Turbulent Density Fluctuations in Thermal Boundary Layers Studied via Molecular Rayleigh scattering

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Spectra of air density fluctuations in the thermal boundary layers over a heated flat plate were measured using a molecular Rayleigh scattering setup. A 12 inch X 9 inch size flat plate was heated over a range of temperatures 0°C ≤ ΔT ≤ 170°C using electrical strip heaters. The heated plate was placed in a low-speed wind tunnel in a range of free-stream velocity of 20 ft/s ≤ U0 ≤ 90 ft/s. At first, care was taken to avoid laminar separation bubble at the plate leading edge. The thermal and velocity boundary layers at different streamwise locations were measured using a hot film probe operated in constant current and constant temperature modes. The temperature distribution on the plate was measured by an infra-red camera. In spite of a relatively high level of free-stream turbulent fluctuations, the relatively low plate Reynolds number 1.2e5 ≤ Re ≤ 5.3e5 ensured laminar flow at unheated condition. Progressively increased heating was found to expedite transition to turbulence. To setup the molecular Rayleigh scattering system a large number of obstacles had to be overcome. In addition to the usual steps, such as the removal of dust particles and the use of photo-electron counting electronics to measure low intensity light, an optical layout was created to minimize stray light contamination. Various efforts were made to reduce the effect of tunnel vibration, and an expanded calibration process had to be applied to relate photo-electron arrival rate to air density. The density fluctuations spectra measured in a fully turbulent zone showed fully developed turbulent spectra while those from the transitional zone showed much lower energy content. To validate the Rayleigh measurements, temperature fluctuations spectra were calculated from density spectra, and then compared with temperature spectra measured with a cold-wire probe operated in constant current mode. The spectra from the trailing edge of the plate were found to be in good agreement, while from the leading edge differed. Various lessons learnt are discussed. In spite of being a work in progress it is believed that the present effort is the first measurement of density fluctuations spectra in a boundary layer flow.