西尾 悠

Abstract This study numerically investigates an early stage of nonlinear interaction for the better understanding of the onset of nonlinear behaviors. Two-dimensional shear flow is chosen as a canonical flow. When two disturbances of different wavenumbers satisfying no resonance condition are initially given, new components appear one after another while the original disturbances grow. The vorticity budget analysis shows that the beat of the two exciting modes plays an essential role in forming the sum and difference nonlinear components, namely the secondary modes. For the nonlinear interaction mechanism, the high vorticity around the center of the shear layer is locally transported in the transverse direction at specific streamwise sections where the amplitude of the vertical velocity fluctuation becomes relatively larger compared to other sections. The distance between these specific sections corresponds to the wavelength of the beat. The vertically dispersed vorticity will then be convected in the horizontal directions by the mean flow. As a result, several regions of concentrated vorticity appear which eventually develop into vortices. The amplification mechanism is found to be the same for both the primary and secondary modes, though the secondary modes arise from the additional perturbation deriving from the initial perturbation.

<title>Abstract</title> Micro air vehicles (MAVs) have been developed for many fields. The MAVs usually receive strong impact from a velocity change in time or space, and facilities for aerodynamic experiments of MAVs under a gusty environment have been required. The present study has developed a gust wind tunnel to generate unsteady and non-uniform flows. We developed a small wind tunnel with eight multi-fans and a shutter mechanism at the upstream of the test section. We controlled the outputs of the fans independently and obtained a linear shear layer with an error of 5 percent. The velocity gradient of the shear layer was from 5 to 8 s−1. The shutter mechanisms provided a longitudinal gust with the velocity change from 2 m/s to 10 m/s within 0.3 seconds.

<title>Abstract</title> The present study investigates the flow field in a rinsing process of a beverage can numerically and experimentally. The three-dimensional Navier-Stokes equations are solved with a finite volume method along with the volume of fluid (VOF) method for free surface. The beverage can set upside down is transported with a constant velocity and rinsed with a water jet ejected from a nozzle below the can. The case of a can at rest is also simulated. The result shows that the ejected water impinges on the can bottom and spreads along the side surface of the can. Then, as it flows down toward the can mouth, its front surface forms splashes. For the stationary can case, after the jet impinges on the can bottom, it almost evenly spreads over the side surface. The water flows downward and becomes branched flows by fingering. The time average of VOF is calculated to visualize the regions rinsed by water. For the case of a moving can, only the top region of the can is rinsed, and the ratio of the rinsed region drops to 29% from 69% for the stationary case. The computed water surfaces qualitatively agree with the experimental result, but the shape of the front surface, such as splashes and fingerings, cannot be resolved with the simulation.

[Open access]

[Open Access]

[Open Access]

Copyright © 2019 ASME. A tube-type gas burner consists of a straight tube with a slit along it and discharges an air-gas mixture through the slit to produce a flame. The flow velocity from the slit depends on the pressure in the tube and the pressure loss at the slit, and it varies in the longitudinal direction of the tube. The resulting uneven flame degrades the quality of the burner. In this study, we develop a one-dimensional theoretical model of the flow in a tube with a slit. To validate the result of the theoretical model, we also conduct experiments and numerical simulations for the same flow field. We applied this theoretical model to a flow in a tube, 1 m length, 40 mm in diameter, with a slit 2.5 mm wide. The end of the tube is closed. We also discuss the effect of the length of the burner on the unevenness.

Copyright © 2019 ASME. Motion of liquid pouring from a beverage can is numerically studied. A liquid is poured from a can which is rotated at a prescribed angular speed. The flow is simulated by solving the unsteady three-dimensional Navier-Stokes equations. An experiment under the same condition is also carried out to validate the computational result. The result shows that, when the can is tipped, the liquid flows over the lid of the can and is once obstructed by the rim of the lid. The numerical result is in good agreement with the experimental result. The effect of condensation formed on a can surface is also considered. The effect of condensation is taken into account by adjusting a contact angle. The liquid pouring from a can trickles down along the can body. The computation reproduces these experimental observations.

[Open Access]

Three-dimensional numerical simulation of the flow around a leading edge of a flat plate when vortices with their axes normal to the flat plate surface is carried out to investigate the leading-edge receptivity to the vortical disturbances. It is shown that the vertical vortices outside the boundary layer are not titled and deformed, contradicting to what was reported in previous studies. It is revealed that the streamwise structures, which are dominant in the boundary layer, are formed due to the velocity field induced by the wall-normal vortices outside the boundary layer. It also is found that neighboring pair of vortices side-by-side in the spanwise direction are connected to each other very close to the wall.