論文

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 ζ − f model. The presently reformulated model alternatively solves the transport equations for the ratio of the Reynolds stress components to turbulent kinetic energy ζij = uiuj/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. ζ − f model, is the appearance of the Reynolds stress production rate (representing an exact formulation) in the ζij equations instead of dissipation rate ε (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 Reτ = 2003 results in a good agreement with available DNS database.

Simultaneous measurement of the fluctuating pressure and velocity field is performed combining 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 with directly measured pressure for validation.

As a part of the laboratory experiment on a microburst, the circumferential instability and the associated instantaneous velocity field of a vortex ring impinging onto a normal wall have been investigated. Vortex rings have been generated at the circular orifice by the impulsive motion of the speaker corn and have been visualized by filling ethylene glycol smoke. The instantaneous velocity fields induced by a vortex ring were measured by employing the PIV in which the cross correlation method was used. The wave number of the circumferential instability increases with an increase of the initial Reynolds number. The wave number at the same initial Reynolds number decreases as the distance from the orifice increases. It was shown that the development of induced secondary vortex ring influences the change of velocity vector fields.

As a part of the laboratory experiment on an isolated microburst, the instantaneous velocity fields of an impulse-driven vortex ring impinging onto a normal wall were investigated. Vortex rings were visualized by filling ethylene glycol smoke. In this paper, as the sequel of the previous paper (Nagata et al., 2000), by employing PIV, the vertical cross-section of the velocity fields was reported. It was shown that the central downward flow as well as the entrainment above the vortex ring agrees qualitatively with numerical simulation of atmospheric microburst (Proctor, 1984). The roles of vortex ring in the velocity field of microburst was discussed. The time evolution of the total circulation in vertical cross section could be fitted to the theoretical equation for that of incompressible viscous vortex.

Experimental study on the turbulent flow in a circular pipe with a semi-circular orifice has been conducted. The velocity was measured by a stereo PIV system that provided the three components of mean velocity in as well as six components of the Reynolds stress on the cross section of the pipe at different streamwise locations. The oncoming flow was confirmed to be a typical fully developed turbulent flow. The flow behind the semi-circular orifice was found to be strongly turbulent, associated with secondary flow. The ambiguity in the experiments was carefully addressed, and it was found that the measurement conducted either with or without the orifice was found to be equally accurate.

In our present study, we compare the turbulent statistics obtained from a vortex method calculation with a finite difference calculation for the turbulent channel flow. The present vortex method is a standard vortex particle method, which uses the core spreading method with spatial adaptation [1] as the diffusion scheme. The Reynolds number was Reb = 5600 and the calculation domain was 3δ × 2δ ×3 δ ×, where δ is the channel half width. The mean velocity profile after one wash-out time is shown in Fig. 1(a), while the mean velocity gradient and Reynolds stress after one wash-out time are shown in Fig. 1(b). The results of the vortex method show an excessive production of Reynolds stress in the viscous layer.

The present study aims at clarifying the turbulent structure developing in a hard disk drive through the measurement of fluctuating pressure on the shroud wall. The measurements have been conducted in two different models, with and without read-write arms, and the results are compared to characterize the effect of the internal structure of turbulent flow on the device. The effect of read-write arms is found to be of primary importance, as it changes the flow structure drastically, with the increasing the arm insertion angle. The most pronounced effect has been found in the gradual change of the characteristic frequency in the power spectrum density of the pressure fluctuation: Both qualitative and quantitative changes are observed.

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.

Complex flow inside a hard disk drive (HDD) was investigated using a simplified 3.5″ model for clarifying the mechanism causing flow-induced vibration. In contrast to the authors' related study 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 complex flow behavior in the disk-to-disk space was investigated with the RWA inserted into the inter-disk space. Flow measurements were carried out with a test rig which consisted of transparent disks, RWA and covers. The measurements were performed at the disk Reynolds number Re d = 1.2 × 105 which corresponds to the rotation speed of 7,700 rpm of a real 3.5″ HDD. Two sets of the flow measurements were performed - the first Reynolds stress components measured along four different lines with the RWA inserted at a shallow angle (experiment I), and the other mean and rms velocity statistics along several selected lines with two different RWA insertion angles (experiment II). The mean velocity and velocity variance were obtained at a spatial resolution of 30 μm along eight different lines perpendicular to the disk surfaces. The high spatial resolution of the results was achieved using a laser Doppler velocity profile sensor with a physical resolution in micrometers and a velocity uncertainty of 0.2 %. In the experiment I, the mean velocity and velocity variance statistics were mostly consistent with the common findings in other studies using axisymmetric models except for the flow behavior in the radial direction at the shroud opening. The secondary flow behavior was likely caused by the shroud opening which was not included 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 exhibit turbulent Couette-like flow in the narrow 1 mm space between the disks and the RWA. © 2013 Springer-Verlag Berlin Heidelberg.

In this study, the effects of Fine Particle Peemng (FPP) treatment on adhesion and proliferation in L929 fibroblast were investigated. FPP treatments were performed on the Ti-6A1-4V alloy specimen by using Ti0 2 (T series) , glass beads (G series) and Alumina (Al series) shot particle. The polished specimen (P series) was also prepaid. Cell culture tests on the specimen were carried out with L929 fibroblast after 6 hours, 1, 3 and 5 days culture. Cell proliferation was evaluated by counting the number of cells by MTT assay. Cell adhesion strength was also measured by counting the number of cells detached by fluid-induced shear stress. The FPP treated surface by using shot particle of varied materials, which have different surface character. The Ti02 series showed good cell proliferation. This was because the protein adsorption of Ti0 2 series encourage cell adhesion. The detachment cells on the specimen were observed after share stress loading. The FPP treated series showed good cell adhesion. This was because the microscopically surface topography encourage cell adhesion.

Bluff body calculation was accelerated by using a special-purpose computer, MDGRAPE-2, that was exclusively designed for molecular dynamics simulations. The three main issues were solved regarding the implementation of the MDGRAPE-2 on vortex methods. These issues were the efficient calculation of the Biot-Savart and stretching equation, the optimization of the table domain, and the round-off error caused by the partially single precision calculation in the MDGRAPE-2. Finally this technique was applied to calculation of flow around a circular cylinder. The drag and lift coefficient was investigated to check the validity of the proposed method.

Reduction of skin-friction drag in a channel flow by traveling wave-like surface heating/cooling is investigated by means of linear analysis and direct numerical simulation. The linear analysis reveals that a downstream traveling wave can reduce the skin-friction drag. Due to non-quadrature between the streamwise and the wall-normal velocity disturbances induced by the buoyancy force, negative Reynolds shear stress is created in the regions near the walls, which contributes to the skin-friction drag reduction. The direct numerical simulation shows that a traveling wave at a high wavenumber slightly reduces the friction drag in a fully developed turbulent channel flow. The velocity fluctuations and the Reynolds shear stress are slightly attenuated by this control. The friction drag increases for larger magnitudes and lower wavenumbers of the traveling waves. Non-zero mean temperature profile is created due to the turbulent heat flux induced by the control.

渦法における粘性拡散モデルの性能評価を目的として、これまで計算例がない一様等方乱流のシミュレーションを行った。乱流エネルギーおよびエンストロフィーの散逸率ならびにスペクトルの分布を調べ、従来頻繁に使用されてきたコアスプレッディングモデルに欠陥があることを指摘し、改良方法を提案した。

We present a new class of C∞-smooth finite element spaces on Cartesian grids, based on a partition of unity approach. We use these spaces to construct smooth approximations of particle fields, i.e., finite sums of weighted Dirac deltas. In order to use the spaces on general domains, we propose a fictitious domain formulation, together with a new high-order accurate stabilization. Stability, convergence, and conservation properties of the scheme are established. Numerical experiments confirm the analysis and show that the Cartesian grid-size σ should be taken proportional to the square-root of the particle spacing h, resulting in significant speed-ups in vortex methods.

An experimental study aiming at lift enhancement of a swept wing by using a plasma actuation driven by repetitive nanosecond-pulse voltage was carried out. The periodic plasma actuation was applied near the apex in order to manipulate the leading edge vortices in the post-stall regime. Wind-tunnel tests using a full-span wing model were conducted for various angles of attack α at a Reynolds number Re≡CmU∞/ν=6.0×105, which was based on the effective chord length Cm and freestream velocity U∞. The repetitive frequency of plasma excitation f was also varied from 20 Hz to 1000 Hz. The aerodynamic forces and moments were obtained as a function of α and f by means of a six component balance. The forces and moments were also compared from asymmetric and symmetric actuation configurations point of view: actuations on one side and on both sides of the leading edges. The increase of the time-averaged lift coefficient CL¯ from 0.807 to 0.842 was observed when the symmetric actuation was applied at the reduced frequency of F+≡fCm/U∞≈0.86. The increase of the lift coefficient was strongly correlated with the increase of drag and pitch-up. The frequency dependency of the lift increase was affected by α. Also, the frequency dependency was changed if asymmetric actuation on a leading edge was applied. The velocity field in the wake was measured by means of stereoscopic particle image velocimetry and was evaluated based on control volume analysis. The results indicated that the change in the intensity of the streamwise vortex led to the lift enhancement.

An active control method for suppression of the rotating stall in a radial vaneless diffuser is proposed. The activator is driven by the prestall pressure fluctuation detected at the shroud wall of the diffuser. In order to extract the characteristic frequency of the prestall signal, an analogue bandpass filter system as well as a digital signal processor (DSP) system based on an auto-regressive model are adopted and the results obtained using the two systems are compared. The latter realizes a real-time response to the varying frequency of the pressure fluctuation of the prestall. It is confirmed that the critical flow rate is decreased by 12% by the DSP method, yielding a further extension of the operation range of the vaneless diffuser as compared to that of the filter system. The phase angle is found to be an important parameter for the optimum operation of the present control system.

The effects of periodic perturbation on turbulent separated flow in a two-dimensional asymmetric diffuser were investigated. Measurements of velocity fields were undertaken using a LDV, and data reduction was conducted on a time-averaged as well as ensemble-averaged manner that made the mean-periodic-turbulent decomposition possible. Effects of imposed perturbation on the velocity field strongly depend on the frequency of the perturbation, and the optimum frequency ranges between 0.01≤St≤0.04, with St being the perturbation frequency normalized by the channel height at the diffuser inlet and the maximum inlet velocity. At the optimum frequency, the effect of perturbation appears as an organized velocity fluctuation along a separated shear flow, resulting in the enhancement of momentum transport, which is the primary effect of imposed perturbation. An increase in the turbulent Reynolds stress component in the recirculation region is also observed, indicating the secondary effect of the organized fluid motion due to periodic perturbation.

The weak-equilibrium condition, which is the basis in the development of algebraic Reynolds stress models is assessed in the prediction of fully developed turbulent channel flow under the influence of system rotation. The budget of the various terms of the exact transport equation for the anisotropy tensor is evaluated by using a DNS database. Two diffusive transport constraints are evaluated by using the DNS data. The results show that neither of them can hold for the near-wall region. An asymptotic analysis of the near-wall behavior is performed and an alternative form of the diffusive transport constraint is proposed. The analysis shows that the proposed alternative diffusive transport constraint has the potential to improve the predictive ability of the resultant ARSM. © 2008 Springer Science+Business Media B.V.

The internal separated flow associated with remarkable mean secondary flow is investigated both experimentally and computationally. The computational 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. On the other hand, some over-proportional magnitude of the velocity fluctuation across the mean flow direction is found by velocity measurement by stereo PIV. 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.