Characteristics of Flow Structures in the Wake of a Bed Mounted Bluff Body in Shallow Open Channels

Abstract

The characteristics of the flow structures observed in the wake of a bluff body mounted vertically on the bed and normal to the flow in a shallow open channel are investigated using detached eddy simulation (DES). The flow structures in the shallow wake are identified using the خ»2criterion. A distinctive feature in the timeaveraged flow field, referred to as the owl face of the first kind, is observed. The position of this spiraling structure is stable at locations close to the bed, while its rotation sense switches from stable inward to unstable outward spiraling as it moves toward the free surface, where the bed friction becomes insignificant and the flow develops into a traditional twodimensional (2D) wake. A threedimensional (3D) structure resulting from a horizontally oriented secondary rollup process is observed immediately downstream of the base of the bluff body in the center of the nearwake region. In addition to the horseshoe vortex, a new structure that wraps around the bluff body in the toe region is identified, referred to as a collar vortex. The presence of the coherent structures in the nearbed region is highlighted and their influence on the wake region is discussed.