In the present study, a two-way coupling Eulerian-Lagrangian approach is developed to assess the cavitation erosion risk in an axisymmetric nozzle. The macroscopic cavitation structures are simulated using the large eddy simulation along with the volume of fluid method. The compressible Rayleigh-Plesset equation and the bubble motion equation are introduced to resolve the microscopic bubble dynamics. The calculated results agree favorably with the experimental data and can capture more flow details, which is associated with the potential erosion risk. Based on the bubble information in multi-scale cavitating flow, a new asymmetric bubble collapse model is proposed to calculate the impact pressure, which is then used to quantitatively assess the cavitation erosion risk in the nozzle. The results show that, compared with the traditional Euler method, the location and value of the potential maximum cavitation erosion risk predicted by this new method are closer to the experimental measurement. The advantages of the newly proposed method are further elaborated systematically. The study found that the high environmental pressure triggered by the collapse of shedding clouds can cause the near-wall bubbles to shrink and even collapse, releasing impulsive pressure, which directly damages the material surface. This phenomenon is considered to be closer to the actual cavitation erosion process. Finally, analyzing the relationship between multi-scale cavitation structures and erosion risk reveals that the high risk of cavitation erosion is mainly due to the oscillation and collapse of near-wall bubbles which are generated near the attached cavity closure line or surrounding the shedding clouds.

Numerical prediction of cavitation erosion risk in an axisymmetric nozzle using a multi-scale approach

Numerical study on formation of a splash sheet induced by an oscillating bubble in extreme vicinity to a water surface

Tsunamis generated by underwater volcanic eruptions are physically modeled in a large three-dimensional wave basin. A unique pneumatic volcanic tsunami generator (VTG) was deployed at the bottom of the wave basin to generate volcanic tsunamis with repeatable source parameters under controlled physical conditions. The volcanic Froude number defined with the VTG eruption velocity and water depth allows to physically model real-world events from slow mud-volcanoes to explosive eruptions. The VTG generates radial N-waves with prescribed vertical stroke motions in the wave basin. Initial three-dimensional water surfaces are reconstructed for the daylighting scenarios. Smooth dome shapes are observed during the submarine volcanic eruption and tsunami wave generation, which is followed by a trough formation at the source. A concentric vertical spike is observed for a specific range of water depths, which is generated by superposition of an inward propagating circular bore on top of the wave generator. The spike can be clustered with different ranges of a dimensionless VTG parameter. With an increasing dimensionless parameter, the spike pattern transitions through three distinct categories: smooth spike, rough spike, and splash spike. The dimensionless spike height and the dimensionless vertical velocity of the spike tip are dependent on the dimensionless VTG parameters. The maximum values of the dimensionless spike height and spike tip velocity are observed in the rough spike regime among all tested experimental scenarios.

https://pubs.aip.org/aip/pof/article-pdf/doi/10.1063/5.0147970/18014271/066605_1_5.0147970.pdf

Physical modeling of spikes during the volcanic tsunami generation

Focused jets have been widely studied owing to the abundance of attractive flow phenomena and industrial applications, whereas annular focused jets are less studied. This study combines experiments, numerical simulations, and analytical modeling to investigate the effect of the contact angle on the generation position and focusing efficiency of annular focused jets between parallel plates. In the experiment, a pulsed laser generates a cavitation bubble inside the droplet, and the rapidly expanding cavitation bubble drives an annular-focused jet on the droplet surface. Changing the plate wettability creates different contact angles and droplet surface shapes between the droplet and plates, which modulates the position and focusing efficiency of the annular jet. Based on the jet singularity theory and by neglecting gravity, the derived formula for the jet position offset is found to depend only on the contact angle, which is in good agreement with the experimental and numerical simulation results. Combined with numerical simulations to analyze the flow characteristics of the droplets between the parallel plates, a new calculation method for the jet focusing efficiency is proposed. Interestingly, when the liquid surface radius is small, the focusing efficiency can be improved by adjusting the contact angle to make the jet position closer to the flat plate, whereas the same operation reduces the focusing efficiency when the radius is large. The study of annular jets can expand the scope of traditional jet research and has the potential to provide new approaches for applications such as high-throughput inkjet printing and liquid transfer. 

Harisena Febriadi

Papers

Effect of contact angles on dynamical characteristics of the annular focused jet between parallel plates