Search

High Froude-number open-channel flows can entrain significant volumes of air, a phenomenon that occurs continuously in spillways, in free-falling jets and in hydraulic jumps, or as localized events, notably at the toe of hydraulic jumps or in plunging jets. Within these flows, turbulence generates millions of bubbles and droplets as well as highly distorted wavy air–water interfaces. This phenomenon is crucial from a design perspective, as it influences the behaviour of high-velocity flows, potentially impairing the safety of dam operations. This review examines recent scientific and engineering progress, highlighting foundational studies and emerging developments. Notable advances have been achieved in the past decades through improved sampling of flows and the development of physics-based models. Current challenges are also identified for instrumentation, numerical modelling and (up)scaling that hinder the formulation of fundamental theories, which are instrumental for improving predictive models, able to offer robust support for the design of large hydraulic structures at prototype scale. [ABSTRACT FROM AUTHOR]

Large wood increases the morphological and hydraulic complexity of rivers, yet it may block and modify the flood discharge capacity of hydraulic structures. To assess the related risk, blockage probability estimation for hydraulic structures such as reservoir spillways is needed. This work presents unstudied parameters for blockage of large wood with a reservoir‐type approach flow, where the inflow velocity has a negligible magnitude. Experiments were conducted in a channel with an ogee crested spillway equipped with piers, representing a commonly used hydraulic structure. Artificial stems were used to systematically evaluate the influence of stem length and stem draft on the blocking process. Different hydraulic conditions were evaluated by changing the water level in the reservoir. The head at the spillway crest with respect to stem draft was found to be a key parameter for blockage probability estimation at a spillway. Additionally, stem length was related to the bay width in the estimation of blockage. Larger heads tend to reduce the blocking probability of large wood, for a given stem draft, while increasing the relative stem length tends to increase the blocking probability. A logistic regression model is provided to estimate large wood blockage probability at ogee crested spillways with piers. Finally, recommendations for engineering practice are presented. Key Points: A logistic regression model was developed to estimate individual stem blockage probability at ogee crested spillways with piersThe blockage probability depends markedly on the hydraulic head and stem draft, and, to a lesser extent on the stem length and bay widthHeads greater than 1.8 times the stem draft can ensure blocking probabilities smaller than 0.20 at ogee crested spillways with piers [ABSTRACT FROM AUTHOR]

The step aerator for stepped chutes was recently introduced. Its purpose is to preaerate the zone between the spillway crest and the point of incipient bottom aeration to counter cavitation damage for relatively large unit discharges. The shape of the step aerator is optimized herein by hydraulic modeling to design the air supply and to indicate the aerator blockage characteristics. The shape and the performance of the improved step aerator are described in terms of air supply and the development of air-water flow on the stepped chute. Further, step insets are considered to reduce the spray formation on stepped chutes for small unit discharges, in combination with the step aerator. The performance of the step insets was investigated and their effect on the spray formation analyzed. The step aerator generally increases the maximum spray height at smallest discharges as compared to the standard stepped chute without step aerator. Insets over the first five steps significantly reduce spray relative to standard stepped chutes. [ABSTRACT FROM AUTHOR]

Scour downstream of ski jumps may be avoided by jet deflection to an area where the energy dissipation is accomplished. The main purpose of this experimental study was the analysis of the jet air entrainment downstream of a ski jump, both for pure water and preaerated approach flow conditions. A systematic variation of the Froude number and the flow depth in the approach flow channel resulted in a range of discharge characteristics, whereas the geometry of the ski jump was maintained for all tests. The air concentration profile was measured at different locations downstream from the ski jump to evaluate the: (1) jet air concentration distribution; (2) location of minimum air concentration along the mixture flow jet and development of the minimum and the cross-sectional average air concentrations; (3) jet trajectories; and (4) process of air entrainment characteristics and jet disintegration. The results demonstrate the significant effect of the approach flow Froude number, the approach flow depth, and of preaeration on jet disintegration. [ABSTRACT FROM AUTHOR]

The upstream reach of stepped spillway flows may become prone to cavitation damage for large specific discharges because of the absence of air close to its bottom, until the point of bottom self-aeration is reached. This study considers the effect of two aerator types located at the first vertical step face to add air to the chute bottom. Compared to standard stepped spillway flow, considerable differences may be observed closely downstream of the aerator, whereas no significant deviations occur in the far-downstream chute reach. The characteristics of bottom air concentration curves on stepped chutes are investigated with an experimental approach. The results are then compared with flows on both smooth chutes and standard stepped chutes. The data analysis results in design equations that may be applied to usual stepped spillways of chute angles around 50°. In addition, a sinusoidal variation of air concentration about the average value as a novel phenomenon is described relating to a local instability in the minimum bottom air concentration reach. [ABSTRACT FROM AUTHOR]

The article offers information related to effect of driftwood on the discharge efficiency of piano key weirs (PKWs) and labyrinth weirs. Topics discussed include discussion on dam operators using PKWs on their spillways, effect of driftwood on the discharge capacity than classical weir types and effective hydrographs during a flood event and its influence on the behavior of the debris at the weir.

The article discusses the highlights of a discussion on the discharge coefficient for free and submerged flow over Piano Key weirs. The participants limited their data sets to two scenarios, which includes tests not respecting the scale effects criterion that were ignored while all other tests were considered. Head-discharge measurements of prototype related case study model tests were taken into account.

The article discusses the reply of the authors on the commentary made to their paper about chute aerators. They emphasize that the comments are right on claiming that the air entrainment coefficient is commonly used to describe the aeration capacity of aerators. They stress the basis of the discrepancy observed in the method used for a better determination of the chute reach protected. They also assert the efficiency of the proposal to increase local bottom slope at the reattachment point.