The present paper focuses on the numerical simulation of the interaction of laser-generated bubbles with a free surface, including comparison of the results with instances from high-speed videos of the experiment. The Volume Of Fluid method was employed for tracking liquid and gas phases while compressibility effects were introduced with appropriate equations of state for each phase. Initial conditions of the bubble pressure were estimated through the traditional Rayleigh Plesset equation. The simulated bubble expands in a non-spherically symmetric way due to the interference of the free surface, obtaining an oval shape at the maximum size. During collapse, a jet with mushroom cap is formed at the axis of symmetry with the same direction as the gravity vector, which splits the initial bubble to an agglomeration of toroidal structures. Overall, the simulation results are in agreement with the experimental images, both quantitatively and qualitatively, while pressure waves are predicted both during the expansion and the collapse of the bubble. Minor discrepancies in the jet velocity and collapse rate are found and are attributed to the thermodynamic closure of the gas inside the bubble.

The collapse of a cavitation bubble near a rigid boundary induces a high-speed transient jet accelerating liquid onto the boundary. The shear flow produced by this event has many applications, examples of which are surface cleaning, cell membrane poration and enhanced cooling. Yet the magnitude and spatio-temporal distribution of the wall shear stress are not well understood, neither experimentally nor by simulations. Here we solve the flow in the boundary layer using an axisymmetric compressible volume-of-fluid solver from the OpenFOAM framework and discuss the resulting wall shear stress generated for a non-dimensional distance, $\unicode[STIX]{x1D6FE}=1.0$ ( $\unicode[STIX]{x1D6FE}=h/R_{max}$ , where $h$ is the distance of the initial bubble centre to the boundary, and $R_{max}$ is the maximum spherical equivalent radius of the bubble). The calculation of the wall shear stress is found to be reliable once the flow region with constant shear rate in the boundary layer is determined. Very high wall shear stresses of 100 kPa are found during the early spreading of the jet, followed by complex flows composed of annular stagnation rings and secondary vortices. Although the simulated bubble dynamics agrees very well with experiments, we obtain only qualitative agreement with experiments due to inherent experimental challenges.

The present paper focuses on the simulation of the expansion and aspherical collapse of a laser-generated bubble subjected to an acceleration field and comparison of the results with instances from high-speed videos. The interaction of the liquid and gas is handled with the volume of fluid method. Compressibility effects have been included for each phase to predict the propagation of pressure waves. Initial conditions were estimated through the Rayleigh Plesset equation, based on the maximum bubble size and collapse time. The simulation predictions indicate that during the expansion the bubble shape is very close to spherical. On the other hand, during the collapse the bubble point closest to the bottom of the container develops a slightly higher collapse velocity than the rest of the bubble surface. Over time, this causes momentum focusing and leads to a positive feedback mechanism that amplifies the collapse locally. At the latest collapse stages, a jet is formed at the axis of symmetry, with opposite direction to the acceleration vector, reaching velocities of even 300 m/s. The simulation results agree with the observed bubble evolution and pattern from the experiments, obtained using high speed imaging, showing the collapse mechanism in great detail and clarity.

The aim of this article is to assess the impact of turbulence and cavitation models on the prediction of diesel injector nozzle flow. Two nozzles are examined, an enlarged one, operating at incipient cavitation, and an industrial injector tip, operating at developed cavitation. The turbulence model employed includes the re-normalization group k–ε, realizable k–ε and k–ω shear stress transport Reynolds-averaged Navier–Stokes models; linear pressure–strain Reynolds stress model and the wall adapting local eddy viscosity large eddy simulation model. The results indicate that all Reynolds-averaged Navier–Stokes and the Reynolds stress turbulence models have failed to predict cavitation inception due to their limitation to resolve adequately the low pressure existing inside vortex cores, which is responsible for cavitation development in this particular flow configuration. Moreover, Reynolds-averaged Navier–Stokes models failed to predict unsteady cavitation phenomena in the industrial injector. However, the wall adapting local eddy viscosity large eddy simulation model was able to predict incipient and developed cavitation, while also capturing the shear layer instability, vortex shedding and cavitating vortex formation. Furthermore, the performance of two cavitation methodologies is discussed within the large eddy simulation framework. In particular, a barotropic model and a mixture model based on the asymptotic Rayleigh–Plesset equation of bubble dynamics have been tested. The results indicate that although the solved equations and phase change formulation are different in these models, the predicted cavitation and flow field were very similar at incipient cavitation conditions. At developed cavitation conditions, standard cavitation models may predict unrealistically high liquid tension, so modifications may be essential. It is also concluded that accurate turbulence representation is crucial for cavitation in nozzle flows.