases 1 or 2 order of magnitudes, then it oscillates but doesn't decrease further. I have checked the Residuals, the cells where the mass unbalance doesn't decrease, are located near the end of lower wake. I have tested the code for simulating cavity driven flow, it works fine. Does anybody have any suggestion? What shall I do? Does the flow have unsteady nature at high Reynolds numbers? 
yes, after Re=700 800 the flow is transitional and it has no sense the 2D laminar solution. You can search for a paper of Armaly et al. on JFM

Quote:
For Re=600, 800 the flow is not anymore steady and is definitely unsteady. So it is normal that your code can not find a steady solution of a flow which is unsteady. So implement unsteady version of your solver accounting for time derivative. refine the grid because you increase the Reynolds and rerun it,it should be fine. 
Thanks a lot
I have another question regarding the flow over a backward step: How can I calculate the reattachment region length? 
The reattatchment point is usually defined by the position where the wall shear stress (its streamwise component) changes sign.

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