Influence of bed proximity on the three-dimensional characteristics of the wake of a sharp-edged bluff body

Abstract

The gap flow effect in a wake is investigated to develop an improved picture of the formation of fluid structures via a numerical simulation of flow past a bluff body with two different clearances from the bed. These two cases are compared with the no-gap case which is considered as a reference case. The transient three-dimensional Navier-Stokes equations are numerically solved using a finite volume approach with the detached eddy simulation as the turbulence model. The effect of the free surface is included in the model by using the volume of fluid method. The fluid structures that are generated in the wake are identified using the λ2-criterion. It is found that the gap flow influences the formation of various types of fluid structures in the wake region. The horseshoe vortex appears to be attenuated as the gap size increases. The turbulent structures at the core of the wake appear to be disorganized and have the ability to extend further in the transverse direction in the absence of the horseshoe vortex. A new structure is identified in the wake flow when the gap size exceeds a threshold value. This structure acts to enhance the positive wall-normal velocity and accelerate the restoration of the free surface to its original position at shorter distances downstream of the bluff body. The lateral entrainment to the wake region is also enhanced as the gap is introduced in the wake flow.