CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM

vortex shedding frequency

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

Like Tree8Likes

Reply
 
LinkBack Thread Tools Display Modes
Old   May 28, 2010, 02:13
Default vortex shedding frequency
  #1
Senior Member
 
NAVEEN.K.M
Join Date: Mar 2009
Location: Bangalore, Karnataka, india
Posts: 114
Rep Power: 9
naveen is on a distinguished road
hi everybody,

i am working on flow around a circular cylinder of diameter 2m using OpenFOAM 1.4.1 and 1.5 versions past 1 month. I am getting pressure and velocity contours correctly in both the versions.I am facing difficult to get the vortex shedding frequency in OpenFOAM.

Can you please suggest me how to get the strouhal number and vortex shedding frequency in OpenFOAM 1.5.

Flow conditions (laminar flow):

Reynolds number---------> 150 (based on cylinder diameter)
velocity------------------------>1 m/s
viscosity------>0.01333 m2/sec(based on cylinder diameter)
diameter of cylinder--------> 2 meters
OpenFOAM version-------->1.4.1, 1.5
solver----------------------------->icoFoam

can you please give me a suggestion how to get these values using the probes function and FFT.

waiting for your response

Regards,

Naveen
Mehrez likes this.
naveen is offline   Reply With Quote

Old   July 19, 2010, 02:10
Default
  #2
Senior Member
 
Jie
Join Date: Jan 2010
Location: Australia
Posts: 123
Rep Power: 8
jiejie is on a distinguished road
Please correct me if I am wrong.

You can catch the time series data for Cd or Cl and use FFT to get the spectra to identify the St.
Mehrez likes this.
jiejie is offline   Reply With Quote

Old   July 19, 2010, 02:58
Default vortex shedding frequency
  #3
Senior Member
 
NAVEEN.K.M
Join Date: Mar 2009
Location: Bangalore, Karnataka, india
Posts: 114
Rep Power: 9
naveen is on a distinguished road
hi jiejie,

Thanks for your reply..........

I have successfully completed that work 1 month back using OpenFOAM...anyway thanks for your kind response...


Regards

Naveen
naveen is offline   Reply With Quote

Old   December 2, 2010, 16:44
Default FFT for a sound spectrum
  #4
New Member
 
Bertrand Delvaux
Join Date: Apr 2010
Posts: 29
Rep Power: 8
Bertrand is on a distinguished road
Hello,

I read you are talking about FFT from probes. I would need this for my case.
How do we proceed FFT in OpenFOAM, or is there any documentation I could find on this?

Thank you!

Bertrand
Bertrand is offline   Reply With Quote

Old   March 19, 2011, 11:44
Default strouhal number
  #5
Member
 
s.rasoul_varedi
Join Date: Feb 2010
Posts: 82
Rep Power: 7
desert_1250 is an unknown quantity at this point
Send a message via Yahoo to desert_1250
hi all of foamers,

can any one tell me how to calculate strouhal number or shedding frequency?
( OF 1.5-dev)

waiting for your response...
tanks
Mehrez likes this.
desert_1250 is offline   Reply With Quote

Old   March 20, 2011, 08:22
Default
  #6
Member
 
Patricio Bohorquez
Join Date: Mar 2009
Location: Jaén, Spain
Posts: 94
Rep Power: 9
pbohorquez is on a distinguished road
You could proceed as described in the following paper:

Bohorquez, P., Sanmiguel-Rojas, E., Sevilla, A., Jiménez-González, J., Martínez-Bazán, C. Stability and dynamics of the laminar wake past a slender blunt-based axisymmetric body. Journal of Fluid Mechanics (in press)
pbohorquez is offline   Reply With Quote

Old   March 20, 2011, 18:32
Default
  #7
Member
 
s.rasoul_varedi
Join Date: Feb 2010
Posts: 82
Rep Power: 7
desert_1250 is an unknown quantity at this point
Send a message via Yahoo to desert_1250
hi dear pbohorquez
thanks for your paper atachment, i think that it is very helpful for me but :

i am working on flow around a circular cylinder of diameter 0.025 & Re=150 using OpenFOAM 1.5-dev versions & get pressure and velocity contours correctly,
i have two question:
1) when i want to calculate Drag and lift coeff with add a related code to controlDict, the answer of this work not validate. why??( answer of drag is very low, while drag in Re=150 must be nearly 1)
2) how can i calculate vortex shedding frequency
3)if have another related refrence for my work, i am very glad that atached link for me

i chose this parameter for calculate cd & cl =>
Aref=0.025
Lref=0.0128
magUinf=0.006
rho=1000
liftDir=(0 1 0)
dragDir=(1 0 0)


tank a lot
waiting...
Rasoul
desert_1250 is offline   Reply With Quote

Old   March 21, 2011, 06:49
Default
  #8
Member
 
Patricio Bohorquez
Join Date: Mar 2009
Location: Jaén, Spain
Posts: 94
Rep Power: 9
pbohorquez is on a distinguished road
To compute the St number you should include in the controlDict the 'sets' and generate during the numerical simulations the samples of the velocity field at the point you are interested in. Then read the information you generated with some software. I employed Matlab to read the data and ezfft to compute the Fourier transform.

I did not calculate the Drag and Lift, so I can not help.
pbohorquez is offline   Reply With Quote

Old   December 8, 2012, 23:59
Question flow around cylinder..
  #9
New Member
 
rishitosh's Avatar
 
Rishitosh Ranjan
Join Date: Dec 2012
Location: Trichirapalli, Tamilnadu, India
Posts: 18
Rep Power: 5
rishitosh is on a distinguished road
hiii..

i m also new in dis CFD area...
i m working on flow over two side by side cylinders... @ Re= 100 and 200..
i got good results..

but i want to find out Strouhal number for all cases...

i came to know by finding shedding frequency with help of FFT, using cofficient of lift data, in fluent 13.0 we can calculate Strouhal number...

but i m getting helpess..

how to use FFT to find shedding frequency....?????
-Rishi
Mehrez likes this.
rishitosh is offline   Reply With Quote

Old   May 17, 2014, 10:12
Default
  #10
Member
 
Mehrez
Join Date: Nov 2012
Location: Bordeaux, France
Posts: 79
Rep Power: 5
Mehrez is on a distinguished road
Hi dear all,
Rishitosh, did you find how to compute the St nmbr ?
Thanks,

Mehrez
alirezahd likes this.
Mehrez is offline   Reply With Quote

Old   April 6, 2015, 11:05
Default
  #11
Senior Member
 
ok
Join Date: Oct 2013
Posts: 338
Rep Power: 5
Maimouna is on a distinguished road
Hi,

any one find the answer? Provide me please. How to add ''sets'' in controlDict file and using fft?

Any answer will be appreciated.

Regards

Maimouna
Maimouna is offline   Reply With Quote

Old   April 9, 2015, 11:10
Default
  #12
Member
 
Eric Robertson
Join Date: Jul 2012
Posts: 95
Rep Power: 5
msuaeronautics is on a distinguished road
Hi guys,

Based on my experience in sampling the wake to get the Strouhal number in OpenFOAM, the best way to do it is to compute the velocity spectra at certain points in the wake. I prefer to use points in the separated shear layer (the regimes I've studied are Re > 10K) but it depends on your case. I've tried using Cd and Cl but these methods seem a bit less optimal in finding all of the relevant frequencies.

1. Use the controlDict 'probes' feature to get a time history of your instantaneous velocity. You'll want to sample at every timestep.
2. Subtract the mean from your instantaneous to get U'.
3. Feed your U' history into a program like MATLAB. I find that the plomb() function gives best results, but there are others.
4. The result gives you a spectra in wave number space, which you will have to convert into frequencies.
Mehrez and utkunun like this.
msuaeronautics is offline   Reply With Quote

Old   May 5, 2016, 12:17
Default
  #13
Senior Member
 
Ariel Edesess
Join Date: Aug 2015
Posts: 101
Rep Power: 2
arieljeds is on a distinguished road
Hi @msuaeronautics, I know that this thread is kind of old so I'm just hoping to be seen here :-).

First of all, thank you for your clear explanation. I am understanding most of it but I have a couple of more specific questions on implementation. I am simulating waves moving past a surface piercing cylinder (interested in free surface effects) using waveFoam.

1) I have set up a line of probes in the wake (from y = -5 to y = 5 for a cylinder of diameter 6 with the origin at (0,0,0) and y is the direction perpendicular to the oncoming waves, z is the vertical direction). I have velocity, pressure, vorticity from the probes.

You said to subtract the mean from the instantaneous. Do you mean to subtract a mean value from the freestream? Or else, do you mean to calculate the mean velocity within the wake for the whole time series and subtract that value from the individual instantaneous velocities?

Thank you!

Ariel
arieljeds is offline   Reply With Quote

Old   May 5, 2016, 12:28
Default
  #14
Member
 
Eric Robertson
Join Date: Jul 2012
Posts: 95
Rep Power: 5
msuaeronautics is on a distinguished road
Quote:
Originally Posted by arieljeds View Post
Hi @msuaeronautics, I know that this thread is kind of old so I'm just hoping to be seen here :-).

First of all, thank you for your clear explanation. I am understanding most of it but I have a couple of more specific questions on implementation. I am simulating waves moving past a surface piercing cylinder (interested in free surface effects) using waveFoam.

1) I have set up a line of probes in the wake (from y = -5 to y = 5 for a cylinder of diameter 6 with the origin at (0,0,0) and y is the direction perpendicular to the oncoming waves, z is the vertical direction). I have velocity, pressure, vorticity from the probes.

You said to subtract the mean from the instantaneous. Do you mean to subtract a mean value from the freestream? Or else, do you mean to calculate the mean velocity within the wake for the whole time series and subtract that value from the individual instantaneous velocities?

Thank you!

Ariel
Hi Ariel,

The latter is correct. Compute the mean velocity over the time series and compute U - UMean, where U is your individual instantaneous value. So, for every point in the domain you have one value of UMean and multiple values of U.
msuaeronautics is offline   Reply With Quote

Old   May 5, 2016, 12:37
Default
  #15
Senior Member
 
Ariel Edesess
Join Date: Aug 2015
Posts: 101
Rep Power: 2
arieljeds is on a distinguished road
Hi @msuaeronautics,

Thank you for your very quick reply! Ok, I think that I understand but just to be very clear as this has been confusing me for a while now:

For each probe location, I should compute the Umean for that location, which would give one UMean value and a time series of instantaneous U, which I would then subtract my one value of UMean for that location from.

I would then have a U' for each separate probe.

From there, (if I am indeed understanding this correctly), how do I decide the location to take the U' from? Or is this again an average of all of the positions, which I guess would give me an average UMean over all the locations and an average instantaneous U from all locations, and therefore an average U'?

I'm sorry if this is a very stupid question, I take a long time to very thoroughly understand something and I appreciate your explanations!

Cheers,
Ariel
arieljeds is offline   Reply With Quote

Old   May 5, 2016, 12:41
Default
  #16
Senior Member
 
Ariel Edesess
Join Date: Aug 2015
Posts: 101
Rep Power: 2
arieljeds is on a distinguished road
Actually I just wanted to add that I am computing the UMean using the fieldAverage function object, so possibly I am already doing part of what needs to be done?
arieljeds is offline   Reply With Quote

Old   May 5, 2016, 12:46
Default
  #17
Member
 
Eric Robertson
Join Date: Jul 2012
Posts: 95
Rep Power: 5
msuaeronautics is on a distinguished road
Quote:
Originally Posted by arieljeds View Post
Hi @msuaeronautics,

Thank you for your very quick reply! Ok, I think that I understand but just to be very clear as this has been confusing me for a while now:

For each probe location, I should compute the Umean for that location, which would give one UMean value and a time series of instantaneous U, which I would then subtract my one value of UMean for that location from.

I would then have a U' for each separate probe.

From there, (if I am indeed understanding this correctly), how do I decide the location to take the U' from? Or is this again an average of all of the positions, which I guess would give me an average UMean over all the locations and an average instantaneous U from all locations, and therefore an average U'?

I'm sorry if this is a very stupid question, I take a long time to very thoroughly understand something and I appreciate your explanations!

Cheers,
Ariel
No problem!

The simple answer is that you compute the frequency analysis per probe. Your analysis is a pointwise thing. Let's say you have probes at A= (1,1,1) and B= (2,2,2).

Probe A will have
1 UMean value
N_TIMESTEPS U values

As well,

Probe B will have
1 UMean value
N_TIMESTEPS U values

Compute U(t) - UMean at both probes A and B, which will each give you

For Probe A
U'(t) at location (1,1,1) over N_TIMESTEPS

For Probe B
U'(t) at location (2,2,2) over N_TIMESTEPS

So, you are correct. But:

You will subsequently have two different frequency spectrums- one at (1,1,1) and at (2,2,2). The one you choose to present is up to you!
msuaeronautics is offline   Reply With Quote

Old   May 5, 2016, 12:48
Default
  #18
Member
 
Eric Robertson
Join Date: Jul 2012
Posts: 95
Rep Power: 5
msuaeronautics is on a distinguished road
Quote:
Originally Posted by arieljeds View Post
Actually I just wanted to add that I am computing the UMean using the fieldAverage function object, so possibly I am already doing part of what needs to be done?
You can do it this way, or you can extract all of your U(t) values (instantaneous velocity) per probe and compute the average. The fieldAverage tool is probably 'better.'
msuaeronautics is offline   Reply With Quote

Old   May 5, 2016, 12:48
Default
  #19
Senior Member
 
Ariel Edesess
Join Date: Aug 2015
Posts: 101
Rep Power: 2
arieljeds is on a distinguished road
Ah ha! Thank you! That was an excellent and clear explanation, I think it makes perfect sense now! Now, to apply it :-)

I am guessing that, hopefully, probe A and probe B should be more or less the same, all being well.

Thanks again, I really appreciate it!

Ariel
msuaeronautics likes this.
arieljeds is offline   Reply With Quote

Old   May 10, 2016, 10:36
Default
  #20
Senior Member
 
Ariel Edesess
Join Date: Aug 2015
Posts: 101
Rep Power: 2
arieljeds is on a distinguished road
Hi @msuaeronautics,

I have been applying what the instructions you gave for St but I'm not trusting my results as the Strouhal number I'm getting seems to be very high. Im still just trying to verify a case for Re = 40 and here are the steps I took:

1) Calculate the average U for each probe location
2) Subtract the average U from each instantaneous U for Uprime
3) Calculate the PSD using the following command [s,fs] = plomb(uPrime,time)
4) I then assumed that the max value given by the previous command was the shedding frequency.
5) Calculate the Strouhal number using the (shedding frequency (max value of previous step) * diameter)/uInf, where I took uInf from a probe located upstream of the cylinder...

I'm sure this isn't quite right though because this gives me a Strouhal number = 0.6722, which is obviously way too high (St should be around 0 for Re = 40)...

Am I misunderstanding something here? (probably especially in steps 4 & 5!)

Cheers,
Ariel
arieljeds is offline   Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
time step and vortex shedding frequency kevin Main CFD Forum 16 November 19, 2009 07:00
Vortex shedding frequency stays unaltered Nelson Main CFD Forum 3 March 5, 2009 11:04
How to measure vortex shedding frequency? Sham FLUENT 2 July 31, 2007 20:20
How to measure vortex shedding frequency Sham FLUENT 1 July 30, 2007 23:49
Frequency of vortex shedding Giuseppe FLUENT 1 January 10, 2006 09:41


All times are GMT -4. The time now is 19:38.