An experimental investigation of the vertical temperature structure of homogeneous stratified shear turbulence

• Published in: Journal of fluid mechanics 2000-12, Vol.425, p.1-29, Article S0022112000002111
• Main Authors: KELLER, KURT H., VAN ATTA, CHARLES W.
• Format: Article
• Language: English
• Subjects: Earth, ocean, space; Exact sciences and technology; External geophysics; Meteorology; Other topics in atmospheric geophysics
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/S0022112000002111

The vertical temperature structure of homogeneous stratified shear turbulence is investigated using new rapid vertical temperature measurements in a thermally stratified wind tunnel. Six cases of gradient Richardson number, Rig = N2/(dŪ/dz)2, where N is the Brunt–Väisälä frequency (N2 = (g/t¯)dt¯/dz), are studied, spanning a range 0.015 [les ] Rig [les ] 0.5. Three- to five-hundred high-resolution temperature profiles are made for several streamwise stations for each case of Rig. These measurements are supplemented with standard fixed-point, Eulerian measurements of streamwise and vertical velocity fluctuations and temperature fluctuations and with an eight-point vertical rake of temperature probes using standard hot-wire and cold-wire techniques. Vertical profiles uniquely enable the computation of available potential energy (APE), Thorpe scales (LTh), and the diapycnal flux (ϕd), as well as one-dimensional vertical wavenumber temperature spectra. These quantities are compared with Eulerian measurements of turbulent kinetic energy (KE), potential energy (PE), and buoyancy flux. It is found that the one-dimensional vertical wavenumber temperature spectrum contains more energy at smaller scales compared to the horizontal spectrum, owing in part to shear distortion, which leads to larger mean square vertical gradients of fluctuating temperature as compared to mean square horizontal gradients. The combination of shear and stratification, especially for cases where the turbulence decays with evolution, accelerates the evolution toward small-scale anisotropy compared to just shear or just stratification. It is found that in highly stratified cases, the diapycnal flux can persist after buoyancy flux has collapsed to negligible values, indicating enhanced heat transfer without turbulent mixing. For low Rig, large-scale vertical advection creates both high local temperature gradients and regions of static instability. Associated with the regions of instability is APE, which grows relative to KE for the least stratified cases. For high Rig, the turbulence evolves to a wavelike state, containing some counter gradient fluxes and unstable patches. This wavelike state has higher heat flux efficiency than the more turbulent states owing to the low dissipation but relatively high diapycnal flux.