In this study, optimal nozzle shape for a cleaning device to removes micro-meter sized particles is considered. This device ejects high-speed air jet into a solid surface and removes the particles adhering the surface. To improve removal performance, it has a nozzle containing a triangular cavity inducing high turbulence. It is important to select optimal nozzle shape to give high removal performance and reduce nozzle manufacturing cost. For the optimal selection, we propose new nozzle shapes with inverted triangular cavity and estimate the removal performance by numerical analysis. The analysis indicates comparable removal moment between conventional nozzle shape and the new shape. The new shape has great advantage in reducing nozzle manufacturing cost.

The Magnus effect is a phenomenon in which lift occurs in a rotating sphere or cylinder, and it is partially applied in engineering. In addition, a negative Magnus effect has been confirmed in which the lift that should originally be in a uniform direction is generated in the opposite direction. The Magnus effect is caused by the asymmetric separation of the boundary layer due to the difference in relative velocity between the upper and lower surfaces of the cylinder due to rotation. The negative Magnus effect is a phenomenon that occurs when the boundary layer on one side causes a turbulent transition due to the difference in relative velocity between the upper and lower surfaces. Therefore, it is important to measure the boundary layer on the surface of a rotating cylinder to elucidate these phenomena. In this study, developed a hot-wire anemometer probe that can measure the boundary layer, and conducted an experiment to measure the velocity boundary layer around a cylinder. As a result, we were able to measure the velocity distribution and turbulence intensity distribution of a very thin boundary layer generated in a stationary cylinder. And it was confirmed that small scale disturbances occurred locally in the peeled shear layer.

To investigate the effect of Reynolds number on high Schmidt number scalar statistics, we measured the dye concentration fluctuation in liquid phase turbulent axisymmetric jet for various Reynolds numbers. The dye concentration fluctuations were measured by LIF method using the optical fiber probe with a high spatial resolution of 2.5 μm, which is equivalent to the minimum scale of concentration fluctuations. It was found that the concentration fluctuation power spectrum shows the -1 power law in the high wavenumber region regardless of Reynolds number. It was also shown that the relative concentration fluctuation intensity on the jet central axis does not depend on Reynolds numbers at high Reynolds numbers.

The three-dimensional compressible Navier-Stokes equations are numerically solved to study effect of the velocity components of inflow disturbance on suppression of Mach waves radiated from a supersonic round jet at high convective Mach number. Inflow disturbances of only u_r or u_θ or u_x velocity components are compared. The numerical results show that inflow disturbance of only the u_θ component and u_r component than the case of only the u_x component is easily accepted into the jet as the third helical mode. In addition, the jet is efficiently diffused by inflow disturbances of only u_θ or u_r component with small energy, and the radiated Mach wave is suppressed.

In recent years, many researchers have used a plasma actuator as novel flow control device. We provided the micro plasma actuator with electrode size of less than 1 mm. A sinusoidal micro plasma actuator induced streamwise vortex on the surface of airfoil and promoted the momentum mixing between the main flow and the boundary layer. Newly developed multi-stage plasma actuator was composed of four sinusoidal curved electrodes with rectangular and staggered arrangements. On separation suppression, PIV analysis was conducted to investigated the flow field around NACA 63₁-012 airfoil. It was found that the staggered multi-stage plasma actuator had a greater effect on the momentum mixing in the airfoil separation than that of rectangular arrangement. In addition, drag reduction rate measured by wind tunnel balance was higher in the staggered multi-stage curved plasma actuator.

The flow field around moving airfoil capable of flexible elastic deformation has become a focus of attention in recent years. These flow fields may be understood as a fluid-structure interaction (FSI) problem, and the motion and deformation of elastic airfoils, as well as the associated vortex flow phenomena in their vicinity, are complicated. Many studies on the flow filed around the elastic moving body have also carried out. The macro scale vortex structure and the dynamic forces acting on the elastic moving airfoil have been understood. However, the growth process of the vorticity in a vicinity of the wall of an elastic airfoil has not been clarified sufficiently. In this study, the authors investigate the growth process of the vorticity in a vicinity of the wall of an elastic airfoil by FSI simulation using ANSYS 17.0 / ANSYS CFX 17.0. Especially, the authors focus on the relationship between the spatial gradient of the wall of the elastic airfoils and the growth process of the vorticity in a vicinity of the wall.

The sweeping jet from a fluidic oscillator has attracted attention as an active flow control device. Many studies on flow control techniques using a sweeping jet have been conducted. However, interaction between the sweeping jet and the main flow is expected to occur, and a complex vortex structure will be generated. The authors have reported the flow characteristics generated by interactions between the sweeping jet from the fluidic oscillator and the main flow based on stereo particle image velocimetry (PIV) measurement. Finally, the sweeping jet from the fluidic oscillator spreads out over a wider range due to its interaction with the main flow. At the high-velocity rate, V_j/V₀, the high-velocity jets at the center of the sweeping jet are surrounded by low-velocity jets with a strong rotational component at the outside. The purpose of the present study is to clarify the development process of the detailed vortex structure generated by the interaction between the sweeping jet from the fluidic oscillator and the main flow.

The purpose of this study is to clarify the effects of disturbance frequency and Reynolds number on the turbulence characteristics in the wake of flat parallel plates with periodic disturbance. In such a wake, it is known that the dominant frequency of velocity fluctuation in the near wake and it in the far wake are different. Therefore, we decided to visualize the vortex structure in the wake by using the phase averaging method based on each dominant frequency. As a result, it was found that the vortex structures change dynamically in the downstream direction because of vortex merging or disappearance.

A wake without reverse flow like a flat plate wake is a very unstable flow, and the vortex structure in such a wake is affected by various disturbances in main flow. In this study, the vortex formation process in the wake of the flat parallel plates with periodic disturbance was shown by using flow visualization technique in order to control the wake structures by disturbance frequency. As a result, it was confirmed that disturbance frequency, particularly frequency combinations in the case of bimodal disturbance affect wake periodicity, vortex size, vortex shape and downstream location of vortex formation / deformation.

In this research, the relationship between the laminar boundary layer thickness on the end wall and the Karman vortex frequency characteristics is investigated using the autocorrelation coefficient. The diameters of circular cylinder is 9 mm and the aspect ratio is kept at 25. The relative boundary layer thickness is altered between 0.37 and 0.82. The Reynolds number is kept at 3000. The measurement position is 4 times the diameter downstream from the cylinder center.

As the result, the autocorrelation coefficient of the cylinder wake depends on the boundary layer thickness. In the case of thick boundary layer thickness 0.82, the vortex shedding has fluctuations and autocorrelation coefficient decays faster than the thin boundary layer thickness 0.37.

A magnetic suspension system for a wind-tunnel model is developed. The goal of this development is to clarify flow fields around flying body in unstable motion. This system noncontactly supports a wind-tunnel model that includes a neodymium magnet. The model has two-degree of freedom, pitch rotation and translation in lift direction, during magnetic levitation in air flow of the wind-tunnel. This study reports the conceptual design of the magnetic suspension system which consists of magnetic circuit and a feedback control system.

The purpose of this study is to control the jet diffusion by using the deforming nozzle. The exit shape of polypropylene nozzle can change from square shape into cruciform one smoothly and periodically. To catch the flow feature, the flow visualization was carried out by using the Laser Light Sheet and the high-speed videocamera. Moreover the velocity measurement of the jet flow was conducted using a PIV method, by changing the nozzle shape variously. The basic characteristics of the jet flow, such as the velocity distribution and the half value width have been obtained.

As a result, it has been found that the diffusion of the jet issuing from the deforming nozzle whose exit shape changes to cruciform from square and furthermore returns to square periodically and continuously is promoted in the downstream region, comparing with the case of the square nozzle jet.

The study aims to remove the motor to rotate the cylindrical blade which generates the power of the vertical axis type Magnus wind turbine. The driving force is the Magnus force generated by rotating cylindrical blades. In this field Challenergy is now a leading company and has improved power generation efficiency by attaching a flat plate behind a cylindrical wing. Our previous research confirmed the effect of the plate last year. The research also found out two countermeasures to improve power generation efficiency. One is to install protection plates at both ends of the circular cylinder blades to suppress wingtip vortices, and the other is to attach a curved wing behind a cylindrical blade. However, since the present windmill consumes the electric power to rotate the cylindrical blade, this now results in a major cause of lowering the power generation efficiency. Therefore, in this research, we have developed vertical axis type Magnus wind turbine which does not require any power by optimizing the shape of cylindrical blade. The wind tunnel experiment showed that the net power generation of the windmill can be achieved only under the limited condition, taking the power consumption of the motor into consideration. The experimental test found that the turbo sail type is the most effective of several models in removing the motor to rotate the cylindrical blade. CFD analysis clarified the mechanism that the turbo sail type is able to improve power generation efficiency.

When considering a wind farm (or a tidal farm) consisting of lots of vertical axis turbines, in order to obtain the high power density (output/site area), it is very important to get the faster-recovery in flow speed of each rotor wake. Three-dimensional computational fluid dynamics analysis targeting a two-bladed vertical axis wind turbine (diameter: 0.75 m, height: 0.5 m) was conducted to investigate the effects of both the slant rotational axis and arms on the recovery of wake, under the condition of tip speed ratio of 3. The wake of the rotor with the slant rotational axis of 25° was recovered faster than that of the armless rotor. In the case without inclination, the wake recovery appeared to be almost the same between the rotors with and without the arms. In the case with inclination, however, the wake recovery of the rotor with arms was delayed in comparison with the armless rotor. In the case of axis-inclination of 25°, the vertical velocity component of 10% of the main flow was observed in the wake of the armless rotor; on the other hand, the rise in that component was suppressed to 5% in the wake of the rotor with arms.

This work compares long-term measurements of wind data by a profiling Lidar against those obtained from towermounted sonic anemometer. The Lidar-measured wind speeds show good agreement with those measured using the sonic anemometer, with the slope of regression line being 1.0 and R² > 0.99. Comparison of the turbulence intensity obtained from the 90th percentile of the standard deviation distribution shows that the Lidar-measured turbulence intensities are mostly larger (by 2% or less) than those measured by the sonic anemometer. The PDF values of peak wind speed (V^) show that when V^ is larger than 9 m/s, the PDFs are almost the same for both devices. Finally, the wind speed distributions are used to compute power distributions for the NREL 5-MW reference wind turbine. The difference in the occurrence frequency between the sonic anemometer and Lidar measurements for all power production bins is less than 1%. Therefore, the study shows that profiling Lidars can be employed in wind energy applications, as they can measure wind resource with acceptable accuracy.

A twin-impulse turbine for bi-directional flow has been developed for wave energy converter. However, the previous studies described that the mean efficiency of the twin turbine would be much lower than that of impulse turbine for a unidirectional flow, since a portion of airflow gets through the turbine under reverse flow whose efficiency is very low. For that reason, a fluidic diode was adopted in the twin impulse turbine in order to reduce the air flow through the turbine under reverse flow. In this study, we investigated the rectification effect of the fluidic diode when a blunt body was provided with a bypass. Computational fluid dynamics (CFD) analysis were conducted to investigate the effect of fluidic diodes on the turbine performance. In the CFD analysis, RANS equations are used as governing equations and the standard k-ε model is used as the turbulence model. The computational domain is composed of a circular tube and fluidic diode and meshed with an approximately 1.5 million mesh elements. As a result, it was found that the rectification effect of the fluidic diode was enhanced by installing a conical bypass with a bypass width ratio of d/D=0.088 and an angle of 44 degrees of a blunt body.

Wave energy can be converted into electrical energy by using wave energy converter. In a wave energy plant with oscillating water column (OWC), the water column in the air chamber oscillates with the sea wave motion and it causes an alternating airflow inside the chamber. The airflow then rotates an air turbine connected to a generator. A sail wing turbine has been proposed for OWC wave energy conversion by the authors. The sail shape changes in the reciprocating airflow and it always rotates in the same direction. In this study, in order to achieve a further improvement in the performance of the sail wing turbine, the guide vanes were installed before and after the rotor and the effect of guide vane on the performance was investigated numerically by computational fluid dynamics (CFD) analysis under steady flow conditions. As a result, the effect and usefulness of the sail wing turbine with guide vane for bi-directional airflow was clarified. Further, it was found that the peak efficiency η_P=0.455 of the sail wing turbine with guide vane was shown at turning angle of guide vane δ=80° and flow coefficient φ=1.58.

A straight-bladed vertical-axis turbine for wave energy conversion has been proposed in order to develop a novel air turbine suitable for oscillating water column based on wave energy plant. The objective of this study is to show the effect of guide-vane geometry on the performance of straight-bladed vertical-axis turbine. The experimental study was carried out by a wind tunnel. The rotor has four straight blades with a profile of NACA0018, a chord length of 80.5 mm, a pitch diameter of 460 mm and a blade width of 490 mm. In this study, the performances of three types of guide-vanes (i.e. an arc, a semi-arc, and a semi-arc having a straight part) were compared by experiments.

Cylindrical wings with smooth surface, longitudinal grooves and spiral fins are used to clarify the performance of the magnus wind turbine. Number of revolutions of wind turbine was measured by using a wind tunnel at wind speed of 4~40 m/s and a cylinder rotation at 400~1600 rpm. As the results, it was revealed that a spiral fin has a wind turbine rotation with the lowest wind velocity and a longitudinal groove cylinder has a highest rotation number of wind turbine.

We try to solve the problem of efficiency drops on Savonius wind turbine at higher tip speed ratios using twisted buckets, which are expected to increase drag acting on the advancing bucket and to decrease drag acting on the returning bucket. These effects may be given by three-dimensional flow structure due to twisted buckets. Flow visualization were performed around four types of twisted Savonius wind turbine models and velocity distributions were obtained by PIV. Phase-averaged velocity field in the rotating angle range of 0° to 180° was used to reconstruct three-dimensional velocity distribution considering geometric symmetry of the turbine and periodicity of flow structures appearing in each phase of the turbine. Pressure distribution was then estimated by solving Poisson equation of pressure from the PIV data. From the result, two major effects of twisted buckets were clarified; one is modification of the drag force on each bucket, and the other is interaction with effect of the gap between two buckets producing a function to reduce drag on the returning bucket.

If a rectangular cylinder with zero attack angle is used in a wind turbine, it never rotates. However, we found that it can rotate as a wind turbine when a ring plate is located in the wake region. The blade is driven by the lift force caused by a necklace type of the longitudinal vortex formed at a cross section of the blade and the ring plate. In this study, the effect of the form factors, such as a gap between the blade and the ring plate and the attack angle of the blade, on the characteristic of the turbine is experimentally examined. As a result, we confirmed that the rectangular cylinder could be a wind turbine blade and the suitable attack angle to maximize the performance existed.

In order to estimate influence of wake interaction on power coefficient of offshore wind farms, large-scale numerical flow simulations were conducted with LES. The biggest issues of flow simulations for offshore wind farm is to solve the high-Reynolds number turbulent flow at the order of 107 around rotational blades and multi time scale phenomena for the interaction of wake flow and turbulent boundary layer. A large-scale numerical analysis exceeding 600 million elements capable of properly analyzing the vorticial structure of the wake were conducted over a million-time steps with a grid resolution sufficient to realize the turning flow at the blade leading edge. It was found that in the case of the wind turbine is arranged at downstream of 3 times the diameter, the power coefficient of the downstream wind turbine is reduced to about 16% compared to that of the upstream wind turbine.

A wind turbine has been developed to a more sophisticated design that take account of many design factors. For example, the effect of wind velocity fluctuation at the installation site is considered to obtain larger wind power from unsteady wind than one from steady wind. This paper describes the performance evaluation of horizontal axis wind turbine in low turbulent intensity natural wind using an original low-cost multi-fan active control turbulence wind tunnel. In particular, this research focuses on the effects of integral scale of turbulence and blade type on wind turbine performance. The performances of original NACA and MEL wind turbine rotors are evaluated in the natural wind of mean velocity 7 m/s, turbulent intensity 2% with varying integral scale 5 m, 10 m and 20 m. As the results, the power coefficient of the MEL rotor in the natural wind with integral scale 20 m is about 1.1 (110%) times as large as one in steady wind. Furthermore, integral scale has larger effect on the MEL rotor than NACA. In addition, we show the Large Eddy Simulation (LES) results of the flow field around the NACA and MEL wind turbine rotors in steady wind of mean velocity 7 m/s and 1800 min^-1 (tip speed ratio equals to 2.6) using the commercial software ANSYS CFX.

The rotor of Wells turbine consists of several symmetrical airfoils positioned around a hub with their chord planes normal to rotational axis. In general, two-dimensional blade is used as the airfoil of Wells turbine. In the present paper the attention is focused on the effect of plasma actuator on stall characteristics of two-dimensional airfoil. We measured the lift characteristics acted on the airfoil with plasma actuator near the trailing edge of the airfoil. The stall attack angle of plasma actuator was larger than that of normal airfoil. The plasma actuator was effective to improve the stall characteristics of airfoil. This characteristics depended on the actuator location, the electrode length of plasma actuator. The stall attack angle of plasma actuator mounted on the suction side of airfoil was larger than that of pressure side.

In order to reduce the power generation of the CSP (concentrated solar power) plants, an economical and technical solution in the heat storage is a key point. In the previous study, an TES (thermal energy storage) system which using air as HTF (heat transfer fluid) and stone as the storage material have been developed. This paper describes the CFD study on a TES system with stone using high-temperature air as HTF which hybridized with CL-CSP (Cross Liner-CSP). The numerical simulation is performed using commercial CFD software ANSYS Fluent using a pilot-scale TES model. The influencing factors which affect the thermal stratification, such as the porosity and mass flow rate, are taken into consideration. Furthermore, the thermocline thickness is discussed for the CFD simulation result to evaluate the performance of charging and discharging of TES tank.

Magnus wind turbine is a wind turbine that uses Magnus effect to generate electricity. Magnus wind turbine is wind turbine that uses Magnus effect to generate electricity. Magnus wind turbines are unique wind turbine systems that rotate with cylinders instead of using the more common propeller-type blades. In previous study, the lift force was measured by using three models (cylinder with no fins, cylinder with straight fins and cylinder with spiral fins) in a wind tunnel testing and it was confirmed that the high lift force was generated by the cylinder with spiral fins. However, the effect of spiral fins on the flow field has not yet been clarified. In this study, in order to investigate the effect of spiral fins on three-dimensional flow field, the fluid force measurements and flow visualization were performed. The spiral fin causes the spanwise flow attached to the surface of the rotating cylinder and this three-dimensional flow enhances the Magnus effect.

An energy harvesting from ocean current or tidal flow is a promising energy source for accomplishing sustainable development goals (SDGs). It is important to develop a minimal environmental impact system to expand opportunities of the energy harvesting. Although a fluttering power generation from ocean current or tidal flow is one of the methods to convert the flow energy to electricity, there are some difficulties to convert from the fluttering vibration to electricity. In this report, we considered whether it would be possible to convert ocean current kinetic energy into electric energy without any environmental loading. We suggest that a novel idea to convert the vibration induced by current flow to circulating flow to generate electrical energy. The circulating flow was generated with a phase difference between a U-shaped soft tube movement. Furthermore, the flow rate shows increasing as the phase differences between the soft tube. We have also constructed fundamental equations to explain this phenomenon. It is expected that the circulation flow can be induced by fluttering the tube using the vibrational energy obtained from the sea.

In order to clarify the effects of freestream turbulence on blade performance, wind tunnel experiments by using turbulence grids, fractal grids and an active turbulence generators were conducted. The results showed that it is possible to investigate the flow field around airfoil is placed in a strong turbulence field with the inertial sub range in the frequency domain such as an atmospheric turbulence by using the active turbulence generator. In this experimental conditions, the Reynolds number around 10⁵, it was confirmed that when the intensity of turbulence increases, the stall angle increases and the sudden drop in lift after the stall is suppressed. It was also found that the drag increases when the freestream turbulence becomes large. From this result, it can be seen that blade performance of the offshore wind turbines is strongly affect by free-stream turbulence.

In order to investigate effects of free stream turbulence on the aerodynamic performance of the NACA0012 at the Reynolds number 1.48×10⁵, a numerical analysis was performed with a turbulence grids installed upstream of the airfoil. Since the simulation domain has sufficient resolution to reproduce the grid turbulence upstream of the airfoil, it is possible to realize a homogenous isotropic turbulence flow such as the atmospheric turbulence or wake flow of turbines. The generated turbulent flow field showed the same characteristics of turbulence attenuation as the previous study, and the inertia sub-ranges were observed in the velocity fluctuation spectra. In the case of an uniform flow without inflow turbulence, the separation bubbles are formed on the blade suction surface, and there make a step in the pressure distribution near the leading edge. On the other hand, in the case of inflow turbulence exist, the separation bubbles disappeared and the flow around the suction surface is distributed along the airfoil. In this Reynolds number and the angle of attack is 6 °, inflow turbulence increases the lift coefficient and decrees the drag coefficient.

We expanded the blade element momentum theory (BEM) for the prediction of the broadband noise of a horizontal axis wind turbine. For the prediction of the broadband noise, the acoustic radiation from the turbulent boundary layers was applied. From the results of the wind tunnel test, NACA0018 generated the humped noise in the attached flow condition, whereas the noise spectra in the separated flow condition made the broadband noise. In this prediction methodology, the noise level of the wind turbine could be predicted by the model size of the isolated blade and the driving condition of the objective wind turbine. At this time, the relative velocity and the angle of attack became the important parameters. We pointed out that the humped noise source in the wind turbine was made from the mid-span to the blade tip on the impeller based on this methodology.

Wind farms are widely used for the wind power generation, because of the cost benefits. For the design of the farm, accurate estimations of the velocity field in the turbine wake and its effects on the downstream turbines are imperative, especially for the optimization of the wind turbine placement. The present study proposes an analysis method for horizontal axis wind turbines, which can be applied for the estimations, with relatively high accuracy and low calculation load, based on the vortex method. A wake panel emitted from the trailing edge of the blade vortex panel is replaced by a pair of orthogonal vortex elements through a virtual vortex sheet, and those vortex elements are synthesized to a discrete vortex element, which reduces the number of vortex elements, resulting in the reduced calculation load. In addition, further reduction of the elements is applied by synthesizing the vortex elements in the far wake region. The present paper explains the analysis method and shows some calculated results in order to evaluate the validity of the method.

Spinner anemometer is generally an ultrasonic wind direction anemometer mounted on the spinner of horizontal axis wind turbine. Research is currently being conducted as a method to estimate wind direction and velocity far from the wind turbine upstream. However, to estimate upstream wind speed, it is necessary to clarify relationship between wind speed upstream of wind turbine and vicinity of spinner. In this research, the flow field upstream of wind turbine and the flow field around spinner at the time of yaw angle change were measured by laser Doppler velocimeter. From obtained three-dimensional velocity component, we constructed relational expression to estimate upstream wind velocity from flow field observed around the spinner.

Velocity field induced by the wake vortices of the rotor blades definitely deforms the turbulent structure of the incoming wind into the rotor, although the conventional turbulent models usually introduce the time series of velocity fluctuation on the rotor plane directly using the spectrum and the coherence functions for the atmospheric turbulence, as the input for the models. The present study aims to clarify the effects of velocity field induced by the turbine on the turbulent structure of the flow incoming into the rotor, by numerical analysis. The induced velocity field and its spatial gradient in the upstream region of the rotor have been calculated by using the model based on the asymptotic acceleration potential theory, which can deduces the time scales of the induced velocity field. The calculation model can also research the frequency characteristics of the velocity fluctuations, which are experienced by the rotor rotating in the turbulent field. It has been found that, the time scales and frequency characteristics of the flows substantially vary in the rotational and radial directions, and are different from those of the atmospheric turbulence.

The purpose of this research is to improve performance efficiency by changing the blade shape of a horizontal axis type wind turbine driven by longitudinal vortex．It is a wind turbine driven by using longitudinal vortex generated around the vicinity of blades．The driving principle is different from conventional wind turbine，and it has the characteristics of low rotation and high torque．Regarding this research，research on cylindrical wings is underway， but cylindrical wing is in a shape that tends to be subject to resistance in the direction of rotation，and sufficient driving force cannot be obtained．Therefore，we will examine the shape of the wing that can maintain the characteristics of longitudinal vortex and reduce resistance．For this purpose，wind tunnel experiments were conducted using the actual wind turbine．The power coefficient and torque coefficient were determined and evaluated．Also，to clarify the effects of longitudinal vortex and the phenomena around the new wing，the analysis was performed using the numerical fluid analysis software STAR-CCM +．Finally，We can clarified that the shape of the wing can generate a longitudinal vortex while receiving drag from the mainstream．

The environmental problem and related energy problem are recently the serious problem all over the world. The possible use of ocean current as an energy resource begins to draw attention in the renewable energy. In this study the contra-rotating turbines are adopted as an ocean current power generator. Firstly, single rotor horizontal axis ocean current turbine performance estimated by Computational Fluid Dynamics (CFD) is carried out. Predicted results show good agreements with experimental results. Next, the water tank test to investigate the influence of upstream and downstream turbine aerodynamic interaction is performed. The downstream turbine rotation speed is 25 % slower than upstream turbine. Finally, wake flow distributions are evaluated by the CFD analysis of contra-rotating turbine using the result of the water tank test. Faster wake recovery behind contra-rotating turbine is confirmed from the calculated results.

This study focuses on the turbine performance and internal flow field of a newly developed cross-flow turbine which has a cavity and a guide wall. The cavity connects at the edge downstream an outside nozzle wall, and the guide wall connects the cavity. In typical cross-flow turbine, the circumferential velocity component is active in the outflow from a runner. The developed turbine can change the velocity component to radial direction by the cavity and the guide wall for performance improvement by enhancement of differential angular momentum and size reduction. The authors revealed that the relationship between internal flow fields and the runner performance by lab test and CFD analysis. As a result, performance tests revealed the maximum and the partial load turbine efficiency under the conditions of various guide vane opening and runner rotation speed. The efficiency becomes the maximum at guide vane opening ratio of 86%. The minimum partial load efficiency is approximately 45% of the best efficiency for the guide vane opening ratio of 14% when the runner rotation speed is constant. Flow visualization results by the experiment and CFD analysis showed a significant role in changing the outflow direction by the guide wall and cavity.

Wind turbines in cold climates are defined as wind turbine operating in an environment where the temperature is 0 or lower. In this paper, the aerodynamic characteristics is clarified by the airfoil performance test of icing airfoil model. With the use of these results, numerical analysis was carried out to clearly the influence of icing airfoil on wind turbine performance and load. As result of the analysis, the rated power with icing airfoil is obtained at higher wind speed than rated one, and maximum value of thrust with icing airfoil becomes lower than clean airfoil.

Darrieus wind turbine is categorized to vertical axis wind turbine driven by lift force. This study aims to improve performance of the wind turbine by preventing dynamic stall at low tip speed ratios. Turbo-sail airfoils, so-called, is one of the devices which can reduce stall state by blowing thin tangential jet on concave surface of wings. A small model of Darrieus wind turbine which has turbo-sail airfoils as its blade was built and tested in a wind tunnel. Stall controlling performance of turbo-sail airfoils applied for Darrieus wind turbine was investigated by visualizing flow field around the blade. From phase- averaged PIV data, it revealed that dynamic stall vortex was suppressed and attached flow state was achieved by the stall controlling. Pressure field was obtained from phase-averaged PIV by solving Poisson’s equation. It revealed that strong negative pressure area beside suction surface of blade was greatly reduced by the stall controlling. Also, local power coefficient at each visualized phase angle were evaluated and improvement of local power coefficient by the stall controlling was confirmed.

The glazing flow and the bias flow normally increase the acoustic resistance of perforated plate with back cavity. However, in some conditions, the acoustic resistance decreases and the cavity tone may be occurred. In this paper, several examples concerning on such sound and flow interaction are examined and the mechanism of sound attenuation and sound amplification by sound and flow interaction is discussed. When sound wave impinges on vortices, the sound energy is once transferred to vortex energy and attenuated. The intensified vortex generates new sound. Whether the sound is attenuated or amplified, depends on which is larger, the firstly attenuated sound or secondary generated sound. When the frequency of the impinging sound is close to the frequency of fluid dynamic oscillation, the vortices are easy to move and the acoustic resistance decreases. When the acoustic resistance is negative, the fluid resonant oscillation (cavity tone) is occurred.

The longitudinal vortex generated around the front pillar driving is one of the main noise sources of a car. It is very important to clarify the generation mechanism of aerodynamic noise caused by longitudinal vortices. There have been some studies on the aerodynamic noise of the longitudinal vortex, but there still remain sound generation mechanisms to be solved. It was experimentally found that there exist sound sources at the apex and the rear end of delta wing in our previous study. The paper aims to clarify the relationship between frequency characteristics and sound sources. This study employed RANS (Reynolds Averaged Navier-Stokes Simulation) for steady analysis. Unsteady numerical analysis was conducted, based on initial values obtained by RANS. For the accurate simulation, eddy viscosity model is employed as Menter SST k-ω model and turbulent model is LES (Large Eddy Simulation). Total grid number used is 120 million. As a result, the predicted values quite agree with experimented ones in the far field, which means that the analytical method is valid. To identify the sound source of the broadband sound, the time derivatives of pressure fluctuations on the delta wing surface were calculated. The time derivatives of pressure fluctuation are high around the apex of the delta wing in each frequency band, and the broadband sound source is considered to be around the apex of the delta wing. To identify the narrow band sound sources, the coherence between far field sound and time derivatives of pressure fluctuations around the rear end of the wing was conducted. Strong coherence was found at a frequency of 100 Hz very close to the narrowband peak frequency. Narrow-band sound source is considered to be around the rear end of a delta wing.

Understanding the vortex flow mechanism of cavity tones is desired to control tones in aeroacoustic field. With respect to cavity tones in deep cavity with acoustic resonances the mechanism of vortex flow has not been made clear enough. When the circulation of the flow around the upstream side edge of the cavity exceeds the predetermined value, the phenomenon that the vortex might flow out is the cause of the cavity tones. It would be the estimated mechanism that the particle velocity of the resonant sound wave influences on the increase of the circulation around the upstream edge. We have tried to simulate the practical phenomenon by simple numerical calculation with this assumption mechanism. As a result of trial and error, it became possible to explain the phenomenon of cavity tones with resonance by the assumption mechanism.

Longitudinal vortices generated around automobiles and airplanes are regarded as one of the most dominant aerodynamic noise sources. There have been so many studies to reveal the generation mechanism of aerodynamic noise produced by longitudinal vortex. However, it has not yet been clarified that how longitudinal vortices have been generated and how the vortices produce aerodynamic noise. The paper aims to reproduce longitudinal vortices with a simple delta wing, and to identify noise sources by the beamforming technique, and to clarify aerodynamic sound generation mechanism. The beamforming test was conducted in JAXA (Japan Aerospace Exploration Agency) wind tunnel. The wing was immersed in a uniform flow with an attack angle 15 degrees in the measurement section and the far field noise produced by longitudinal vortex was measured by the far field microphone and the microphone array with 96 microphones installed in the anechoic chamber. The beamforming technique identified two noise sources. One is located at the tip and the other at the rear end of the wing. Regarding noise frequency characteristics, aerodynamic noise consists of two bands, narrow and broad one. It was found that narrow and broad band frequency noise increase in proportion to 4.8th power and 6.5th power of the uniform airflow velocity, respectively. Narrow band frequency noise is generated at the rear end of the wing whereas broad band one is at the tip of the wing. CFD analysis clearly visualized unsteadily fluctuating motions of vortices in the vicinity of the tip of the wing. The tip has the strongest vorticity which becomes weaker and weaker in the rearward direction. It is therefore considered that unsteady motion of vortices with the stronger vorticity at the tip induce the larger level of pressure fluctuations with broad band frequencies. This leads to noise generation of broad band frequency at the tip of the wing.

To clarify the mechanism for aerodynamic sound to be generated from an airfoil in a flow with turbulence, numerical simulations have been carried out for a flow around an airfoil subjected to the wake of a circular cylinder. The test airfoil has the NACA0012 profile with a chord length of 150 mm and a spanwise length of 7.5 mm, and it is set at angle of attack of 9 degrees. The wind velocity is set to 30 m/s, which results in an airfoil Reynolds number of 3.0×10⁵. The circular cylinder is set 100 mm upstream of the leading edge of the airfoil. The numerical simulations are composed of large eddy simulation (LES) and aeroacoustical simulations that solves the Lighthill equation in the frequency domain with the Lighthill tensor computed by the afore-mentioned LES as the acoustical source terms. The sound pressure level is also computed by the Curle’s equation with the fluctuation in the fluid force computed by the LES as the acoustical source. When they flow near the leading edge of the airfoil, the vortices in the cylinder wake are stretched due to the acceleration of the main flow, forms strong source of sound, and radiated intense sound in the upstream direction.

A small axial-flow fan utilized in cooling systems of electronic equipment is often installed in a casing with slit structures. However, in some installation conditions, the ventilation through the slits is prevented due to obstructions. In this study, the effects of slit structures of the casing on performance and aerodynamic sound were investigated. The fan with casings of different slit intervals were installed in a rectangular duct. When the casing slit was completely blocked, the pneumatic power by the fan was decreased around a design flow rate. The results of numerical simulations performed along with the experiments indicated that the flow towards the tip of each blade appears for the casing without slit structures, where the flow separation occurs at the leading edge of the blade and the wake becomes more turbulent. The experiments showed that the velocity fluctuations measured between the blades and struts are intensified in broadband frequency range by the turbulence in the wake. Also, the measured sound pressure levels at the harmonic frequencies of the blade passing frequency (BPF) were lower and the broadband level was higher for the casing without slits than for the casing with slits, respectively. The power of the velocity fluctuations at BPF and harmonic frequencies in the wake was found to be also decreased by closing slits. In addition, the experimental results with a different slit interval showed that the sound pressure levels at the harmonic frequencies can be reduced with the appropriate slit interval while preventing the performance degradation.

In this study, it is clarified the influence of the total number and resolution of the computational grids on the accuracy of the numerical analysis by using the large eddy simulation for a small box fan. It is analyzed the accuracy by using six cases of computational grids with different resolutions and total numbers of the grids. These grids have different resolution in the casing strut and the grid type near the blade-wall surface. As a result, application of the prism-type mesh near the blade-wall surface has been found crucial for resolving thin laminar boundary layers, and thus improving the prediction accuracy of the performance curve. Far field sound prediction are performed by Curle’s equation⁽⁵⁾ or acoustic analysis with calculated fluid force.