HIGH FREESTREAM TURBULENCE EFFECTS ON TURBULENT BOUNDARY-LAYERS

High freestream turbulence levels significantly alter the characterist ics of turbulent boundary layers. Numerous studies have been conducted with freestreams having turbulence levels of 7 percent or less, but s tudies using turbulence levels greater than 10 percent have been essen tially limited to the effects on wall shear stress and heat transfer. This paper presents measurements of the boundary layer statistics for the interaction between a turbulent boundary layer and a freestream wi th turbulence levels ranging from 10 to 20 percent. The boundary layer statistics reported in this paper include mean and mts velocities, ve locity correlation coefficients, length scales, and power spectra. Alt hough the freestream turbulent eddies penetrate into the boundary laye r at high freestream turbulence levels, as shown through spectra and l ength scale measurements, the mean velocity profile still exhibits a l og-linear region. Direct measurements of total shear stress (turbulent shear stress and viscous shear stress) confirm the validity of the lo g-late at high freestream turbulence levels. Velocity defects in the o uter region of the boundary layer were significantly decreased resulti ng in negative wake parameters. Fluctuating ms velocities were only af fected when the freestream turbulence levels exceeded the levels of th e boundary layer generated mts velocities. Length scales and power spe ctra measurements showed large scale turbulent eddies penetrate to wit hin y(+) = 15 of the wall.