Initial behavior and the subsequent motion of a bubble in liquid nitrogen are investigated experimentally using high-speed photography. A bubble is generated by focusing a pulsed ruby laser into liquid nitrogen at 78.0 K, changing the ambient pressures up to 253.2 kPa which corresponds to the applied pressure (or overpressure), Δp, being 147.1 kPa. When the energy level of the laser beam at the focus exceeds an irradiance threshold, for instance 5.4×1011 W/cm2 for Δp=4.9 kPa, the optical breakdown occurs in the liquid nitrogen, followed by a series of high-speed phenomena such as plasma formation, shock wave emission, and vapor bubble generation. It is found that during a very short period after the plasma formation a bubble grows nonspherically reflecting from the plasma shape, but the bubble volume itself varies with time in the same way for all cases performed in the present experiment. The liquid inertia is a dominant factor affecting the bubble growth, while the thermal effect becomes remarkable during the bubble collapse, resulting in the retardation of the bubble motion. The characteristic behavior of a laser-induced cavitation bubble in liquid nitrogen is significantly influenced by the phase change of vapor at the bubble surface as well as by the vapor pressure inside the bubble. Immediately after the bubble rebound, instabilities are amplified over the bubble surface similar to those caused in the water case.

This paper presents some modifications of the gamma-based model developed by K.M. Shyue (J. Comput. Phys., 1998, 142, 208-242). In deriving process of the original model, pressure equilibrium and an isentropic condition are not involved explicitly. Therefore, to adopt these physical conditions, we improved evolution equations for problem-dependent material quantities (e.g. ratio of specific heat). As a result, a non-conservative term associated with the pressure relaxation was added to the equations. The modifications have no effect on an oscillation-free property of the original model. Furthermore, because division by a volume fraction is not included in the present model, an inherent property of mixture-type models is maintained. To capture contact discontinuity sharply, we utilized an approximate Riemann solver HLLC with a third-order MUSCL interpolation. Finally, we carried out one-dimensional numerical tests in order to evaluate the influence on accuracy. Numerical results show that the modified model has good capability for simulating low Mach number flow problems.

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