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^circ 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
Highlights
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
In contrast to Schanderl et al (2017), who investigated the flow field in front of a wall-mounted cylinder on a flat plate, we study the flow field inside a scour hole around a circular cylinder at the same and at a lower Reynolds number
By assuming the dissipation rate to be in the order of magnitude of 0.1u3sym∕D, the spatial resolution of the S-particle image velocimetry (PIV) data would increase to 9 K for Re = 39,000
Summary
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. The boundary-layer approaches the scour hole from upstream leading to flow separation at the scour rim that generates vorticity (Melville and Raudkivi 1977; Graf and Istiarto 2002; Dey and Raikar 2007; Kirkil et al 2008). The remaining studies, were either intrusive such as hot wire anemometer (Melville and Raudkivi 1977) and submerged ADV (Dey and Raikar 2007), or the experimental set-up was simplified by attaching a half-sided cylinder to the side wall of a transparent flume (Unger and Hager 2007) It is unclear how the flow dynamics were affected by the presence of the measurement device itself or by the transparent solid wall in particular in the symmetry plane upstream of the cylinder where the strength of the horseshoe vortex was identified as being the largest.
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