A Comparative Study between Sand- and Gravel-Bed Open Channel Flows in the Wake Region of a Bed-Mounted Horizontal Cylinder

Kalpana Devi,Ram Balachandar,Prashanth Reddy Hanmaiahgari,Jaan H Pu

doi:10.3390/fluids6070239

Abstract

In nature, environmental and geophysical flows frequently encounter submerged cylindrical bodies on a rough bed. The flows around the cylindrical bodies on the rough bed are very complicated as the flow field in these cases will be a function of bed roughness apart from the diameter of the cylinder and the flow velocity. In addition, the sand-bed roughness has different effects on the flow compared to the gravel-bed roughness due to differences in the roughness heights. Therefore, the main objective of this article is to compare the mean velocities and turbulent flow properties in the wake region of a horizontal bed-mounted cylinder over the sand-bed with that over the gravel-bed. Three experimental runs, two for the sand-bed and one for the gravel-bed with similar physical and hydraulic conditions, were recorded to fulfil this purpose. The Acoustic Doppler Velocimetry (ADV) probe was used for measuring the three-dimensional (3D) instantaneous velocity data. This comparative study shows that the magnitude of mean streamwise flow velocity, streamwise Reynolds normal stress, and Reynolds shear stress are reduced on the gravel-bed compared to the sand-bed. Conversely, the vertical velocities and vertical Reynolds normal stress are higher on the gravel-bed than the sand-bed.

Highlights

Turbulent flows over rough surfaces occur in engineering applications and natural sciences
The Reynolds Shear Stress (RSS) peaks values are declining along the streamwise direction downstream of the cylinder over both the sand-bed and gravel-bed. This finding on the gravel-bed is similar to the flow pattern downstream of the rough forward-facing step (Wu et al [31]). This analysis conveys a clear understanding that gravel-bed roughness in comparison to sand-bed roughness significantly modifies the turbulence levels downstream of a horizontally mounted cylinder
The higher values of streamwise velocity, vertical velocity, Reynolds Normal Stress (RNS), and RSS were observed in the case of a higher Reynolds number flow over the sand-bed due to a higher level of turbulence

Summary

Introduction

Turbulent flows over rough surfaces occur in engineering applications and natural sciences. Determining the roughness effects on the turbulent flows in engineering applications is very important since the mechanisms of production, diffusion, and energy transfer between the mean and the turbulent fields, especially in the near-wall region, are influenced by surface roughness. There has been considerable work carried out to understand the dynamics of turbulent flows over rough surfaces. Very recently, a few researchers have focused on illustrating the realistic roughness effects on various features of turbulent flows [7,8]. The realistic roughness is quite different from the regular (or ‘modelled’) roughness. It is described by a wider spectrum of wavelengths and random distribution of structures of each scale

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Conclusion