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 bluff body. A numerical comparison for the fluid structures observed in a time-averaged field of the shallow-wake flow behind a bluff body 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 turbulence 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 distinctive 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' development at the near-surface location in the far-wake and on the streamtraces in front of the bluff body. 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.