Flow over backward facing step

[RoyJoy]

Greetings all. I was wondering if anyone would be able to answer a question for me with regard to flow over a backward facing step. Is there anyway to characterize the ratio H/X for the reattachment of flow at some downstream point given only the H and without the use of numerical modeling? Thank you so much for your help.

 

[btrueblood]

This ref. may get you close, or at least give you some more references to look at...

Driver, D.M. and Seegmiller, H.L., "Features of a Reattaching Turbulent Shear Layer in Divergent Channel Flow," AIAA Journal, Vol. 23, No. 2, Feb. 1985, pp. 163-171.

The paper is referenced by some numerical studies of the problem you are looking at (the numerical studies are for air flow).

"and without the use of numerical modeling? "

The numerical models I've seen (NASA/AF NPARC and WIND CFD codes) do a fairly poor job of predicting reattachment distance. You (will) do yourself credit to find as much published experimental data as possible, and hope to correlate to your specific case.

 

[GregLocock]

Kay and Nedderman (Intro to Fluid Mechanics) talk about a critical depth at the step, but carefully ignore what happens after the step.

If you know the stream velocity at the step (which is higher than in the preceeding channel, and for which the equation is well known, doesn't Bernouilli and so on give a reasonable approximation (and if not, why not?)

Anyway a google search for 'flow over a weir' gave many leads.

My bet is some crusty old sewage engineer has compiled the chart you are looking for, if you are lucky it'll be in King's "Handbook of Hydraulics", which I don't have.






Cheers

Greg Locock

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