File:ADV fft length.png

Summary

Contours represent the log of the fft-length required to resolve the non-dimensional wavenumber [rad/m] indicated in each panel's title. The contours are plotted as a function of [math]\displaystyle{ \varepsilon }[/math] and speed past the sensor u. The fft-length controls the lowest frequency that is resolved by the spectra. Panel (a) provides a bit of breathing room for fitting the inertial subrange given the sparse number of spectral observations at low wavenumber. The middle panel (b) shows the fft-length to resolve 1 decade's worth of inertial subrange. The bottom panel (c) shows the fft-length that begins to resolve the viscous subrange i.e., the end of the inertial subrange. It can be used to gauge the fft-length for computing shear probe spectra to estimate [math]\displaystyle{ \varepsilon }[/math] from the viscous subrange.