THE EROSIVE POTENTIAL OF THE HORSESHOE VORTEX SYSTEM AND ITS RELATION TO DIMENSIONAL AND DIMENSIONLESS PARAMETERS

Abstract:

The turbulent flow of air or water around an object can result in scour and erosion of the underlying substrate. The fluid mechanics of this process is explained the dynamics of the Horseshoe Vortex (HV) system that forms as a result of the flow separation induced by the object. In this work, the characteristics of the HV system in front of a wall-mounted circular cylinder have been studied experimentally to improve understanding of these complex dynamics as a function of relevant dimensionless parameters. The flow velocity field within the plane of symmetry, upstream of the cylinder, was measured using Particle Image Velocimetry (PIV) and vorticity fields were obtained from this. A range of natural flow conditions were obtained when Froude number (Fr) was varied from subcritical to supercritical conditions over the range 0.015 to 2.46, and the influence of other governing parameters such as the cylinder-based and depth-based Reynolds numbers was also investigated systematically. The results of a Quadrant analysis show the variation in turbulent stress contributions from positions underneath and upstream of the HV system. Proper Orthogonal Decomposition (POD) was used to elucidate the size and nature of the observed flow structure. The results show that the flow depth (h) has the primary control on the location of the HV system, while the magnitude of the peak wall stresses is controlled by cylinder diameter, Fr, and flow depth. That the controls include a mixture of dimensionless and dimensional factors highlights some of the complexity of translating laboratory results into practical recommendations for the study of natural systems, or for effective modelling with eddy-resolving numeric.