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.

An underwater spark is able to induce a cavitation bubble, and this principle has been utilized to make cavitation bubble generators for several decades. In this paper, an improved instrument for generating spark-induced cavitation bubbles is described in detail. The voltage time history inside the instrument is measured to show the working process and principle. Cavitation bubbles are generated by the instrument and recorded by a high-speed camera. The radius time history of the bubble is obtained using an image processing algorithm. The ratio of its minimum radius to its maximum radius reaches ~0.2, which indicates that there is little undissolved gas in the bubble. With the radius time history, the velocity fields around the bubbles were calculated by the 1D continuity flow equation, and the pressure fields were calculated by the 1D Euler equation. One cavitation bubble is chosen and discussed in detail. The velocity and pressure on the bubble interface achieve their maximums (~25 m/s and ~1.2 MPa, respectively) at the same time, when the radius is at its minimum (~1 mm). Some statistical results are also presented to show the effect of the instrument.

Cavitation erosion is a fundamental concern in many fields of engineering, e.g. the flow separation zone in high speed flow and the blade area of hydraulic machinery. However, the influence of the physical characteristics of the environmental medium in these engineering facilities on cavitation erosion is still unclear. In this paper, we adopted underwater low-voltage discharge technology to generate cavitation bubbles, and studied the mesoscale causes of material erosion in solutions with different surface tensions. It is found that with the decrease of liquid surface tension, the radial velocity of cavitation bubbles decreases significantly during the shrinkage of the cavitation bubble and the minimum radius to which the cavitation bubble shrinks increases, the micro-jet generated by the cavitation bubble develops more slowly, and the impact strength of cavitation bubble collapses on the wall decreases. The findings reveal new physical mechanisms of the cavitation-induced material erosion in liquids with different surface tensions at the macroscopic level. These will provide significant guidance for cavitation prevention in various applications such as aeration cavitation-alleviation, hydraulic machinery, ultrasonic cleaning and the medical industry.

In recent years, the hydropower development of the lower Mekong River has accelerated, accompanied by habitat loss and fragmentation. We conducted two experiments using video recordings and traps to evaluate the effectiveness of a nature-like rock ramp bypass in the Sesan River in 2019 and 2021. The results show that the fishway provides both upstream and downstream passage for at least 24 non-salmonid species of fish. The vast majority of fish choose to ascend from July to September, especially in August, and hardly between October and November. The fish inside the fishway prefer to move during the daytime (6:00–18:00), especially during August and September. An excessive water depth at the entrance can lower the number of ascending fish, whereas a higher water depth at the exit can cause the opposite result. Nevertheless, the size of fish monitored exhibits a decreasing trend, suggesting the nature-like bypass cannot completely mitigate the impact caused by this impassable Sesan Ⅱ dam. Therefore, a quantitative assessment of the bypass is highly encouraged, whereas the selection of the tracked fish species and experimental period requires considerable deliberation. This study alleviates the dilemma of insufficient fishway evaluation in tropical countries, which can provide researchers with data support on future non-salmonid fishway designs.