論文

ダウンバーストの実験室モデルによる流れ場計測を行った。粒子画像流速系データに対し子午面上のアンサンブル平均を行い、乱れの生成、輸送に関する基礎的な知見を得た。実際の気象観測結果と良い一致を示した。

コリオリ力を受ける乱流境界層の熱伝達特性について実験を行い、回転パラメータの変化によりたて渦が発生し、伝熱効果が促進されることを明らかにした。

周期的な外乱を受けるバックステップ流れをPIV計測し、周期平均処理を施すことで大規模渦構造を抽出し、その運動を解析した。

Vertical-axis hydrokinetic turbines offer a high-power-density solution for marine energy, yet their performance is constrained by complex wake interactions. This study utilizes time-resolved particle image velocimetry (PIV) to investigate the near-blade vortical fields of a tandem turbine system. By capturing the impingement of upstream trailing-edge vortices on downstream blades, we elucidate how dynamic stall and the subsequent shedding of the leading-edge vortex (LEV) modulate lift and torque in highsolidity rotors.

This study investigates the Reynolds stress transport process in surfactant turbulent pipe flow, aiming to elucidate how surfactants affect turbulent momentum transfer. Utilizing stereoscopic particle image velocimetry (PIV), we captured the instantaneous velocity field across the pipe’s cross-section. To comprehensively analyze the Reynolds stress transport, both velocity and pressure gradient fields were essential. We derived the pressure gradient field from velocity data by numerically solving the Poisson equation. Our findings reveal a significant reduction, approximately 20%, in Reynolds shear stress attributable to the presence of surfactants. Moreover, we computed the production term and velocity-pressure gradient term from the measured velocity field. While the production term balances with the velocity-pressure gradient term for both water and surfactant flows, these terms are notably smaller for surfactant-laden flows. This suggests that the suppression of Reynolds shear stress is primarily driven by the suppression of the production term. Additionally, the velocity-pressure gradient term is also diminished to maintain equilibrium within the Reynolds stress equation, suggesting the reduction of the redistribution of energy between normal stress components which is a key mechanism of maintaining the Reynolds stress. These insights deepen our understanding of how surfactants modulate turbulent momentum transport in pipe flow.

This paper investigates the rotational lift production of translating and rotating wings within a small insect’s Reynolds number range. Using the Reynolds number 1200 of a bumblebee, three wing section profiles were studied: a circular cylinder model as a reference for a blunt body for which the well-known Magnus effect will occur, a flat plate model as a reference for a sharp body for which the Kramer effect will occur, and finally, an elliptical cylinder model as a transition case. Direct force measurement and particle image velocimetry (PIV) experiments were performed to measure the lift produced and the surrounding flow velocity, and the Kutta–Joukowski theorem was applied to analyze the PIV results. The Kutta–Joukowski theorem gives the relationship between lift and circulation on a body moving at constant speed in a real fluid with some constant density. The experimental results were analyzed and verified by comparing them to the computational results. In general, there is reasonable agreement between the experimental and computational results, confirming that the Magnus effect is observed for the circular cylinder model and no Kramer effect is observed for the flat plate model. The elliptical cylinder model does not appear to be blunt enough for the Magnus effect to occur, and it is not sharp enough for the Kramer effect to occur.

A fast vortex method has been developed by using a special-purpose computer, MDGRAPE-3. MDGRAPE-3, successor of MDGRAPE-2, has been applied to the same calculations (Sheel et al. in Comput. Fluids 36:1319-1326, 2007) and the improvement in speed was 1000 times faster when compared with the ordinary PC Xeon 5160 (3.0 GHz) for N = 10 6. The simultaneous use of the fast multipole method, Pseudo-particle multipole method with the special-purpose computers gives further acceleration of vortex method calculations. In addition, performances and efficiency have been investigated carefully. Further possibility has been investigated carefully and compared the present result with the previous one. © 2009 Springer-Verlag.

Propeller geometry significantly influences the performance and efficiency of propulsion systems in watercraft. This study investigates the thrust enhancement potential of mini screw-type propellers on a laboratory-scaled watercraft through computational fluid dynamics (CFD) simulations conducted using OpenFOAM. The unsteady Reynolds-Averaged Navier-Stokes (RANS) equations, coupled with the k-omega-SST turbulence model, are applied to simulate fluid-structure interactions. The sliding mesh technique is utilized to model the rotation of the propellers with high fidelity. The Reynolds number, based on propeller diameter, is approximately 50,000. Seven distinct propeller geometries are analyzed, with variations in the number of blades and helical pitch angles. Performance evaluation focus on thrust force and the presentation of wake flow features for each design. The findings highlight the relationship between propeller geometry and hydrodynamic performance, offering insights into wake structures and flow dynamics. This study provides a systematic approach to propeller design for laboratory-scaled watercraft, contributing to the broader understanding of thrust enhancement and propulsion efficiency in small-scale experimental systems.

In disk drive systems, flow-induced vibration, often attributed to turbulent fluid motion, presents a significant challenge due to its wide-ranging frequency spectra. Helium-filled enclosures have been introduced to mitigate this issue by reducing the force exerted by fluid motion, owing to the lower gas density compared to air. This density reduction leads to a decrease in the inertia force relative to the viscous force, lowering the Reynolds number by nearly an order of magnitude and resulting in an anticipated transition from turbulent to laminar flow. However, the transitional regime poses challenges, potentially causing intermittently alternating flow motion between turbulent and laminar states, characterized by velocity fluctuations of larger magnitude. Our experimental study involved a series of tests with a system comprising five 3.5-inch disks enclosed within a cylindrical casing. Varying the disk rotation speed from 600 rpm to 7200 rpm resulted in Reynolds numbers ranging from about 1×104 to 1×105. While air-filled configurations consistently exhibited fully turbulent flow, helium-filled setups demonstrated laminar characteristics at lower Reynolds numbers. Our findings suggest transitional behavior at intermediate Reynolds number ranges, indicating a shift from stable to unstable flow patterns. These observations underscore the potential challenges associated with helium-filled disk systems.

Cross-flow turbines have been recently re-discovered in the fields of wind and tidal energy thanks to the high power density they can provide when arranged in arrays. In this paper, we investigate the mechanism of torque generation and maintenance of a pair of miniature Marine Hydrokinetic Turbines via Particle Image Velocimetry, providing new insights on the influence of vortex shedding onto the lift generation process. The velocity field around the blades was measured for two different tip speed ratios and across various configurations of the devices, to then evaluate the forces involved by performing a momentum balance over moving control volumes. Our findings suggest that a lift force was first generated, in line with the airfoil theory, by the flow acceleration on the inner surface of the blade, and was subsequently maintained after the onset of dynamic stall by the low pressure associated with a leading-edge vortex. Furthermore, increases in tip speed ratio and blockage effect were observed to positively impact vortex strength and torque production. The results were compared with two-dimensional CFD studies for the purpose of validating the control volume method for torque estimation, as well as discussing and quantifying the 3D effect.

In 2011, John Dabiri proposed the use of counter-rotating vertical-axis wind turbines to achieve enhanced power output per unit area of a wind farm. Since then, various studies in the wind energy and marine hydrokinetic (MHK) literature have been dedicated to pairs of vertical axis turbines in both co-rotating and counter-rotating configurations, in terms of their power production, wake characterization, and optimal array design. Previous experimental works suggest an enhancement of up to 27.9% in the system power coefficient of pair configurations compared to a single turbine. Additionally, previous numerical studies have indicated that the increased power output is correlated with higher torque on the turbine blades which correspondingly produces a stronger leading edge vortex. This paper presents an extended investigation into a pair of laboratory scaled cross-flow hydrokinetic turbines in counter-rotating configurations. Experiments were conducted to observe, compare, and discuss the leading edge vortex shedding from the turbine blades during their positive torque phase. The turbines operated in a small water flume at the diameter-based Reynolds number of 22,000 with a 0.316 m/s freestream velocity and 4% turbulent intensity. Using a monoscopic particle image velocimetry setup, multiple realizations of the water flow around each blade at their positive torque phase were recorded and phase-averaged. Results show consistent leading vortex shedding at these turbine angles while a correlation between the turbine power performance and the vortex size and strength was observed.