Investigation of the effect of a smooth strip on rough-wall turbulent boundary layers

Abstract

A series of experiments was conducted to study the effects of a smooth strip on heat transfer and fluid dynamics in the turbulent boundary layer on an otherwise rough surface. The first 0.9m of the test section is rough, followed by 0.1-m smooth strip, and the remaining 1.4m is rough. The rough surface is composed of 1.27-mm diameter hemi-spheres spaced 2-diameters apart in staggered arrays. The experiments include measurements of Stanton number distributions as well as mean temperature, mean velocity, and turbulence intensity profiles. The results are compared with previously published data from experiments with a rough leading portion and a smooth final portion and from experiments on an all-rough surface. Over the smooth strip, Stanton number decreases by almost 45% relative to the all-rough value; however, the Stanton number distribution recovers typical rough-wall behavior in a short distance. The Stanton number measurements are compared with predictions using the discrete-element method. In general, the agreement is excellent. Mean velocity and turbulence intensity profiles show the flow downstream of the strip to rapidly attain rough-wall behavior in the near region, while requiring more distance to exhibit a complete rough-wall behavior.