Evaluation of Equilibrium Turbulence for a Hypersonic Boundary Layer at Nonadiabatic Wall Conditions

This paper presents an experimental study to characterize the compressible turbulent boundary layer produced along a flat plate in the NASA Langley 8-Foot High Temperature Tunnel and determine the test conditions necessary to achieve equilibrium turbulence. In addition, the present study extends the data base for equilibrium compressible turbulent boundary layers over quasi-isothermal walls which are far from the adiabatic wall temperature. The measurements consist of pilot pressure, static pressure, and total temperature distributions in the boundary layer. A flat plate measuring 9.7 feet long and 4.3, feet wide was used for the study to provide a naturally turbulent boundary layer which is suitably thick for probing. In addition, surface measurements consisting of heat transfer and pressure distributions were obtained. The tests were conducted at a nominal free­ stream Mach number of 6.5, total temperatures of 2700 and 3300 °R, and angles of attack of 5 and 13 degrees. The corresponding nominal boundary-layer edge Mach numbers were 6.2 and 5.0. The nominal ratios of adiabatic wall temperature to cold wall temperature were 4.4 and 5.4 and the momentum thickness Reynolds numbers at the boundary-layer probe locations ranged from 400 to 7800. The results of this study indicate that momentum thickness Reynolds numbers of at least 4000 are required to obtain an equilibrium turbulent boundary layer along a flat plate in the Langley 8-Foot High Temperature Tunnel. This evaluation is based primarily on the behavior of shape factors calculated from the velocity and density distributions which were inferred from the pressure and temperature measurements in the boundary layer. These results are generally supported by comparisons. made with the standard incompressible velocity distributions given by Coles using the compressible transformation of van Driest.