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-   -   Vortex shedding frequency stays unaltered (http://www.cfd-online.com/Forums/main/16430-vortex-shedding-frequency-stays-unaltered.html)

Nelson March 4, 2009 14:04

Vortex shedding frequency stays unaltered
 
Hey guys, I'm having problem with the vortex shedding frequency, basically, all I'm doing is increasing the average fluid flow speed from 31.5 (cm/s) to 63 (cm/s), therefore, it's a really low reynolds number.

now I have my timestep of both 0.03 sec, 0.015 and 0.01 sec respectively, and the quality of the result from 0.03 sec isn't really fine, so it's not discussed here, however, my vortex shedding frequency stays 3.7 no matter it's 0.015 sec or 0.03 sec.

At the moment I am comparing the experimental data to the experimental ones, but it doesn't seems to be optimistic, therefore, if there is anyone who can help, it would be deeply appreciated!!!

thank you Nels

Julien March 5, 2009 07:54

Re: Vortex shedding frequency stays unaltered
 
What is the Strouhal number you are predicting? How many timesteps do you have per vortex shedding period for the smallest timestep you tested?

Nels March 5, 2009 10:25

Re: Vortex shedding frequency stays unaltered
 
Hey Julien, thanks for the reply, I am getting 3.7 hz for the 31.5 cm/s which is the same as the literature, however, as velocity doubles, the shedding frequency should doubles, which is the problem, it stays constant as 3.7 Hz.

And the Strouhal number I'm looking for is around 0.1-ish.

for the smallest timestep (i.e: 0.01 sec), I have 23 timesteps per vortex shedding, which shouldn't be the case as I have doubled the free stream velocity setting.

Thanks


Julien March 5, 2009 11:04

Re: Vortex shedding frequency stays unaltered
 
23 points per period is reasonable. What order accuracy timestepping are you using?

Is there any chance that your code is normalizing out the velocity somewhere?

Other than that, I have no idea.


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