# Stouhal number

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 February 10, 2010, 08:01 Stouhal number #1 New Member   Mehr Join Date: Feb 2010 Posts: 5 Rep Power: 9 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 Senior Member   Jouke de Baar Join Date: Oct 2009 Posts: 126 Rep Power: 10 "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 x-velocity, 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. Mehrez likes this.

 February 11, 2010, 03:21 Vortex shedding #3 New Member   Mehr Join Date: Feb 2010 Posts: 5 Rep Power: 9 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 Senior Member   Jouke de Baar Join Date: Oct 2009 Posts: 126 Rep Power: 10 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.cfd-online.com/Forums/flu...ure-point.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 pressure-time-history of a downstream point). Also see: Martinuzzi & Havel, Vortex shedding from two surface-mounted cubes in tandem, Int J Heat and Fluid Flow, June 20004

 February 11, 2010, 06:55 #5 New Member   Mehr Join Date: Feb 2010 Posts: 5 Rep Power: 9 I will check them all.

 February 12, 2010, 04:29 Pressure average #6 New Member   Mehr Join Date: Feb 2010 Posts: 5 Rep Power: 9 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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Quote:
 Originally Posted by jack1980 2. I suppose you mean a time averaged pressure in a point? See: http://www.cfd-online.com/Forums/fluent/40392-how-can-i-monitor-pressure-point.html . Then read the file in excel and take the average.
Quote:
 Originally Posted by caspian_mehr 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.
Since I am not working with Fluent anymore, if I remember you can work with something like: Solve/monitor/surface, choose Area Weighted Average
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May 17, 2014, 13:06
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Sami
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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:
 Originally Posted by jack1980 "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 x-velocity, 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.

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