Journal of Hydraulic Engineering

Due to their small footprint and efficient hydraulic performance, piano key weirs (PKWs) have been gaining growing attention in the past two decades. Besides the primary geometric parameters like magnification ratio and weir height, the crest shape is another crucial factor influencing the overall performance of this type of hydraulic structure. Beyond discharge aspects, the present study investigates how the flow characteristics, energy dissipation, and scale effects of a piano key weir are influenced by different crest configurations. Overall, results indicate that crest shape majorly affects the nappe trajectory and has significant influence on the hydraulic characteristics of a PKW. The downstream crest shape was observed to be the key element governing the outflow characteristic at low heads, whereas the crest shape in the upstream side was found to be more critical for upstream relative heads within the range of 0.4–0.6. With the resulting nappe behavior, half-round crest was found to provide the highest discharge capacity, while flat crest was the most efficient configuration in terms of energy dissipation. Furthermore, relative to rectangular crests, models with rounded crests were observed to have more significant scale effects at low heads and deviated downstream flow regimes. To facilitate the practical design of such structure, empirical equations were developed to predict the discharge coefficients and downstream residual energies.

Free-surface aeration is a key characteristic of high-velocity flows on spillways. Despite their common use as conveyance structures, there is a research gap in the understanding of flows across spillways with small invert roughness. To address this gap, this study investigated flows down a moderate sloped spillway with four different microrough invert configurations, providing novel information on flow patterns, flow properties, and flow resistance. Visual observations identified distinctive differences between free-surface roughness and air entrainment, where the former increased with invert roughness and occurred consistently upstream of the intersection of the turbulent boundary layer with the free-surface. The evolution of velocities, air concentrations, and interface frequencies along the chutes was documented. All velocity distributions were well represented by a log law with wake function and uniform free-surface layer. A semianalytical model was able to predict air concentration and interface frequency distributions for all experiments with a characteristic length that was linked closely to the mean air concentration. Analysis of the flow resistance provided previously missing design data and suggested that researchers need to be cautious when reporting microrough invert conditions in order to avoid overestimating their flow resistance. A comparative analysis of the air–water flow properties for the three microroughness configurations showed that the air concentration distributions were well scaled with a Froude similitude, and that the number of free-surface perturbations and entrained bubbles was affected by scale effects.

Velocity distributions in supercritical open-channel flows over stepped cavities have traditionally been described using a power-law approach or theoretical solutions of plane mixing layers. These approaches were found to be valid either above the step edges or above/within step cavities, but no generalized model is available. In this study, a four-layered velocity model is proposed, which combines different physical concepts, including the mixing layer, log-layer, wake function, and free-stream layer. This multilayer model was applied to previous experimental stepped chute data, providing novel opportunities to comparatively assess the relative contribution of individual physical effects on the velocity profile. Model parameters provided insights into flow hydrodynamics, comprising mixing layer length scales and shear velocities. Equations for Reynolds shear stresses within the different layers were formulated using an eddy viscosity concept, while normal stresses and turbulent kinetic energy compared well to semitheoretical open-channel flow equations.

We make use of the partial slip boundary condition, commonly introduced to investigate steady flows, to evaluate the oscillatory flow generated close to a rough sandy bottom by propagating surface waves. Moderate values of the Reynolds number are considered such that the flow regime is laminar. However, an eddy viscosity is introduced to take into account the momentum transfer induced by the small vortices shed by the roughness/grain elements and the effect that these vortices have on the mixing processes. Because of the oscillatory forcing, the slip velocity at the bottom turns out to be characterized by both an amplitude and a phase that are evaluated by fitting the model results to experimental measurements. Moreover, a reasonable form of the ’eddy viscosity’ is chosen on the basis of physical arguments. A comparison between the model predictions and the experimental measurements supports the proposed approach.

Reservoir dams play a significant role in society and the economy. Low-level outlets (LLOs) are key safety devices providing reservoir drawdown for maintenance and emergency purposes, sediment flushing, and release of environmental flow. High velocities and turbulence levels of the free-surface flow lead to air entrainment and air transport along the LLO tunnel, resulting in subatmospheric air pressures. In addition, shockwaves formed downstream of the gate may lead to a complete filling of the tunnel cross section, possibly resulting in slug flow and flow pulsations, which should be avoided for operational safety. In this study, physical model tests were performed to investigate the effects of gate opening and hydraulic head on shockwave patterns. Especially for large contraction Froude numbers, the shockwaves were strongly aerated, resulting in a complex air–water flow pattern. The results provide insights into the formation and propagation of shockwaves, contributing to an improved design of LLO tunnels.