研究発表

離散渦法により3次元キャビティー流れの解析を行った。渦法は物体周りの流れのような自由境界を有する場への適用が主に行われており、固体壁で囲まれた領域の解析における性能評価が十分に行われてこなかったところ、固体壁面境界の取り扱いの重要性について新たな考え方を示した。

The internal separated flow associated with remarkable mean secondary flow is investigated. The numerical investigation indicates that the commercially available RANS model group and LES provide obvious difference in terms of the prediction of the flow reattachment and recovery. The stereo PIV measurement indicates over-proportional magnitude of the velocity fluctuation across the mean flow direction, and it is obviously challenging for LES to capture the extremely complex flow pattern. The recirculation region in either side of the orifice seems to be the source of the streamwise vortex that governs the development of the flow near the flow reattachment point. The considered flow serves as a useful test case for both RANS modeling and LES.

Simultaneous measurement of the fluctuating pressure and velocity field is performed using a miniature static-pressure probe and a stereo-PIV. The static-pressure probe is placed just behind the velocity measuring plane illuminated by the laser sheet, and the instantaneous velocity field and the fluctuating pressure are measured simultaneously in a turbulent wing-tip vortex flow. The instantaneous pressure field is estimated by numerically solving a discrete two-dimensional Poisson equation for pressure. The result is compared to directly measured pressure for validation.

乱流中で速度変動と圧力変動を同時に計測する手法を開発し、その性能評価を行った。速度変動と圧力変動の相関係数を求めたところ、主流方向成分と圧力変動の相関よりも横方向速度成分と圧力変動の相関のほうがプローブの形状等の影響をより強く受けることを指摘し、その対策について調べた結果を報告した。

HDDモデル内部で発達する乱流場において圧力センサーを用いた変動圧力計測を行った。ディスクの表面と裏面に2本のセンサーを配置して同時計測を行うことで、ディスクの表裏の圧力差を求めた。圧力差の変動スペクトルとディスク振動のスペクトルとの対比により、圧力差が加振力として作用する可能性を示した。

完全発達した管内流が半円形のオリフィスにより剥離・再循環領域を形成する流動場を、ステレオPIVにより計測し、高速度カメラによる撮影で可視化を行った。内部剥離は二次流れを伴う複雑な様相を呈すること、オリフィスの前後に形成される再循環領域から発生する渦が二次流れの原因となることなどが明らかにされた。

Complex flow inside a hard-disk drive (HDD) was investigated using a simplified 3.5” model to clarify the mechanism causing flow-induced vibration. In contrast to most HDD studies in the past, our model had a non-axisymmetric geometry equipped with a shroud opening and a read/write arm (RWA). The model is designed to serve as a benchmark to study HDD flows both in experiments and in numerical simulations. The secondary flow behavior was likely caused by the shroud opening which wasn’t used in most of the models in the past. In experiment II, the mean velocity and velocity variance were successfully measured through examination of the flow above and below the RWA in the space between the rotating disks. The resulting velocity statistics show turbulent Couette-like flow in the narrow 1 mm space between the disks and the RWA. In addition to the flow measurements, deflections of components in the model were evaluated during operation, which will provide precise boundary conditions for future numerical simulations.

We report a detailed measurement of flow velocities in a simplified, transparent 3.5” hard-disk drive (HDD) model.The simplified geometry of the new model allows systematic flow investigation both in experiments and in numerical simulations. A focus is put on the flow behaviors in the disk-to-disk space with the RWA being inserted. The velocity statistics obtained in the narrow space of 500 μm by means of a novel laser Doppler velocity profile sensor are presented and discussed in this extended abstract.

Large eddy simulations of the turbulent flow between shrouded co-rotating disks, representing a simplified model of hard disk drives, are performed. The computation domain extended a region covering the both sides of a disk, so that the fluctuating pressure field surrounding the disk is simulated. Besides, the influence of the shroud geometry is investigated by comparing the flat shroud wall and the one with the ribs. In the case of flat shroud, the fluctuating pressure on the upper- and lower-surface of a disk indicated a strong correlation with the fluid motion travelling across the disk-tip clearance. In the ribbed shroud case, however, the organized flow structure that is observed in the flat shroud case disappeared, and the fluctuating pressure acting on the surface of the disk is remarkably diminished.

The vortical uid motion caused by a heaving elastic plate was experimentally investigated. The thin plate was oscillated in the direction normal to its plane in the water at rest. The vortices generated at the tip of the plate were observed at the mid-span plane and the velocity eld was measured by means of a particle image velocimetry (PIV). The measurements were undertaken at different frequencies of the oscillation relative to the resonance frequency of the plate. As a result, phase-averaged momentum ux through these control surfaces illuminated the effect of relative frequency.

渦度場の時間発展に着目する渦法では、渦度分布を有する渦要素の重ね合わせで渦度場を表現し、渦要素の運動をラグランジュ的に解析する。外部流れのように渦度が領域内の限られた範囲にだけ存在する場合に有利であるが、反面、領域全体が渦度で満たされる内部流れではその検証が十分になされていない。本研究では基礎的な問題として時間とともに発達するチャネル流れを取り上げ、渦度輸送方程式では取り扱うことのできない圧力勾配項を合理的に表現する手法について考察する。

過去の実験から得られた知見に基づき，シュ ラウド壁面が平滑なものと，リブ付きシュラウドの2 つのモデ ルについて積層回転円盤間に発達する乱流のLES を行い，流体 運動と圧力変動について知見を得ることを目的とする．その際， 1 枚のディスク上下の両面に発達する流れ場の相互干渉を再現 するため，計算領域を3 枚のディスクに囲まれた2 つの隣接す る領域とした

半円オリフィスを含む管内流は，パラメータが管内径口のみ で表わされる単純な形状でありながら，二次流れやはく離・再付 着が発生する複雑な流れとなることがわかっており，また，流 入・流出境界条件が一意に定まるため，乱流モデルの評価とし て適切なテストケースになると期待される．

昆虫や鳥類の羽ばたき飛行における翼の柔軟性の効果は，エ ネルギー効率向上，MAV の機構単純化の観点から，研究が盛ん に行われている．本研究では，単純な翼のモデルとして並進 往復(ヒービング) 運動する弾性平板を用い，周囲に誘起される 流れをPIV で計測することで，平板の柔軟性の効果を調査する．

Flow inside a hard-disk drive (HDD) is investigated using a simplified 3.5” HDD model with read-and-write arm (RWA) and shroud opening. The model was designed to serve as a benchmark of HDD flow investigation both in experiments and in numerical simulations. In the present study, complex flow behaviors in the narrow disk-to-disk space are focused with the RWA inserted into the space. Especially, the flow behavior near the RWA is of great interest. The velocity statistics are acquired at four different locations using a velocity profile sensor with a spatial resolution of 5 μm along the axial direction. The measurement was carried out at the disk Reynolds number ReD=2.4×105, which corresponds to the rotation speed of 7700 rpm at a real 3.5” HDD for desktop computers. The velocity profile sensor successfully resolved the velocities between the narrow areas without disturbing the flow.

The present study considers a reformulation of the Durbin’s (1993) ERM-based second-moment closure model aiming at reduction of the numerical stiffness originating from the wall boundary conditions. The reformulation performed represents an analogy to the procedure Hanjalic et al. (2004) proposed when deriving the eddy-viscosity-based zeta-f model. The presently reformulated model alternatively solves the transport equations for the ratio of the Reynolds stress components to turbulent kinetic energy zeta_ij = u_i u_j/k, instead of equations governing the Reynolds stress tensor. It is believed that the boundary conditions of the newly derived elliptic relaxation equations will contribute to the numerical robustness of the model with respect to the immediate wall vicinity. Another advantage, analogously to the Hanjalic’s et al. zeta-f model, is the appearance of the Reynolds stress production rate (representing an exact formulation) in the zeta_ij equations instead of dissipation rate epsilon (originating from the corresponding model equation). Application of the present model formulation to a plane channel flow in a range of Reynolds numbers up to Re_t = 2003 results in a good agreement with available DNS database.

An experimental investigation is conducted into the effect of the shape of a miniature static-pressure probe on the velocity-pressure correlation measurement. Ten static-pressure probes, with systematically-changed probe diameters and lengths, are compared in combination with an X-type hot-wire probe to examine the accuracy of the simultaneous measurement of the fluctuating velocity and pressure. The effect of the probe shape on the distance between the probes, which is necessary to avoid interference, is also investigated for the various pressure probes, and it is found that the change in probe diameter has the primary influence on the probe interference. In the velocity-pressure correlation measurements, the results of the pressure fluctuation p' and the velocity-pressure correlation up are not affected by the probe shape. On the other hand, it is indicated that the measurement of vp is extremely sensitive to the configuration of the pressure holes including its orientation.

The gap flow in the tip clearance of a hard disk drive model has been investigated with optical measurment techniques. The flow was driven by co-rotating disks inside a cylindrical enclosure in order to simulate a hard disk drive for data storage devices. The main focus of the investigation was the understanding of the complex flow behaviors in the narrow region between the disk tip and the outer shroud wall, which is supposed to be one of the causes of flow induced vibrations of the disks.