Numerical modelling of an acoustically-driven bubble collapse near a solid boundary

The acoustically-driven collapse of a bubble results from the oscillating position of an ultrasound transducer face in a liquid medium. Existing fully compressible models of bubble collapse have been applied to represent the Rayleigh collapse and the shock-induced collapse. These applications may not adequately represent the conditions associated with the acoustically-driven collapse. The current work presents a fully compressible model that is the first to capture the collapse of a bubble set in a liquid medium subjected to an ultrasound transducer. The oscillating transducer face is represented by an immersed moving reflective boundary. The flow is simulated using a conservative interface capturing method, which includes the use of a high-order WENO reconstruction, a maximum-principle-satisfying and positivity-preserving limiter, and the HLLC approximate Riemann flux. The numerical method is verified by quantitative comparison to the benchmark shock-bubble problem. A simulation is conducted of an acoustically-driven collapse of an initially spherical bubble near a solid boundary (wall). The radius of the bubble at the beginning of the collapse, R-max = 40.56 mm, is given by the Rayleigh-Plesset growth from a R-0 = 10 mu m bubble. The distance from the centre of the bubble to the wall (standoff distance) is S = 1.1R(max). The acoustic field resulted from a transducer face oscillating with a frequency of 30 kHz and a displacement amplitude of 0.4174 mu m, with a subsequent acoustic pressure amplitude of p(A) approximate to 120 kPa. The collapse shape is found to be qualitatively consistent with previous work and a jet is observed after 4.7 mu s. The resulting maximum pressure at the wall is about 6.7 MPa. We compare the wall pressures of the acoustically-driven collapse to the Rayleigh collapse. Compared to the acoustically-driven collapse model, the Rayleigh collapse can easily overestimate or underestimate the resulting maximum wall pressure depending on the choice of the initial pressure distribution.