A test of isotropy and Taylor's hypothesis in the atmospheric boundary layer

Abstract

This study attempts to determine the scales of turbulence in a high Reynolds number shear flow near which transition to isotropy occurs and the scales for which Taylor's hypothesis is applicable. The flow studied was the wind near height x ₃ = 2 m above a flat land surface. Four hot-wire anemometers were mounted in a three-dimensional array with equal separations between 1.8 m and 2 cm in three different directions. Theoretical cross-spectra were computed from the observed spectra of downwind velocity fluctuations assuming isotropy and Taylor's hypothesis. Comparison between these and the observed cross-spectra revealed that the turbulence in the flow studied was consistent with both assumptions provided k _1x3&> 20, where k ₁ is the radian wavenumber; this was the lower bound to which no departure from isotropy could be detected by the experiment. For 4 ≦ k _1x3≦ 20, the observations are consistent with symmetry of the turbulence about the downstream direction. That part of Taylor's hypothesis relating observed frequency at a stationary sensor to the downstream wavenumber component appears to be justified within experimental error for k _1x3& > 3.