The Smoothed Particle Method is applied to the gas dynamics in the astrophysical problems and the radiative transfer problem for the direct visualization of gas flows. The fluid is treated as an ensemble of spatially extended Lagrangian particles, which have density distribution of the Gaussian kernel. The Smoothed Particle Hydrodynamics (SPH) describes the motion of each particle in the Lagrangian approach and then calculates the physical quantities from the distribution of particles. Hence SPH is particularly suited for the simulation of highly distorted flow. The Smoothed Particle Rendering (SPR) integrates the ray equation through the opaque medium and calculates the contribution of scattered light. The opacity represents the density scalar and the emissivity is derived by the temperature field. The validity of SPR indicates the possibility to simulate the radiation hydrodynamics. We will show four SPH applications visualized by SPR.

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The large-amplitude to-and-fro swinging of the separation point of a flow on a convex wall, which was found in axisymmetric geometry under a certain flow condition, is found also in two-dimensional flow geometry. The swinging of separation point appears itself when the boundary layer upstream is nearly laminar, but the main stream contains considerable disturbances. In the present experiment, the main stream disturbance is given by a thin circular cylinder placed upstream, perpendicular to the flow but parallel with the wall. Through analysis of velocity fluctuations recorded simultaneously at different points, the followings are concluded : The boundary layer on the wall influenced by main stream disturbances undergoes transition to turbulence locally and intermittently. When and only when the turbulent region thus created reaches the convex part of the wall, the separation point is shifted far downstream. The turbulent region is initiated at the location or further downstream, where the wake disturbances of the cylinder starts to touch with the boundary layer. The turbulent region develops itself while convecting downstream. In its initial phase it has a laterally elongated plan-form, which remains virtually unchanged over a considerable distance. When the peak turbulence level inside reaches some threshold, however, its longitudinal dimension increases quickly, and its plan-form soon becomes longitudinally elongated.

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The present paper deals with the two-dimensional flow field around a square cylinder moving slowly with a constant velocity along the central line of channel. The effects of the channel side-wall and the fluid elasticity to the flow field were studied by the flow visualization and the numerical analysis. The expansion of streamlines in the vicinity of the square cylinder is suppressed by the approach of the channel side-wall and the length of twin-vortices in the rear of square cylinder decreases with a decrease of the ratio of the channel breadth to the side length of square cylinder. The fluid elasticity in viscoelastic fluids suppresses the expansion of streamlines in the rear part of the square cylinder and makes rear twin-vortices smaller. Besides, computational experiments based on Maxwell model were carried out in the ranges of 10-50 of Reynolds number and 0-0.10 of Weissenberg number. It was shown that the length of rear twin-vortices increased with an increase of Reynolds number and decreased with an increase of Weissenberg number.

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In order to clarify the formation of Kármán vortex street, the present paper examines experimentally the disturbance growth in the formation region behind a circular cylinder at a Reynolds number, 3.2×10³. At this Reynolds number, it is found that the disturbance growth is governed initially by the instability of the shear layers close to the separation points and subsequently by that of the wake profile downstream. Although these two kinds of instabilities really contribute to the disturbance growth leading to the formation of Kármán vortex street, the wake-type instability is found to be more dominant in terms of high growth rate and sharp frequency-selectivity. These observations are not in harmony with the recent work by Unal & Rockwell, who emphasized the importance of the initial disturbance behaviour in the region mainly governed by the instability of the separated shear layer. To obtain a better understanding of the formation of Kármán vortex street, it is highly desired to clarify more details of disturbance structure and behaviour in the process.

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Under high freestream turbulence, unsteady flow separation can occur at the leading edge of a thin airfoil to lead to the generation of discrete vortices, even at zero incident angle. In the present study, the critical amplitude of external disturbances for its occurrence is investigated by applying acoustic forcing for a flat-plate airfoil. The results show that even at the literally sharp leading-edge, the intensity of external disturbance for the excitation of vortices is as high as 5% of U_∞ in terms of v (normal-to-plate) fluctuation.

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