Heat Transfer and Fluid Dynamics Measurements in Accelerated Rough-Wall Boundary Layer

Abstract

The combined effects of freestream acceleration and surface roughness on heat transfer and fluid dynamics in the turbulent boundary layer were investigated experimentally. The experiments included a variety of flow conditions ranging from aerodynamically-smooth through transitionally-rough to fully-rough boundary layers with accelerations ranging from moderate to modestly strong. Two well-defined rough surfaces composed of 1.27 mm diameter hemispheres spaced 2 and 4 diameters apart, respectively, in staggered arrays on otherwise smooth surfaces were used as the test surfaces. The first 1.5 m of the test section had zero-pressure gradient followed by a 0.4 m accelerated region with the remaining 0.4 m adjusted to zero-pressure gradient. The Stanton number for the rough-wall experiments decreased or increased for accelerated rough-wall cases compared to zero-pressure gradient cases depending on flow conditions. For fully-rough boundary layers, Stanton numbers increased with acceleration compared to zero-pressure gradient at the same x-position. For aerodynamically-smooth and transitionally-rough boundary-layer flows, the effect of acceleration was not similar to that of fully-rough flows and was highly dependent upon the flow conditions. The acceleration caused a decrease in the relative turbulence level over the rough surface. The profiles of u′2¯ for the accelerated runs were lower than those of zero-pressure gradient cases, and a substantial decrease in the Reynolds shear stress (−u′v′¯) component was observed when acceleration was applied.