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May 12, 2016, 14:24 

#21  
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Eric Robertson
Join Date: Jul 2012
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Quote:
In addition, are you using a sufficient sample frequency? 

May 13, 2016, 10:33 

#22 
Senior Member
ArielJ
Join Date: Aug 2015
Posts: 127
Rep Power: 7 
Hi @msuaeronatics,
Thank you again for your response. In reading, I also started to suspect this. However, I now need to understand how to do it using those coefficients. You mention using the time history of Cd ... do you mean Cl for this? And, if so, does this imply that the following steps should be taken: 1) Calculate Cl from the pressure distribution around the cylinder (I have probes set up around the cylinder, I also am taking surface information from the surfaces function object, but I am not confident in using this information). I also have forces set up to calculate the lift force, Fl, however I haven't verified if this method works for the waveFoam solver involving freesurface flow. So, once again, I don't fully trust my values for Fl. 2) Assuming the Fl value over time is correct, calculate Cl over time, and then calculate an average ClMean, subtract that from the instantaneous Cl for a Cl', take the FFT of this value for the shedding frequency, fs, which I should then use to calculate the Strouhal number, using the ambient velocity measured near the inlet. Again, I don't know if the method listed above is the correct method, but this is how I am understanding it currently, please feel free to correct. In regards to your second question, the simple answer is "I don't know". Terrible answer, I know. I'm trying to understand better the sampling frequency and how to change it but at this point, this is another concept I am not confident on. Apologies for the very long posting, your help has been greatly appreciated. Best regards, Ariel 

May 16, 2016, 19:07 

#23  
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Eric Robertson
Join Date: Jul 2012
Posts: 95
Rep Power: 11 
Quote:
In addition, I believe sampling the history of instantaneous CL should work fine. Calculating a mean may not be beneficial to you at your Reynolds number, which might feature only really lowfrequency behavior. A simple FFT of the CL history should give you what you're looking for. Then you nondimensionalize by Strouhal number in the manner you described. 

May 17, 2016, 09:12 

#24 
Senior Member
ArielJ
Join Date: Aug 2015
Posts: 127
Rep Power: 7 
Thank you again, I think I have a much better understanding of what to do. Again, the challenge now is to apply it. You've been very helpful, thanks a lot!


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