
[Sponsors] 
May 28, 2010, 02:13 
vortex shedding frequency

#1 
Senior Member
NAVEEN.K.M
Join Date: Mar 2009
Location: Bangalore, Karnataka, india
Posts: 114
Rep Power: 9 
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 

July 19, 2010, 02:10 

#2 
Senior Member
Jie
Join Date: Jan 2010
Location: Australia
Posts: 123
Rep Power: 9 
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. 

July 19, 2010, 02:58 
vortex shedding frequency

#3 
Senior Member
NAVEEN.K.M
Join Date: Mar 2009
Location: Bangalore, Karnataka, india
Posts: 114
Rep Power: 9 
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 

December 2, 2010, 16:44 
FFT for a sound spectrum

#4 
New Member
Bertrand Delvaux
Join Date: Apr 2010
Posts: 29
Rep Power: 8 
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 

March 19, 2011, 11:44 
strouhal number

#5 
Member

hi all of foamers,
can any one tell me how to calculate strouhal number or shedding frequency? ( OF 1.5dev) waiting for your response... tanks 

March 20, 2011, 08:22 

#6 
Member
Patricio Bohorquez
Join Date: Mar 2009
Location: Jaén, Spain
Posts: 95
Rep Power: 9 
You could proceed as described in the following paper:
Bohorquez, P., SanmiguelRojas, E., Sevilla, A., JiménezGonzález, J., MartínezBazán, C. Stability and dynamics of the laminar wake past a slender bluntbased axisymmetric body. Journal of Fluid Mechanics (in press) 

March 20, 2011, 18:32 

#7 
Member

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 1.5dev 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 

March 21, 2011, 06:49 

#8 
Member
Patricio Bohorquez
Join Date: Mar 2009
Location: Jaén, Spain
Posts: 95
Rep Power: 9 
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. 

December 8, 2012, 23:59 
flow around cylinder..

#9 
New Member
Rishitosh Ranjan
Join Date: Dec 2012
Location: Trichirapalli, Tamilnadu, India
Posts: 18
Rep Power: 6 
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 

May 17, 2014, 10:12 

#10 
Member
Mehrez
Join Date: Nov 2012
Location: Bordeaux, France
Posts: 80
Rep Power: 6 
Hi dear all,
Rishitosh, did you find how to compute the St nmbr ? Thanks, Mehrez 

April 6, 2015, 11:05 

#11 
Senior Member
ok
Join Date: Oct 2013
Posts: 345
Rep Power: 6 
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 

April 9, 2015, 11:10 

#12 
Member
Eric Robertson
Join Date: Jul 2012
Posts: 95
Rep Power: 6 
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. 

May 5, 2016, 12:17 

#13 
Senior Member
ArielJ
Join Date: Aug 2015
Posts: 126
Rep Power: 3 
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 

May 5, 2016, 12:28 

#14  
Member
Eric Robertson
Join Date: Jul 2012
Posts: 95
Rep Power: 6 
Quote:
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. 

May 5, 2016, 12:37 

#15 
Senior Member
ArielJ
Join Date: Aug 2015
Posts: 126
Rep Power: 3 
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 

May 5, 2016, 12:41 

#16 
Senior Member
ArielJ
Join Date: Aug 2015
Posts: 126
Rep Power: 3 
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?


May 5, 2016, 12:46 

#17  
Member
Eric Robertson
Join Date: Jul 2012
Posts: 95
Rep Power: 6 
Quote:
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! 

May 5, 2016, 12:48 

#18 
Member
Eric Robertson
Join Date: Jul 2012
Posts: 95
Rep Power: 6 
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.'


May 5, 2016, 12:48 

#19 
Senior Member
ArielJ
Join Date: Aug 2015
Posts: 126
Rep Power: 3 
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 

May 10, 2016, 10:36 

#20 
Senior Member
ArielJ
Join Date: Aug 2015
Posts: 126
Rep Power: 3 
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 

Thread Tools  
Display Modes  


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 