Koustuv Debnath

Bridge Pier Scour in Clay-Sand Mixed Sediments at Near-Threshold Velocity for Sand

Local scour at circular bridge piers embedded in a clay-sand-mixed bed was investigated in laboratory flume experiments. The effects of clay content, water content, and sand size on maximum equilibrium scour depth, equilibrium scour hole geometry, scouring process, and time variation of scour were studied at velocities close to the threshold velocities for the sand in the clay-sand mixture. It was observed that clay content and water content were the key parameters that effect the scouring process, scour hole geometry, and maximum equilibrium scour depth. The bridge pier scouring process in clay-sand mixtures involved different dominating modes for removal of sediment from scour hole: chunks-of-aggregates, aggregate-by-aggregate, and particle-by-particle. Regression-based equations for estimation of nondimensional maximum scour depth and scour hole diameter for piers embedded in clay-sand mixtures having clay content of <40% and water content of <40% were proposed as functions of pier Froude number, clay content, water content, and bed shear strength.

Turbulence Statistics of Wave-Current Flow over a Submerged Cube

AbstractThis paper describes an experimental study carried out in a laboratory flume to investigate the interaction of a surface wave with a unidirectional current over a submerged cubic obstacle. The three-dimensional velocity field was measured using an acoustic Doppler velocimeter (ADV). The results highlight the changes induced in the mean velocity profile, turbulent intensity, and Reynolds shear stress in a plane of symmetry from the superposition of surface waves of different frequencies. Modifications in the mean velocities, turbulence intensities, and Reynolds shear stresses with respect to the flat surface case, in the vicinity of the cube, are explored. This study also investigates the dominant turbulent bursting event that contributes to the Reynolds shear stress in the near-bed flow influenced by the cube. The results show that near the boundary, the contributions to the total shear stress from ejection and sweep are dominant. However, away from the boundary, the outward and inward interaction...

Changes in turbulent flow structure over rough-bed under combined wave-current motions

Abstract This paper presents the result of an experimental study carried out in a laboratory flume to investigate the interaction of surface wave and unidirectional current over cube-mounted rough-bed. The cubes were made of wood and were positioned at the channel-bed with different spacing. The study particularly focuses on the changes induced in the spatially averaged velocity profile, turbulent intensity and Reynolds stress due to the superposition of waves of different frequencies on current-induced flow. The spatially averaged velocity profile shows two distinct regions: the linear or exponential distribution below the roughness tops and logarithmic profile above the roughness top. The profile of streamwise turbulent intensity is similar to that of current-only flow except in the region near the water surface. In contrast, the profile of bottom-normal turbulent intensity is almost constant over the entire depth. Below the roughness crest, the form-induced stress changes significantly with switching from negative to a positive value with change in roughness spacing, but is not affected much by the presence of surface wave.

Local scour around abutment in clay/sand-mixed cohesive sediment bed

New data on scouring around vertical and wing wall abutments embedded in clay/sand-mixed beds are reported and the mechanism of the scouring process is described. In total, 87 experimental runs on vertical wall abutments and 30 experimental runs on 45° wing wall abutments in cohesive beds were conducted. The effect of clay content, water content, bed shear strength and pier Froude number on maximum equilibrium scour depth, scouring process, and time variation of scour were investigated. Further, equations for estimation non-dimensional maximum scour depth for vertical wall abutments embedded in clay–sand mixtures are proposed as functions of pier Froude number, clay content (C), water content (Wc), and bed shear strength for 0.35 ⩽ C ⩽ 1; 0.19 ⩽ Wc ⩽ 0.44. In addition, shape correction factor for 45° wing wall abutments in cohesive sediment beds is evaluated.

Turbulent flow characteristics in a rectangular channel under the influence of cube-mounted rough bed

Abstract This study was undertaken to examine the effect of bottom roughness on the first- and the second-order turbulence statistics at a high Reynolds number. The artificially generated roughness was consists of cubes equally placed to one another along a longitudinal and lateral direction and the height of the cubes (k) occupies 12.5% of the depth of flow (h). Two roughness separations P/k = 5 and 9 were examined and compared with the result that obtained over a flat surface. Here, P is the pitch between consecutive roughness elements. For each case, three-component velocity measurements were obtained using a micro-acoustic Doppler velocimeter at the flume centerline which allows examination of the large variations of the turbulence intensities, Reynolds shear stress, eddy viscosity, and mixing length below the roughness top. In this region, the flow is more directly influenced by the shear layers from the preceding cube. Spectra analysis was also performed to obtain the oscillation pattern that affects the turbulent properties below and above the roughness.

Discussion of “Boundary layer development over non-uniform sand rough bed channel”, by Anurag Sharma and Bimlesh Kumar (2017)

Abstract This discussion provides highlights with regard to determination of the appropriate length for boundary layer development over non-uniform sand bed channels with and without seepage. It is demonstrated that the compound effects of Kármán’s coefficient κ and virtual origin should be accounted to quantify the behaviour of roughness length over non-uniform sand bed channels with seepage.

Higher order turbulent flow characteristics of oscillatory flow over a wall-mounted obstacle

Abstract The paper presents an experimental study in a laboratory flume to investigate the combined wave–current flow over a wall-mounted cubic obstacle. Detailed velocity data were collected using three-dimensional (3D) micro-acoustic Doppler velocimeter from upstream to downstream of the cube along the centerline of the flume. This study examines the spectra, co-spectra, and turbulence kinetic energy flux analysis under the influence of cube in the turbulent boundary layer. It is observed that stream-wise spectral and co-spectral energy distributions vary with the addition of surface wave. Particular attention is also given to understand the behavior of higher order moments to explicitly account for turbulent momentum transport characteristics over wall-mounted cube under wave–current interactions.

Flume study on contraction scour in clay–sand mixed cohesive bed

Abstract Flow constriction, whether man-made or natural, occurs whenever there is a significant reduction of the width of natural flow and hence the induced beds shear stress increases considerably and consequently the bed sediment within the contraction get scoured. The problem is often idealized as a rectangular long contraction consisting of a contracted zone, length of which is greater than the transverse width of the flow. From the literature survey, it is revealed that several empirical models were proposed to compute the maximum equilibrium scour depth at contraction. In almost all the reported investigations, bed sediment used were non-cohesive sand, except one or two where bed sediment was made up of clay and completely cohesive in nature. In the present investigation, an attempt has been made to study the scour in long contraction for the sediment comprises the mixture of clay and fine sand with varying proportions. A total of 30 experimental runs in laboratory flume on clay–sand mixed cohesive sediment were carried out to study the effect of varying clay fraction, approach flow velocity and contraction ratio, on maximum equilibrium scour depth and its location within long contraction.

Pier scour within long contraction in cohesive sediment bed

In the present study, an attempt has been made to study the bridge pier scour embedded long contraction for the bed sediment prepared by a mixture of clay and fine sand with varying proportions having a specific range of antecedent moisture content. Results particularly focused on the clay–sand mixed cohesive bed at varying clay fractions of the sediment bed, approach flow velocity, contraction ratio and different pier shapes, on maximum equilibrium scour depth for pier scour within long contraction. Further, regression based equations for the estimation of non-dimensional maximum scour depth for piers within long contraction in clay–sand mixed cohesive bed embedded were proposed.

Turbulence study over submerged obstacles on waves and currents combined flow

Abstract The paper explores an experimental study carried out in a laboratory flume to investigate the interaction of surface wave and unidirectional current over a pair of hemispherical obstacles with relative spacing, L/d = 4 where L is center to center spacing and d is the obstacle height. The effect of surface waves and current interaction on the mean velocity profile, turbulence intensities and Reynolds shear stress of an open channel flow under the influence of two hemispherical obstacles has been presented in this paper. The components of velocity along three different directions were measured using 3D micro-acoustic Doppler velocimeter. The experiments were conducted for Reynolds number 58800. Quadrant analysis has been adopted to investigate the turbulence bursting events contributing to the Reynolds shear stress in the near-bed region. It is observed that closed to the top of the obstacles, the contribution to the total shear stress due to ejection and sweeping events changes with the superposition of waves.