CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Stouhal number

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree1Likes
  • 1 Post By jack1980

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   February 10, 2010, 08:01
Question Stouhal number
  #1
New Member
 
Mehr
Join Date: Feb 2010
Posts: 5
Rep Power: 16
caspian_mehr is on a distinguished road
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.
caspian_mehr is offline   Reply With Quote

Old   February 10, 2010, 10:55
Default
  #2
Senior Member
 
Jouke de Baar
Join Date: Oct 2009
Posts: 127
Rep Power: 16
jack1980 is on a distinguished road
"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.
jack1980 is offline   Reply With Quote

Old   February 11, 2010, 03:21
Question Vortex shedding
  #3
New Member
 
Mehr
Join Date: Feb 2010
Posts: 5
Rep Power: 16
caspian_mehr is on a distinguished road
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
caspian_mehr is offline   Reply With Quote

Old   February 11, 2010, 05:52
Default
  #4
Senior Member
 
Jouke de Baar
Join Date: Oct 2009
Posts: 127
Rep Power: 16
jack1980 is on a distinguished road
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
jack1980 is offline   Reply With Quote

Old   February 11, 2010, 06:55
Default
  #5
New Member
 
Mehr
Join Date: Feb 2010
Posts: 5
Rep Power: 16
caspian_mehr is on a distinguished road
I will check them all.
caspian_mehr is offline   Reply With Quote

Old   February 12, 2010, 04:29
Question Pressure average
  #6
New Member
 
Mehr
Join Date: Feb 2010
Posts: 5
Rep Power: 16
caspian_mehr is on a distinguished road
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.
caspian_mehr is offline   Reply With Quote

Old   February 12, 2010, 05:49
Default
  #7
Super Moderator
 
-mAx-'s Avatar
 
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 3,297
Rep Power: 41
-mAx- will become famous soon enough
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.
Quote:
Originally Posted by caspian_mehr View Post
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
__________________
In memory of my friend Hervé: CFD engineer & freerider
-mAx- is offline   Reply With Quote

Old   May 17, 2014, 14:06
Exclamation
  #8
Member
 
Sami
Join Date: Nov 2012
Location: Cap Town, South Africa
Posts: 87
Rep Power: 13
Mehrez is on a distinguished road
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.
Mehrez is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
[mesh manipulation] Mesh Refinement Luiz Eduardo Bittencourt Sampaio (Sampaio) OpenFOAM Meshing & Mesh Conversion 42 January 8, 2017 13:55
DecomposePar unequal number of shared faces maka OpenFOAM Pre-Processing 6 August 12, 2010 10:01
[blockMesh] BlockMeshmergePatchPairs hjasak OpenFOAM Meshing & Mesh Conversion 11 August 15, 2008 08:36
Unaligned accesses on IA64 andre OpenFOAM 5 June 23, 2008 11:37
[Commercial meshers] Trimmed cell and embedded refinement mesh conversion issues michele OpenFOAM Meshing & Mesh Conversion 2 July 15, 2005 05:15


All times are GMT -4. The time now is 07:02.