論文

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.

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. ◦

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.

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.

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.

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.

The experimental study has been conducted on the influence of the periodic perturbation on the separated flow over a backward-facing step. In order to investigate the large-scale eddy motion, which is evolved from the step edge due to the periodic suction and injection, the Particle Image Velocimeter has been introduced. The effect of the perturbation is obvious in reduction of the reattachment length, providing at most the reduction by over 30% against that of the steady flow condition. The investigation on the turbulent transport term in the mean momentum equation reveals the increase in the individual components of Reynolds stress is responsible for the activated momentum transfer across the shear layer, which in turn results in the earlier flow reattachment. The general tendency is in good accordance with other studies in past, though the relative level of the turbulent shear stress is somewhat low. Further investigation on this point is necessary.

In the present study, an experimental investigation is conducted into the effect of the shape of a miniature staticpressure probe on the velocity-pressure correlation measurement. Ten static-pressure probes, with systematicallychanged 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.

For realistic turbulent flow simulation, quantitative representation of turbulent flow dynamics is desired. In the present study, two-dimensional homogeneous isotropic turbulence is simulated by using 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., core spreading model and Moving Particle Semi-implicit (MPS) Laplacian model, are compared. For the former model, marge and addition process 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. Computational time is remarkably reduced by using Fast Multipole Method (FMM) while retaining accuracy.

クラスレートハイドレートの表面と周囲流体中の間に起こる物質伝達率を可視化による推定と数値解析の両面から求め、ハイドレート膜厚と物質伝達率が反比例の関係にあることを明らかにした。

Large eddy simulations of turbulent flow between shrouded co-rotating disks, representing a simplified model of a hard disk drive, are performed. The computation domain surrounds a complete disk and is bounded at top and bottom by half a disk. Therefore, it is possible to compute the fluctuating pressure field surrounding the middle disk. Also, the influence of the shroud geometry is taken into account by comparing a flat shroud wall and a wall with rib chambers. In the flat shroud case, the fluctuating pressure on the upper- and lower-surface of a disk indicates a strong correlation with fluid motion travelling across the disk-tip clearance region. However, in the ribbed shroud case the organized flow structure that is observed in the flat shroud case disappears and the fluctuating pressure acting on the surface of the disk is remarkably diminished. © 2013 Springer Science+Business Media Dordrecht.

Three-dimensional Reynolds-averaged Navier-Stokes (RANS) solutions from CFX were utilized to investigate the leakage flow characteristics in the labyrinth honeycomb seal of steam turbines. At first, the accuracy and reliability of the utilized RANS approach was demonstrated using the published experimental data of the honeycomb seal. It showed that the utilized numerical method has sufficient precision to predict the leakage performance in seals. Then a range of sealing clearances, cell diameters, cell depths, rotation speeds, and pressure ratios were investigated to determine how these factors affect the leakage flow rate of the labyrinth honeycomb seal. The computed leakage flow rate increased with increasing sealing clearance and pressure ratios. Furthermore, the results show that the studied labyrinth honeycomb seal has the optimum sealing performance in the case of honeycomb cell diameter equals labyrinth step width, and the ratio of the honeycomb cell depth to honeycomb cell diameter is 0.93 under the designed condition. The flow pattern of each case is also illustrated to describe the leakage flow characteristics in labyrinth honeycomb seals. © 2010 by ASME.

Effects of clearance sizes on the pressure and leakage characteristics in a shaft labyrinth brush seal were numerically investigated using Reynolds-averaged Navier-Stokes (RANS) and non-Darcian porous medium model solutions. The leakage flow characteristics in the referenced design shaft labyrinth seal of turbomachinery was initially studied using the RANS solution. The initial shaft labyrinth seal was redesigned into the multi-stage labyrinth brush seal. The reliability and accuracy of the RANS and the non-Darcian porous medium model for leakage flow in brush seals were demonstrated through comparison of the experimental data. Then, the leakage flowrate of the redesigned shaft labyrinth brush seal was calculated at four sizes of bristle pack tip clearances (0, 0.1, 0.3, and 0.5 mm) and compared with the referenced shaft labyrinth seal. The numerical results show that the leakage flowrate increases rapidly with increasing clearance between the bristle pack tip and the rotor surface for the shaft labyrinth brush seal. The leakage flow pattern in the referenced labyrinth seal and the redesigned shaft labyrinth brush seal was also illustrated. © 2008 IMechE.

An attempt is made for simultaneous measurements of velocity and pressure in three-dimensional turbulent flows by means of a miniature pressure probe combined with a triple hot-film anemometer. A turbulent wing-tip vortex is chosen for the test case because of the strong fluctuation of velocity and pressure near its core region. Measurements are undertaken in the near field. The static pressure fluctuation and the velocity-pressure correlation are experimentally investigated. Although they showed distribution that is consistent with those in previous reports, the influence of probe interference needs be substantially reduced.

Flow around a circular cylinder having a porous surface is studied numerically by means of direct numerical simulation (DNS) and large eddy simulation (LES). The flow in the porous media is represented by a spatially-averaged (i.e., macroscopic) model. The parameters of the most effective porous media are found from a two-dimensional parametric test at the Reynolds number, based on the free stream velocity and the diameter of cylinder, of Re = 1000. The lift force fluctuation is notably reduced in the case of thicker porous surface. The DNS of three-dimensional flow at Re = 1000 reveals that the porous surface works to stabilize the shear layer. The LES at a very high Reynolds number (Re = 105) shows that the vortex shedding is completely suppressed by the porous media, which supports the experimental observation by Sueki et al. (2007). Simulations at different Reynolds numbers show that the stabilization effect works better at higher Reynolds numbers.

The spatio-temporal pressure-velocity correlation in a turbulent boundary layer is investigated so as to understand the link between pressure fluctuations and turbulent coherent structures. A new experimental set-up is developed to measure the pressure fluctuations at the wall and in the field and, simultaneously, the velocity field by stereoscopic particle image velocimetry. The present measurement area covers the whole boundary layer thickness, and the spatial resolution of the measurement is good enough to assess the representative length scales of the flow. The Reynolds number effect is quantified from the data at Reθ=7300, 10 000, 18 000. The spatio-temporal three-dimensional structures of the pressure-velocity correlations, Rpu, Rpv and Rpw, are evaluated. The wall pressure fluctuations are closely coupled with coherent structures which occupy a large region of the boundary layer in the wall-normal and spanwise directions and up to 10δ/Ue in time, where δ and Ue denote the boundary layer thickness and the free stream velocity. Reynolds number effects are mainly observed on the size and intensity of the pressure-velocity correlations. Conditioning the correlations on the pressure signal sign shows different types of flow phenomena linked to the positive and negative pressure events. For the wall pressure, positive pressure fluctuations appear to be correlated with the leading edge of a large sweeping motion of splatting type followed by a large ejection. The negative pressure fluctuations are linked to a localized ejection upstream, followed by a large sweeping motion downstream. For the pressure fluctuations in the field, in addition to the structures observed with the wall pressure, the pressure-velocity correlations exhibit a significant correlation in a region very extended in time. Such long structures appear to be independent of the one observed at the wall and to grow significantly in time with the Reynolds number when scaling with external variables. When conditioned by the pressure sign, clear ejection and sweeping motions are observed with associated streamwise vortical structures at a scale of the order of 0.2δ. These structures can be linked to the large-scale motion and very-large-scale motion previously observed by different authors and seem to organize in a scheme analogous to the near-wall cycle, but at a much larger scale.

This paper reports on the influence of Coriolis force on the heat transfer characteristics in rotating laminar boundary layer. The experiments have been conducted for the mean flow velocity U8=4.0m/s and the angular velocity Ω=4.6 rad/s, and the heat flux has been applied uniformly over the plate. Liquid crystal method is applied to measuring the surface temperature distribution. The results indicate that heat transfer has been enhanced on the pressure surface. The velcity measurements show that Coriolis instability induces the counter-rotating longitudinal vortices which argument the heat transfer from the pressure surface. On the other hand, the heat transfer from the suction surface remains unchanged as compared to the no-rotating case, which is due to the Coriolis force that stabilizes the boundary layer. As a consequence, the averaged heat transfer coefficient is higher on the pressure surface than that on the suction surface and the stationary wall by approximately 40%.

Turbulent flow in a hard disk drive model which contained two disks and a RW (read-and-write) arm was numerically analyzed at a disk rotation Reynolds number of about 105. The evaluation of the fluctuating torque revealed that the RW arm may be oscillated at a frequency which had only weak correlation with the disk rotation speed. The fluctuating pressure was stronger on the leeward surface of the RW than that of the windward. It was further indicated that the fluctuating torque on the RW arm was weakly correlated with the surface pressure fluctuation on the shroud wall.

Measurement of the turbulent velocity field have been performed in a newly designed low-speed two-dimensional mixing layer facility. Mean and fluctuating velocity in the streamwise direction are measured by an I-type hot-wire anemometer, and the dissipation rate is evaluated from the time derivative of the fluctuating velocity signal based on the Taylor hypothesis and under the assumption of local isotropy of turbulence. The moments of velocity fluctuation up to the third order agree well with previous experiments and DNS, when normalized by the local turbulence velocity scale; the region of self-similarity in terms of the relative turbulence structure is found to be larger than the conventional definition of the global self similarity region where the turbulence velocity scale no longer varies in the streamwise direction. The dissipation rate can also be well correlated with the recent DNS result, when the Taylor micro length scale and local turbulence velocity scale are chosen as the characteristic length and velocity scales, respectively.

An algebraic model of the turbulent transport of Reynolds stress is proposed, based on an examination of the budget of the transport equation for the third-moment of fluctuating velocity. Combined with a basic version of the second-moment closure, the new model is applied to the calculation of a two-dimensional free mixing layer. The results show a slight modification of the conventional gradient transport model for the turbulent diffusion, and yield a better agreement with experimental results in terms of the amplitude of the third-moment. The inclusion of a simple model for the pressure diffusion term indicates the possibility of further improvement of the turbulent transfer process in free flows.