The radiated acoustic pressure and time scales of a spherical bubble

Numerical simulations of violent bubble dynamics are often associated with numerical instabilities at the end of collapse, when a shock wave is emitted. Based on the Keller-Miksis equation, we show that this is caused by two time scales associated with the phenomenon. Nonsingular equations are thus formed based on asymptotic expansion theory and the time derivatives of the bubble radius are shown to have algebraic singularities in the Mach number. The period of oscillation is shown to divide into two asymptotic layers: a long and short time scale. The short time scale, on which significant acoustic radiation is emitted from the bubble, has been determined to be (R) over bar (max)([(p) over bar (infinity) - (p) over bar (v)]/rho c(2))(1/(3 kappa))/c, where c is the speed of sound in the liquid, (R) over bar (max) the maximum bubble radius, rho the liquid density, (p) over bar (infinity) the hydrostatic pressure of the liquid, (p) over bar (v) the vapour pressure of the liquid and kappa the polytropic index of the bubble gas. Using the scalings for this short time scale, the radiated acoustic pressure scale has been deduced to be rho c(2)(R) over bar (max)([(p) over bar (infinity)-(p) over bar (v)]/rho c(2))(1/(3 kappa))/R, where R is the radial distance from the bubble centre to the point of measurement. The results are validated by comparison with experimental results.

Automated summary

Numerical simulations of violent bubble dynamics often encounter numerical instabilities due to two time scales associated with the phenomenon, as revealed by the Keller-Miksis equation. The short time scale for significant acoustic radiation emission from the bubble has been determined and validated through comparison with experimental results.