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 Li December 17, 2008 08:24

The spectrum of eddy sizes

Hi, I read it from a book of Turbulence phenomena: A mean fluctuation velocity alone does not completely define a turbulent fluid; one must also consider the spectrum of eddy sizes'. But would you tell me how the spectrum of eddy sizes are determined. I measured velocity with LDV, would you tell me if I can get the eddy sizes from the experiment data? How? Thank you very much!

And I give my best wish to you all in 2009. Merry Christmas and Happy New Year.

Best Regards, Li

 agg December 17, 2008 17:34

Re: The spectrum of eddy sizes

You can compute two-point correlations at different points in the domain and plot it. This should tell you the eddy sizes in your flow.

i.e. compute f = <u'v'>/[sqrt(<u'u'><v'v'>)] and plot it

 Li December 18, 2008 04:47

Re: The spectrum of eddy sizes

Hi agg, Thank you very much! I will try.

 Ferramis December 24, 2008 04:44

Re: The spectrum of eddy sizes

Well if you used LDV to acquire your velocity samples, then there is only the way over the time domain, ie. plot the autocorrelation and extract the integral time scale and then relate it to a length scale by use of taylors frozen turbulence. However remember that you don't have equidistant timesteps, so there might be a need for some interpolation of the timeseries (if sufficiently fast sampled) in order to get a sampling frequency which then can be used for the computation. Or plot the energy spectra in the same manner against wave number (also by use of Taylors frozen turbulence hypothesis) and locate the position of highest energy content. All this under the assumption that Taylors hypothesis works in your flow case. 2-point correlations will be hard to do unless you had to measurements volumes with your LDV system, which I doubt.

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