Other
Scientific paper
Jun 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988phdt........36l&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF CALIFORNIA, SAN DIEGO, 1988.Source: Dissertation Abstracts International, Volume: 49-07, Section:
Other
12
Scientific paper
The decay of grid generated turbulence in the presence of strong thermal stratification is studied in a continuously stratified, open-loop wind tunnel. The air flow facility uses a novel design to produce temperature gradients in excess of 200^circC/m and Brunt-Vaisala frequencies up to 2.5 s^ {-1}. Thermal anemometers for the stimultaneous measurement of vertical and streamwise velocity fluctuations and fluctuating temperature are described. A new cold-wire bridge design is given and accurate procedures for the thermal calibration of hot wires are developed. The accuracy and signal resolution of the instruments is considered. The data include one-point statistical measurements through moments of fourth order and associated power- and cross-spectra. Cross-channel phase measurements are used to analyze the scales of correlation of velocity and temperature. The turbulence is characterized as a two-scale process dominated by buoyancy forces at large scales of motion and dissipative effects at small scales. Buoyancy effects on the vertical heat flux, the vertical velocity, the scalar variance, and the dissipation rates are scaled with a buoyancy time scale to produce universal curves. Important features of the flow development are characterized by an overturning length scale (associated with potential energy) and a buoyancy length scale (associated with kinetic energy). Spectra of velocity and temperature satisfy universal equilibrium scaling at high wavenumbers but show buoyancy effects at small wavenumbers. A new universal equilibrium scaling is developed for the cross-spectrum. The flow is seen to remain isotropic at high wavenumbers over the entire range of turbulent decay studied. Cospectral and phase data are used to validate the two-scale model of the turbulence. The flow may show large-scale re-stratification while active turbulence persists at smaller scales. The vanishing of the vertical transport does not represent extinction of turbulent motion. No internal wave effects are observed at any stage of the decay. Comparisons to other studies are given and the implications of the present results for thermal design are discussed.
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