In this review, the gas flows around a small particle and the forces on it are discussed on the basis of the kinetic theory of gases. Recent numerical analyses using the original Boltzmann equation are presented in detail. Comparisons of these recent theoretical results with some existing experimental data or the results based on the model equation are also discussed.

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Numerical Wind Tunnel (NWT) is a parallel computer introduced to the National Aerospace Laboratory at 1993. It is constructed by 140 processor elements (PEs), each of which is still a vector computer having 256 MB of main memory and a peak speed of 1.7 GFLOPS. Therefore, the whole NWT has the aggregate memory of 35 GB and the overall peak speed of 236 GFLOPS. Computing speed of a practical CFD program, however, should be a rather lower value than the peak speed, because in the practical calculation, data transfers among PEs are often required in the NWT and the speed of the transfer is remarkably slower than that of the calculation conducted only in the local memory of PE. As an example of a parallel computation using the NWT, a Fourier spectral method used in a direct numerical simulation of homogeneous isotropic turbulence and its parallel algorithm for the application to the NWT are described. As a result, the simulation with 512³ mesh points has been conducted by 128 PEs of the NWT and attained a computing speed of 90.3 GFLOPS. From the numerical result obtained by the simulation, various dynamical properties of turbulence, such as Kolmogorov spectrum, the tube-like concentration of the intense vorticity, are analysed.

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In the propagation process of strongly nonlinear acoustic waves and shock waves, there exist several intriguing nonlinear phenomena which present striking contrasts to the known results from the weakly nonlinear theory. In this review the attention is focused on acoustic streaming among the strongly nonlinear phenomena in the problems of plane wave, spherical wave, and the wave emitted from a circular piston. Strongly nonlinear plane wave radiated by harmonic oscillation of a plate develops into an asymmetrical sawtooth-like wave in the near field, and this leads to the generation of streaming in the direction of wave propagation, whereas according to the weakly nonlinear theory the wave evolves into the sawtooth wave of symmetric profile and streaming does not occur. Streaming continues to reduce the density of the gas in the near field, which suggests that a steady state may not be realized. Strongly nonlinear spherical wave can also excite streaming and hence the density of the gas near the sound source is reduced as well as in the plane wave problem. Strongly nonlinear wave radiated by a circular piston induces strong streaming which forms a vortex-ring-like flow pattern. The location of the core of vortex-like flow almost coincides with a high vorticity region generated by a multiple interference of shock waves.

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The success of a future supersonic combustion ramjet engine highly depends on the efficiency of fuel-air mixing. As the compressibility suppresses the growth of mixing layers with increasing the convective Mach number, it is crucial to develop the techniques for enhancing the supersonic mixing. To see how the compressibility hampers the turbulent motions and how to alleviate its effects, the present paper first analyzes important previous results for the growth of supersonic mixing layers. We then propose to use streamwise vortices to enhance the supersonic mixing, and further conduct experiments of introducing streamwise vortices into a supersonic wake at a Mach number 2.5. The experiments verify that streamwise vortices can be easily generated. The results (schlieren photographs) also show that although the streamwise vortices introduced are rather stable downstream a high rate of fluid entrainment into the vortices can be obtained even during their formation, indicating the capability of enhancing the supersonic mixing. We further consider the generation of small-scale eddies through the breakdown of the streamwise vortices (with various combinations of intensities, scales, rotations and in spanwise raw configurations). It is proposed to cause their breakdown by small scale eddies (and/or smaller scale streamwise vortices) embedded in them at their initial formation, by their mutual interaction in spanwise raw configurations, by their interaction with incident weak shock waves, and by their own instabilities.

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