Mean characteristics of fluid structures in shallow-wake flows

An enhanced picture of the formation of fluid structures including the effect of the free surface and the bed is captured via a numerical solution of flow past a bed mounted bluffbody. A numerical comparison for the fluid structures observed in a time-averaged field of the shallow-wake flow behind a bluffbody utilizing the rigid-lid assumption and volume of fluid method is carried out in this study to bring forth the shortcomings in previous studies. The transient three-dimensional governing Navier–Stokes equations are computationally solved using a finite volume technique with Detached Eddy Simulation as a turbu- lence model. A flat wave model is used with the volume of fluid method to simulate the free surface of the water-air interface. The flow structures in the shallow-wake are qualitatively identified using the λ2 -criterion. In addition to the well-known features present in traditional shallow flows, other distinc- tive features are captured at the near-bed and near-surface locations. The formation of these structures and their dynamics are addressed in this paper. It is found that use of the volume of fluid method, which provides a more accurate representation of the free surface, has a significant impact on the structures’ de- velopment at the near-surface location in the far-wake and on the streamtraces in front of the bluffbody. Unique streamwise oriented structures are captured in the near-bed and free surface, which transport the fluid particles from the centre of the wake to the outer region. To the contrary, the fluid structures with the rigid-lid assumption are not completely developed near the free surface

Effect of Reynolds Number on Wall-normal Turbulence Intensity in a Smooth and Rough Open Channel Using both Outer and Inner Scaling

Sudden change of bed condition is frequent in open channel flow. Change of bed condition affects the turbulence characteristics in both streamwise and wall-normal direction. Understanding the turbulence intensity in open channel flow is of vital importance to the modeling of sediment transport and resuspension, bed formation, entrainment, and the exchange of energy and momentum. A comprehensive study was carried out to understand the extent of the effect of Reynolds number and bed roughness on different turbulence characteristics in an open channel flow. Four different bed conditions (impervious smooth bed, impervious continuous rough bed, pervious rough sand bed, and impervious distributed roughness) and two different Reynolds numbers were adopted for this cause. The effect of bed roughness on different turbulence characteristics is seen to be prevalent for most of the flow depth. Effect of Reynolds number on different turbulence characteristics is also evident for flow over different bed, but the extent varies on bed condition. Although the same sand grain is used to create the different rough bed conditions, the difference in turbulence characteristics is an indication that specific geometry of the roughness has an influence on turbulence characteristics. Roughness increases the contribution of the extreme turbulent events which produces very large instantaneous Reynolds shear stress and can potentially influence the sediment transport, resuspension of pollutant from bed and alter the nutrient composition, which eventually affect the sustainability of benthic organisms

The Impact of Ice Cover and Sediment Nonuniformity on Erosion Around Hydraulic Structures

Based on two case studies, the impact of ice cover on local scour around bridge piers is presented in this chapter. Bed material with different grain sizes is used and ice covers with different roughness is used to study the scour characteristics. The impact of nonuniformity of sediment is also investigated. Results show that with the increase in densimetric Froude number, there is a corresponding increase in the dimensionless scour depth. For nonuniform sediment, due to the formation of an armor layer, less maximum scour depth was noted around bridge foundation structures compared to uniformly distributed sediment. The increase in ice cover roughness results in a larger scour depth and geometry. The results indicate that it is imperative to pay attention to the impact of ice cover on the scour around hydraulic structures

Flow Over A Wall-Mounted Finite-Length Square Cylinder

A conventional way of studying the flow characteristics of flow over a bluff body is by extracting the velocity distribution in horizontal planes crossing the bluff body near the bed, mid-height and close to the free-end location. The intention of selecting the planes at these levels is to examine the horseshoe vortex and the bed effect, flow separation at the mid-height, and the downwash flow from the free-end. In some cases, where a relatively short cylinder is studied along with a taller cylinder, the mid-height level of each may present very different flow structures. It is proposed in this study that a better way of selecting the planes of interest could be based on the pressure distribution imposed on the front surface of the bluff body by the approaching flow. This paper uses a sample numerical study of wall-mounted finite-length square cylinder with three aspect ratios of 1, 2 and 4, immersed in the air flow with a thin boundary layer. The horizontal planes based on the front face pressure distribution are selected and examined to show the advantage of the proposed method