I want to plot the energy spectra of the streamwise velocity fluctuation versus the wavenumbers using matlab. How can I extract the wavenumbers from the hotwire data, is it by applying FFT to the results and then using dspdata.psd(Data,Frequencies)?
Try using the pwelch command to give power spectral density as a function of frequency. From there wave number is k=2*pi*f/c where c is the convection velocity.
I don't know which kind of data can you get from hotwire. But in numerical simulation, if we get the fluctuations of velocity, the we can do FFT on the fluctuations in matlab to get the Turbulent energy spectra with the wavenumbers (in space ) or with the frequencies (in temporal ).
@Ric: Thank you pwelch works, just wondering how you calculate the convection velocity when there is no heat transfer?
@Senthil: It is turbulent
@Guiquan: I'm wondering how using FFT gives you the Turbulent energy spectra?As I know, FFT converts the time domain into the frequency and the magnitude of the converted data doesn't represent the fluctuation energy!
There are two methods to get turbulent kinetic energy by FFT, one is from Rii(correlation coefficient) and the other is by integral a function of FFT(u_prim) in fourier space, u_prim can be got from (u-u_ave), so u_prim is a function of time, and the fluctuation energy is a function of u_prim.
Hi Iman, generally you have to use two hot wires and do the cross-correlation. However, downstream of a circular cylinder, the convection velocity of the vortices is generally 0.8U, where U is the free-stream velocity. See Williamson 1988.
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