論文

一様等方性乱流場を渦法により計算し、従来行なわれることのなかったDNSとの直接的な比較を通じて、双方の手法の比較を行なうとともに、渦法でこれまで評価が十分にされなかったエネルギー保存性などについて検討を加えた。

従来の代数応力モデルを回転系に適用した際に問題となる座標系依存性について調べ、特に壁面近傍における拡散項の振る舞いに大きな問題が発生することを示した。また、改良モデルを提案し、DNSデータベースによる評価を行なった。

完全発達したチャネル乱流にスパン方向に軸を持つ回転が加わった場合に発生する乱流構造の変化について系統的なDNSを実行した。従来行なわれることのなかった高い回転数での結果によると、スパン方向と流れ方向に成分を持つ強い渦構造が発達することが認められた。

固気混相乱流中における固体粒子の運動を評価する目的でLESを実行し、主流中にせん断が含まれる場合に発生する浮力の効果について詳細に調べた。

For realistic turbulent flow simulations, quantitative representation of turbulent flow dynamics is desired. In the present study, two-dimensional homogeneous isotropic turbulence is simulated by using a grid-free vortex method to focus on the viscous dissipation process. The results are compared with those of spectral DNS. Two viscous diffusion models, i.e., a core spreading model and Moving Particle Semi-implicit(MPS) Laplacian model, are compared. For the former model, merging and insertion of particles are incorporated to ensure uniform distribution of vortex elements. It is shown that the MPS Laplacian model is superior to the conventional core spreading model in terms of the decay rate of enstrophy and energy spectra. Furthermore, the computational time is remarkably reduced by using a Fast Multipole Method(FMM), while retaining accuracy.

慶應義塾大学大学院理工学研究科に設置された先端科学技術国際コースの事例を紹介し、その経験に基づき日本への留学生の質を確保するために必要な事項について論及した。

We developed a technique for the measurement of fluctuating hydraulic-pressure at an arbitrary point in turbulent flow. A pressure-measuring system was composed of a miniature static-pressure probe (SP-probe), a strain-gagetype pressure transducer, and a tube connecting them. A model equation of dynamic response of the pressuremeasuring system was introduced, and a calibration technique for determining parameters in the model equation was developed. Effects of the size of the SP-probe, length of the tube, and sensitivity of the pressure sensor were systematically investigated using several SP-probes, tubes, and pressure transducers with different sizes, lengths, and sensitivities, and fluctuating hydraulic-pressure measurement was performed in a wake of a circular cylinder

The paper introduced a ducted water curent turbine triple helix with very low head less than 2 m and water current at river or in the ocean. The expected power is between 100 kW and 1000 kW. The differ-like passage was installed in a water tunnel and its effect was investigated both experimentally and numerically. The PIV measurement provided a useful information to evaluate the relationship between the power coefficient and flow field behind the turbine.

The flow field generated by a sinusoidally flapping rigid flat plate was measured using stereo particle image velocimetry to estimate the pressure field on two cross sections of the flapping plate. The pressure field was estimated by integrating the three–dimensional Poisson equation derived from the Euler equation. The solution of this Poisson equation is obtained as a function of the measured phase-averaged velocity field. Simultaneous to PIV measurements, the torque on the flapping axis, which depicts the forces acting on the wing, was measured using strain gauges. The maximum torque coefficients, observed from both methods, of each cycle are delayed from the maximum acceleration of the plate. One of the factors of the phase delay is attributed to the separation of vortex structure along the edges of the plate. After the separation point the increase of pressure drag is delayed relative to the flapping motion. At the same phase, vortex structure is generated on the leeward side of the plate at the free end. Low pressure region was observed on the leeward side of the plate which further add to the pressure drag.

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 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 torque estimation, although only on a partial surface of the plate, is within reasonable agreement with the measured torque.

We have developed a probe-system for simultaneous measurement of three velocity components and pressure in turbulent flows. A miniature total pressure probe is placed adjacent to the sensors of a triple hot-film probe in order to achieve the spatial resolution which is equivalent to that of the triple hot-film probe itself. The instantaneous static pressure is calculated from measured velocity and total pressure by means of a newly developed processing method based on the Bernoulli equation for unsteady flows. The measurements were undertaken in a turbulent wing-tip vortex flow. The look-up table method is employed for the calibration of the hot-film probe so accurate velocity data could be obtained over a wide range of the flow-attack angles. It is also demonstrated that the present probe-system is capable of measuring fluctuations in both velocity and pressure in the 20-650 Hz frequency range. The distribution of the fluctuating pressure obtained by this indirect method is in good agreement with the results from direct measurements of static pressure, demonstrating the promising performance of the present method. Furthermore, an improvement in the ability to make measurements of the velocity-pressure correlation across the wing-tip vortex is achieved. This improvement is possible because the effects of lateral velocity components are properly taken into account in the present formulation. The investigation regarding the transport equation budget for turbulent kinetic energy shows an anomalous structure of turbulence in this flow, mainly due to the meandering of the vortex, and the measurement of pressure diffusion is found to play an important role in the characterization of this kind of flow. © 2010 Springer Science+Business Media B.V.

We have developed a probe-system for simultaneous measurement of three velocity components and pressure in turbulent flows. A miniature total pressure probe is placed adjacent to the sensors of a triple hot-film probe in order to achieve the spatial resolution which is equivalent to that of the triple hot-film probe itself. The investigation regarding the transport equation budget for turbulent kinetic energy shows an anomalous structure of turbulence in this flow, mainly due to the meandering of the vortex, and the measurement of pressure diffusion is found to play an important role in the characterization of this kind of flow.

Vortex methods are a group of Lagrangian and semi-Lagrangian methods based on the vorticity-streamfunction or vorticity-velocity formulation of the Navier-Stokes equation, and provide an interesting alternative to grid based methods for external flows dominated by unsteady vortical motion. In the present review article, we will assess the advantages and disadvantages of vortex methods for the simulation of incompressible turbulent flows, based on the speed and accuracy benchmarks that have been performed recently. Our goal is to objectively and quantitatively evaluate the performance of this non-standard method, by directly comparing it's speed and accuracy against finite difference and pseudo-spectral DNS codes under identical calculation conditions. We also present examples of vortex methods in engineering applications of turbulent flows.

The fast multipole method (FMM) for periodic boundary conditions is extended to periodic boundary conditions with shear. Using this shear periodic FMM, the pure Lagrangian vortex method is applied to the calculation of a homogeneous shear turbulence, and compared with a finite difference code using identical calculation conditions. The isosurface plots and the anisotropy tensors reflect the ability of the present vortex method to handle strongly sheared turbulence.

We have performed experiments in a turbulent mixing layer with periodic forcing introduced by a Piezo Film Actuator (PFA). Three dierent lengths of PFAs have been used, and the eects of various combinations of forcing amplitudes and frequencies are investigated. By forcing at the natural and its first sub-harmonic frequencies, two counter-rotating vortices are clearly observed in one period of forcing. By forcing at second sub-harmonic frequency, the vortical structure is found only in the downstream region.

乱流境界層における局所壁面せん断応力の光学的計測方法を提唱し、光散乱理論に基づき性能評価を行った。

単一の渦輪が平板に衝突する際に、平板上で2次渦輪が発生することがその後の流れ場の発達に影響を与えることを流れの可視化とPIV計測によって明らかにした。

3次元一様等方性乱流にたいして渦法を適用し、周期境界条件の取り扱い、粘性散逸の評価により計算方法の健全性を評価した。

乱流壁面せん断応力計測用ホットフィルム周辺の乱流場をDNSで解析し、ホットフィルムの動特性におよぼす表面物性の影響、乱流構造と特徴的な信号の関係などを明らかにした。

パルス状の信号で発生させた単一の渦輪が平面に垂直に衝突する際に形成される流れ場について流れの可視化とPIV計測により考察した。