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February 10, 2010, 08:01 
Stouhal number

#1 
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Mehr
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Hi everybody.
I am studying vortex shedding around a block. I need to find Strouhal number. Somebody told me i should do this by monitoring vortex. Is there an accurate method in fluent to measure it? Another question about ftt. How can I use it? I read user guide but I couldn't find how could I read or prepare it to use. Sorry for my weak English. 

February 10, 2010, 10:55 

#2 
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Jouke de Baar
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"The ... Strouhal number is given by St = f D/U, where D is the cylinder diameter ...and U is the freestream velocity ..."
"The easiest way to compute the frequency is to use the time between peaks of the xvelocity, pressure, or lift coefficent. (The total drag coefficient should not be used to compute the frequency as it feels the influence of both vortices and will give a frequency twice that of the individual vortex shedding frequency.)" From: http://www.grc.nasa.gov/WWW/wind/val...es/Study1.html So I suppose you can simply use the frequency of your lift coefficient. 

February 11, 2010, 03:21 
Vortex shedding

#3 
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Mehr
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Thanks for your help.
At first let me tell you about my case. I am studying laminar and turbulence flow around two blocks in series. So I want to know the effect of distance and blockage ratio on vortex. The blockage ratio is .2 up to .7. 1. How can I get " time averaged streamline" in Fluent? 2. Is it possible to calculate "averaged pressure" or other field? how? 3. How can I calculate " time series of velocity"? 4. Is there some easy way to calculate " critical Reynolds"? Regards 

February 11, 2010, 05:52 

#4 
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Jouke de Baar
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I see. Haven't been looking at this problem before, but found some things that might be usefull.
1. I don't know if there is such a thing. I think that by definition a streamline is taken at a distinct instant of time. 2. I suppose you mean a time averaged pressure in a point? See: http://www.cfdonline.com/Forums/flu...urepoint.html . Then read the file in excel and take the average. 3. Again, same link, but use velocity. 4. Is this where vortex shedding starts? Two initial thoughts: I suppose this is the Re where the cl starts oscillating; so plot cl amp against Re and look for a sudden increase? Also, doesn't the cd initially drop when you increase the Re; but then, I'd expect it to grow again when vortices start shedding; so plod cd against Re and look for the minimum? Also, in this case I think you might have three Strouhal numbers. One for each of the obstacles (determine from cl1 and cl2 frequency) and one total Strouhal number (determine from frequency in pressuretimehistory of a downstream point). Also see: Martinuzzi & Havel, Vortex shedding from two surfacemounted cubes in tandem, Int J Heat and Fluid Flow, June 20004 

February 11, 2010, 06:55 

#5 
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Mehr
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I will check them all.


February 12, 2010, 04:29 
Pressure average

#6 
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Mehr
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Thanks for your help
What I need for pressure is not just in one single point. As I said I want to have an average pressure along the channel. (I mean my two blocks are in a channel and i need to see pressure behavior along the channel, other words in the flow direction. Thanks in advance. 

February 12, 2010, 05:49 

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Maxime Perelli
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May 17, 2014, 13:06 

#8  
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Mehrez
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Hello dear Jack,
Thank you for this interesting ref, however I have a question about the St number computed from a point in the downstream. In the ref, they use the x component of the velocity, But I have noticed that the y component of the velocity starts to oscillate for a Reynolds number lower than Re for which the x component starts to oscillate. Do you think that I should compute the frequency from the y component of the velocity ? Thank you, Best regards Mehrez Quote:


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