Structure of sheared turbulent fields

Abstract

Small scale turbulent temperature and velocity measurements were made in a variety of turbulent shear flows in the atmosphere and laboratory, in air and in water. Nonzero skewness values of the streamwise temperature gradient were observed in every case, with a sign equal to that of the gradient dotted with the cross product of the mean temperature gradient and mean shear vectors. Large temperature jumps were found across sharp shear zones at the outer boundaries of large scale eddies rotating with the mean vorticity of the shear flow, giving a distinctive ramp-like structure to the temperature signals which seems to be a characteristic feature of scalar fields mixed by sheared turbulence. The streamwise gradient of the vertical velocity in the atmospheric boundary layer over the ocean was found to have positive skewness. Thus, both velocity and temperature fields are locally anisotropic in the same sense as the anisotropic mean quantities.