Presentations

In the hard disk drive (HDD), the high-speed rotation of the disk generate turbulent air flow. The turbulent flow causes vibration of the disks and RW arm. The flow induced vibration (FIV) is a major obstacle in improving the performance and reliability of the HDD. Therefore, the investigation on the details of the flow structures and its relationship with FIV are of great engineering imp ortance. In this research, fluctuating pressure measurement on the inner wall of simplified HDD model was performed changing arm insertion angle. Accompanying changes in the arm insertion angle, changes in the flow structure and in the frequency of the pressure fluctuation spectrum were confirmed.

Splitting-free vorticity redistribution method was developed in Kirchhart (4) in order to avoid viscous-splitting when modeling the diffusion term of the vorticity transport equation. The method instead approximates the Laplacian directly. New scheme such as small neighborhoods significantly reduced the method’s computational cost. In the assessment of this method to a radial symmetric vorticity field, fine agreement with the previous calculation done by Beale and Majda (5) in the case of inviscid was confirmed. The second order convergence to the exact solution was also observed. Then, the convergence of the results to the inviscid case for vanishing viscosity was verified.

We present the torque and power measurement of laboratory-scale counter-rotating vertical-axis hydrokinetic turbines, built around a magnetic hysteresis brake as the speed controller and a Hall-effect sensor as the rotational speed transducer. A couple of straight-three-bladed turbines were linked through a transmission of spur gears and timing belt and coupled to the electronic instrumentation via flexible shaft couplers. A total of 8 experiments in 2 configurations were conducted in the water channel facility (4-m long, 0.3-m wide, and 0.15-m deep). Power generation of the turbines (0.06-m rotor diameter) were measured and compared with that of single turbines of the same size. The wakes generated by these experiments were also measured by particle image velocimetry (PIV) and numerically simulated by unsteady Reynolds-averaged Navier-Stokes (URANS) simulation using OpenFOAM. Preliminary results from wake measurement indicated the mechanism of enhanced power production behind the counter-rotating configuration of vertical-axis turbines.

An experimental study was conducted for self-propelling fin plates in the fluid at rest. The plates were oscillated at constant amplitude and frequency in the direction normal to its plane, and self-propelled in a devise similar to a merry-go-round. The self-propelling speed and vibration characteristics were measured for modeled caudal fins which had two variable geometric parameters: The length of leading edge Llead, and the chord length Lchord along the centerline. The experiments were undertaken for fins of 9 different shapes at the forcing frequency varying from 1.0 Hz to 10 Hz, resulting in the self-propelling speed of nearly 300 mm/s at most. It was found that Llead solely affects the magnitude of trailing edge amplitude and self-propelling speed, while Lchord influences the Vibration mode of the plates. The effect of elasticity has been found to be of primary importance, and there are possibilities to maximize the self-propelling speed by choosing an appropriate combination of the shape and material of the fin and the forcing frequency.

The behavior of horseshoe vortex around a circular cylinder mounted on the bottom of turbulent boundary layer was experimentally investigated. The velocity field was measured by a PIV for the Reynolds number, based on the oncoming flow velocity and the cylinder diameter, of ReD=11,500 on the mid-plane in front of the cylinder. The time-averaged streamlines and vorticity distribution as well as the Q-criteria confirmed the existence of the fluctuating vortices. The PDF of fluctuating velocity beneath the vortices indicated a good agreement with previous studies. The temporal variation of the horseshoe vortex was also analyzed by observing the x-t diagram of u at different planes parallel to the bottom wall. It was confirmed that the horse-shoe vortex meanders within a certain range in the streamwise direction, while it disappears or merges with tertiary vortices at random in time.

The horse-shoe vortex appearing in front of the circular cylinder placed on the boundary layer was numerically investigated. The unsteady, three-dimensional feature of the flow field was captured by a lattice-Boltzmann method, where more than 300 million grids were used. The visualized instantaneous flow field indicated a complex vortex structure before and behind the cylinder. The spectrum analysis on the fluctuating pressure in these vortices indicated, however, there are only weak correlations between the horse-shoe vortex in front of the cylinder and the shed vortices in the wake. A strong down-ward flow in front of the cylinder was generated as a consequence of the horse-shoe vortex. The wall shear stress on the bottom wall was found to be maximum around the corner of the cylinder, as a consequence of the flow acceleration due to the down-ward flow which was related to the generation of the horse-shoe vortex.

パイプライン留置から動脈瘤の治癒に至る機序は完全に理解されていない。前床突起部など屈曲が強い血管での治療成績が一定せず、コイル塞栓を併用するなどの姑息的解決策が取られている。今回我々は血管屈曲が整流効果に与える影響を検証した。 血管屈曲が整流効果に与える影響は大きく、メッシュ密度と同様に重要なパラメーターとして考慮する必要がある。