Hisao Azuma

Three-dimensional large-amplitude drop oscillations: experiments and theoretical analysis

Three-dimensional large-amplitude oscillations of a mercury drop were obtained by electrical excitation in low gravity using a drop tower. Multi-lobed (from three to six lobes) and polyhedral (including tetrahedral, hexahedral, octahedral and dodecahedral) oscillations were obtained as well as axisymmetric oscillation patterns. The relationship between the oscillation patterns and their frequencies was obtained, and it was found that polyhedral oscillations are due to the nonlinear interaction of waves. A mathematical model of three-dimensional forced oscillations of a liquid drop is proposed and compared with experimental results. The equations of drop motion are derived by applying the variation principle to the Lagrangian of the drop motion, assuming moderate deformation. The model takes the form of a nonlinear Mathieu equation, which expresses the relationships between deformation amplitude and the driving forces magnitude and frequency

Effect of gravitational acceleration on temperature wave propagation in a critical fluid

Temperature propagation near the critical point of a classical fluid is investigated theoretically. The governing equations of thermal energy transfer near the critical point are introduced and a linear analysis is carried out. The dispersion relation between the angular frequency and the wave number is obtained and the wave characteristics are discussed. The effect of gravitational acceleration on the temperature wave propagation is made clear. Through this analysis, the following results were obtained; (1) The propagation speed of temperature waves is γ/(ρ0κT),where γ, ρ0, and κT are, respectively, the ratio of specific heats, the density, and the isothermal compressibility, with or without gravity if the wavelength is larger than 10−3.(2) The amplitude of wave increases with time in the antigravitational direction and decreases in the gravitational direction but the decay time is long if the wave number is small. (3) Waves decay quickly if the wave number is larger than 104.

Computer simulation of oscillatory Marangoni flow

Abstract Numerical simulations of thermocapillary convection of low-Prandtl number melts in a rectangular cavity subject to a horizontal temperature gradient under zero gravity are performed. Marangoni numbers range from 100 to 1000 and mesh size range from 25 × 100 to 50 × 200. The simulation results show that the flow pattern is strongly influenced by the mesh size of the simulation, if the Marangoni number exceeds 300. When the finest mesh is used, the oscillatory Mangoni convection appears under low-Marangoni number, such as 400. If the Marangoni number is higher than 700, three flow patterns appear with changing mesh size. The first is high frequency oscillation, the second is multi-frequency oscillation and the last is slow frequency oscillation.

On the heat transfer mechanisms in microgravity nucleate boiling

Abstract Since the existing data shows inconsistent trends of heat transfer characteristics, the methods of data acquisition and of analysis for the clarification of heat transfer mechanisms were developed to obtain the knowledge universally applied to microgravity nucleate boiling. Based on the information about the bubble structure, the possibility of steady-state heat transfer, the heat transfer enhancement and deterioration, and the behavior of microlayer underneath bubbles, a predictive method of heat transfer was introduced by using measured microlayer thickness.

Study on secondary bubble creation induced by bubble coalescence

Abstract The mechanism of secondary bubble creation induced by bubble coalescence, observed during drop tower experiments, is studied. To carry out detailed investigations of the creation phenomena, 2-dimensional numerical simulations are performed. The simulation results agree fairly well with the experimental ones and indicate that the size ratio A and the non-dimensional surface tension St play the most important role in the phenomena. Considering energy distributions of the phenomena, it is found that the criteria of the creation can be well described by two simple equations.

Results of Space Experiment on Bubble Interaction in a thermal gradient

An experiment observing bubble behavior in a thermal gradient was conducted on board the Shuttle (SL-J). Bubble behavior showed very strong interaction between the bubbles. Bubbles were attracted to a bubble attached to a hot wall due to the thermocapillary flow associated with it, making a line along the thermal gradient. Groups of bubbles in a thermal gradient moved more slowly than that expected for a single bubble, but showed a higher speed near the hot wall due to the attraction mentioned above.

Free surface fluid behavior in a two-dimensional container with arbitrary configuration (computer simulation and experiment using drop tower)☆

Abstract A new computer program for problems of two-dimensional time-dependent fluid flow in containers with free surface is presented. The program has been developed to provide a wider range of capabilities than other methods for treating those problems. In order to examine the capability of our program, two types of experiments are made by using a drop tower and these results are compared with those of computer simulations. Reasonably good agreements are obtained.