# Backward Facing Step Flow

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 July 14, 2013, 10:51 Backward Facing Step Flow #1 Member   Ardalan Join Date: Feb 2010 Location: Baku Posts: 50 Rep Power: 9 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 x-y 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? ramakant likes this.

 July 14, 2013, 15:15 #2 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 3,819 Rep Power: 42 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

August 18, 2013, 11:58
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Join Date: Aug 2011
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Quote:
 Originally Posted by Ardalan Hello to everyone - Iterative solution to obtain steady solution (no time marching)

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.

refine the grid because you increase the Reynolds and re-run it,it should be fine.

 August 21, 2013, 08:54 #4 Member   Ardalan Join Date: Feb 2010 Location: Baku Posts: 50 Rep Power: 9 Thanks a lot I have another question regarding the flow over a backward step: How can I calculate the reattachment region length?

 August 21, 2013, 09:22 #5 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,693 Rep Power: 26 The reattatchment point is usually defined by the position where the wall shear stress (its streamwise component) changes sign.

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