Bubble collapse is one of the leading causes for the cavitation erosion of submerged structures. For better understanding of the destructive mechanism of cavitation, high-fidelity simulation is performed to simulate the complete process of single bubble collapse near a planar solid wall. The wave propagation method with the approximate Riemann solver Harten Lax and van Leer Contact is adopted to solve the compressible two-phase five-equation model. We implement fifth-order weighted essentially non-oscillatory scheme with the block-structured adaptive mesh method to resolve shock waves and moving interface with high-resolution. We simulate single bubble collapsing in free-field to validate the present numerical methods and solver. Our results (e.g., averaged bubble-interior pressure and the radius variation) are found in excellent agreement with the theoretical Keller–Miksis solutions. In this study, the shock wave transmitted inside the bubble and the water-hammer shock formed in the liquid are under quantitative investigation. Numerical results reveal that the interactions between the shock wave and bubble interface give rise to peak pressures of liquid phase, and the initial stand-off distances have important influence on shock wave pattern, wall peak pressure, and bubble dynamics.

The excellent drag reduction effect of the bubble drag reduction technique has been proved through many experiments since it was proposed. In this paper, the authors investigate the bubble-turbulence interaction and the corresponding drag reduction effect with a two-way coupled Euler–Lagrange code. The liquid phase is simulated by using a large eddy simulation method with the immersed bubbles treated using a nonlinear collision model to accurately simulate the bubble–wall interaction. A Gaussian distributed method is adopted to obtain the void fraction and interphase forces in the two-way coupled algorithm. Two typical wall-bounded turbulent flow problems (turbulent channel flow and boundary layer flow) are simulated to validate the accuracy and stability in bubbly flows and investigate the drag reduction mechanism. First, the effect of bubbles on the turbulent flow is studied in the channel flow cases in which the bubbles are observed attaching to the upper plate and swaying in the spanwise direction. In this case, Reynolds stress near the wall is decreased, which contributes to the drag reduction. Moreover, drag reduction of a turbulent boundary layer flow with bubble injection is studied in which the drag reduction under different air flow rates is in good agreement with experimental results. The contribution of turbulence and different liquid forces to the migration of bubbles away from the wall is investigated. The bubble trajectory in the turbulent boundary layer is divided into three distinct stages and discussed in detail finally.

Breaking bow waves entrain massive gas that generate ambient noise and produce spray and bubbly wake with whitecap. This study aims to give a quantitative description of the flow structures and bubble formation during the breaking process. We consider the breaking bow waves induced by a surface-piercing flat plate and perform simulations based on an in-house code. We employ a conservative coupled level-set and volume of fluid method to capture violent variation of the liquid–gas interface. A robust immersed boundary method is adopted to model the motion of the plate. To resolve very small flow structures associated with the wave breaking process with the available computational resources, a block-structured adaptive mesh refinement strategy is used. It is found that the predicted wave characteristics, such as wave height, wave crest location, and wave profile, are consistent with the experiment. A wide range of flow phenomena, including the thin liquid sheet, jet overturning, and splash-ups are well reproduced by the present simulation. In addition, we implement a bubble-droplet detection program to track single bubbles, and the characteristics of bubble cloud (entrained air volume, spatial distribution, and penetration depth) can be analyzed quantitatively. Three typical bubble creation mechanisms for the air entrainment process of the breaking bow wave are reported, and ensemble-averaged statistics of the bubble size distribution are presented. We also quantify the evolution of the bubble distribution and discuss the power-law scaling during the bow wave breaking process.