The dynamics of bubbles inertially collapsing in water near solid objects have been the subject of numerous studies in the context of cavitation erosion. While non-spherical bubble collapse, re-entrant jet dynamics and emitted shock waves have received significant interest, less is known about the temperatures thereby produced and their possible connection to damage. In this article, we use highly resolved numerical simulations of a single bubble inertially collapsing near a rigid surface to measure the temperatures produced in the fluid and estimate those in the solid, as well as to identify the responsible mechanisms. In particular, we find that elevated temperatures along the wall can be produced by one of two mechanisms, depending on the initial stand-off distance of the bubble from the wall and the driving pressure: for bubbles initially far from the wall, the shock generated by the bubble collapse is the source of the high temperature, while bubbles starting initially closer migrate towards the wall and eventually come into contact with it. A scaling is introduced to describe the maximum fluid temperature along the wall as a function of the initial stand-off distance and driving pressure. To predict the temperature of the solid, we develop a semianalytical heat transfer model, which supports recent experimental observations that elevated temperatures achieved during collapse could play a role in cavitation damage to soft heat-sensitive materials.

Direct Numerical Simulations of multiphase flows have progressed rapidly over the last decade and it is now possible to simulate, for example, the motion of hundreds of deformable bubbles in turbulent flows. The availability of results from such simulations should help advance the development of new and improved closure relations and models of the average or large-scale flows. We review recent results for bubbly flow in vertical channels, discuss the difference between upflow and downflow and the effect of the bubble deformability and how the resulting insight allowed us to produce a simple description of the large scale flow, for certain flow conditions. We then discuss the need for the development of numerical methods for more complex situations, such as where the flow creates spontaneous thin films and threads, or where additional physical processes take place at a rate that is very different from the fluid flow. Recent work on capturing localized small-scale processes using embedded analytical models, focusing on the mass transfer from bubbles in liquids with low mass diffusivity, suggests one approach. We conclude by discussing immediate needs for progress on the theoretical framework for describing the large-scale motion of multiphase flows and the need for multiscale methods to capture physical processes taking place at diverse length and time scales.

In the computation of multiphase flow with mass transfer, the large disparity between the length and time scale of the mass transfer and the fluid flow demand excessive grid resolution for fully resolved simulation of such flow. We have developed a subscale description for the mass transfer in bubbly flow to alleviate the grid requirement needed at the interface where the mass gets transferred from one side to the other.In this fluid dynamics video, a simulation of the mass transfer from buoyant bubbles is done using a Front Tracking method for the tracking of interface and a subscale description for the transfer of mass from the bubble into the domain. After the mass is transferred from the bubble into the domain, mass is followed by solving an advection-diffusion equation on a relatively coarse Cartesian grid. More detail about the method can be found in our paper [1]. This simulation shows 13 moving bubbles in a periodic domain,

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