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Numerical studies on detonation wave propagation in rotating detonation engine and its propulsive performance with one- and multi-step chemistries of a hydrogen-based mixture are presented. The computational codes were developed based on the three-dimensional Euler equations coupled with source terms that incorporate high-temperature chemical reactions. The governing equations were discretized using Roe scheme-based finite volume method for spatial terms and second-order Runge-Kutta method for temporal terms. One-dimensional detonation simulations with one- and multi-step chemistries of a hydrogen-air mixture were performed to verify the computational codes and chemical mechanisms. In two-dimensional simulations, detonation waves rotating in a rectangular chamber were investigated to understand its flowfield characteristics, where the detailed flowfield structure observed in the experiments was successfully captured. Three-dimensional simulations of two-waved rotating detonation engine with an annular chamber were performed to evaluate its propulsive performance in the form of thrust and specific impulse. It was shown that rotating detonation engine produced constant thrust after the flowfield in the chamber was stabilized, which is a major difference from pulse detonation engine that generates repetitive and intermittent thrust.

Stream finishing is accepted as one of the post processing operations. It is not only capable of grinding but also offers polishing of additive manufactured components, having advantages of larger material removal rates and controllable toolpath. We have developed a stream finishing model through semi quantitative prediction via computational fluid dynamics (CFD) simulations. The scheme couples the granular flow field with the material removal scheme by solving the granular flow using a continuum-based method. For the rheology, the media viscosity is determined to resolve the flow field so the pressure induced by the media and the material removal rate can be predicted. The model calibration involves developing a tribometer and using it to measure the media pressure for several scenarios based on the rotational speed of the drum (30 rpm), radial distances of the tribometer (100, 250, 400 mm), submerged depths (100, 200, 250 mm) and its glancing angles (0, 15°, 30°, 45°, 60°, 75°, 90°). The work is extended to study the media flow for a simplified square work piece. The results indicate that the particle velocities on the surface of the work piece predicted by simulations are comparable to those of experiments. They show similar patterns and magnitudes for the parameters tested, which demonstrate the capability of the model to correctly predict the granular flow field.

This paper presents a study to describe the behaviour of a non-equilibrium bubble in a fluid (Fluid 1) that is in contact with another fluid (Fluid 2). Fluid 2 is assumed to incorporate some elastic properties, which are modelled through a pressure term at the fluid–fluid interface. The Laplace equation is assumed to be valid in both fluids and the boundary integral method is employed to simulate the dynamics of the bubble and the fluid–fluid interface. Interesting characteristic phenomena concerning bubble oscillations and the deformation of the fluid–fluid interface are studied for a range of parameters (distance from the fluid–fluid interface, density ratios of the two fluids and elastic properties of Fluid 2). Some of the phenomena observed are jet formation in the bubble, bubble splitting, a ring bubble separating from the main bubble, mushroom-shaped bubbles and the dynamic elevation of the elastic interface. Most of these phenomena are only observed when Fluid 2 possesses some elastic properties (besides the usual formation of a high speed liquid jet). Comparisons with experimental observations confirm the validity of our simulations.

We propose a methodology based on the boundary element method (BEM) to simulate pressure pulse–bubble interaction. The pulse resembles a shock wave and is in the form of a step pulse function incorporated into the Bernoulli equation. Compressibility effects of the water surrounding the bubble are neglected, and the dynamic response of the bubble to the impinging pulse is assumed to be mainly inertia controlled. The interaction induces the formation of a high-speed jet that penetrates the bubble. Results show that bubble shape, collapse time and jet velocity are in good agreement with other numerical models and experiments, and the method is more computationally efficient.

Ultrasonic cavitation bubble phenomena play a key role in numerous medical procedures such as ultrasound-assisted lipoplasty, phacoemulsification, lithotripsy, brain tumor surgery, muscle and bone therapies and intraocular or transdermal drug delivery. This study investigates numerically the interaction of a bubble with a bio-material (fat, skin, cornea, brain, muscle, cartilage or bone) involved in the treatments mentioned when subjected to an ultrasound field. A range of frequencies is used to study the bubble behavior in terms of its growth and collapse shapes, and the maximum jet velocity attained. Simulation results show complex dynamic behaviors of the bubble. In several cases a jet is formed directed away from the bio-material while in others, toward it. In certain cases, the bubble eventually breaks into two, with or without the formation of opposite penetrating jets. Very high maximum velocities of jets directing away or toward the bio-materials can be observed in some cases (700 to 900 ms−1). (E-mail: mpekbc@nus.edu.sg)

Experimental observations and numerical simulations (based on the boundary element method) concerning an oscillating bubble near a flexible (thin) membrane are presented in this paper. The bubbles are created using an underwater electrical spark discharge. It is shown that the presence of a membrane can have a profound influence on the behavior of a bubble.

The effect of nozzle shape, angle and length on the propulsive performance of continuously rotating detonation engine is numerically investigated in a three-dimensional annular chamber with the one-step chemical kinetics of a hydrogen and air mixture. The thrust and specific impulse of the rotating detonation engine with four different nozzles are evaluated to find out the best performance nozzle shape. This chosen nozzle shape is used as a baseline nozzle. In addition to the description of a detailed flowfield structure in the chamber with the baseline nozzle, the changes in flowfield properties at the chamber inlet and nozzle exit due to nozzle shapes are also investigated. With the baseline nozzle, the evaluation on the effect of nozzle angle and length is performed. The best engine performance and less total pressure loss are achieved in the divergent nozzle with the nozzle angle of 10 ◦ and the nozzle length of 0.04 m. However, the rotating detonation engine has the negligible nozzle effect on the propulsive performance, compared with that for pulse detonation engine.

Detonation waves successively rotating in an annular chamber are numerically investigated to understand an overall ∞owfleld structure in the combustion chamber of an engine and its basic operation with a continuous fuel injection. This study leads to further investigation into continuously rotating detonation wave, which is the backbone in developing a rotating detonation-based propulsion system. A computational code developed is based on multi-dimensional Euler equations with source terms due to chemical reactions. Spatial terms in governing equations are discretized with a flnite volume method and a MUSCLbased Roe scheme, while temporal terms are discretized with a second-order, three-step Runge-Kutta method. Source terms are treated with a time-operator splitting method in order to isolate stifiness. The detonation is modeled with the one-step chemical reaction of a hydrogen and air mixture. A detailed ∞owfleld structure including detonation properties is presented in two- and three-dimensional annular chamber. The propulsive parameters of a rotational detonation engine are evaluated and its comparison of one- and two-waved detonation engine is performed in a three-dimensional chamber.

The dynamic interaction of a shockwave (modelled as a pressure pulse) with an initially spherically oscillating bubble is investigated. Upon the shockwave impact, the bubble deforms non-spherically and the flow field surrounding the bubble is determined with potential flow theory using the boundary-element method (BEM). The primary advantage of this method is its computational efficiency. The simulation process is repeated until the two opposite sides of the bubble surface collide with each other (i.e. the formation of a jet along the shockwave propagation direction). The collapse time of the bubble, its shape and the velocity of the jet are calculated. Moreover, the impact pressure is estimated based on water-hammer pressure theory. The Kelvin impulse, kinetic energy and bubble displacement (all at the moment of jet impact) are also determined. Overall, the simulated results compare favourably with experimental observations of lithotripter shockwave interaction with single bubbles (using laser-induced bubbles at various oscillation stages). The simulations confirm the experimental observation that the most intense collapse, with the highest jet velocity and impact pressure, occurs for bubbles with intermediate size during the contraction phase when the collapse time of the bubble is approximately equal to the compressive pulse duration of the shock wave. Under this condition, the maximum amount of energy of the incident shockwave is transferred to the collapsing bubble. Further, the effect of the bubble contents (ideal gas with different initial pressures) and the initial conditions of the bubble (initially oscillating vs. non-oscillating) on the dynamics of the shockwave-bubble interaction are discussed.

High intensity soundwaves, such as shockwaves and HIFU, are widely used in biomedical applications, for example, extracorporeal shockwaves lithotripsy (ESWL) and HIFU prostate cancer treatment. The high pressure in the tissue and nearby fluid often causes formation of cavitation bubbles. Our previous simulation results via BEM approach show complex bubble ultrasound interactions: in certain cases a jet is formed directed away from the nearby tissue while in others, towards it [Fong et al., Ultrasound Biol. Med., 32(6), 925–942, 2006]. In the present work, pulsed ultrasound microbubble interaction is studied using the same code to provide further understanding to the jetting behavior of the bubbles near (different) biomaterial surface which is critical to the success of tissue ablation and the minimization of auxiliary damages. Separately, we have also simulated shockwave bubble interaction using the BEM code with excellent concurrence to other compressible numerical schemes such as arbitrary Lagrangian‐Eulerian and free Lagrange methods [Klaseboer et al., Comput. Methods Appl. Mech. Eng. 195, 4287–4302 (2006)] and experiments. It is suggested that the (primary) dynamic response of the bubble to soundwaves is mainly inertia controlled since compressibility of the fluid is not modeled.

A study of the interaction between a membrane and a submerged oscillating bubble is presented. Though the behavior of such a bubble near an elastic (relatively thick) boundary has been studied by several authors, much less attention is focused on the behavior of such a bubble near a (thin) elastic membrane. For membranes, it is the curvature and not the deflection that is responsible for a pressure buildup in the fluid close to the bubble. Due to this difference in physics, it is not a certainty if the dynamics of bubbles near a deformable elastic boundary vis-a-vis a membrane would exhibit any similarity. Our intent is a systematic study on the latter, which can be exploited in future work (e.g., in biomedical applications where elastic membranes are often involved). Experimental observations of transient bubble interaction with a thin elastic membrane are presented and the dynamics of the bubble in the vicinity of the membrane are compared to the boundary element method simulations. The bubble is genera...