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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本論文は将来のスクラムジェットエンジンに直接係わる超音速混合の促進制御の問題を扱っている.まず, 超音速混合層に関する従来の重要な研究成果に基づき, 超音速域で乱流混合が抑制される機構について考察し, 超音速混合の促進に適した流れの構造として縦渦が有効であることを導き, さらに主流マッハ数M=2.5の超音速後流中への縦渦導入の実験を行った.そして, 極めて速やかな縦渦形成が超音速流の特徴を生かした簡便な手法で達成されること確認し, 縦渦形成段階で非圧縮流の場合に匹敵する連行 (エントレインメント) 量の得られる可能性が高いこと, 縦渦が比較的安定であるため混合を一層促進するには縦渦を小スケールの渦に崩壊させる必要があること, などを示し, さらに縦渦崩壊の手法について考察してその具体的な手法を提案している.

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