Adverse and Favourable Pressure Gradient Turbulent Flows Over Smooth and Rough Surfaces

Abstract

An experimental investigation was undertaken to study the salient features of adverse and favourable pressure gradient turbulent flows over a smooth wall and gravel roughness in asymmetric diverging and converging channels. Reference experiments were also performed in a parallel walled channel for which the pressure gradient was nearly zero. A high resolution particle image velocimetry system was used to conduct the velocity measurements. From these measurements, both one-point and two-point statistics were extracted and used to determine the effects of combined roughness and pressure gradient on the turbulence structure. It was found that adverse pressure gradient and surface roughness increased the turbulence intensities and Reynolds shear stress over the entire boundary layer, while favourable pressure gradient increased the turbulent intensities in the wall region and decreased the turbulence level in the outer layer. The Reynolds shear stress was decreased substantially by the favourable pressure gradient resulting in a considerable decay in the levels of the stress ratios over the smooth surface and gravel roughness. The distributions of the turbulent diffusion terms show considerable transport of turbulent kinetic energy and Reynolds shear stress towards the wall in the presence of adverse pressure gradient and surface roughness, while these terms are attenuated by favourable pressure gradient. In the diverging channel, it was found that surface roughness increases the spatial extents of the two-point streamwise velocity auto-correlation contour in the inner layer and increases the extents of the wall-normal velocity correlation in the outer layer.