Modal Analysis of Turbulent Flow near an Inclined Bank–Longitudinal Structure Junction

Abstract

The present study examines the effect of upstream channel bank angle on the turbulent flow characteristics and the underlying coherent structures associated with the development of local scour in the vicinity of the leading edge of a longitudinal structure. Three laboratory experiments with channel bank angles of 28°, 45°, and 65° were carried out. Flow measurements were obtained employing a stereoscopic particle image velocimetry (SPIV) system. The characteristics of underlying energetic flow features in each of the experiments were investigated by using the proper orthogonal decomposition (POD) and optimized dynamic mode decomposition (optDMD) techniques. The SPIV results indicated that for a moderate bank slope θ=28°, the horseshoe vortex (HV) amplifies turbulent kinetic energy and, consistently, bed shear stresses that consequently contribute to the local scour development. As the channel bank angle increases, and thus the degree of flow obstruction decreases, the intensity of HV declines and eventually diminishes for θ≥65°. Thus, the erosional capacity of the flow is reduced. The modal analyses specified that with the increase of channel bank angle, the flow energy is distributed among a larger number of POD modes and the similarity between the POD and DMD modes decreases. The relatively low energy content of the POD and DMD modes suggests the aperiodic behavior of the main necklace vortex and verifies the significant reduction in its coherency with the channel bank slope.