Vapor explosion phenomena and recent experimental studies on the microscopic mechanism of them are described.
The vapor explosion accidents in metal and pulp industries are reviewed. The explosions occurred in the SL-1 reactor and the Chernobyl power reactor station are also said as the vapor explosion. The experimental results are summarized and the necessitive four stages for the occurrence of a large scale vapor explosion are shown. The spontaneous nucleation theory and the thermal detonation theory, which are the typical models of the vapor explosion, are explained.
Vapor explosion experiments using single drops of molten LiNO₃ submerged in ethyl alcohol are described. High-speed films of explosion configuration are compared with the induced pressure traces and the pressure is found to be the control factor of the phenomena. The propagation stage, which is important for a large scale vapor explosion, is photographed in plural drops experiments. A qualitative model of vapor explosion is presented.

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Examples of pattern formation problems in fluid mechanics, such as the self-induced oscillation of liquid surface, the vortex street formation in a mixing layer and the thermal convection, are explained on the stand point of a new scientific activity concerning the basic study of pattern formation. Then, the recently established society, The Society for Science on Form, Japan, is introduced, and an interest in this reseach field is provoked among readers of this Journal.

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A flow produced by a vortex pair approaching a circular cylinder was numerically analyzed by using the discrete vortex methods due to the Cheer's model and due to the modified ones proposed in this paper.
The two major features of the flowfield obtained by the previous experiment are the generation of the secondary vortices of large scales and the characteristic motions of the vortex pair widening largely their mutual distance. These features were found to be well simulated by the vortex methods. Moreover, the tertiary vortices were predicted to arise by the present simulation, which were confirmed to appear by the smoke visualization.
Further, we discussed regarding adequate time intervals of the generation of successive discrete vortices on the solid surface, which is a problem arising from the discrete vortex method itself. Also, a modified “Cloud In Cell” method was proposed for the better expression of the vorticity field to transform the discrete vortex field to a continuous field.

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In the air flow behind a heated grid, the fluctuations of velocity and temperature and the correlation between them are separated by a single hot-wire method and the various characteristics of turbulence are measured. The positive correlation exists between the velocity and temperature fluctuations and the characteristics of the turbulent velocity field are subjected to the influences of the temperature fluctuation. These facts can not be accepted from the standpoint of assuming the flow field behind the grid to be isotropic. Excepting the above mentioned phenomena, the measured turbulence characteristics vary in the streamwise direction in good coincidence with the isotropic turbulence theory.

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Effects of a flexible wall on the flow disturbance are studied experimentally using thin-walled rubber tubes of different lengths conveying steady inflow of water. Experiments are performed for the tubes surrounded by air, water and glycerol. Fluctuations of the flow velocity and displacements of the outer tube wall are measured by a hot-film anemometer and a gap senser around the laminar-turbulent transition. The results show that when the tube length and surrounding media are varied, the critical Reynolds number for rubber tubes are higher than that for rigid tubes in each case, and suggest that the wall flexibility can suppress the occurrence of turbulence in the flow. Negative correlations between the maximum frequency of flow velocity and wall displacement are observed from the spectrum analysis. The results also suggest that there is likely an optimum condition of frequency of wall vibrations which produce maximum reduction of flow disturbance. Then, it can be inferred that the wall vibration plays an important role to govern the occurrence and growth of disturbance of flow.

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Nonlinear behaviour of the two-dimensional disturbances in a plane Poiseuille flow is investigated by three different methods. Firstly, nonlinear neutral surfaces are calculated by the Newton-Raphson method to resolve the question of Itoh (1986) why his result does not coincide with that of Herbert (1976). Secondly, we perform numerical simulation to show that the nonlinear neutral surfaces obtained by the Newton-Raphson method are attainable asymptotically by the dynamical solution starting from an arbitrary initial condition. Lastly, we investigate the nonlinear development of disturbances by weakly nonlinear stability theory and compare the results with those obtained by the numerical simulation.

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The present paper deals with the axisymmetrical flow field around a circular disk moving slowly along the axis of a circular cylinder vessel containing a viscous liquid. It is solved numerically by the finite difference method. The effects of Reynolds number of the disk motion and the side-wall of the cylinder vessel on the flow field around the disk were discussed on the basis of numerical calculations. The expansion phenomenon of stream lines in the vicinity of the disk is suppressed by the approach of the side-wall of the cylinder vessel and the length of the vortex region in the rear of disk decreases with an increase of the diameter ratio of disk to cylinder. By applying the energy dissipation method to the flow field, the drag on a circular disk was calculated in the ranges of 0.01-50 of Reynolds number and 0.1-0.67 of the diameter ratio. The drag on a circular disk at the same Reynolds number increases with an increase of the diameter ratio. Breach's formula about the drag on a disk of infinitesimal thickness in the unbounded viscous liquid is extended to the formula in the bounded viscous liquid on the assumption that three coefficients in his formula are dependent on the diameter ratio. And, the new formula for the drag on a disk of infinitesimal thickness considering the side-wall effect of a circular cylinder vessel was proposed on the basis of the present calculations.

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