Journal of Hydraulic Engineering

Abstract

Low-level outlets are essential flood discharge structures for high-head dams. High-speed free-surface flows releasing into tunnels lead to substantial air discharge, and the air supply is important for gate vibration mitigation, cavitation protection by aeration, and slug flow transition phenomena. An additional challenge is the excessive wind noise due to low air supply efficiency, threatening staff working conditions in flood discharge operations. However, the current understanding of the effects of air vent sizes and locations on the air supply efficiency is still limited. In the present study, based on a series of experimental tests, air vent designs of long flood discharge tunnels, including a single vent and two combined working vents, are analyzed with respect to the air discharge variation. For air vent designs with a small vent size and an installation location downstream, the air discharge into the tunnel is low, and the air supply efficiency is low even for two combined working vents. Equations for the air supply efficiency are proposed, which enables quantifying the ventilation performance, and related vent parameters are recommended for favorable air ventilation designs. The present investigation provides a reference for the flood discharge tunnel design and ventilation operation assessment in hydraulic engineering.

Practical Applications

Air ventilation design is an important part of flood discharge tunnels and water supply and drainage systems. The single air vent sizes, locations, and combined working vents are considered in the present studies. The single air vent sizes, air vent downstream locations, and two vent distances are effect factors, providing reasonable assessment methods to predict the air supply efficiency for different structural and hydraulic conditions. A reasonable hydraulic design can obtain high air supply efficiency and regulate air discharge distributions in multiple air vents. The guidelines in the present analyses are effective to optimize the multiple air ventilation system and to obtain safety operations and acceptable staff working conditions.

In the stratified environment, systematic experiments are carried out to understand the impacts of rigid vegetation on the evolution of lock–release gravity currents, including the propagation process and entrainment characteristics. A theoretical model is developed to predict the gravity currents completely entering the vegetation-drag-dominated regime and verified by the experimental data. The adjustment of rigid vegetation on the current propagation is limited by the strongly stratified ambient fluid. With the coupling of the stratified ambient fluid and rigid vegetation, responses of current frontal velocity to the change of any single environmental factor are not monotonic. The phenomenon of finger-like intrusion, excited by the stratified environment and promoted by the rigid vegetation, provides a nonnegligible contribution to the current entrainment. In the homogeneous environment, the entrainment occurring in the current head caused by the wakes behind submerged patches is stronger than that of the Rayleigh–Taylor instability, while the situation is reversed when the ambient fluid is stratified.

Incorporating a guide vane-type spiral flow generator can effectively resolve particle deposition issues during long-distance pipeline conveyance. Combining model experiments and computational fluid dynamics (CFD) results under different guide vane height conditions, the paper investigated the structure of the circumferential velocity and the generated rotational effects of the guide vane-type spiral flow generator. Due to the combined effects of the guide vane constraint and water inertia, an envelope surface with a zero circumferential velocity exists along the inner edge of the guide vanes and in the direction of water flow. The envelope surface divides the circumferential flow velocity distribution into internal negative and external positive velocity zones. This paper discovered that the distribution of the envelope surface in the radial and axial directions is affected by both the height of the guide vanes and the Reynolds number. The rotational intensity of the fluid particles around the axis was characterized by the circumferential moment of momentum. When the relative height of the guide vane was 0.35, the highest percentage of circumferential flow velocity in the flow field was reached, while the rotational kinetic energy conversion efficiency was maximum.

Accurate estimation of local scour depth has long been considered to be an important step for the design of bridge piers. Methods currently serving in different countries are generally based on equilibrium equations, which have been found to either overestimate or underestimate the local scour depth. In this study, we proposed a new design method for clear-water local scour depth based on temporal evolution characteristics. We collected data from 67 experiments from the referenced literature and conducted four in-house experiments. Using factor analysis and trial calculation, we derived a new temporal equation for local scour depth evolution. Comparison showed that the maximum predicting error of the total 71 experiments was reduced from 162% to 534% using four existing temporal equations to 34% using the new equation. The corresponding design method was illustrated by an idealized scenario. In addition to its understandability and usability, the new method had better flexibility and fault tolerance, because it did not involve the concept of equilibrium scour depth, which lacked an objective and universal criterion.

The impacts of moving bedload particles on hydraulic structures or natural bedrock in rivers can cause substantial and sometimes fast erosion, with implications for structural stability and the morphodynamics of channels. For a given set of hydraulic and sediment load conditions (termed erosivity), erosion rates are modulated by the resistance of the substrate, which is quantified in a scaling parameter termed erodibility. In this technical note, we describe in detail an experimental setup and protocol to measure the relative erodibility of natural rocks under fluvial impact erosion. The experiments were designed to achieve four central aims: to provide (1) comparable experimental conditions by keeping erosivity as constant as possible; (2) cheap, easy-to-build, and easy-to-use experimental devices; (3) a straightforward and reproducible experimental protocol; and (4) avoid special needs in terms of space, equipment, connections or fixtures. We suggest that our design and approach can serve to provide a standardized protocol for measuring comparable data on erodibility under fluvial impact erosion of natural rocks and manmade materials in the laboratory.