Strouhal number for sphere and high Re

Freddeflopp · October 13, 2016, 04:56

Dear members and experts,
I need to roughly approximate the strouhal-number for a sphere in free flow.
The free velocity of surrounding air is 10-45 m/s, e.g. a high Re above 1e6

The literature and works I've found mainly treats much lower Re

Looking for St number for cylinders does not seem to be appropriate, since I believe the number should be considerably higher for a sphere in this Re region.

What is a good approximation of St in this region?

Any help is much appreciated

best regards,
Fred

mome · October 18, 2016, 11:34

Hi, I'm not very knowledgeable about this, but I thought there is an upper critical Reynolds number of about ~3e5 for vortex shedding of spheres no? Could be wrong but what if you don't have large scale oscillations at all?

I'm sure you can find plenty on it..

Freddeflopp · October 19, 2016, 17:18

Thanks for the reply!

Yes, you are 100% correct. However, there seems to be a re-establishment of the turbulent vortex street when Re get higher than 3.5e6 (for cylinders) according to lectures and some researchers. see for example slide 5 in the link below (they refer to J. Lienhard):

web.mit.edu/13.42/VIV_2004.ppt

However, I cant find any result concerning spheres. In this case low frequencies were unwanted, so I used a cylinder worst case approximation with St=0.2 and got low, but acceptable frequencies.

I would be interesting to know if the re-establishment only is valid for cylinders.

mome · October 20, 2016, 07:41

yeah I recall these coherent structures around cylinders, don't know if something like that exists for spheres and after a 5 minute google I agree that there is not so much work at Re>10e6..
I have the impression there are no observations of that phenomenon though. I found info that beyond 3.7e5 the estabished vortex pair is steady. And a report that shows peaks in the spectrum at up to 1.2e6 attributes this to a vibration in their frame

maybe you can find more! Good luck
Would be interesting to know..
M.

October 13, 2016, 04:56	Strouhal number for sphere and high Re	#1
Freddeflopp New Member Join Date: Oct 2016 Posts: 2 Rep Power: 0	Dear members and experts, I need to roughly approximate the strouhal-number for a sphere in free flow. The free velocity of surrounding air is 10-45 m/s, e.g. a high Re above 1e6 The literature and works I've found mainly treats much lower Re Looking for St number for cylinders does not seem to be appropriate, since I believe the number should be considerably higher for a sphere in this Re region. What is a good approximation of St in this region? Any help is much appreciated best regards, Fred

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October 18, 2016, 11:34		#2
mome Member Join Date: May 2014 Posts: 30 Rep Power: 11	Hi, I'm not very knowledgeable about this, but I thought there is an upper critical Reynolds number of about ~3e5 for vortex shedding of spheres no? Could be wrong but what if you don't have large scale oscillations at all? I'm sure you can find plenty on it..

October 19, 2016, 17:18		#3
Freddeflopp New Member Join Date: Oct 2016 Posts: 2 Rep Power: 0	Thanks for the reply! Yes, you are 100% correct. However, there seems to be a re-establishment of the turbulent vortex street when Re get higher than 3.5e6 (for cylinders) according to lectures and some researchers. see for example slide 5 in the link below (they refer to J. Lienhard): web.mit.edu/13.42/VIV_2004.ppt However, I cant find any result concerning spheres. In this case low frequencies were unwanted, so I used a cylinder worst case approximation with St=0.2 and got low, but acceptable frequencies. I would be interesting to know if the re-establishment only is valid for cylinders.

October 20, 2016, 07:41		#4
mome Member Join Date: May 2014 Posts: 30 Rep Power: 11	yeah I recall these coherent structures around cylinders, don't know if something like that exists for spheres and after a 5 minute google I agree that there is not so much work at Re>10e6.. I have the impression there are no observations of that phenomenon though. I found info that beyond 3.7e5 the estabished vortex pair is steady. And a report that shows peaks in the spectrum at up to 1.2e6 attributes this to a vibration in their frame maybe you can find more! Good luck Would be interesting to know.. M.