To reveal the potential mechanism of rainfall on the diffusion of microplastic pollution, a drop impact experiment was conducted on the microplastic particle layer. High-speed photography was used to capture the flow regime after drop impact on the microplastic particle layer, and a flow zone map was established to determine the threshold between each flow regime. The influence of characteristic parameters on the flow regime after drop impact was also analyzed. The results indicate that the flow regime of drops after impacting the layer of microplastic particles includes bouncing, spreading, coronal splashing, and prompt splashing. Increasing the Reynolds number Re₀, Weber number We₀, relative size D^* of microplastic particles, or relative thickness h^* of particle layers can promote the transition of the flow regime from bouncing to spreading and then to coronal splashing or prompt splashing. A dimensionless number M=D^*h^*We₀Re₀^-2/5 is proposed based on Buckingham π theorem for identifying the threshold between different flow regimes. The mechanism of drop impact on the splashing of microplastic particles includes forward collision and viscous carrying. The former exists in all flow states, while the latter exists in bouncing, coronal, and prompt splashing flow states. Compared with viscous carrying, forward collision driving is more likely to cause large-scale propagation of microplastic particles.

To prevent the semi-submerged jet in ski-jump stepped spillways, which leads to abnormal pressure distributions, severe water wing phenomena and structural vibrations in the upstream step section, a combination of physical model tests and theoretical analysis was used, and three types of test models were designed to study the influence of installing ventilation shafts and extending the inlet horizontal section on the flow pattern and hydraulic characteristics of the ski-jump section. The results show that installing ventilation shafts and extending the inlet horizontal section can significantly improve the ski-jump flow pattern, effectively mitigate the water wing phenomenon, and successfully eliminate adverse hydraulic conditions such as negative pressure. The maximum operating discharge for the ski-jump stepped spillway and the minimum length of the aeration basin were determined, providing a reference for the structural design and operation of ski-jump stepped spillways.

To investigate the evolution characteristics of the air pocket during the geyser process, a numerical simulation of the release of an entrapped air pocket in a drainage pipe system under double-sided inflow conditions was conducted. The geyser formation mechanisms, air discharge characteristics, and air-water energy evolutions were analyzed. The results indicate that the entry and subsequent release of an entrapped air pocket into the vertical shaft trigger an air pocket-driven geyser, followed by the occurrence of a rapid-filling geyser. Depending on whether these two processes operate independently or interactively, the geyser process can be classified as either a separated type or a hybrid type. A higher dimensionless pipeline pressure difference P^* or a smaller dimensionless initial air pocket volume V_a^* is more likely to trigger a hybrid geyser process. The air pocket discharge ratio during the air pocket-driven geyser is approximately 2%-47%; it first increases and then decreases with increasing P^*, and it decreases with increasing V_a^*. The air pocket discharge ratio during a rapid-filling geyser is approximately 0-5%, and it is minimally affected by P^* and V_a^*. During the geyser process, the peak kinetic energy of the air occurs in the air pocket-driven geyser stage, while the peak kinetic energy of the water in the vertical shaft occurs in the rapid-filling geyser stage. The geyser intensity decreases with increasing P^* and increases with increasing V_a^*. An air pocket-driven geyser occurs when the dimensionless maximum kinetic energy of the air per unit mass exceeds 1.5; a rapid-filling geyser occurs when the dimensionless maximum kinetic energy of the water per unit mass exceeds 0.5.