Flow Field in a Skimming Flow Over a Vertical Drop Without End-Sill

Abstract

The flow structure in the shear layer and in the recirculation zone of a skimming flow downstream of a vertical drop without end-sill measured using high speed particle image velocimetry (HSPIV) and flow visualization method is presented. The interface between the sliding jet and the recirculation zone (zone below sliding jet) was enhanced through non-ventilation condition between the drop structure and the jet. The flow field measured through HSPIV was used to represent the characteristics of mean streamwise velocity in the shear layer and mean horizontal velocity in the recirculation zone. With the growth of shear layer as the jet slides down over the recirculation zone, the momentum exchange from the sliding jet into the recirculation zone via the shear layer increases along with energy loss. Hence, it was observed that the amount of energy dissipated in the skimming flow at the drop structure without ventilation is greater than that with ventilation by an average value of 50% for Y c / H ≥ 0.2 (where Y c = critical depth and H = drop height). However, the flow structure in the recirculation zone is found to be analogous to that of turbulent plane wall jet. The nonlinear regression analysis is used to fit the regressed velocity profiles to the measured HSPIV mean velocity distributions. Further, the appropriate characteristic velocity and length scales are selected to attain the unique similarity profiles both in the shear layer and in the recirculation zone of the skimming flow. The selection of the characteristic scales is also discussed. The similarity profiles are well comparable with those of napped flow without end-sill and with ventilation as well as of skimming flow with end-sill and without ventilation. It is interesting to observe that, the proposed similarity profiles for the shear layer also map the data of backward-facing step flow and cavity shear flow. In addition, the turbulence characteristics in the shear layer, including turbulence intensities, turbulent kinetic energy, viscous and Reynolds shear stresses, and turbulence energy-budget balance, are illustrated in detail. From the variation of turbulence production it is observed that near the drop structure the energy exchange is from the chaotic recirculation zone to the sliding-jet flow, while in the later part it is reversed. Furthermore, the analysis of turbulence energy-budget balance indicates very significant role of turbulence production, pressure diffusion and turbulence diffusion as compared with turbulence advection that has very minor role in turbulence energy-budget balance for the central part of the shear layer.

Application of SIM, HSPIV, BTM, and BIV Techniques for Evaluations of a Two-Phase Air–Water Chute Aerator Flow

“…The feature of the approach flow upstream the offset highlights increase of U 0 (i.e., its positive convective acceleration) and the corresponding decrease of h 0 . The offset acts like a drop structure such that the change of the approach flow into a projecting free jet commences near the brink of the offset, around which depression of the free surface occurs (Lin et al [34,38,[45][46][47]). Detailed velocity field and profiles of the approach flow and free jet for −4.0 cm ≤ X ≤ 9.0 cm, along with the mass-conservation check using 2D continuity equation, is found in Lin et al [38].…”