Behaviors of cavitation bubbles driven by high-intensity ultrasound

In a multi-bubble system, the bubble behavior is modulated by the primary acoustic field and the secondary acoustic field. To explore the translational motion of bubbles in cavitation liquids containing high-concentration cavitation nuclei,evolutions of bubbles are recorded by a high-speed camera, and translational trajectories of several representative bubbles are traced. It is found that translational motion of bubbles is always accompanied by the fragmentation and coalescence of bubbles, and for bubbles smaller than 10 μm, the possibility of bubble coalescence is enhanced when the spacing of bubbles is less than 30 μm. The measured signals and their spectra show the presence of strong negative pressure, broadband noise,and various harmonics, which implies that multiple interactions of bubbles appear in the region of high-intensity cavitation.Due to the strong coupling effect, the interaction between bubbles is random. A simplified triple-bubble model is developed to explore the interaction patterns of bubbles affected by the surrounding bubbles. Patterns of bubble interaction, such as attraction, repulsion, stable spacing, and rebound of bubbles, can be predicted by the theoretical analysis, and the obtained results are in good agreement with experimental observations. Mass exchange between the liquid and bubbles as well as absorption in the cavitation nuclei also plays an important role in multi-bubble cavitation, which may account for the weakening of the radial oscillations of bubbles.

THE MOTION OF A SMALL BUBBLE IN WAVES

The motion of a single spherical small bubble due to buoyancy in the ideal fluid with waves is investigated theoretically and experimentally in this article. Assuming that the bubble has no effect on the wave field, equations of a bubble motion are obtained and solved. It is found that the nonlinear effect increases with the increase of the bubble radius and the rising time. The rising time and the motion orbit are given by calculations and experiments. When the radius of a bubble is smaller than 0.5mm and the distance from the free surface is greater than the wave height, the results of the present theory are in close agreement with measurements.

An improved view-factor method including plasma filling for angular distribution of radiation temperature from a laser-driven hohlraum

Angular distribution of radiation temperature from a laser-driven hohlraum is vital for investigations on the radiation field inside the hohlraum, code validation, and predication of drive on the capsule in indirect-drive inertial confinement fusion. A modified version of the view-factor method including plasma filling is proposed, which improves the accuracy of the description of angular distribution of radiation temperature. Firstly, the radial velocity of the gold bubble motion is scaled from a simple data-based model in a gas-filled hohlraum experiment performed on a hundreds of kJ laser facility in China. Then, an equivalent radiative volume model is advanced to approximately characterize the contribution of the blow-off bubble in the new view-factor method incorporate into IRAD3D. The simulation shows reasonable agreement with experimental measurements in a gas-filled hollow hohlraum. Furthermore, the influence of the electron density and temperature distribution, and bubble velocity, is analyzed. The value of the method is that it can be used as an approximate 'first-look' at hohlraum energy balance prior to a more detailed radiation hydrodynamic modeling.

THE MOTION OF A BUBBLE IN WAVES

The motion of a single spherical bubble due to buoyancy in the ideal fluid with waves is studied in this article. Assuming that the bubble has no effect on the wave field, equations of a bubble motion are attained. The equations are solved. It is found that the nonlinear effect increases with the increase of the bubble radius. Since the difference between the frequencies of gravity waves and the eigenfrequency of bubbles is great for small bubbles, their oscillations can not be coupled. During the rising of a bubble, the oscillation of the bubble decays very fast, so it can be ignored. The rising time and the motion orbit are given. When the wave is high and the bubble is small, the spiral orbit may appear.

Imaging of Structure Evolution in Solidifying Al–Bi Immiscible Alloys by Synchrotron Radiography

Using synchrotron X-ray imaging technique,the segregation evolution in solidifying Al-10 wt% Bi immiscible alloys was investigated at different cooling rates.Irrespective of the cooling rate,most of the Bi solute appeared at the upper part of the sample after solidification.The reason for this Bi enrichment phenomenon is different for different cooling rates.Besides Marangoni motion,positive segregation,which has rarely been noticed before,can also make Bi solute transfer to the hot top zone.It is also found that,bubbles（or pores） appear in solidifying Al-10 wt% Bi alloys,and the number of bubbles（or pores） increases with the increase of the cooling rate,while the size of the bubbles（or pores） decreases.