  Citation: Heydari, N.; Diplas, P. Three-Dimensional Turbulent Flow Characteristics Near the Leading Edge of a Longitudinal Structure in the Presence of an Inclined Channel Bank. Water 2022, 14, 3524. https:// doi.org/10.3390/w14213524 Academic Editor: Jennifer G. Duan Received: 12 October 2022 Accepted: 1 November 2022 Published: 3 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). water Article Three-Dimensional Turbulent Flow Characteristics Near the Leading Edge of a Longitudinal Structure in the Presence of an Inclined Channel Bank Nasser Heydari and Panayiotis Diplas * Department of Civil and Environmental Engineering, Lehigh University, Bethlehem, PA 18015, USA * Correspondence: pad313@lehigh.edu Abstract: The present work investigates turbulent flow structures and behavior near the leading edge of a longitudinal flow obstruction in an open channel with an inclined bank. A volumetric particle image velocimetry (VPIV) was employed to collect velocity data. The results indicate that a relatively moderate channel bank angle (θ = 28 ◦ ) does not prevent the formation of a junction vortex (JV) system. Indeed, it is found that the JV system develops over the channel bank and extends to the leading edge of the flow obstruction. It is demonstrated that the main factor that strengthens the primary junction vortex (JV1) at the tip of the protrusion is the pronounced downward flow. The probability density functions (pdfs) of the velocity fluctuations specify that the JV1 oscillates aperiodically between the so-called zero-flow and back flow modes. This explains the amplification of TKE at its core. It is shown that the velocity difference between the peaks of the pdf increases from over the channel bank towards the tip of the flow obstruction. The aperiodic behavior of the JV1 is confirmed via the proper orthogonal decomposition (POD) technique. Specifically, it is demonstrated that the leading POD modes, associated with the JV system, contain lower energy content compared to periodic flows. The time-averaged vertical vorticity field verifies the presence of a tornado like vortex structure near the upstream face of the retaining wall. Finally, the results suggest that the largest bed shear stress values in the mean flow are located near the tip of the protrusion. Keywords: local scour; sediment transport; junction vortex; horseshoe vortex; modal decomposition; particle image velocimetry 1. Introduction Flow past bank protrusions is of paramount importance in many river engineering applications. The most common type of such structures are bridge abutments, groynes, and spur dikes. These structure are built to minimize bank erosion or to enhance river habitats [1,2]. Local flow dynamics are modified in the presence of bank protrusions. In extreme cases, this leads to the formation of very complex, large-scale energetic coherent structures. When they are situated, or are advected, to near a channel bed the boundary shear stress and the pressure fluctuations are amplified. As a result, when the bed is erodible sediment particles are entrained. This ultimately causes development of a scour hole which can undermine the integrity of infrastructure components [3]. Previous experimental [4–6] and numerical [7,8] studies dealing with turbulent flows around bridge abutments and spur dikes, mounted on one of the vertical sidewalls of a channel, have identified highly turbulent 3D flows within their proxies that consist of turbulent structures over a wide range of scales. The dominant coherent flow structures and relevant features around the foregoing obstructions include: the junction vortex (JV) system and the recirculating flow near the upstream base of the protrusion, the separated shear layer that originates from the leading edge of the obstruction, and the wake flow past the protrusion. It is important to note that the JV system around flow obstructions attached to one side of a channel is typically referred to as a horseshoe vortex system in Water 2022, 14, 3524. https://doi.org/10.3390/w14213524 https://www.mdpi.com/journal/water