Transitions between smooth and rough surfaces in turbulent channel flows for d- and k-type rough elements

TL;DR

This experimental study investigates how transitions between smooth and rough surfaces in turbulent channel flows affect flow dynamics, revealing oscillations and vortex behaviors specific to different roughness types.

Contribution

It provides new experimental data on flow transitions over d- and k-type rough elements, highlighting oscillations in turbulence and vortex dynamics not previously documented.

Findings

Higher oscillations in Reynolds stress for k-type roughness

Fluid ejection observed from cavities to upper flow layers

Proposed relation between vortex angular velocity and turbulence oscillations

Abstract

This paper presents an experimental study on the transition from smooth to rough walls, and back to the smooth one, in turbulent closed-conduit flows. These transitions cause a shift on flow velocity profiles that changes their parameters when compared to the flow over a smooth wall. Different water flow rates were imposed in a closed conduit of rectangular cross section, where rough elements consisting of cavities of d- and k-type were positioned covering a part of the bottom wall of the test section. Reynolds numbers based on the channel half-height were moderate, varying between 7800 and 9600, and the regime upstream of the rough elements was hydraulically smooth. Experimental data for this specific case remain scarce and the involved physics rests to be understood. The flow field was measured by low frequency PIV (particle image velocimetry) and by flow visualization, the latter using a continuous 0.1 W laser, a high-speed camera, and scripts written by the authors. From the instantaneous fields measured with PIV, the mean velocities, fluctuations, shear stresses and turbulence production were computed. The results show the presence of oscillations in Reynolds stress and turbulence production, that are higher for the k-type roughness and were not shown in previous experimental works. From the high-speed movies, the angular velocities and frequencies of vortices in the cavities were computed, and the occurrence of fluid ejection from the cavities to upper layers of the flow was observed. A relation between the angular velocities of inner-cavities vortices and the oscillations in Reynolds stress and turbulence production is proposed.