FLOW AND HEAT-TRANSFER BEHAVIOR IN TRANSITIONAL BOUNDARY-LAYERS WITH STREAMWISE ACCELERATION

The effects of streamwise acceleration on a two-dimensional heated bou ndary layer undergoing natural laminar-turbulent transition were inves tigated with detailed measurements of momentum and thermal transport p henomena. Tests were conducted over a heated flat wall with zero press ure-gradient and three levels of streamwise acceleration: K = (nu/(U) over bar(infinity)(2)) (d (U) over bar(infinity)/dx) = 0.07, 0.16, and 0.25 x 10(-6). Free-stream turbulence intensities were maintained at approximately 0.5 percent for the baseline case and 0.4 percent for th e accelerating cases. A miniature three-wire probe was used to measure mean velocity and temperature profiles, Reynolds stresses, and Reynol ds heat fluxes. Transition onset and end were in,ferred from Stanton n umbers and skin-friction coefficients. The results indicate that mild acceleration delays transition onset and increases transition length b oth in terms of distance, x, and Reynolds number based on x. Transitio n onset and length are relatively insensitive to acceleration in terms of momentum thickness Reynolds number. This is supported by the bound ary layer thickness and integral parameters, which indicate that a fav orable pressure gradient suppresses boundary layer growth and developm ent in the transition region. Heat transfer rates and temperature prof iles in the late-transition and early-turbulent regions lag behind the development of wall shear stress and velocity profiles. This lag incr eases as K increases, indicating that the evolution of the heat transp ort is slower than that of the momentum transport. Comparison ofthe ev olution of rms temperature fluctuations to the evolution of Reynolds n ormal stresses indicates a similar lag in the rms temperature fluctuat ions.