論文 - 小尾　晋之介

Numerical Study on the Hydrodynamic Performance of a Flexible Caudal Fin with Different Trailing-Edge Shapes

Khin M.H.W., Obi S.

Biomimetics 9 （ 7 ）  2024年07月

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This paper presents a three-dimensional fluid-structure-coupled simulation of a flexible caudal fin with different trailing-edge shapes. The influences of caudal-fin shape on hydrodynamic performance are investigated by comparing the results of a simplified model of a square caudal fin with forked and deeply forked caudal fins under a wider range of non-dimensional flapping frequency, 0.6 < f* < 1.5, where f* is the ratio of flapping frequency to the natural frequency of each caudal fin, i.e., f* = f/fn. The leading edge of each caudal fin is forced to oscillate vertically in a water tank with zero free-stream conditions. The numerical results show that the amount of forking in the geometry of the caudal fin has significant effects on its hydrodynamic performance. A comparison of thrust coefficients shows that the square caudal fin has a greater thrust coefficient in the non-dimensional frequency range of 0.6 < f* < 1.2, while the deeply forked caudal fin generates higher thrust when 1.2 < f* < 1.5. In terms of propulsive efficiency, the square caudal fin is more efficient when 0.6 < f* < 0.9, while the propulsive efficiency of a deeply forked caudal fin is significantly enhanced when 0.9 < f* < 1.5. Based on our results, the deeply forked caudal fin has greater thrust coefficients and a higher propulsive efficiency in a higher frequency range than the natural frequency of each caudal fin. The thrust characteristics and flow fields around each caudal fin are investigated in detail.

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COMPARATIVE ANALYSIS OF VORTEX FIELD MEASUREMENT AROUND MHK TURBINE BLADES USING PIV AND CFD TECHNIQUES

Hafdaoui A., Tanaka S., Doan M.N., Obi S.

ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) 8 2024年

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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.

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REVISITING THE FLOW INSTABILITY OF SHROUDED COROTATING DISK SYSTEMS

Watanabe A., Obi S.

ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) 5 2024年

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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.

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Three-Dimensional Hydrodynamic Analysis of a Flexible Caudal Fin

Khin M.H.W., Obi S.

Applied Sciences (Switzerland) （Applied Sciences (Switzerland)）  12 （ 24 ）  2022年12月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り

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A 3D fluid–structure coupled simulation of a square flexible flapper, the basic model of a caudal fin, is performed to visualize the flow field around the caudal fin. A plate immersed in a water tank is driven to oscillate vertically by its leading edge. A quantitative analysis of the thrust generated by the plate, which is difficult to explore experimentally, is performed over a range of non-dimensional flapping frequencies 0.93 (Formula presented.) 1.47 to explore the mechanism of thrust generation in more detail. Comparisons are made between three different flapping frequencies around the structural resonance. Numerical results at different flapping frequencies provide a reasonable estimate of the trailing edge amplitude and phase lag of the motion of the plate’s leading and trailing edges. The pressure distribution and deformation of the plate are analyzed to estimate the time evolution of the maximum and minimum thrust generation during the flapping period. Variations in pressure distribution on the plate surface are mainly due to the displacement of the trailing edge relative to the leading edge. Thrust is mainly provided by the pressure difference at the trailing edge. The maximum thrust was found to correspond to the maximum relative deformation of the trailing edge. The optimum frequency (Formula presented.) = 1.2 corresponding to the maximum thrust generation does not coincide with the structural resonance frequency, but remains at a frequency slightly higher than the resonance. These results indicate that the relative deformation of the plate plays an important role in the estimation of the flow field and the associated thrust generation. The numerical results may provide new guidelines for the design of robotic underwater vehicles.

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FLUID FLOW INDUCED BY AN ELASTIC PLATE IN HEAVING MOTION

Khin M.H.W., Kato K., Sung H.J., Obi S.

ASEAN Engineering Journal （ASEAN Engineering Journal）  12 （ 3 ） 1 - 9 2022年08月

研究論文（学術雑誌）, 共著, 最終著者

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We perform three-dimensional simulations of an elastic plate heaving in a sinusoidal motion with different frequencies to explore the effects of the forcing heaving frequency on the hydrodynamic force generation and flow structures. It is aimed to simulate the motion of a rectangular elastic plate, representing a simplified flexible wing or fin, placed in an open water tank as in a corresponding experimental study. The top edge of the plate oscillates sinusoidally in water at rest. Simulations are conducted for the fluid structure interaction of an elastic plate with a special version of the open-source library C++ OpenFOAM, foam-extend-3.2. The plate oscillates at different heaving frequencies in a range of 3.5 Hz and 4.5 Hz. Experiments are carried out to validate our simulations. The results are in a good agreement with the experiment in terms of the representation of the resonant frequency and the induced hydrodynamic forces. It is found that the hydrodynamic force and propulsive efficiency are mainly affected by the elastic deformation and forcing heaving frequencies. The generated thrust is observed to be significantly enhanced at the resonant frequency, while the propulsive efficiency is increased at the heaving frequency which is greater than the resonant frequency. The results of our simulations point to the importance of resonant flapping frequency for considering the optimal heaving frequency to achieve the best performance and to get the improved thrust force, which are crucial for the locomotion of birds, insects, fishes, flapping-based micro air and underwater vehicles.

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Numerical study of the dynamic stall effect on a pair of cross-flow hydrokinetic turbines and associated torque enhancement due to flow blockage

Doan M.N., Obi S.

Journal of Marine Science and Engineering （Journal of Marine Science and Engineering）  9 （ 8 ）  2021年08月

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An open-source 2D Reynolds-averaged Navier–Stokes (RANS) simulation model was presented and applied for a laboratory-scaled cross-flow hydrokinetic turbine and a twin turbine system in counter-rotating configurations. The computational fluid dynamics (CFD) model was compared with previously published experimental results and then used to study the turbine power output and relevant flow fields at four blockage ratios. The dynamic stall effect and related leading edge vortex (LEV) structures were observed, discussed, and correlated with the power output. The results provided insights into the blockage effect from a different perspective: The physics behind the production and maintenance of lift on the turbine blade at different blockage ratios. The model was then applied to counter-rotating configurations of the turbines and similar analyses of the torque production and maintenance were conducted. Depending on the direction of movement of the other turbine, the blade of interest could either produce higher torque or create more energy loss. For both of the scenarios where a blade interacted with the channel wall or another blade, the key behind torque enhancement was forcing the flow through its suction side and manipulating the LEV.

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Flow field measurement of laboratory-scaled cross-flow hydrokinetic turbines: Part ii—the near-wake of twin turbines in counter-rotating configurations

Doan M.N., Kawata T., Obi S.

Journal of Marine Science and Engineering （Journal of Marine Science and Engineering）  9 （ 7 ）  2021年07月

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Cross-flow hydrokinetic turbines have sparked interest among fluid dynamicists for their potential for power enhancement in paired configuration. Following the first part of a laboratory-scaled turbine wake measurement project, this second part presents a monoscopic particle image velocimetry measurement of the near-wake of two cross-flow hydrokinetic turbines in six different counter-rotating configurations. The turbines operated in a small water flume at an average diameter-based Reynolds number of 2 × 104 with the incoming streamwise velocity of 0.316 m/s. The six configurations included two turbine separation distances, two turbine phase angles differences, and two different relative incoming flow angles. Similar to the observation of the single turbine configurations in part I, a correlation between flow field structures and the corresponding power output was observed. Effects of each parameter of the counter-rotating configurations are further discussed, which suggest guidelines for setting up multiple devices in a power farm. This article is accompanied by all full numeric data sets and videos of the results.

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Flow field measurement of laboratory-scaled cross-flow hydrokinetic turbines: Part i—the near-wake of a single turbine

Doan M.N., Kai Y., Kawata T., Obi S.

Journal of Marine Science and Engineering （Journal of Marine Science and Engineering）  9 （ 5 ）  2021年05月

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Recent developments in marine hydrokinetic (MHK) technology have put the cross-flow (often vertical-axis) turbines at the forefront. MHK devices offer alternative solutions for clean marine energy generation as a replacement for traditional hydraulic turbines such as the Francis, Kaplan, and Pelton. Following previous power measurements of laboratory-scaled cross-flow hydrokinetic turbines in different configurations, this article presents studies of the water flow field immediately behind the turbines. Two independent turbines, which operated at an average diameter-based Reynolds number of approximately 0.2 X 105, were driven by a stepper motor at various speeds in a closed circuit water tunnel with a constant freestream velocity of 0.316 m/s. The wakes produced by the three NACA0012 blades of each turbine were recorded with a monoscopic particle image velocimetry technique and analyzed. The flow structures with velocity, vorticity, and kinetic energy fields were correlated with the turbine power production and are discussed herein. Each flow field was decomposed into the time averaged, periodic, and random components for all the cases. The results indicate the key to refining the existed turbine design for enhancement of its power production and serve as a baseline for future comparison with twin turbines in counter-rotating configurations.

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Observation and discussion of leading edge vortex shedding from laboratory-scaled cross-flow hydrokinetic turbines in counter-rotating configurations

Doan M., Kai Y., Kawata T., Alayeto I., Obi S.

American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FEDSM （American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FEDSM）  3 2021年

ISSN  08888116

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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.

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Twin marine hydrokinetic cross-flow turbines in counter rotating configurations: A laboratory-scaled apparatus for power measurement

Doan M.N., Kai Y., Obi S.

Journal of Marine Science and Engineering （Journal of Marine Science and Engineering）  8 （ 11 ） 1 - 16 2020年11月

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This article proposes an experimental apparatus design to measure the power of a cross-flow marine hydrokinetic turbine system operating in a laboratory water tunnel. Data, from one Hall sensor output signal, was processed to capture the three types of torque exerted on the turbines: mechanical loss, brake, and hydrodynamic torque. The method was then applied to compare the power of a twin turbine system in different counter-rotating configurations. Controlled by a hysteresis brake, the tip-speed-ratio was varied in a constant freestream velocity of 0.316 m/s. While the braking torque was independent of the speed, the mechanical loss was found to depend on the system rotational speed and the amount of mass mounted on the mechanical support. In a counter-rotating configuration, the turbines were synchronized through a pair of spur gears and timing pulleys. Operating at the average chord based Reynolds number of 8000, each turbine had three NACA0012 blades mounted at 15 pitch angle. The power coefficient results of 8 turbine configurations showed the tendency of power enhancement of counter-rotating configurations due to blade interaction and increase in blockage ratio. Comparison of the results suggested direct application in a river flow scenario and manipulation of the blade interaction for optimal power production. ◦

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円管中に設置された半円オリフィス後流における界面活性剤水溶液流れのステレオ PIV 計測

河合正郎， 河田卓也， 小尾晋之介

流体工学部門講演会講演論文集, OS01-02 2020年

円筒容器内で回転する積層円盤間での遷移レイノルズ数領域における流体関連振動に関する実験的研究

服部能幸， 河田卓也， 小尾晋之介

流体工学部門講演会講演論文集, OS13-01 2020年

Computational fluid dynamic analysis of a marine hydrokinetic crossflow turbine in low Reynolds number flow

Doan M.N., Alayeto I.H., Kumazawa K., Obi S.

ASME-JSME-KSME 2019 8th Joint Fluids Engineering Conference, AJKFluids 2019 （ASME-JSME-KSME 2019 8th Joint Fluids Engineering Conference, AJKFluids 2019）  2 2019年

ISSN  9780791859032

　概要を見る

This study focuses on surveying different turbulence models and dynamic mesh techniques to simulate a marine hydrokinetic (MHK) crossflow turbine at Re ˜ 7,000. While several research projects have shown that studies of MHK devices in low Re flow could still yield interesting and significant results, existing computational fluid dynamic (CFD) simulations were conducted at the chord based Re of 10 ~ 10 . The wake and power production of a laboratory-scaled MHK crossflow turbine were numerically simulated and compared with relevant experimental data. The vertical axis turbine operated in a small flume with 20% blockage ratio and was fabricated by mounting three NACA0012 (2.54 cm chord length) straight blades at a radius of 3.41 cm and 15 pitch angle. Within OpenFOAM environment, blade-resolved models were built with Spalart-Allmaras, k-omega shear stress transport (SST), and k-kl-omega unsteady Reynolds-averaged Navier-Stokes simulation (URANS) in both two and three dimensions. Results from each model were compared with the experimental power measurement and flow field obtained by monoscopic particle image velocimetry (2D PIV). Additionally, four different techniques for moving the solid boundaries (turbine blades) in the unsteady simulation were presented and compared in terms of solution consistency and required computational power. Overset mesh, time-deforming mesh, and moving immersed boundary are all available in this open source environment, beside the common rotating mesh technique, and possess the potential to be applied to a more complicated configuration of turbines. c 5 6 ?

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Wake characteristics comparison between isolated and pair configurations of marine hydrokinetic crossflow turbines at low Reynolds numbers

Alayeto I.H., Doan M.N., Kumazawa K., Obi S.

ASME-JSME-KSME 2019 8th Joint Fluids Engineering Conference, AJKFluids 2019 （ASME-JSME-KSME 2019 8th Joint Fluids Engineering Conference, AJKFluids 2019）  1 2019年

ISSN  9780791859025

　概要を見る

Wake characteristics and power coefficient of laboratory scaled marine hydrokinetic cross flow turbines were studied both experimentally and numerically. Single and turbine pair configurations were experimentally tested in a 3.5 m long, 0.3 m wide and 0.15 m deep water channel facility. Each turbine was built with three straight NACA0012 blades with chord 2.54 cm and 6.828 cm diameter, corresponding to a solidity of 1.2. The Reynolds number associated with the velocity given by the water pump and blade chord oscillated near 7000. Empirical power measurement was obtained multiplying the average torque by average rotational velocity. These measurements were obtained with a magnetic hysteresis brake utilized as control system and a Hall effect sensor used as speed transducer, respectively. Wake velocity profile was obtained by image processing of Particle Image Velocimetry (PIV) measurements at different positions. The empirical results were contrasted with numerical computational fluid dynamics (CFD) simulations carried out with Salome and OpenFoam. The computational model solved the Unsteady Reynolds Averaged Navier-Stokes (URANS) equations in 2 dimensions using the turbulence models k-? Shear Stress Transport (SST) and Spallart-Allmaras (SA). The experimental and numerical results show a clear difference of power coefficient and wake shape for both turbine configurations. This influence of two nearby blades on the flow can be exploited to obtain higher ratios of power per land area, leading to an increase of the overall generation of a power plant by a careful arrangement.

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Experimental study on modeled caudal fins propelling by elastic deformation

Baba N., Obi S.

American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FEDSM （American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FEDSM）  1 2018年

ISSN  9780791851555

　概要を見る

Copyright © 2018 ASME. The present study proposes a new device for the experiment of self-propelling bodies in the water. As opposed to the studies in the past whose experiments were often carried out in a water channel with a given freestream velocity, the new device allows the model to swim under an actual self-propelling condition. The adopted model mimics the caudal fin of various shapes and made of elastic material, and the self-propelling speed is investigated primarily as a function of the forcing frequency. The influence of the amplitude of forced vibration and the materials of different elasticity is also investigated. The flow field around the model fin has been measured by PIV to characterize the flow pattern produced by the fin-motion.

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Experimental and computational fluid dynamic analysis of laboratory-scaled counter-rotating cross-flow turbines in marine environment

Doan M., Alayeto I., Padricelli C., Obi S., Totsuka Y.

American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FEDSM （American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FEDSM）  2 2018年

ISSN  9780791851562

　概要を見る

Copyright © 2018 ASME Power generation of laboratory-scaled marine hydrokinetic (MHK) cross-flow (vertical axis) turbines in counter-rotating configurations was scrutinized both experimentally and numerically. A tabletop experiment, designed around a magnetic hysteresis brake as the speed controller and a Hall-effect sensor as the speed transducer was built to measure the rotor rotational speed and the hydrodynamic torque generated by the turbine blades. A couple of counter-rotating straight-three-bladed vertical-axis turbines were linked through a transmission of spur gears and timing pulleys/belt and coupled to the electronic instrumentation via flexible shaft couplers. A total of 6 experiments in 3 configurations, with various relative distances and phase angles, were conducted in the water channel facility (3.5 m long, 0.30 m wide, and 0.15 m deep) at rotor diameter base Reynolds number of 20,000. The power curve of the counter-rotating turbines (0.068-m rotor diameter) was measured and compared with that of a single turbine of the same size. Experimental results show the tendency of power production enhancement of different counter-rotating configurations. Additionally, the two-dimensional (2D) turbine wakes and blade hydrodynamic interactions were simulated by the shear stress transport k-omega (SST k-omega) model using OpenFOAM. The computational domain included a stationary region and two rotating regions (for the case of counter-rotating turbines) set at constant angular velocities. The interface between the rotating and stationary region was modeled as separated surface boundaries sliding on each other. Velocity, pressure, turbulent kinetic energy, eddy viscosity, and specific dissipation rate field were interpolated between these boundaries.

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羽ばたき翼の回内・回外運動が揚力発生に及ぼす影響に関する実験的研究

近藤貴之， 小尾晋之介

流体工学部門講演会講演論文集 2018, OS13-6 2018年

弾性変形する平板の形状と自己推進速度に関する実験的研究

池田曉， 馬場直道， 小尾晋之介

流体工学部門講演会講演論文集 2018, OS13-7 2018年

アーム付 HDD モデルにおけるシュラウド内壁面の変動圧力計測

真田航平， 小尾晋之介

IIP 情報・知能・精密機器部門講演会講演論文集 2018, 1C13_1 2018年

乱流境界層に置かれた円柱前部の四面体による馬蹄形渦への影響

吉井慎太郎， 小尾晋之介

流体工学部門講演会講演論文集 2018, OS1-17 2018年