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Old   February 10, 2010, 08:01
Question Stouhal number
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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.
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Old   February 10, 2010, 10:55
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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 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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Old   February 11, 2010, 03:21
Question Vortex shedding
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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
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Old   February 11, 2010, 05:52
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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: How can I monitor pressure at a point? . 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
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Old   February 11, 2010, 06:55
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I will check them all.
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Old   February 12, 2010, 04:29
Question Pressure average
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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.
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Old   February 12, 2010, 05:49
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Quote:
Originally Posted by jack1980 View Post
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.
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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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Old   May 17, 2014, 14:06
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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 View Post
"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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