[naveen]

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

[jiejie]

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.

[naveen]

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

[Bertrand]

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

[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

[pbohorquez]

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)

[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

[pbohorquez]

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.

[rishitosh]

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]

Hi dear all,
Rishitosh, did you find how to compute the St nmbr ?
Thanks,

Mehrez

[Maimouna]

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

[msuaeronautics]

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.

[arieljeds]

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

[msuaeronautics]

Originally Posted by arieljeds

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.

[arieljeds]

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]

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?

[msuaeronautics]

Originally Posted by arieljeds

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]

Originally Posted by arieljeds

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.'

[arieljeds]

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

[arieljeds]

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