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vortex shedding frequency

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Old   May 12, 2016, 14:24
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Eric Robertson
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Originally Posted by arieljeds View Post
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
I think taking a force coefficient approach (i.e. histories of CD or CL) is far more appropriate for your Reynolds number. In other words, instead of analyzing U', use an appropriate history of CD.

In addition, are you using a sufficient sample frequency?
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Old   May 13, 2016, 10:33
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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 free-surface 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
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Old   May 16, 2016, 19:07
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Eric Robertson
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Quote:
Originally Posted by arieljeds View Post
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 free-surface 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
Yes, I think in your case CL would be best to use.

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 low-frequency behavior. A simple FFT of the CL history should give you what you're looking for. Then you non-dimensionalize by Strouhal number in the manner you described.
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Old   May 17, 2016, 09:12
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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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