An experimental investigation of the evolution of turbulent potential and kinetic energies and the vertical temperature structure of homogeneous stably stratified sheared turbulence.

摘要

Homogeneous stably stratified sheared turbulence has been studied in detail using both standard fixed point methods and a new rapid vertical profiling system. A 10-layer thermally stratified wind tunnel generated mean flows in which both the mean temperature gradient and the mean velocity gradient could be adjusted independently. Six cases of mean flows, characterized by the gradient Richardson number, Rig=N2/dU/d z2, were studied, with initial values of Rig = 0.015, 0.055, 0.095, 0.135, 0.25 and 0.5. The turbulence was initiated with a 2.54 cm bi-planar grid. Fixed point measurements of streamwise and vertical velocity fluctuations and temperature were made at several streamwise stations using standard X-wire and cold-wire techniques. Vertical profiles of temperature were made at several streamwise stations using standard cold-wire techniques and a rapid vertical traverse. Multipoint measurements of the temperature field were also made using a vertically aligned 8-point cold-wire rake.;The turbulent kinetic and potential energies were found to have identical, exponential growth rates with respect to the nondimensional time scale, tau = St. Evaluation of the first-order terms in the evolution equations for turbulent kinetic and potential energies show that the Osborn-Cox model [Osborn & Cox (1972)] is generally not valid for these types of flows. A self-preserving solution was derived and shown to predict the exponential growth, and the constant ratio of kinetic and potential energies, for homogeneous stratified shear turbulence.;The vertical structure of the temperature field at low Rig was found to be dominated by high stable gradient regions adjacent to unstable patches. These structures are associated with large scale advective flux of buoyancy. Comparison of vertical and horizontal wavenumber spectra of temperature show that spectral energy in the vertical direction is confined to a narrower band of wavenumbers. This difference leads to a small scale anisotropy, which is controlled by both shear and stratification.;Measurements of Thorpe scales and available potential energy show that the energy associated with overturn patches is much less than the total potential energy and that decay of the turbulence reduces the number of overturn patches but not the scale of the patches.