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-   -   Backward FAcing Step - Frequency (http://www.cfd-online.com/Forums/main/12441-backward-facing-step-frequency.html)

 CFDtoy October 20, 2006 16:20

Backward FAcing Step - Frequency

I am looking at a BFS problem and would like to know if any measurement of the frequency of the backward facing step chamber has been carried out. (Backward facing step system has its own resonant frequency ..the frequency of the chamber itself..)

Could anyone direct me to papers or comment about it?

CFDtoy

 diaw October 21, 2006 00:23

Re: Backward FAcing Step - Frequency

CFDtoy wrote:

Backward facing step system has its own resonant frequency ..the frequency of the chamber itself..

---------

diaw writes:

Could you please clarify this statement a little more? I simulate & research related phenomena & would be interested in the basis for your statement.

desA

 CFDtoy October 22, 2006 12:19

Re: Backward FAcing Step - Frequency

Based on the step height to the length of the chamber it can act on its own as a resonant chamber sort of. I was wondering if anyone has tried to measure the frequency of such backward facing step chambers??

CFDtoy

 Jonas Holdeman October 23, 2006 07:55

Re: Backward FAcing Step - Frequency

Please explain what you mean by "chamber" in the BFS problem. In the standard problem, the downstream duct is uniform and its length is infinite, but truncated for numerical computations with downstream/exit boundary conditions that simulate the infinite length.

In a device like a whistle or organ pipe there is a definable chamber and an inlet/outlet, but a strong interaction between inlet and outlet flow. But that is not the case with the BFS problem.

 CFDtoy October 23, 2006 12:08

Re: Backward FAcing Step - Frequency

Hi Jonas: I guess I ll rephrase the problem. You are right, the backward facing step problem has inf length downstream.

= <--BFS problem----> ================= = =

= Step length defined=

= ..................................... =

= Step height defined=

=====================

If in the case that the downstream exit is confined and the BFS is made to look like a cavity ...what about the frequency of such chambers?

I am sure there is a finite frequency exist for the chamber...based on the step height and length of the duct there...

Can you suggest or comment about finding the resonant frequency of such chamberS?

CFDtoy

 ag October 23, 2006 15:46

Re: Backward FAcing Step - Frequency

There is quite a bit of work that has been done on cavity acoustics - the classic paper describing a reasonably good empirically based frequency estimate is the one by Rossiter. Do a google search on "Rossiter equation" and you should find quite a bit. A backward facing step on the other hand has no resonant frequency (that I'm aware of) because only one frequency source is present (the shear layer) and there is no real feedback path.

 Andrew October 27, 2006 10:21

Re: Backward Facing Step - Frequency

There are in fact two very distinct feedback mechanisms in the mixing layer flow. The first is from acoustic disturbances, with the second produced by fluctuations that are induced by concentrations of vorticity present within the layer itself.

 CFDtoy October 28, 2006 21:31

Re: Backward FAcing Step - Frequency

ag ! That was a great reply ..i found the rossiter eqn and several interesting dynamics and models> Curretly goin thru them..

In the same spirit, can u comment on oscillatory flow into small fluidic channels with these cavity in them ..have u come across any papers onthat ?

Thanks a bunch !

CFDtoy

 ag October 29, 2006 21:51

Re: Backward FAcing Step - Frequency

I haven't done any work in the area of smal fluidic channels. My work has been focused in the area of aircraft bays.

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