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Pressure changes travel at an infinite sound speed in an incompressible ideal fluid, opposite to what happens in a bubbly liquid, where the presence of bubbles adds compressibility such that the sound speed becomes finite. Here, the transmission of a pressure signal through a confined bubble array is studied numerically. An axisymmetric Boundary Integral (BI) code was used to simulate a horizontal array of spherical bubbles inside a cylindrical container. On one end of the container, a piston is able to move in either a sinusoidal or impulsive way to excite the bubbles, whereas the other end of the cylinder is fixed. A one-dimensional model, hereafter called the Multiple Bagnold Problem (MBP) model, was developed to predict the behavior of the 3D system. A good agreement between the model and the simulations was found for initial excitation times and qualitatively good agreement for later times. The MBP model was subsequently used to obtain an expression for the sound speed in monodisperse systems, which depends on the ratio between the driving and the natural bubble frequency. Two regimes were found, one where the pressure signal travels unattenuated and the other where attenuation is present. Subsequently we turn to more complex, bidisperse systems, where the influence of the presence of bubbles of two different sizes is explored. Analytical expressions to predict the eigenfrequencies were obtained for monodisperse and bidisperse alternating systems. For bidisperse stacked systems the eigenfrequencies were computed numerically giving a good agreement with the MBP simulations. Finally, we discuss the propagation of a pressure signal through a polydisperse system.

The presence of gas or vapour bubbles may strongly influence the forces that occur during the impact of a liquid mass onto a solid. Here, we study this effect numerically, in a well-controlled manner, by simulating the short-time interaction between an impacting droplet and a solid surface, mediated by the gas layer between droplet and solid just before collision, in the presence and absence of bubbles. A boundary integral method is used to simulate the falling droplet, the mediating air layer is modeled using lubrication theory, whereas uniform gas bubbles are added to the droplet that obey a polytropic equation of state. We show that the presence of gas bubbles inside the droplet can have a significant influence on the force exerted on the substrate, even before touchdown. This is due to the transmission of load from the solid, through the gas layer and finally into the bubbly droplet, buffering the impact. We simulate different bubble configurations, modifying their number, size, shape and initial position. It is found that larger bubbles, as well as those close to the impact zone, dampen the collision more as compared to small bubbles or the ones that are far from the droplet's surface. In addition, multiple small bubbles are shown to have a similar or even greater effect as a single large bubble.

When objects are forced to flow through constrictions their transport can be frustrated temporarily or permanently due to the formation of arches in the region of the bottleneck. While such systems have been intensively studied in the case of solid particles in a gas phase being forced by gravitational forces, the case of solid particles suspended in a liquid phase, forced by the liquid itself, has received much less attention. In this case, the influence of the liquid flow on the transport efficiency is not well understood yet, leading to several apparently trivial, but yet unanswered questions, e.g., would an increase of the liquid flow improve the transport of particles or worsen it? Although some experimental data is already available, it lacks enough detail to give a complete answer to such a question. Numerical models would be needed to scrutinize the system deeper. In this paper, we study this system making use of an advanced discrete particle solver (MercuryDPM) and an approximated numerical model for the liquid drag and compare the results with experimental data., Comment: arXiv admin note: text overlap with arXiv:2205.04354