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46

Chapter 3

Abstract

Channel Flow

Abdullah Faruque

Roughness Effects on Turbulence

A comprehensive study was carried out to understand the effects of roughness on the turbulence characteristics of flow in an open channel and would be presented in this chapter. Tests were conducted with four different types of bed surface conditions (an impermeable smooth bed, impermeable rough bed, permeable sand bed and an impermeable bed with distributed roughness) and at two different Reynolds number (Re = 47,500 and 31,000). The variables of interest include the mean velocity, turbulence intensity, Reynolds shear stress, shear stress correlation and higher-order moments. Quadrant decomposition was also used to extract the magnitude of the Reynolds shear stress from the turbulent bursting events. The effect of bed roughness on the turbulence characteristics can be seen throughout the depth of flow and thus dispute the 'wall similarity hypothesis'. In comparison to other roughness, distributed roughness shows the greatest effect on both streamwise and vertical turbulence intensities. Velocity triple products that reflects the transportation of turbulent kinetic energy is also seen to be affected by roughness of the channel bed with a variation of 200–300% compared to the flow over smooth bed. To analyze the turbulent bursting events, quadrant decomposition tools were used and found that the roughness affected heavily in the production of extreme turbulent events. The increases of the intensity and frequency of this turbulent burst causes the increase of instantaneous Reynolds shear stress. Transport of the sediment, pollutant suspension from the channel bed, changing the composition of the nutrient in the flow, sustainability of the benthic organisms, entrainment and exchange of energy and momentum are all influenced by this change of Reynolds shear stress. The sand used to form the various bed roughness conditions is same but found that the effect on different turbulence characteristics are different for different roughness. This is a strong indication that the geometric formation of the roughness is the cause of the differences in turbulence characteristics for different roughness formed by the same sand grain.

Keywords: turbulence, open channel flow, roughness, Reynolds shear stress,

Open channel flow comprises a sheared boundary layer like flow [1]. It is in utmost interest for the engineers and researchers to understand the structure and

quadrant analysis, higher-order moment

1.1 Open channel flow: general

1. Introduction

49

Characteristics in an Open
