Investigating the outer scale of turbulence with time domain processing of anemometer data

Abstract

Sonic anemometers are used to study the outer scale in near ground level turbulence. Turbulence is expected to obey a Kolmogorov power spectrum within some inertial range, where the temperature or index of refraction fluctuations decrease as the inverse 11/3rds power of the spatial wavenumber. Below this inertial range (that is for sufficiently small spatial wavenumbers, or equivalently sufficiently large scale sizes) the form of the power spectrum isn’t predicted by theory, but it is expected to roll off. A levelling off of the power spectrum at low spatial frequencies corresponds to a levelling off of the structure function at large spatial separations, and this is the signal sought in the data. Near the ground there is some evidence the outer scale size may be as small as the height above ground. Sonic anemometer data was collected in the summer of 2019 in conjunction with optical turbulence experiments. These experiments showed good agreement between different ways of monitoring turbulence. In these experiments, the sonic anemometers were mostly mounted 2.64 meters above the ground. In this work, the anemometer data is being revisited to study the outer scale. Outer scale effects are quite subtle with optical techniques, which are arranged to be most sensitive to variations in index of refraction within the inertial range precisely in order to avoid inner and outer scale effects. Sonic anemometry usually achieves this by including only nearest neighbor measurements in turbulence estimation, but here we examine the variance of temperature differences across a wide range of baselines in order to study the structure function itself.