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美国政府科技报告
>EFFECT OF DISTRIBUTED THREE-DIMENSIONAL ROUGHNESS AND SURFACE COOLING ON BOUNDARY-LAYER TRANSITION AND LATERAL SPREAD OF TURBULENCE AT SUPERSONIC SPEEDS
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EFFECT OF DISTRIBUTED THREE-DIMENSIONAL ROUGHNESS AND SURFACE COOLING ON BOUNDARY-LAYER TRANSITION AND LATERAL SPREAD OF TURBULENCE AT SUPERSONIC SPEEDS
An investigation was made in the Langley 4- by lf-foot supersonic pressure tunnel at Mach numbers of 1.6l and 2.01 to determine (l) the effect of distributed roughness on boundary-layer transition with the model surface at adiabatic wall temperature and cooled and (2) the effect of surface cooling on the lateral spread of turbulence. Both distributed granular-type and single spherical roughness particles were used, and transition of the boundary layer was determined by hot-wire anemometers. The transition-triggering mechanism of the three-dimensional roughness at supersonic speeds appeared to be the same as that previously observed at subsonic speeds. In fact, the criticalnValue of the roughness Reynolds number parameter (that is,the value at which turbulent spots are initiated by the roughness) was found to be approximately the same at supersonic and subsonic speeds when complete local conditions at the top of the roughness, including density and viscosity, were considered in the formulation of the roughness Reynolds number. For three-dimensional roughness at a Reynolds number less than its critical value, the roughness introduced no disturbances of sufficient magnitude to influence transition. Surface cooling, although providing a theoretical increase in stability to small disturbances, did not increase to any important extent the value of the critical roughness. Reynolds number for three-dimensional roughness part ides.Cooling, therefore, because:of its effect on the boundary-layer thickness, density, and viscosity actually promoted transition due to existing three-dimensional surface roughness for given Mach and Reynolds numbers. The measured lateral spread ofturbulence in the boundary layer appeared to be unaffected by the increased laminar stability derived from the surface cooling.
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