SEPARATED FLOW TRANSITION MECHANISM AND PREDICTION WITH HIGH AND LOW FREESTREAM TURBULENCE UNDER LOW PRESSURE TURBINE CONDITIONS

摘要

A correlation for separated flow transition has been developed for boundary layers subject to initial acceleration followed by an unfavorable pressure gradient. The correlation is based on the measured growth of small disturbances in the pre-transitional boundary layer. These disturbances were identified and quantified through spectral analysis of the wall normal component of velocity. Cases typical of low pressure turbine airfoil conditions, with Reynolds numbers (Re) ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) were considered at low (0.5%) and high (8.7% inlet) freestream turbulence levels. In some cases, two-dimensional rectangular bars were placed at the beginning of the adverse pressure gradient region as passive flow control devices. The dimensionless magnitude of the initial disturbance which begins to grow at the suction peak depends on the freestream turbulence level and the size of any bar applied to the surface. The growth rate depends on the Reynolds number. When the pre-transitional disturbances grow to a sufficient magnitude, transition begins. The new correlation is based on the physics observed in the turbulence spectra, but allows transition prediction using only the Reynolds number, freestream turbulence level and bar height. The correlation has been checked against experimental data from the literature, and allows transition location prediction to within the uncertainty of the experimental measurements. The correlation represents an improvement over previous correlations which accounted for Reynolds number or freestream turbulence effects, but not both.