As a laboratory model of downburst, statistics of turbulent velocity field of a vertical gravitational flow is investigated. By mechanically breaking a thin film placed at the bottom of a cylindrical container, a finite mass of a high-density liquid begins to fall into a stationary low-density liquid, forming a vertical thermal, impinging onto a horizontal ground and diverging radially outward. By employing PIV, ensemble averaged maps of velocity vectors, azimuthal vorticity and turbulent stresses in a meridian plane are obtained. The statistical characteristics in the downdraft-, impinging-and diverging stage are examined. The nature and the role of the circulatory flow are demonstrated.

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Because the computational resources are finite, one needs to truncate the computational domain when he/she simulates a physical problem. This truncation gives rise to non-physical artificial boundaries and one cannot obtain proper solutions unless appropriate boundary conditions on such boundaries are imposed. Practically nonreflecting boundary conditions, which are boundary conditions that prevent the generation of reflections, are of great importance. By the reason of the practical robustness and the simplicity of implementation, the Poinsot-Lele boundary condition is one of the most popular methods for the Navier-Stokes equations right now. Their method is based on Thompson's boundary condition for the Euler equations, which, however, is essentially one-dimensional. Therefore the Poinsot-Lele boundary condition is valid only when the flow is perpendicular to the boundary theoretically. Here we propose a nonreflecting boundary condition for the Euler equations which does not have the assumption on the direction of flow. We also discuss its extension to the Navier-Stokes equations. Our basic idea is to estimate the direction of the flow from numerical data.

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Jets issuing through small holes in a crossflow have proven effective in the control of boundary layer separation. Longitudinal (streamwise) vortices are produced by the interaction between the jet and crossflow. This technique is known as the vortex generator jet method. The beneficial effect of separation control is obtained only if the jets are pitched to the lower wall and skewed with respect to the crossflow direction. Jets in crossflow provide complex interacting flow fields that contain many vortices. The details of near fields of the jet are not cleared for issuing the pitched (inclined) jet. In this study, flow visualization technique was used to aid in understanding the complex three-dimensional flow field in the near field of the jets. The spectral and velocity measurements were carried out by hot-wire anemometry. For the pitched jets, the presence of a wake behind the jet in the crossflow is observed. Furthermore, the origin and formation of vortices in the wake are described and shown to behave quite differently from the normal jet.

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To obtain a better understanding of the vortex sound the present study considers the sound generation by oscillating vortex pair and vortex ring. The vortex pair is such that the two vortex filaments maintain their constant distance, with their mutually sign-different circulations of the same intensity varying in time periodically. For the case of oscillating vortex ring the same time-varying vortex filament maintains a rectangle shape with a constant enclosing area. We have analyzed the far-field sound generated by the present 2D and 3D oscillating vortex systems in the otherwise calm space and obtained closed form expressions for the sound pressure. The present flow models are applied to the Kármán vortex street of a circular cylinder, in particular, to the vortex formation region immediately behind the cylinder in order to obtain the far-field sound. With all those results we have discussed the flow mechanism of vortex pair and vortex ring working as the dipole sound source generating the Aeolian tones.

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