Experimental investigation of the effects of acceleration on flow and heat transfer in the turbulent rough-wall boundary layer

摘要

The effect of freestream acceleration on heat transfer and fluid dynamics in turbulent rough-wall boundary layer is investigated experimentally. The experiments include 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. Some smooth-wall data were taken to serve as base-line data for comparison with the data from rough surfaces. 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 heat transfer results are presented as Stanton number distributions. In addition, boundary layer profiles of mean velocity and Reynolds stresses were obtained using hot-wire anemometry. The data were collected for four different inlet freestream velocities ranging from 5 m/s to 28 m/s. The nominal values of the acceleration parameter ranged from K = 1.4 $times$ 10$sp{-6}$ to 0.3 $times$ 10$sp{-6}.$.;The Stanton number for the rough-wall experiments increased with acceleration compared with zero-pressure gradient values at the same position or Reynolds number. This is directly opposite to the behavior of smooth-wall turbulent boundary layers where the Stanton number decreases with acceleration. For aerodynamically-smooth and transitionally-rough boundary layers, the effect of acceleration is not seen immediately at the beginning of the accelerated region as it is for fully-rough flows. However, as the boundary layer thins under acceleration, the surface becomes relatively rougher resulting in a sharp increase in Stanton number.;Acceleration for both rough and smooth surfaces causes a decrease in the relative turbulence level. The profiles of $overline{usp{/2}},$ $overline{vsp{/2}}$ and $overline{wsp{/2}}$ for the accelerated runs are lower than those of zero-pressure gradient cases, and a substantial decrease in the Reynolds shear stress $(overline{-usp/vsp/})$ component was observed when acceleration is applied.