A method for extracting the turbulence intensity and integral length scale form single-component molecular tagging velocimetry*

This study demonstrates a method to extract the turbulence intensity and integral length scale from single-component molecular tagging velocimetry (1c-MTV) measurements of freestream turbulence. These measurements are challenging because of the very small magnitude of the fluctuating velocities compared to the freestream velocity, and the presence of low-frequency facility unsteadiness as well as measurement white noise. The approach takes advantage of the inherent capabilities of 1c-MTV to measure the streamwise velocity at a very high spatial resolution of 52~$\mu $m over a line that extends 5.3~cm in the cross-stream direction. The resulting data set is equivalent to that which would result from a linear sensor array of 1024 tightly-spaced hot wires, enabling computation of the cross-stream autocorrelation function efficiently. The high-spatial resolution of the measurements allows removal of the white noise contribution to the autocorrelation function, whereas the extended domain of the measurements facilitates rejection of the influence of the low-frequency facility unsteadiness. The ``noise-removed'' autocorrelation function is used to compute the intensity and integral length scale of turbulence. The procedure is applied to grid-generated freestream turbulence.