[shantanukale]

Hello....
I am simulating flow over a circular cylinder using OpenFOAM. Everything is going well except values of cl and cd and Strouhal number.
I have two primary questions:
1. Is there any effect of SampleDict file on calculation of force co-efficients?
2. If there 2 in-line or side-by-side cylinders, how to calculate lift and drag for the two cylinders separately...

Thanks a lot in advance.....

[achyutan]

Hi,

for your first question, answer is NO. force coefficients are calculated by runtime function objects whereas sampling is a part of post processing.

It is possible to calculate lift,drag over individual surfaces separately. It needs slight modification in forces.C/forceCoeffs.C.
However, your problem seems to be different. If you are not getting correct cl, cd values, you have check for convergence and other issues. Plotting Cp would be a better indicator.
If you need help regarding your pursuit for answering 2nd question, I would be ready to help you.

kind regards

[shantanukale]

Thanks a lot for your kind reply..
Could you please tell me how should I calculate Cp in openfoam? I tried using paraview calculator. However, I got only one value. How to calculate other values?
I have one more issue with my simulations. The Strouhal number I calculate from lift co-efficient is exactly the half of expected. I am running non-dimensional simulations. Kindly reply..

Thank you once again.

[achyutan]

Hi,
There are many threads available in the forum regarding Cp calculation from paraview/parafoam. I suggest you better take out the pressure data on the cylinder surface by sampling and calculate Cp in excel/matlab or any other suitable tool.

First check out your Cp profile and check for convergence of your solution by plotting the residuals. There could be other issues which can be reasons for mismatch of your strouhal number.

I guess you must have integrated the pressure over cylinder surface in the paraview calculator and hence you are getting a single value.
kind regards

[shantanukale]

Thank u once again. I found one tutorial in FLUENT. They have multiplied the frequency obtained by 2 to obtain exact Strouhal number. Is that the right way to do?
One more doubt, what should be the changes in the forceCoeffs.C file so that I can compute cl and cd separately for both the cylinders.

Thank You....

[achyutan]

Hi,
Strouhal number definition can differ in the sense that some literature give it as fd/U and in some papers it is defined as fd/2U. So, it depends on which definition u are following for your values to match with validated results.
If you
kind regards

[shantanukale]

Could you please let me know in which case which formula is applicable? I am unable to find any literature which is based on calculation of vortex shedding frequency from cl plot. Also, how to use FFT in OpenFOAM?

Thank you...