Favorable pressure gradient turbulent flow over straight and inclined ribs on both channel walls

Abstract

This paper reports on experimental study of turbulent flows over straight and inclined transverse ribs of square and triangular cross sections attached to the bottom and top walls of an asymmetric converging channel. The pitch-to-height ratio of the ribs was 10. A particle image velocimetry technique was used to conduct extensive velocity measurements at channel midspan and in planes close to the leading and trailing edges of the inclined ribs. From these measurements, spatial averaged profiles of the mean velocity and higher order statistics were obtained to study the effects of rib geometry, pressure gradient, spanwise plane, and rib inclination on the flow characteristics. The results show that rib geometry has no significant effects on the mean flow and turbulent quantities. The roughness effects produced by the straight ribs outweighed pressure gradient effects in the inner region of the flow. As a result, the skin friction coefficient is nearly independent of pressure gradient. The Reynolds shear stress and turbulent transport of the shear stress are also independent of pressure gradient. On the contrary, favorable pressure gradient decreased the Reynolds normal stresses in the outer region and increased the magnitudes of the triple velocity correlations and transport of turbulent kinetic energy (TKE). The three-dimensional secondary motion produced by the inclined ribs distorted the mean flow pattern and substantially diminished the ribs’ effectiveness to augment skin friction and turbulence. For example, the skin friction over the inclined ribs is only 50%–70% of the value measured over the straight ribs. Furthermore, the size of equivalent sand grain required to produce the same amount of drag is one-tenth to one-third of the rib height for the inclined ribs compared to two- to fourfold for the straight ribs. The inclined ribs also reduced the level of the Reynolds stresses, triple velocity correlations, and transport of both the turbulent kinetic energy and Reynolds shear stress compared to the straight ribs. The skewness factors showed that important structural differences exist between flows over straight and inclined ribs.