Flow around a scoured bridge pier: a stereoscopic PIV analysis
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RESEARCH ARTICLE
Flow around a scoured bridge pier: a stereoscopic PIV analysis Ulrich Jenssen1 · Michael Manhart1 Received: 3 February 2020 / Revised: 15 July 2020 / Accepted: 29 August 2020 / Published online: 22 September 2020 © The Author(s) 2020
Abstract We performed stereoscopic particle image velocimetry of the turbulent flow inside a scour hole around a cylinder in a sandy bed. At two planes, symmetry plane and 45◦ with respect to the approach flow, the flow and its turbulence structure were investigated. We used two Reynolds numbers (20, 000 and 39, 000) based on the cylinder diameter and the depth-averaged velocity in the symmetry plane. The flow is characterized by a strong down-flow in front of the cylinder, a large horseshoe vortex inside the scour, and an upstream directed wall jet underneath. The values of vorticity in the horseshoe vortex and of the velocity in the wall jet are larger than in a comparable configuration on a flat bed. Enhanced levels of turbulent kinetic energy are found around the horseshoe vortex and in the shear layer detaching from the rim. The orientation of the main axis of the velocity fluctuations changes when the flow enters the scour hole: from about wall-parallel in the detaching shear layer to vertical at the horseshoe vortex. The production of turbulent kinetic energy shows a maximum upstream of the horseshoe vortex centre with considerable production in the shear layer and in the wall jet underneath the horseshoe vortex. Furthermore, strong wall-parallel velocity fluctuations are visible in this region, and bimodal velocity distributions are found, but not anywhere else. The time-averaged wall-shear stresses are largest under the horseshoe vortex and most likely larger than in a corresponding flat-bed configuration. Graphic abstract
1 Introduction
* Michael Manhart [email protected] 1
Hydromechanics, Technical University of Munich, Arcisstr. 21, 80333 Munich, Germany
Local disturbances of the flow around hydraulic structures such as bridge piers can result in enhanced wall-shear stress levels and the development of local scour holes. Such scour holes can be a severe threat for the safety of the hydraulic structure. The estimation of the depth of a scour hole is a
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difficult task and empirical formulae for the design of pier foundations go in hand with large uncertainties, e.g., (Link et al. 2008). The coupling of a detailed numerical simulation with a model for sediment transport and/or erosion could considerably reduce these uncertainties as Roulund et al. (2005) pointed out. They investigated the scour development in front of a cylinder by a combined experimental/numerical study. Although the sediment transport model was calibrated, they observed a discrepancy of about 15% in the scour depth upstream the cylinder and assigned this deviation to the numerical method to model the erosion process. Similar difficulties have been observed by Khosronejad et al. (2012). They attribute the difficulties in finding a cor
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