STUDY OF FLOW CHARACTERISTICS OVER EXTREMELY ROUGH BED CHANNEL

Abstract

This study concerns with the flow characteristics over channels with extremely high bed roughness. Flow characteristics include the vertical velocity and the shear stress profile, the spatial variability of the Reynolds shear stress, calculation of the shear velocity, and measurement of the Resistance of flow in terms of both Darcy —Weisbach friction factor (f) and the Manning's roughness coefficient (n) and the characteristics of the Auto and Cross correlation coefficient. Experiments were conducted for the flow over extremely rough bed (bed material consisted of boulders, geometric mean of major axes: 13.9 cm, intermediate axes: 10.4 cm. and minor axes: 5.6 cm.) as well as flow over normal gravel bed (with uniform gravel size of lcm.) with same flow parameters (discharges and slope of the channel) for both types of flow, with an aim to compare the results with each other. An acoustic Doppler velocimeter (ADV) was employed to measure the 3D instantaneous velocities in the turbulent flow field. The turbulent characteristics were derived from the instantaneous velocity readings. The experimental results reveal that that the Reynolds shear stress for the flow over extreme rough bed, varies highly in space from positive to negative values also, unlike the flow over normal gravel bed where the variability is quite small in magnitude. Also the shear stress is not maximum near bed for the flow over extreme rough bed like the flow over gravel or sand bed. The maximum shear stress occurs some depth above the bed and reduces downward after that. The shear velocities were calculated using independent methods for both types of flow. Their magnitudes show quite deviation from each other. The Darcy-Weisbach friction factor and Manning's roughness coefficient were also calculated by different methods and turns to be quite difference from each other. x i The auto correlation and cross correlation coefficient for both the flows show allover decreasing trend with time, although for flow over extreme roughness the correlations have much larger value and sustains much longer before turning to almost nil value. The boulders or the large roughness materials create a form drag force which contributes to a part of flow resistance besides the shear stress which makes the flow difficult in nature to understand. The quantification of the drag was not possible here.