Self-aeration in open channel flows occurs owing to free surface air entrainment. Self-aeration development and fully cross-sectional distribution of air concentration are not thoroughly understood. In the present study, an analytical solution for the averaged cross-sectional air concentration in the gradually varying region is established using a simplified mechanism of free surface air entrainment. For a fully cross-sectional distribution of air concentration affected by the channel bottom, a model of a diffusion region without wall restraint is proposed, and two situations are classified based on averaged cross-sectional air concentration. Good agreement between measured data and calculations is obtained, and the computational accuracy of the air concentration distribution near the wall is improved. The results reveal that the channel slope determines the air entrainment quantity, while water flow discharge determines the self-aeration evolution distance. The solutions for the averaged cross-sectional air concentration and the effect of the bottom wall on air diffusion promote air–water flow applications in hydraulic engineering practices.

As a hydrodynamic phenomenon, cavitation is a main concern in many industries such as water conservancy, the chemical industry and medical care. There are many studies on the generation, development and collapse of cavitation bubbles, but there are few studies on the variation of the cyclic impact strength on walls from the collapse of cavitation bubbles. In this paper, a high-speed dynamic acquisition and analysis system and a pressure measuring system are combined to study the impact of a cavitation bubble generated near a wall for various distances between the cavitation bubble and the wall. The results show that (1) with the discriminating criteria of the impact pressure borne by the wall, the critical conditions for the generation of a micro-jet in the collapse process of the cavitation bubbles are obtained, and therefore collapses of cavitation bubbles near the wall are mainly divided into primary impact area collapses, secondary impact area collapses and slow release area collapses; (2) it can be seen from the impact strength of the cavitation bubble collapse on the wall surface that the impact of cavitation bubbles on the wall surface during the first collapse decreases as γ (the dimensionless distance between the cavitation bubble and the wall) increases, but the impact of the second collapse on the wall surface increases first and then decreases sharply. When γ is less than 1.33, the impact on the wall surface is mainly from the first collapse. When γ is between 1.33 and 2.37, the impact on the wall surface is mainly from the second collapse. These conclusions have potential theoretical value for the utilization or prevention and control technologies for cavitation erosion.

The failure of a natural or an artificial dam often leads to catastrophic flooding downstream. Studies have been conducted to investigate the mechanisms of single dam failure and the propagation of dam-break flood wave. However, little attention has been paid to the cascade failure of two or more sequential dams. In this article, consequences of a dam-break flood wave down a channel exerting pressure load on a downstream dam with different backwater effects were studied experimentally. Three typical flood patterns were observed for different downstream reservoir water depths. High precision pressure sensors were employed to measure the pressure load at different heights on the dam. Changes of the maximum pressure load were measured as the bed slope and the upstream and downstream reservoir water depths varied. An empirical formula was obtained by linear regression to predict the maximum pressure load. Additionally, a sinusoidal attenuation function was introduced to describe the time evolution of pressure load exerted on the downstream dam, and its parameters were determined using the measurement data in different cases.

The air bubble entrainment and self–aeration phenomena in free-surface water flows reveal a rich interplay of fundamental science and engineering, and the size distribution of the entrained bubbles enhances the air–water gas flux, improves the gas transfer, and influences the cavitation erosion protection in high–speed flows. In the present study, we investigate the bubble–formation mechanism of free–surface air entrainment and the related microscopic bubble scale in the laboratory. This paper provides a quantitative description of bubble entrainment. The entrapment deformation of the local free surface over a period follows a power–law scaling and entrains a bubble when the entrapped surface becomes enclosed in the unstable movement period. Both the size scale and shape character of the entrapped free surface determine the size and skewness of the distribution of the air bubble. The entrapment deformation process confirms that the instability behaviour of the local air–water interface results in the onset of bubble entrainment. Further research is necessary to elucidate the instability criterion dominated by the interface instability and promote a new understanding of multiphase flow generation and development.

Air supply to free-surface water flows in tunnels is a key safety issue for high-head dams. The present study analyses the effects of approach water velocity and area residual of tunnel cross-section on air demand, using the volume of fluid (VOF) method in combination with the k–ε turbulence model. For a tunnel area residual less than 0.4, the attenuation of air velocity above the free-surface water flow decreases with an increase in water velocity. For larger area residuals, the air velocity distributions remain unchanged. The maximum ratio of mean air velocity to water velocity is obtained if the relative area residual is equal to 0.4. A relationship for the mean air velocity above the water flow is suggested. Without considering the flow aeration condition, the numerical simulations reproduce the air demand in the tunnel flow normally with an average error of ±25%. These results provide reference for air ventilation design of flood discharge tunnels.

Accurate control of both the timing and quantity of injection events is critical for engine performance and emissions. The most serious problem which reduces the accuracy of the control operation in such systems is a time delay of the responsiveness for the opening and closing operation of the electromagnetic valve. Modern electronic control systems should be capable of driving high speed solenoid injectors at a very fast switch frequency with high efficiency and acceptable power requirements. In this paper, the dynamic characteristic of a high speed servo-hydraulic solenoid injector for diesel engine, with different driving circuits and control methods, is investigated. A pre-energizing control strategy based on a dual power supply is applied to speed up the opening response time of the injectors. The effect of free-wheeling circuit on the closing response time is experimentally studied and a reverse-energizing circuit is utilized for achieving a rapid extinction of the operating magnetic field. The solenoid current is measured to get the electrical delay of the injectors. Meanwhile, high-speed photography is employed to detect the total delay (electrical delay, hydraulic delay and mechanical delay) of the injection spray process. The results show that, there is a high correlation between the opening delay and the solenoid current, but the armature of the injector will not move until the current reaches a threshold value. The switching time (both the opening and the closing time) could be further optimized through the proposed driving circuits and control strategies without any change to the injectors. As a result, the interval between two injections can be significantly reduced, which can provide greater flexibility for multi-injections strategies.