Expansion and Collapse of Bubbles in the Central Region of a Streamer

Strong single expansion and collapse of the central bubble in a streamer of an odd number of cavitation bubbles in acetone is numerically studied. The conditions considered are close to those in the well-known experiments on neutron emission during acoustic cavitation of deuterated acetone. Initially, all the bubbles are spherical, with the same radius of 5 m, equidistant from the neighboring ones, with the centers in a straight line. The vapor in the bubbles is in the saturation state at the ambient liquid temperature of 273 K. The liquid pressure varies harmonically with a frequency of 19.3 kHz and an amplitude of 15 bar. The initial spacing of the bubbles is characterized by the ratio of the initial distance between the centers of the neighboring bubbles to the maximum radius of a single bubble during its expansion under similar conditions. It is shown that in the final stage of collapse, a radially converging shock wave arises in the central bubble in the vicinity of its surface. During the convergence, its non-sphericity and that of the pressure and temperature fields behind it rapidly increase. For , the shape of the shock wave until the moment at which some part of the shock wave front enters a small central region with a radius of 2.5 m, keeps close to spherical. As decreases, the shock wave becomes more flattened along the axis of symmetry. With increasing , the maximum pressures and temperatures in the central bubble at the mentioned moment monotonically grow to the values corresponding to a single bubble. Under the growth is fairly slow.