This study investigates the suppression of the so-called hole tone produced when a jet, issued from a circular nozzle or hole in a plate, goes through a similar hole in a second plate. The mean velocity of the air jet u0 was 10 m/s. The nozzle and end plate hole both had a diameter of 51 mm, and the impingement length Lim between the nozzle and end plate was 50?90 mm. We propose a new passive control method of suppressing the tone which might disorganize the feedback loop with an axisymmetric obstacle on the end plate. We find that the effect of the obstacle is well described by the combination (W/Lim, h) where W is the distance from the edge of the end plate hole to the inner wall of the obstacle, and h is the obstacle height. The tone is suppressed when backflows from the obstacle affect the jet shear layers near the nozzle exit. We also do a direct sound computation for a typical case where the tone is successfully suppressed. Axisymmetric uniformity observed in the uncontrolled case is broken almost completely in the controlled case. The destruction is maintained by the process in which three-dimensional vortices in the jet shear layers convect downstream, interact with the obstacle and recursively disturb the inflow from the nozzle exit. We also discuss a transition of flow fields from an original state to a suppressed state when we put the obstacle on the end plate abruptly in the computation.

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The lattice Botlzmann method (LBM) for two-phase flows containing a deformable body with a viscoelastic membrane is improved to simulate circular pipe flows by incorporation of the immersed boundary method. The method is applied to the motion of a biconcave discoidal body in pressure-driven pipe flows. The validation of the red blood cell (RBC) model is demonstrated by investigating the relation between the deformation index and terminal streamwise velocity of the RBC in circular pipe flow. In addition, the behavior of a biconcave discoidal body in constricted pipe flows is simulated under various geometrical conditions. These results indicate that the present method is applicable to simulation of the motion of RBCs in microscale capillary blood vessels.

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The exhaust port of a truck internal combustion engine forms the interface between the combustion engine and the turbocharger. Approximately 30-40% of the energy potential is lost in the exhaust gasses after combustion, which can be partially recuperated in a turbocharger. Hence, energy losses in the connection are highly undesired. However, due to the high occurring velocities and the complex geometry, flow separation, flowstructure formation, and secondary flow motion are the major sources of energy losses. Within the exhaust process, the valves open while the piston continues moving in the combustion camber. This process is often analyzed by modeling the piston and valves at fixed locations, but conserving the total mass flow. Using advanced methods, this process can be simulated numerically in a more accurate manner. This study compares Large Eddy Simulation based data, assessing the implied differences due to the choise of method for simulating the exhaust process from an engine cylinder. A simple case using fixed positions for valve and piston is contrasted with the cases where static valve and moving piston, and moving valve and moving piston are considered, respectively. The generated flow phenomena are compared within the cases.

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Two-dimensional jet impingement heat transfer enhanced by a single rib has been studied by large eddy simulations. Installation of ribs induces flow separation and reattachment, and realize high heat transfer coefficient in the wall jet region. Higher rib-height was found to be effective to make the enhanced heat transfer region larger. Large eddy simulation was found to predict reattachment length correctly, which then resulted in good agreement of local heat transfer coefficients between experiment and simulations except the stagnation and reattachment regions, where over- and under-estimation occurs. In addition, concurrent large eddy simulations for different rib-heights, which are realized by an in-house code, are found effective to examine the effect of rib-height on the formation of turbulent eddy structure passing over the rib, which then forms turbulent shear layer behind the rib and are responsible for the enhancement of turbulent heat transfer around reattachment region.

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This paper is dedicated to the investigation and analysis of wind turbine wake. An experimental work is undertaken in wind tunnel on a horizontal axis wind turbine model. The velocity field in the wake is measured using PIV with synchronization in order to relate velocity and vortices to the rotating blades. The tip vortices are investigated in successive azimuthal planes and the analysis of vortex characteristics permits to express the vortex core diameter, the swirl velocity distribution and the vortex diffusion as functions of the vortex age. The positions of the vortex cores are obtained by a specially developed algorithm based on circulation calculation. The quality of the obtained results permits their use as a reference for validation of numerical computations.

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Butterfly wind turbine (BWT) is a vertical axis wind turbine (VAWT) of a new concept with multiple looped blades making a double-blade structure. A micro BWT model (diameter: D=0.4m, height: H=0.3m) was manufactured by stereolithography to carry out the wind tunnel experiments. Good self-starting characteristics and poor maximum power coefficients of the model were demonstrated by the experiments. The performance prediction based on the Blade Element Momentum (BEM) theory using Quadruple-Multiple Streamtube (QMS) model showed large difference from the experimental results. To investigate the effects of blade section, two-dimensional CFD analyses were carried out for two double-blade rotors of a symmetry or cambered airfoil and were compared with the BEM-QMS predictions for the 2D rotor of the same size. Although the 2D-CFD analysis could not make clear that the BEM simulation with aerodynamic data of a symmetrical blade in a parallel flow would correspond to the performance of a rotor with cambered blades in curvilinear flow, it showed that the cambered blades will generate more torque than the symmetrical blades at least as for the outer rotor.

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Wall pressure measurements were conducted for a 90 degree elbow of which the axis curvature coincided with its inner diameter (125 mm). Reynolds numbers examined were 3.2 X 105 and 5.0 X 105. Results showed that distributions of fluctuating normalized pressures obtained here and those made by Shiraishi et al. for the Reynolds number of 3.25 X 106 [1] coincided within 0.02 of the dynamic pressure. Power spectral density functions of fluctuating pressures mostly exhibited the slope of the minus seven-third law at large frequencies. The peak spectrum with the Strouhal number of 0.5 could be found in the curvature inside downstream of the elbow. They corresponded to the vortex shedding from the boundary layer developed in the inner and aft part of the elbow. The peak intensity having the Strouhal number of 0.5 quantitatively was in accordance with that of the data obtained in the experimental setup that Shiraishi et al. used [2], suggesting that the law of dynamical similarity could be applied with regard to this oscillation.

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2D and 3D fluid simulations for flow around an in-line forced oscillating circular cylinder have been conducted in order to clarify the mechanism of flow induced vibration. Immersed boundary method is used to solve the moving boundary. Additionally, OpenFOAM which is open source CFD-tool is used. The vibration modes of a cylinder are categorized into two excitation region in terms of the reduced velocity. In the first excitation region, it is shown that a low drag force forms by contacting between high pressure region and a circular cylinder. It is considered that a in-line oscillation of a cylinder comes from the contact phenomena. In the second excitation region, it is shown that the time averaged lift drag doesn't reach zero on some oscillating conditions. It is considered that a cross-flow oscillation of a cylinder comes from the phenomena.

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High-Reynolds-number turbulence consists of a hierarchy of multi-scale vortex structures. In order to investigate generation mechanism of the hierarchy relevant to the physical-space energy cascade, we have conducted large-eddy simulations (LES) of incompressible hyperbolic stagnation-point flow driven by steady external body force in the form of quadruple vortices. Under this steady external force, four large-scale vortex tubes are sustained in a periodic domain. The energy cascade process that smaller-scale vortex tubes are created by being stretched in straining regions around larger-scale vortex tubes (Goto 2008, 2012) is identified in the flow. In our LES, such a generation process of smaller vortex tubes by larger-scale strain is observed in a wider inertial range than in direct numerical simulations with the comparable number of grid points. One of the most interesting observations in this flow is that this generation process of smaller vortices takes place in a quasi-periodic manner. For example, global flow quantities such as mean energy and its transfer rate to subgrid scale vary periodically in time with the period of O(10T ), where T is the eddy turnover time, even in the high-Reynolds-number case. In this cycle, the activity of the hierarchy of coherent vortices changes cyclically in time as well. Therefore, this cycle might be regarded as an elementary process of the energy cascade in isotropic turbulence.

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Flying and aquatic animals release a vortex by pitching a wing or a fin. It is well known that vortex motion plays an important role in generation of unsteady fluid force. We intend to elucidate the relationship between unsteady fluid forces and three-dimensional vortex structure in the case of the greater reduced frequency. In the present study, pitch-oscillating motion was carried out as a basic unsteady motion. The vortex structure and its behaviour of a discoid airfoil are investigated during pitch-oscillating. The vertical flow fields were measured using a stereo PIV technique. The vortex growth is observed during pitch-oscillating motion, and large scale vortex is released into the wake close to the airfoil. It was confirmed that two significant vortices are released during one cycle of pitching oscillating and the fluid force shows a peak value at the instant of the vortex shedding.

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In order to obtain high efficiency aircraft wings and to improve the energy efficiency in the fields of eco-friendly transportation, the flow characteristics of the aircraft wing were studied with the change of lift-to-drag ratio(L/D) through the CFD analysis. The design process was focused on generating the high lift force and low drag force as the lift-to-drag ratio was increased. In this paper, various dimple patterns, in this case circular-shaped dimple pattern, circular-shaped dimple pattern having embossed conical structure in it, and the dimple pattern having the satellite dimple ahead of it, were numerically designed. Each dimple distance and its position were changed as the artificial conditions. The numerical analyses were conducted by using a commercial code, ANSYS CFX. Numerical result depending on the lift-to-drag ratio distribution was graphically depicted for various dimple patterns.

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Reduction of aeroacoustic noise from a pantograph head of high-speed trains has become increasingly important. For this purpose, we introduce a flow control method using synthetic jet actuators. The wind tunnel test results show that aeolian tone from the pantograph head can be reduced under the wind velocity of 25m/s, but that under higher free-stream velocity, the reduction effect is less. Therefore, synthetic jet actuators that are compact and can induce higher mixing action are necessary for high-speed flow control. The experimental results obtained in this study will contribute towards the design of synthetic jet actuators applicable for a pantograph head.

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In the case of a rectangular prism with a side ratio of less than 0.5 and D-section (semi-circle) prism, the low-speed galloping appears at lower reduced velocity than the resonant reduced velocity. In this paper, we carried out free-vibration tests of cantilevered rectangular prisms with the side ratio under 0.5 and D-section (semi-circle) prism in a water tunnel. The effects of the side ratio and the turbulence intensity of free-stream on the flow-induced vibration characteristics of prisms were investigated. The response amplitude of a rectangular prism increases with decreasing of the side ratio. The D-section prism vibrates at lower reduced velocity than the rectangular prism with a side ratio of 0.5, which has the same depth as the D-section prism. It is found that the free-vibration characteristics of rectangular and D-section prisms are influenced by high turbulence of the free-stream.

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In this study, simultaneous measurement of axial velocity and concentration is carried out along the centerline axis of turbulent round jet in the self similar region using line scan camera. Line scan camera enables us to measure the light intensity at multiple positions as many as 2048 simultaneously at high acquisition rate as high as 100k Hz. Reynolds number based on nozzle diameter and nozzle exit velocity is relatively low of 3200. Rhodamine B (fluorescent dye) solution is issued from the nozzle into a test water tank. Schmidt number is 2380. Jet axis is illuminated by green line laser and concentration of Rhodamine B solution is measured by LIF method. Instantaneous concentration profile recorded by line scan camera shifts downstream with time proceeding. Correlation image technique is applied to estimate local and instantaneous axial velocity. Spatial resolution of concentration measurement is smaller than Kolmogorov scale but greater than Batchelor scale. Scalar dissipation cannot be measured directly but can be estimated from turbulent scalar flux since velocity and concentration are measured simultaneously. Batchelor spectrum, which is proportional to the inverse of wave number, is observed.

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This paper is concerned with a mathematical model of a simple axisymmetric silencer model consisting of an expansion chamber followed by a tailpipe. The unstable shear layer is modeled via a discrete vortex approach, based on axisymmetric vortex rings. The aeroacoustic model, which is described in the present short paper, is based on the Powell-Howe theory of vortex sound. The boundary integrals, which represent the scattering by the cavity and the tailpipe, are discretized via the boundary element method.

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The effects of end wall conditions and aspect ratios on the surface pressure and the local drag coefficients of a circular cylinder were examined experimentally at blockage ratio 3% and Reynolds number 8000. Both end walls supporting the cylinder were the flat plates with different tip conditions on which two-dimensional transition boundary layers were formed. The ratio of boundary layer thickness to cylinder diameter at the cylinder position was 0.60 or 0.83, and the aspect ratio was varied from 8 to 32. In the case of thick wall boundary layer, the base pressure coefficient along the cylinder axis was almost constant for all aspect ratios except in the vicinity of the end wall, but rapidly rose with decreasing the aspect ratio. In the case of thin wall boundary layer, the base pressure coefficient was little constant along the cylinder axis, but the difference for various aspect ratios is relatively small.

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To clarify the flow mechanism of the separation control on circular cylinder when pulsed plasma actuator is applied, the characteristics of the induced jet from the pulsed plasma actuator was examined from the results of jet thrust measurement and high-speed schlieren flow visualization. The effect of the frequency of pulsed plasma actuator on the flow separation on a circular cylinder model is discussed in the viewpoint if aerodynamic force and the flow field measured with time-resolved PIV at the low speed wind tunnel test. As a result, pulse frequency dependency of the strength of the induced jet is clarified and their direct effect in the separation control performance is observed.

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The characteristics of separation flow past a heated sphere are investigated at around critical Reynolds number in conditions using three-dimensional numerical simulation in which temperature dependence of fluid properties such as density and viscosity is exactly considered. Boussinesq approximation is no longer applicable due to large temperature difference adopted in this study. The result shows that drag coefficient of the heated sphere in drag crisis region becomes larger than that of the unheated case and it increases up to the coefficient found in subcritical region. This is because the temperature difference between the sphere and ambient fluid strongly affects the flow separation points and reattachment resulting in small recovery of the pressure in the wake and reduction of the temporal fluctuation of the lift force acting on the sphere. These effects are considered to attribute to the temperature dependence of fluid properties in the vicinity of the sphere and effect on the transient of the boundary surface in critical region. The order of the buoyancy effect becomes relatively small compared to inertia effect in present Reynolds number region so that the buoyancy does not have a significant effect on the fluid force on the sphere.

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In low Reynolds number region about order of 1,000 to 10,000, aerodynamic characteristics of NACA0012 airfoil shows unique characteristics because of appearance of laminar separation and separation bubble on the airfoil. Lift characteristics, especially, strongly show non-linear characteristics with decreasing of its Reynolds number, and performance of the airfoil becomes worse because of involving the separated flow on the airfoil. We applied a DBD plasma actuator on the airfoil that it aims to control and suppress the separation of boundary layer on the airfoil, and performed measurements of aerodynamic forces and flow field of the airfoil. As the results of measurements, it is clearly confirmed that the aerodynamic characteristics were improved by using the plasma actuator. Moreover, improvement effect on the aerodynamic characteristics show different one due to changing of installed position of the plasma actuator on the airfoil.

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The pressure distribution on a NACA0012 airfoil was measured in order to clarify the flow field around the airfoil in the low-Reynolds-number region from 10,000 to 50,000. In the present study, a wind tunnel and a NACA0012 wing model with 70 static pressure ports were used for the measurements. The pressure on the airfoil surface was measured by a micro-pressure sensor. Based on the obtained pressure distributions, we confirmed in detail the behavior of the separation bubble and observed the change of a short bubble into a long bubble. As the angle of attack was increased, the non-linearity of the aerodynamic characteristics was affected by both changes in the separation point and the length of the separation bubble. We confirmed that the behavior of the separation bubble is affected by flow field phenomena that depend on the Reynolds number.

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A cubic open type chamber was designed and built in order to produce nearly Homogeneous and Isotropic Turbulence field (HIT) at the cube center, with low mean velocity. This was accomplished by mixing eight synthetic jets in ambient atmosphere all pointing towards the center. The power and frequency of excitation of the specially made jet actuators were varied and the flow characteristics of the resulting flow field were monitored. Flow visualization and image processing were employed in order to examine the interaction of a free jet and a smoke plume with the synthetic turbulent field.

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At cross wind conditions trains are subjected to strong moments, lift and side forces which reduce the stability of the vehicle. The cross wind sensitivity of trains is a safety relevant topic and train manufacturers are required to meet safety criteria defined in various national and international regulations. In an ongoing research project the lee-side vortex, sometimes called "delta vortex", which contributes to the critical rolling moments, is investigated in order to find methods to reduce its strength.

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The cold gas dynamic spray (CGDS) process is a promising technology to produce high strength, strong corrosion resistance and excellent wear-resistant coating. This technology is able to equip the manufacturers to meet more stringent requirements of engineering components they produce. In this study, a two-step numerical methodology is developed to simulate the powder-laden flow in the CGDS process. In the first step the gas dynamics involved in CGDS process is studied. It essentially consists of a development of supersonic flow within CGDS nozzle followed by a supersonic jet impinging on the substrate. A particle model is then developed to calculate the particle flight path, and the deposition process. The relationships between the nozzle stand-off distance, spray area and such CGDS operating parameters as nozzle pressure ratio, temperature of carrier gas, particle inject locations are investigated and reported.

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In this study unsteady simulations of the flow through and after a 90° pipe bend has been performed by Large Eddy Simulations (LES). In the passenger car engine there is an abundance of pipes and pipe bends. Since pipes and bends are often situated upstream of important engine components the flow in these needs to be well predicted. This entails that there is a need for accurate pipe flow simulations in order to ensure that the inflow conditions, to the e.g. cylinders or turbocharger, are as close to the experimental values as possible. The flow field is further studied by the use of Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD). It has been found that there is a low frequency oscillation in the strength of the alternately dominant dean vortex at the exit of the pipe bend. This phenomenon is analysed and the mechanism for it is discussed.

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The three-dimensional time-dependent compressible Navier-Stokes equations are numerically solved to study acoustic emission mechanisms in a supersonic round jet at high convective Mach numbers. A 5th-order compact upwind algorithm developed by Deng and Maekawa (1996)[1] is used for spatial derivatives and a 4th-order Runge-Kutta scheme for time advancement. The Navier-Stokes characteristic boundary conditions are used in the streamwise and radial directions and periodic boundary conditions in the azimuthal direction. Numerical results for the convective Mach number Mc = 0.97 are presented (Mc is defined by eq.(13) in Section2). Two different cases were investigated. The first case is the jet forced by the linear unstable modes. The second case is the jet flow forced randomly. The numerical results provide new physical insights into Mach wave generation nature in a supersonic round jet. Upstream disturbance conditions play an important role for the emission of the Mach waves in a supersonic jet. The numerical results show that the jet forced with a pair of first helical modes can indicate the elimination of Mach waves at restricted emission angles due to the interference of these modes. The pressure fluctuations generated by the growth of the opposite helical mode are lineally superposed into the jet near sound filed. Therefore, this result suggests that the direction of Mach wave radiation can be controlled by introducing of optimal helical modes in turbulent jets.

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Disturbances in turbulence are cause and effect of non-linear interactions, so that turbulence itself is known as a non-linear phenomenon. Recently, it was surprisingly found that there exists a linear disturbance excited by a periodic vibration in a two-dimensional turbulent jet. Amplitude of the disturbance normalized by that of the initial disturbance is constant when the initial disturbance amplitude is not considerably strong. In this study, artificial periodical disturbance from a wall surface is introduced to a flat-plate turbulent boundary layer, and development of the disturbance is investigated using a hot wire anemometry. The periodical component, extracted from the streamwise velocity signals with ensemble-average technique, indicates that the existence of a linear disturbance in a turbulent boundary layer, where strong non-linear interaction among turbulent disturbances occurs. It is worth noting that the excited disturbance structure is similar to a hairpin vortex that is generally observed in turbulent shear flows, suggesting that the point-like initial disturbance generates an artificial hairpin vortex.

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The paper presents current developments in both experimental and numerical identification of two-phase flow phenomena in transcritical CO2 ejectors. Neither explicit identification/classification of the flow patterns nor determination of velocity field for the CO2 ejector passages, based on experimental methods of flow visualization, was reported so far. First trials for determination of spatial fields for the Mach number, pressure, and density inside the CO2 ejector passages, modelled with a validated CFD tool, were reviewed. The influence of the phase transition relaxation on the converging-diverging motive nozzle performance was analysed. Insufficient qualitative and quantitative identification of the phenomena influencing the mass, momentum, and energy transfer was discussed.

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For the thermal design of supercritical water reactors, it is necessary to develop an analysis method to correctly predict turbulent heat transfer characteristics in subchannels of fuel bundles. Spacers are set into the subchannels to keep a distance between adjacent fuel rods. The turbulent heat transfer generally enhances by reduction of cross-sectional area in the subchannels due to existence of the spacers. However, since thermo-physical properties of supercritical fluids drastically changes at the vicinity of the pseudocritical point, enhancement of the turbulent heat transfer depends on the thermal design. Then, the Japan Atomic Energy Agency is developing an analysis method to predict thermal-hydraulic characteristics of the supercritical fluids. The heat transfer calculations were performed under the condition of a subchannel with a specer surrounded by four fuel rods. The present study describes an increase of the turbulent heat transfer coefficient in the subchannel.

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It is recently revealed that there exists a linear disturbance in a two-dimensional turbulent jet. In this study, theoretical explanation for the linear disturbance was attempted based on disturbance equations, and velocity measurements using an x-type hot-wire anemometer and their comparison to a linear stability theory based on the parallel flow assumption were performed. An periodical-average was applied for the velocity data to extract a disturbance excited by a periodical initial disturbance, then it was confirmed linearity of the excited disturbance to the initial disturbance for low intensity of the initial disturbance. The growth rate of the excited disturbance is in qualitative agreement with the linear stability theory as well as the disturbance distribution. The disturbance equations for the periodic components of disturbances includes the second-order product terms of the random components, and the third-order and the fourth-order product terms appear in the high-order disturbance equations. The experimental data indicate that these terms are proportional to the initial disturbance amplitude until the initial disturbance is so strong that the excited disturbance is non-linear behavior, indicating that the disturbance equations are linear for an infinitesimal periodic component. This well explains existence of the linear disturbance in the turbulent flows.

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We improve a spatial resolution of concentration measurement system based on the light absorption spectrometric method. The light source of the system is upgraded from a halogen lump to light-emitting diodes (LEDs). We also develop an optical fiber probe with a smaller sampling volume than that of the former system. The performance of the concentration measurement system is tested in a turbulent planar liquid jet with a second-order chemical reaction. The concentrations of chemical product and non-reactive species are measured. The results show that the mean concentrations measured by the new system are consistent with those measured by the former system. However, the new system enables to measure the concentration fluctuations at higher frequencies than the former system.

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Effects of the cylinder wake in a freestream on statistical properties of a turbulent boundary layer over a flat plate are investigated experimentally by using a wind tunnel. The cylinder is installed horizontally above the tripping wire and the effects of its wake on the zero-pressure-gradient turbulent boundary layer are investigated. The results show that the logarithmic law for the time-averaged mean velocity is still applicable for the turbulent boundary layer interacted with the cylinder wake. However, the turbulent intensities in the outer region (y+ > 500) and the Reynolds shear stress in the entire region of the boundary layer are decreased by the cylinder wake.

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An experimental investigation of the turbulence structure generated by regular and fractal grids (hereinafter referred to as regular grid turbulence (RGT) and fractal grid turbulence (FGT), respectively) in a water channel is tried by means of stereo particle image velocimetry (stereo PIV). The conventional rectangular array grid and the square-type fractal grid with a fractal iteration of N = 4 [1] are used. The Reynolds number based on the effective mesh size is 2;500 for both cases. Two high-speed video cameras are used to measure threecomponent velocities in a laser sheet. The preliminary results show that the vertical profile of the mean streamwise velocity in FGT is larger in the central area of the water tunnel than in the off-central area, whereas that in RGT is uniform cross-sectionally. The turbulence intensity in FGT is larger than that in RGT.

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In this study, effect of nano-fibrillated cellulose, whose water suspension is non-Newtonian fluid, on the transitional channel flow was investigated. Skin friction measurements and flow visualization of channel flows of single-phase water and the nano-fibrillated cellulose suspension were performed. Drag reduction was realized for the concentrated nano-fibrillated cellulose suspension of 400 ppm though the reduction range of Reynolds number is limited only for the transitional flows. The nano-fibrillated cellulose suspending into water causes transition delay in two-dimensional channel flow not only in terms of simple viscous increase due to the nano-fibrillated cellulose addition but also in comparison against the apparent Reynolds number.

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Natural convection of air over an upward-facing horizontal two-dimensional heated plate is investigated by means of direct numerical simulation (DNS). The Rayleigh number based on the length of the plate and the representative temperature difference is 108 in the present study. A plume uprising from the central region of the plate induces two-dimensional boundary layer flow over the plate from its edge to the center. This boundary layer flow develops from laminar state with the ambient quiescent fluid entrained. In the present study, special attention is focused on turbulence structure of separated boundary layer flow under a practically important high Rayleigh number condition. According to the preliminary study of threedimensional thermal instability, spanwise temperature disturbance within a narrow band region along the edge is employed to encourage spontaneous flow separation and transition to turbulence. Turbulence structure near the center of the plate is statistically discussed.

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This paper presents an integrated model of a computational modes and the experimental study on a gas turbine internal blade cooling problem. A sharp bent channel with several different rib arrangements were investigated and the flow and heat transfer behaviours are analysed in various aspects of turbulence characteristics.

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The work investigates the effect of turbulent intensity (Tu) on the force and wake of a NACA0012 airfoil at chord Reynolds number Rec = 5.3X103 and 2X104. Lift and drag coefficients (CL and CD) on and flow fields around the airfoil were measured with Tu varied from 0.6% to 6.0%. Four Rec regimes are proposed based on the characteristics of the maximum lift coefficient (CL,max), i.e., ultra-low (Rec < 104), low (104 〜 3X105), moderate (3X105 〜 5X106) and high (> 5X106) Rec. It is noted that at Rec = 5.3X103 (ultra-low Rec regime) the stall is absent for Tu = 0.6% but occurs for Tu = 2.6% and 6.0%. In the low Rec regime, the Tu influence wanes and the critical Rec, above which stall occurs, decreases with increasing Tu. When the airfoil angle (α) of attack varies from 0° to 30°, at low Tu (0.6%) the shear layers over the airfoil are mostly laminar in the ultra-low Rec regime but involve transition in the low Rec regime. The effect of increasing Tu on flow bears similarity to that of increasing Rec, albeit with a difference. The flow separation shifts upstream with increasing Rec but downstream with increasing Tu.

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A new pipe test facility for investigations on high Reynolds number pipe flow turbulence (Rem≦ 1.5 X 106) was setup, namely the CoLaPipe. The facility is designed to be closed-return with two available test sections providing a length-to-diameter ratio of L/D = 148 and L/D = 79. We present intensive measurements on the development of the flow from laminar to turbulent flow state with respect to natural transition as well as artificial transition. Our focus is on the detection of the fully developed turbulent flow state due to the characteristic behavior of the mean flow and the higher order statistics, e.g. turbulence intensity, skewness and flatness. The influence of tripping devices, i.e. orifices with different blockage ratios, on this evolution is of great interest, respectively. Here the question arises whether the turbulent flow state can be induced earlier in streamwise direction or not, which is important for further experiments, like CICLoPE. As the turbulent flow state is important due to structural observations and the deduced scaling behavior, we present additional experimental results to shed light on some open questions, i.e. inner peak behavior. All presented measurements are conducted on a hydraulically smooth surface by using single-wire probes working in constant-temperature hot-wire anemometry mode.

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Active Flow Control (AFC) experiments were performed at the California Institute of Technology and at The University of Arizona on a generic airplane vertical tail model and on swept back wings. They proved the effectiveness of sweeping jets in improving the control authority of a rudder and the lift generation of flaps. The results indicated that a momentum coefficient (Cμ) of the order of 1% could increase the lift or the side force by 30-50%. However sparsely distributed actuators providing a collective Cμ ? 0.1% were able to increase these forces by 20%. This result is achieved by reducing the spanwise flow on the swept back wing or rudder and its success is attributed to the large sweep back angle of the test articles.

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Mixing of a jet stream with an ambient fluid depends on several factors. The importance of these factors can be expressed in form of dimensionless numbers, relating the time-scales of the different processes to each other. The different time-scales depend in the properties of the fluids in question (i.e. diffusivity), the geometrical set-up (e.g. nozzle shape, shear-layer thickness) and the operating conditions (laminar/turbulent and accelerating/decelerating or intermittent jet). Turbulence has enhanced mass and momentum transfer due to the generation of eddies of different scales. Similar effect can be attained by generating larger scale vortices by introducing intermittent injection. Intermittent injection enhances entrainment. The results show that the declaration phase contributes most to this process. The mechanism of vorticity generation for single phase and multi-phase jets, under different conditions, is discussed.

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As supersonic aircraft and their turbojet engines become more powerful they emit more noise. The principal physical difference between the jets emanating from supersonic jets and those from subsonic jets is the presence of shocks in the supersonic one. This paper summarizes a study of noise reduction technologies applied to supersonic jets. The measurements are performed with a simulated exhaust of a supersonic nozzle representative of supersonic aircraft. The nozzle has a design Mach number of 1.56 and is examined at design and off-design conditions. Several components of noise are preset including mixing noise, screech, broadband shock associated noise and crackle. Chevrons and fluidic injection by microjets and a combination of them are shown to reduce the noise generated by the main jet. These techniques provide significant reduction in jet noise. PIV provides detailed information of the flow and brings out the physics of the noise production and reduction process.

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This study presents recent research activities on coherent structures in combustor flows employing linear hydrodynamic stability theory. Large-scale coherent structures play an important role in swirling combustor flows. On the one hand isothermal swirling jets undergoing vortex breakdown are susceptible to self-excited flow oscillations. They manifest in a precessing vortex core and synchronized growth of large-scale helical vortical structures. On the other hand, thermoacoustic oscillations are often related to axisymmetric flow structures that are driven by the acoustic field. Despite the qualitative difference between the self-excited helical instabilities and the acoustically forced axisymmetric instabilities, the linear analysis is capable to describe both phenomena with a astonishing accuracy. The proposed theoretical framework allows for a systematic analysis of the dominant flow dynamics in the turbulent combustor flow and their interplay with the flame. This is demonstrated by considering two examples: a swirl-stabilized flame featuring a precessing vortex core that becomes suppressed with the addition of steam, and a swirl-stabilized flame subjected to strong axial forcing mimicking thermo-acoustic oscillations.

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Investigations of the definition and randomness of turbulence were reviewed. The Kolmogorov complexity measure of randomness was then introduced. Numerical and graphic data in the mixing layer formed downstream of a two-dimensional nozzle exit were compressed with the aid of a compression program. Approximated Kolmogorov complexity, AK, and normalized compression distance, NCD, were obtained. The AK indicated the regularity of the laminar flow and the randomness of the turbulent flow quantitatively. The NCD of the numerical value varied with data length. Between the same data, it approached zero, yet, between different data, it approached unity as the data length increased. The NCD of the numerical value in the natural transition process in the mixing layer increased monotonically downstream. Thus, the NCD appears to be the measure of the transition process.

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In point-source models for solid particles, the differences of velocity and angular velocity between the object and surrounding fluid are usually considered in the traditional equation of motion. But the curvature of streamline of undisturbed flow has not been considered enough. It suggests that the lift force is ignored in a free vortex, resulting in the poor accuracy for the prediction of particle behavior in turbulence. We investigate the motion of a spherical particle in two different types of background undisturbed flow, namely free and forced vortices, by the direct numerical simulation. For the simulation, we proposed an outflow boundary condition to deal with the circular flow within a limited domain size. To observe the rotating motion of the particle, the particle center was fixed, and freely-rotatable condition and nonrotatable condition were dealt with. By investigating the drag force, lift force and torque acting on the particle, the effect of the geometric parameter of the vortices was clarified. Then we proposed a new correlation equation of the angular velocity. In addition, a general formula for the lift coefficient was suggested taking into account the curvature of the streamlines of the undisturbed flow, the vorticity and the angular velocity.Keywords: Solid Particle, Rotation, Lift Force, Streamline Curvature, Wake, Direct Numerical Simulation

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The influence of Stokes number on the evolution of the distributions of particle concentration and velocity from their exit values is reported for a co-flowing jet issuing from a long pipe. The data are obtained by simultaneous Planar Nephelometry and Particle Image Velocimetry using four cameras to provide high resolution through the first 30 jet diameters and also correction for optical attenuation. These data provide much more detailed information than is available from previous measurements. These provide new understanding of how the Stokes number influences the flow at the jet exit plane and how this influence propagates throughout the jet.

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Many researchers have studied the flow around single/plural rotating disks with infinite/finite radii. These flow problems can be found out in various industrial applications such as axial compressors, vaneless diffusers, multiple-disk pumps, disk/dram-brake systems. In general, these types of flow are likely to entail non-axisymmetric secondary flows, which occasionally cause disk vibrations and noises. In the present study, the author especially deal with the flow between co-rotating disks, namely, the disks which rotate co-axially in the same direction at the same angular velocity, with a narrow gap enclosed by a stationary shroud at their circumferences. This flow is modeled on that inside the random-access disk storage devices of computers. The author experimentally and numerically shows that this flow is very complicated with three-dimensionality and turbulence, despite the simplicity in its imposed boundary conditions of geometry.

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Active control of flow separation has been investigated to reduce energy losses in fluid machinery. In order to improve the lift drag ratio in unsteady flow, feedback flow separation control was investigated by using a dielectric barrier discharge plasma actuator (DBD-PA) and a fiber Bragg grating flow sensor (FBG-FS) under periodic flow separation by oscillating the wind tunnel test section wall. Tangential jets were generated from sheet-type DBD-PAs and string-type DBD-PAs. Active control of the tip leakage flow was also demonstrated in 2D and 3D models of the turbine blade tips by using string-type and ring-type DBD-PAs embedded in the casing wall. DBD plasma was formed selectively in the regions where the turbine blade tips come very close to the casing wall. This reduces the turbulent intensity caused by the tip leakage flow by simply applying high voltages at radio frequencies to the DBD-PA.

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The jet flow phenomena are not only interesting but also important in the field of fluid and thermo dynamics and engineering. Moreover, they are used widely in various kinds of industries, for example, for mixing-, diffusing-, heating-, cooling-, and transporting-processes. The flow characteristics of jet flows are influenced considerably by the nozzle geometry. The mean and fluctuating velocity profiles at the nozzle exit are quite different by the nozzle geometry, and these affect largely on the jet flow characteristics in the downstream. In this paper, the effects of the nozzle geometry on the jet flow characteristics, especially mixing-, spreading-, diffusing-, and heat-transfercharacteristics for the pipe-, orifice-, notched orifice-, petal shaped-, and others-nozzles are discussed, and their some applications are shown.

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This paper introduces thermoacoustic instability of a quiescent gas enclosed in a channel (or a tube) caused by thermoviscous diffusion under an axial temperature gradient. This instability may be regarded as that of heat flowing in background. Once the heat flow becomes unstable, it entails motions of the gas, which spread as acoustic waves. If the instability is promoted by an acoustic resonance of the gas column, then there emerge its oscillations. They are called thermoacoustic oscillations and exploited as a novel means of heat engines. To consider the instability, a thermoacoustic-wave equation is first introduced. This describes a spatial and temporal behavior of an excess pressure in the channel. While the equation is valid generally in linear theory, it may be approximated for short- and long-time behaviors. Linear stability analysis is briefly outlined and marginal conditions of instability are considered. As the amplitude of oscillations becomes higher, nonlinear effects tend to suppress the linear growth. The thermoacoustic oscillations observed in reality occur as outcome of a balance between linear instability and weak nonlinearity. Origins of nonlinearity may be classified into the one as wave and the other as vortex. The former concerns with emergence of an acoustic streaming and an acoustic shock wave, while the latter concerns with formation of jets, wakes and separated flows. Discussions are given on these problems.

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In this paper we present results concerning the spatial development of energy spectra E11(f) and their associated integral and Taylor scales in conjunction with the spatial developments of vorticity, strain and production rates of vorticity and strain obtained from Direct Numerical Simulations of spatially developing grid-generated turbulence. We use a fractal square grid and a single mesh grid where the mesh is similar to the largest square on the fractal square grid. We find two adjacent but physically different regions in these flows relatively close to the grid: one where the Q-R diagram has not yet formed its well-known, presumed universal, tear-drop shape [14] but E11(f) is not too far from 〜f-5/3 over a decade of a frequency range which is set by inlet conditions rather than Kolmogorov scalings: and one where the Q-R diagram immediately adopts the well-known tear-drop shape but E11(f) is just about proportional to 〜f-5/3 over nearly a decade of frequencies. In the present fractal grid simulation, the first region gives rise, as one moves downstream, to the non-equilibrium behaviour Cε 〜1/Reλ whilst the second region leads to Cε = Const.

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