論文

Turbulent flow measurements were conducted between two bluff bodies set in uniform flow in tandem arrangement. The velocity field obtained with PIV was averaged with respect to either time or phase of periodic pressure oscillation induced by vortex shedding from the bluff body, i.e., Reynolds decomposition or three-level decomposition. The Reynolds stresses caused by periodic fluid motion was found excessively large compared with those related to turbulent fluctuations in the entire flow field in observation. The PIV data were used to solve the discrete Poisson equation of instantaneous pressure. The effect of organized vortex motion caused strong correlations between velocity and pressure gradient when observed from the framework of the Reynolds averaging. © 2006 Elsevier Inc. All rights reserved.

As a laboratory model of downburst, the statistics of a turbulent velocity field of a vertical gravitational flow is investigated. By mechanically breaking a thin film fixed 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. It impinges onto the horizontal ground and then diverges radially outward. By employing particle image velocimetry, the ensemble-averaged maps of velocity vectors, azimuthal vorticity, and turbulent stresses in a meridian plane are obtained. The statistical characteristics in the downdraft stage, impinging stage and diverging stage are examined. The nature and the role of the circulatory flow are demonstrated. The results show reasonable agreement with the actual downbursts observed in the atmosphere. Based on these results, the windshear hazard index for aircraft encountering a downburst is evaluated. Copyright © 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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.

The method to estimate the dynamic load of a flapping wing by the integration of pressure on the wing's surface is discussed. The flapping wing was modeled as a plate flapping sinusoidally in hovering condition. The flow around the flapping plate was measured using stereo PIV on multiple measurement planes along the out-of-plane direction. The phase-averaged velocity field of each measurement plane was calculated so that three-dimensional analyses could be applied. The phase-averaged pressure field was obtained from the integration of the three-dimensional Poisson equation for pressure using the available information acquired from stereo PIV measurements. The pressure field is visualized on the measurement planes. In this study, the estimated load was the torque of the axis of rotation. This torque was compared with the result from strain gauge measurements. The torque estimation, although only on a partial surface of the plate, is within reasonable agreement with the measured torque. The integration of the Poisson equation based on stereo PIV measurements and estimations of the torque shows that an increase in the torque at the start of a flapping stroke is caused by the stagnation on the surface of the plate from the flow that is induced by the leading-edge vortices. © 2011 Springer-Verlag.

The present study reports on a series of direct numerical simulation of turbulent rotating channel flows at rotation numbers higher than those previously reported. The bulk Reynolds number (Re) was set approximately constant at 5800, while the non-dimensional rotation number Ro was increased up to 5.79. The flow visualization showed that the structure of the enlarged eddies on the pressure side, and the orientation of the structure was inclined towards downstream. The investigation of the budget of Reynolds stress transport equation indicated that the pressure transport of Reynolds stress and re-distribution term were extraordinary large at higher Ro. This led the conventional decomposition of velocity-pressure correlation terms to being not appropriate in this flow. It is suggested that the Reynolds stress modeling should better target the correlation between velocity and pressure gradient, or pressure diffusion and re-distribution terms must be handled with the same efforts, which indeed increases the task of modeling. Copyright © 2007 by ASME.

非対称平面ディフューザに発生する剥離流れについて、レイノルズ数を系統的に変化させてLDV計測を行い、その影響を調べた。

レイノルズ応力方程式モデルを複雑乱流に適用する際に問題となる数値解析的不安定性を解消するスキームを開発し、その検証を行った。(6頁)

平板境界層に発達する固気混相流の境界層において、熱伝達特性と流れ場の性格及び固体粒子群の挙動との相関を実験的に解明した。(8頁)

Turbulent transport of Reynolds stress by triple moment of fluctuating velocity and pressure-velocity correlation are evaluated from a DNS for wake of a rectangular cylinder located in a uniform flow. The turbulent diffusion transport of turbulent kinetic energy hardly correlates with its gradient vector, indicating that the generally accepted gradient diffusion model is inadequate. It is inferred that the separate modeling of the individual Reynolds stress component provides a better possibility for modeling the turbulent transport in this flow, because of the strong departure from the equilibrium state of turbulence. The possibility of the direct modeling of the pressure gradient-velocity correlation is discussed.

ピンホール型LGMによる乱流境界層の壁面せん断応力計測に際し、DNSによる速度データを用いて統計量の健全性を評価した。

回転多層円盤間流れの空力学的不安定性と円盤の弾性振動の関係について種々のパラメータを変化させ実験を行い、両者の相互作用からなる連成振動系のメカニズムを考察した。

マウスの肺胞が疾患を持つ場合と正常の場合で圧力インピーダンス特性が異なることに着目し、新生児の診断への応用について考察した。

回転境界層内熱伝達特性を、主流が層流と乱流の場合について実験により調べ、縦渦の発生による熱伝達の促進は乱流状態では見られないことを明らかにした。

周期的なかく乱をバックステップ乱流に導入した際に再循環領域に発達する渦構造を、周期平均処理を施したPIVデータから同定し、乱流応力の生成における役割について考察した。

回転風洞内に発達する層流境界層の熱伝達特性を感音液晶を用いて計測する手法を開発し、コリオリ力により発生する縦渦構造の存在により強く影響を受けることを実験的に明らかにした

Airfoil flow separation control using plasma actuator driven by repetitive nanosecond pulse voltage was experimentally investigated. The pressure distribution on an airfoil surface was measured by means of a liquid manometer, which is free from electromagnetic interference of the plasma. By integrating the pressure distribution, the lift coefficient was computed and the effects of the input voltage amplitude and repetitive frequency were evaluated. The results show two different manners of the lift increment depending on the angle of attack. At the pre-stall and stall angle, the flow is steady and the lift increment does not depend on the frequency. A strong hysteresis effect is also observed, i.e., once the lift increases due to the plasma actuation, it is still increased even after the actuation stops. At the post-stall angle, the flow is unsteady and the lift increment becomes significant with actuation at frequencies related to inherent flow instabilities for the shear layer and wake, which are determined by the spectrum analysis based on hot-wire measurements. At the preference frequency ranges, there is a certain voltage amplitude depending on the angle of attack, at which the extended plasma layer results in an optimum increase of lift. © 2014 Springer-Verlag Berlin Heidelberg.

Simultaneous measurement of fluctuating velocity and pressure by a static-pressure probe and a hot-wire probe was performed in the near wake of a circular cylinder, in order to strengthen reliability of the measurement technique. Effect of geometry of the static-pressure probe was systematically investigated, and validity of the measurement results was addressed by quantitative comparison with reference data by a large-eddy simulation. Interference between the probes was found to mainly depend on the diameter of the pressure probe and only weakly on the length. A certain time lag between the velocity and pressure signals was detected in the experiment, and the measurement results of velocity-pressure correlation up ̄ and vp ̄ obtained with the correction of the time lag were in good agreement with the computational results. It was also found that the measurement of vp ̄ is extremely sensitive to a small time lag between the velocity and pressure signals, while that of up ̄ is not. © 2014 Springer-Verlag Berlin Heidelberg.

The complex flow features inside hard disk drive models are investigated in an axisymmetric and a semi-open shroud configurations. For the axisymmetric case, we have employed both experimental and computational approaches. The experiment focuses on both flow dynamics and the disk vibration, where measurements of the fluctuating pressure and velocity are undertaken at some representative points. The correlation between the disk vibration and the fluctuating pressure in the turbulent flow between disks is evident from the spectral analysis. The experimentally observed fluctuating pressure and velocity are partly due to the disk vibration and its contribution could be estimated by comparing the experiment with the results of a large eddy simulation. For the semi-open shroud case, although the characteristic peaks attributable to the large-scale vortical structure are still observed in the power spectra, the pressure fluctuation and the disk vibration are suppressed when the arm is inserted. © 2011 Elsevier Inc.

The flow in the tip clearance of a hard disk drive model has been investigated with laser Doppler techniques. The flow was driven by co-rotating disks inside a cylindrical enclosure in order to simulate a hard disk drive used for data storage devices. The main focus of the investigation was on the understanding of complex flow behavior in the narrow gap region between the disk tip and the outer shroud wall, which is supposed to be one of the causes of flow induced vibration of the disks. Experiments in the past have never been able to examine this region because of the lack of the spatial resolution of sensors in the highly three-dimensional flow in the region. In the present investigation, the flow velocity in the tip clearance region was measured with optical measurement techniques for the first time. The flow behaviors are investigated for four different conditions with two different gap widths and two different shapes of the shroud walls with and without ribs. The velocity measurements were taken both with conventional laser Doppler velocimetry and using a laser Doppler velocity profile sensor with a spatial resolution in the micrometer range. The circumferential velocity component was measured along the axial and radial directions. The steep gradients of the circumferential mean velocity in both directions were successfully captured with a high spatial resolution, which was achieved by the velocity profile sensor. From the supplementary investigations, the existence of vortex structures in the tip clearance region was confirmed with a dependence on the shroud gap width and the shroud shape. The interactions of the two boundary layers seem to be the source of the complex three-dimensional behaviors of the flow in this region. © 2010 Springer-Verlag.

A novel experimental technique to measure static pressure fluctuation was applied in order to evaluate velocity-pressure correlation in a turbulent mixing layer. The developing region of the mixing layer was found to be out of equilibrium state, indicating the oscillatory velocity fluctuation due to vortex shedding and remarkable production of turbulence. The contribution of pressure-related terms in the transport equation of the Reynolds stress is found to be well captured by the present method. A simple model for pressure diffusion by Lumley gave a satisfactory approximation when the flow approached the self-similar state. However, the analogy between pressure diffusion and turbulent diffusion becomes weaker when the flow exhibits characteristics which are not found in the equilibrium state. © 2006 Elsevier Inc. All rights reserved.