We propose a modified low-Reynolds-number k-ε turbulence model, in which anisotropy of the Reynolds normal stresses is considered, for viscoelastic fluids and have tested it to predict the drag-reduced turbulent channel flow. In order to obtain the constitutive stresses more accurately, we solved numerically the averaged constitutive equation based on the Giesekus model. This equation includes several terms that pertain to the correlation between velocity and constitutive-stress fluctuations and require to be modeled. We employ a model, in which the Reynolds stress and the viscoelastic stress were used to estimate the correlation terms, and hence it is important to predict the Reynolds normal stress precisely. In this study, we propose an algebraic equation to model the anisotropic Reynolds normal stresses using a damping function of the eddy viscosity. The present proposal allows the constitutive stresses to be predicted well.

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Direct numerical simulations are performed in order to investigate the development of flow field and the friction drag in a subsonic plane boundary layer with a backward-facing step. The boundary layer is forced by normal velocity disturbance of various frequencies at the end of the back-step. The numerical results show that the large spanwise vortexes formed behind the backward-facing step and the observed growth rate of momentum thickness are smaller than flat plate turbulent boundary layer in this region.

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It is known that the sinusoidal oscillating wall in the turbulent channel flows yielded the significant drag reduction. In this study, we conducted the LES of turbulent channel flow with spanwise traveling wavy wall. We investigated the relation between the drag reduction ratio and the wavy wall parameters such as wave amplitude and period.

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In multistage turbomachinary, complex unsteady flow phenomena are inherent due to the bladerow interaction. Its effect is non-negligible for the performance and the stability. Therefore, in this study, we performed large-scale full-annulus DES calculations to investigate the unsteady flow phenomena in a multistage axial compressor. The simulation showed that the large-scale fluctuation of blade torque was caused by the interaction between the wake from the upstream stator and the leading edge separation on the blade suction surface.

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In turbomacinery, secondary flows such as horseshoe vortex and leakage vortex induce the aerodynamic loss. Stacking is applied to turbine blades for reducing the secondary flows. The purpose of this paper is to conduct numerical simulations of flow fields around the rotor and stator blade rows in a multi-stage low pressure steam turbine. Flows through stacked blade rows are calculated and compared to those of the original blade rows.

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This paper presents numerical investigations for unsteady flow fields in supersonic turbine stages for a Rocket Engine. Two dimensional analyses were conducted using spatial high-order scheme CFD code "Numerical Turbine". Analysis cases were single stage cases with two back pressure conditions, 1.5 stages case and 2 stages case. Results indicate pressure increase toward LE at suction side of 1^<st> Rotor blade. This pressure increase is caused because rotor blade passes through nozzle TE direct shock wave and reflect shock wave. Furthermore, vortex is created at near rotor blade suction side and is convected near suction side. As a result, pressure decrease toward downstream at suction side of 1^<st> Rotor blade occurs. Influences of back pressure are limited at rotor outlet region due to flow chokes at rotor passage near outlet.

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This paper presents experimental and numerical simulation study of single turbine stage flow field with injected purge air. To investigate the influence of the purge air from the Rim-seal cavity, we conducted 5-Hole probe and Slant-type hot wire probe traverse measurement downstream of the rotor blade. Those measurement results were used to validate the numerical simulation data then we described the flow field of the rotor passage and cavity domain. In the case of injected purge air, Rim-seal cavity flow field tend to non-uniform and we observed unsteady vorticity distribution around cavity outlet. Those flow filed entrained streamwise vorticity and entropy generation rate in the rotor passage.

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This paper details experimental and numerical studies on effectiveness in controlling boundary layer separation with the performance of dimples on a low-pressure turbine airfoil. The profile loss due to boundary layer separation in LPT has a significant contribution to its overall loss, especially in high stage loading case. Hence it becomes important to study control methods of the boundary layer separation. The aerodynamic performance of the airfoil is investigated in a linear cascade test facility. The boundary layer on the suction surface is measured using a hot wire probe. RANS simulations are also carried out to gain a better understanding of the flow field around the airfoil. The experiments show that the proposed method can reduce the size of the separated flow region, and the numerical simulations reveal the vortical structure caused by the dimples on the suction surface.

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Recent researches have indicated that flow fields near rotating stall inception in a low-speed axial compressor rotor with two different tip clearances are different from each other. In the large tip clearance case, rotating disturbance appears and disappears repeatedly at interval of several tens or hundreds of rotation. It is thought that the stall inception process in the large tip clearance case is dominated by the breakdown of the rotor tip leakage vortex, but it needs more investigation and analysis to explain. In this study, the evolution process of rotating disturbance in a low-speed axial compressor has been investigated by using techniques of experimental measurement and statistical method.

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A technique for high-accuracy predicting performance curve instability in pumps and clarification of the phenomenon in order to control the instability are required for designing high performance pumps. We analyzed internal flow of a test mixed-flow pump by using Large Eddy Simulation (LES) and clarified that euler's head drop caused by stall near the tip of impeller blade is a dominant factor of this test pump's instability. When the head drops, stall fields cover whole blade passages. Angular momentum drop caused by the stall on leading edge exceeds angular momentum increment caused by decreasing in flow rate on trailing edge. The stall cell rotates between impeller blade passages every almost 20 revolution on the occasion of the head drop. The stall field in the blade passage grows caused by increasing attack angle, and the blade passage is recovered from the stall caused by decreasing pressure gradient and increasing flow rate.

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Fans and blowers are used in various industrial fields. The most of the aims of the development is high efficiency and low noise. In this research the effect of blade shape of the axial fan on the performance and the noise is investigated. The wing shape and wing form is improved from the original fan by using CFD and experiment. The improved blade using Controlled-Diffusion-Airfoil profile and sweep and skew shape proved to improve the efficiency and reduce the noise. The effect of leading edge curvature radius of airfoil on the performance of the improved fan is investigated by CFD.

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In this study, with a focus on the rotating stall phenomenon in the vaneless diffuser passage, PIV is conducted to understand the flow structure. Unlike most of other studies, a centrifugal compressor operates at higher tip speed is selected as measurement application. Phase locked measurement was utilized to compensate for the lack of time resolution and both impeller rotation and wall pressure fluctuation were used as trigger source. Considering results obtained at different span height cross sections, the whole flow structure is clearly visualized.

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This study experimentally examines unstable flow phenomena occur in a five-stage centrifugal blower equipped with inlet guide vanes (IGV) before the first stage. Six high response pressure transducers were mounted on several parts of the blower to detect a surge and a rotating stall. We confirmed three types of pressure fluctuation in low flowrate region; deep surge, spike-like rotating stall, and mild surge. At 4.5 percent IGV opening condition, mild surge was observed in the steep negative slope region of the total system head curve, where the whole compression system was supposed to be stable. We considered that mild surge was caused by a balance between a stabilization effect of the first stage with prewhirl and a destabilization effect of the other stages that had no prewhirl and stalled.

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Transonic centrifugal compressor impellers for automotive turbochargers operate from near surge limit to choke limit. In order to improve operating range of compressors, it is indispensable to understand flow physics of unsteady flow phenomena at off-design condition. In this study, the unsteady flow phenomena in a transonic centrifugal compressor impeller at impeller stall condition have been investigated by DES(Detached Eddy Simulation). The numerical simulation results showed that shock-vortex interaction generated large low speed regions in the impeller and the low speed regions make unsteady flow field at the near disturbance onset point. At the disturbance onset point, due to the tip leakage spiral-type vortex breakdown, unsteadiness appeared in the vortical structures. Moreover, at the developed disturbance point, low speed regions rotated in the impeller with time.

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The system requirements of the axial flow fan are various. It is often seen that the axial flow fans are used under the inadequate operating condition. In recent equipment design, the cases where the obstacles are distributed in upstream of the fan for downsizing are increasing. In the present study, an attempt is made to clarify the influence of the upstream obstacle on the performance characteristics of axial flow fan. Furthermore, the flow instabilities around the fan with the obstacle are discussed.

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In order to clarify the relationship between performances and internal flows in a low specific-speed diagonal flow fan, the experimental and numerical investigations ware carried out. The test fan is radial-outflow type diagonal flow fan similar to centrifugal fan and its specific-speed is 670[min^<-1>, m^3/min, m]. The velocity fields at rotor outlet were measured using a slant hotwire probe and the wall pressure on the outer casing was found out using a highly responded pressure transducer. The numerical simulations were carried out at the same flow condition. As results, the fan performance was very good near the design point. Near design flowrate, it is found that tip leakage vortex was occurred. At excessive flowrate, it is found that separated flow was occurred on blade pressure surface.

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The blade thickness of centrifugal impellers is relatively thin and the leading edge must be shaped into the arc or the ellipse with a small radius of curvature. As a consequence, it becomes difficult to generate the smooth computational grid without skewness in the process of CFD analysis. The modification of leading edge into the edge shape makes it possible to easily generate the computational grid with reduced skewness. However, the edge shaped leading edge may promote the leading edge separation and the development of boundary layer on the blade surface. In this study, the flow in three types of centrifugal compressor impeller which are different in the shape of blade leading edge was analyzed numerically. The computed results clarified the influences of the modification of blade leading edge shape on the flow behavior in the impeller passage and the aerodynamic performance of impeller.

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The operating range of a centrifugal compressor is usually limited by the choke and the surge. The surge is related to the compressor system but it deeply relates to the inlet stall of impeller and/or the diffuser stall. Therefore, three flow rates of the velocity distributions were measured using a PIV in the impeller inlet. The correlations between the swirl flow and the reverse flow in the impeller inlet were discussed using velocity distributions of dynamic PIV system.

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A multi-objective optimization technique is applied for design of a low solidity cascade diffuser (LSD) in centrifugal blowers. An optimization code is operated by a meta-model assisted evolutionary algorithm. Both of the pressure static coefficient at design flow rate and small flow rate are applied as objective functions. Optimized result shows good improvement of the lift characteristics, and it is confirmed by an experimental test rig. It is found that a small tip clearance of the LSD is effective for range enhancement without deteriorating the diffuser performance.

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Recent aircraft morphing program envisions an aircraft without conventional control surfaces. Instead of moving control surfaces, the vehicle control systems may be implemented with a combination of propulsive forces, micro-surface-effectors, and fluidic devices dynamically operated by an intelligent flight control system. In the present work, first of all, the accuracy of a high-order LES turbulent model, developed by the author is verified by applying it to predict the experimental drag and lift coefficient for (NACA4412) airfoil. Finally, the work described in this paper focused on the aerodynamic virtual shaping of an (NACA0012) symmetric airfoil at zero angle of attack, using only a thin jet without additional fixtures. The effects of virtual shaping are investigated systematically with jet actuation at different chord-wise locations, and different ratio of jet and free-stream velocity. The jet was used to alter the aerodynamic shape of a symmetric airfoil to enhance lift and/or to reduce drag.

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The cooling technology for the high temperature gas turbine blade is the one of the most important issues for its development. In this research the method for the heat transfer enhancement and the loss reduction of the rib turbulator and the return passage in the gas turbine blade is investigated. The combination of inclination direction of the rib turbulator for the inlet and outlet straight duct is optimized by the experiment and CFD. Using the result of the former, the rib arrangement in the returnpassage is also optimized by the experiment and CFD.

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Conjugate heat transfer simulations were conducted for the first stage rotor blade in an actual gas turbine and compared with experimental results. The objectives of this research are to validate prediction accuracy of temperature distribution on the rotor blade surface and find key factors to improve the prediction accuracy. These simulations were carried out at the similar condition as gas turbine operation. Temperature distributions predicted by the simulations reasonably agreed with those of the experiment near the rotor leading edge part. However, temperature distribution on the pressure side was different between the simulation and experiment. In the internal cooling flow passage, main stream leans toward one side of walls due to Coriolis force and the temperature on this side became lower than the experimental results. It is important for obtaining better prediction accuracy of heat transfer to solve the complex flow phenomena caused by several vortices generated by rotation, flow turning, or the rib-turbulator in the internal cooling flow region.

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A coupling analysis of thermal convection problems is performed in this work. By approximating the material derivative along the trajectory of fluid particle, the characteristic curve (CC) method can be considered. The most attractive advantage of this method is the symmetry of the linear system, which enables some classic symmetric linear iterative solvers, like the conjugate gradient (CG) method or the minimal residual method (MINRES), to be used to solve the interface problem of the domain decomposition system. Applications to industrial problems are demonstrated to show the effectiveness of our approach.

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In order to improve efficiency of micro gas turbine system, it is imperative to decrease compression work for introducing air. The present study aims to develop an evaporative cooling system applied to a micro centrifugal compressor in order to reduce the compression work. In this paper, the effects of mist introducing conditions, such as mist quantity, mist introducing position in inlet duct, and inlet air temperature, are examined experimentally. In experimental setup, a number of ultrasonic vibrators, which can provide water mist of 3μm number mean diameter, are installed in the mist introducing duct. Experimental results show that the inlet air temperature has noted effects on compressor performance, through the effects on the quantity of evaporated water. The present evaporative cooling system has been proven quite valuable at high inlet temperature condition. There exists a trend that reduction rate of compression work depends strongly on the quantity of evaporated water, which is correlated with the inlet air temperature. Concerning about the introducing position of the mist in the duct, the introduction closer to compressor inlet seems to have a bit of advantage for improving compressor performance than the introduction far upstream.

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Radial fan is mainly used as a motor cooling fan for railway vehicle and industrial use. This fan is low efficiency compared with axial flow fan, and the windage loss is large. In recent years, the high output and totally enclosed motor is increasing, and totally enclosed motor with external cooling fan is increasing especially. However, the rate of a windage loss increases at the time of a high rotation speed, and lowers efficiency. Although it is mathematized experimentally and is grasped quantitatively about windage loss of the rotating disk, the fan with blades is not generalized for the complexity of form. This paper describes the result evaluated by CFD about windage loss and mass flow.

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To improve the efficiency and to reduce the noise of a centrifugal blower, we have studied the influence of the number of blades in a vaned diffuser on aerodynamics and noise level. We have found that both the noise level and the dominating order of BPF changes with the number of blades. The change in noise level due to blade number is closely related to the characteristics of resonance. At non-resonant points, it is found from our experiments that noise change due to the number of revolution becomes smaller with the increase of blade number.

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Rotary blood pumps consisting of hydrodynamic bearings have been developed to realize a long-term mechanical circulatory/respiratory support system. An axial flow pump was developed for an implantable left ventricular assist device, designed to work for at least five years without maintenance. An extracorporeal centrifugal pump was developed for cardiopulmonary support system, designed to work for as long as one month. Both systems demonstrated sufficient hydrodynamic performances and biocompatibility in the chronic animal experiments. The mechanical destruction of blood cells due to high rotation of the impellers were within an acceptable range.

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In recent years, blood pumps are used as alternative therapy of heart transplant, however, the characteristics is affected by the biological circulation dynamic state. In this study, a load testing machine for blood pumps have been developed with the aim of clarifying the blood pump evaluation criteria. This simulator has capability to reproduce biological circulation of a cardiac pulsation, blood pressure fluctuation and blockage of blood vessel. The water solution of glycerol is used as working fluid in order to simulate human blood in this paper, and evaluation and examination of a simulator are performed from the centrifugal blood pump response.

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The contra-rotating axial flow pump with two rotors rotating reversely, had been proved with remarkable performance benefits as well as the significant interaction between blade rows in the our previous experiments. A new type of rear rotor, aiming at the reduction of blade rows interaction and the secondary flow suppression, showed the favorable performance both in EFD and CFD method at the design condition, while an unfavorable positive slope was found on the performance curve in the partial flow rate range. In the present research, to further understand the mechanism of the positive slope, internal flow fields of contra-rotating rotors at partial flow rates were fully investigated by means of experimental measurements and numerical simulations.

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An application of contra-rotating rotors to axial flow pump is effective downsizing the pump system. On the other hand, it might cause a significant blade rows interaction between two rotors. Therefore, it should be clarified for stable operation of pump system. This paper presents an experimental investigation of blade rows interaction by casing wall static pressure measurement of a contra-rotating axial flow pump, whose rear rotor has been designed by low speed design method. From the results, the pressure field in the front rotor is found to be significantly influenced by the rear rotor and the flow field of front rotor changes due to the pressure interaction.

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A double-blade centrifugal pump can reduce radial thrust and securing passage particle diameter as compared with Single-blade centrifugal pump, because the impeller is symmetry geometrically. However, many things have not been solved about the radial and axial thrust of a double-blade centrifugal pump. Therefore, to improve pump reliability, it is necessary to quantitatively understand unsteady hydrodynamic force and clarify the behavior and mechanism of the thrust generated. The radial and axial thrust acting on three kinds of double-blade centrifugal pumps having different casing throat area were investigated by numerical analysis in this research. As a result, the effects of the casing throat area that gave the radial and axial thrust were clarified.

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Mechanical seal is often applied to separate turbine from pump in rocket turbopumps. Turbopump has suffered from rotor vibration problems. In this study, we measure rotordynamic characteristic of mechanical seal under whirling motion. Rotordynamic force of this seal was measured by the rotordynamic test stand (JARTS) with active magnetic bearings. It was found that the mechanical seal may have desatbilizing effect under forward whirling motion.

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The objective of this research is to investigate the loss characteristics of torque converter (T/C) using computational fluid dynamics. The performance of a torque converter has been one of important improvement for an automatic transmission equipped with automobiles. Therefore, clarifying the loss characteristics of T/C is a key factor to improve its performance and efficiency. The simulation result verified that the discharge angle of turbine blade has much effect on the loss characteristics of T/C.

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The automotive torque converter is one of the key components of automatic transmission, which transfers power from the engine to the gear system. The occurrence of cavitation in the torque converter should be avoided, due to which significant noises and mechanical vibrations might be generated. Although the cavitation is suppressed by imposing the charge pressure on the torque converter, it is necessary to understand the occurrence of cavitation and its behavior in torque converter elements. It is known that the occurrence of cavitation is the most susceptible on stator at the stall condition with zero turbine velocity condition. In the present study, we observe cavitation behaviors in an automotive torque converter at the stall condition by means of the transparent model. At the same time, we measure pressure fluctuations in the torque converter, and investigate a relationship between the occurrence of cavitation and the pressure fluctuation.

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The thermodynamic effect which affects the cavitation performance of a cavitating centrifugal pump was investigated experimentally using liquid nitrogen. To measure the pump cavitation performance, a test setup which could carry out experiments using both liquid nitrogen and cold water was constructed. The test pump was a centrifugal type magnetic pump, and two impellers, which differed in cavitation performance, were used in experiments. The experimental results indicated that cavitation performance using liquid nitrogen was better than that using cold water due to the thermodynamic effect of cavitation. The estimated temperature depression due to the thermodynamic effect decreased with a decreasing flow coefficient. And it was shown that the estimated temperature depression on the low cavitation performance impeller was larger than that on the high cavitation performance impeller at the same flow coefficient. Moreover, nondimensional thermodynamic parameter Σ* of present study shows a large value comparing with other previous studies. It was indicated that the thermodynamic effect of the cavitation using the centrifugal pump on present study is stronger than that of other previous studies.

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Cavitating flow around a Clark Y 11.7% hydrofoil in a water tunnel is studied experimentally in order to understand the cavitation performance of hydrofoil and its relation to cavitation behaviors. It is confirmed that, at the angle of attack of 8.0 degree, as the cavitation number is decreased, the lift estimated from time-averaged pressure distributions around the hydrofoil slightly increases from that in the non-cavitating condition just before the sudden breakdown, which has been reported by the past literatures. This time, lift and drag forces are directly measured in various cavitating numbers and angles of attack.

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A large-eddy simulation (LES) with one-equation dynamic (OD) model was performed for turbulent cavitating flows around a Clark-Y 11.7% hydrofoil. Our results for time-averaged lift and drag were compared with experimental and computational data in previous studies. In terms of the breakdown properties, our method improved the reproductivity of the experimental evidence. We investigate turbulent structures of these flows, and we confirmed an importance of resolving near the surface region in order to reproduce the breakdown properties.

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Cavitation is the most harmful phenomena for the hydraulic turbo machinery. To avoid the performance degradation by cavitation, an inducer is installed upstream of the main impeller. The pumps for general purposes sometimes need inducer to compensate the lack of NPSH of the main impeller. The purpose of this study is to examine the behavior of back flow and mechanism of head drop. Experiments and CFD analysis were carried out for inducer, centrifugal impeller (C.I.) and C.I. with inducer at design flow rate and low flow rate.

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The cavitaion surge phenomenon was experimentally investigated for three types of flat plate helical inducer with different solidities and numbers of blades. It was found that the cavitation surge of three-bladed inducer occurs in larger NPSH range than that of two-bladed inducer with the identical solidity. In terms of the effect of solidity, it was found that the inducer with larger solidity suffered from the cavitation surge in smaller NPSH range, probably due to the reduced blade loading. In three bladed inducers, many additional instabilities were observed such as forward rotating cavitation, attached cavitation, surge like pulsation and so on.

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In the liquid propellant rockets, POGO instability can occur through the fluctuation of propellant supply to the engine, the thrust fluctuation, and structural vibration. For the prediction of this instability, it is required to provide unsteady characteristics of the pump represented as the transfer matrices correlating the upstream and downstream pressure and flow rate fluctuations. In the present study, the flow rate fluctuation is evaluated from the fluctuation of pressure difference at different locations assuming that the fluctuation is caused by the inertia of the flow rate fluctuation. The present paper describes the results of experiments performed in some situations. For the validation, transfer functions obtained by experiments are compared with Rubin's result.

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In order to investigate fundamental characteristics of three-dimensional corner separations of decelerating cascade flow, a channel flow around a NACA65 blade in a decelerating flow by turning was examined. Five-hole probe survey showed that the exit flow of the blade had a few similar points to the cascade flow in that the accumulation of the low-energy fluid on the corner part, the secondary flow and the pitchwise periodicity of the wake. PIV measurement showed that the focus-type separation was seen in the momentary streamline along the suction surface of the blade, whereas no apparent separation was seen in the flow along the endwalls.

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Micro-pumps are very important for advanced micro fluidic systems which have been well investigated during the last decades. We developed a valve-less micro-pump with one diffuser/nozzle shaped channel and a variable volume chamber which produces an oscillating flow. Micro-pumps require at least total pump head of 40 kPa to apply micro fluidic devices. In order to achieve above pump head, optimization of channel shape is required by investigation of internal flow for the valve-less micro-pump. In this study, flow visualization of internal-flow for the valve-less micro-pump had done.

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Pressure sensitive paint (PSP) is a useful noncontact measurement tool to obtain continuous pressure distribution on solid surfaces. Application of PSP to analysis of complicated flow field in turbomachinery is strongly desired. However, low pressure sensitivity for low-speed flow fields and temperature dependence of PSP makes significant error for pressure measurement, especially for application of PSP to turbomachines with small pressure rise. In this research we applied dual-layer PSP/TSP to measurement of surface pressure of the suction surface of a propeller fan rotor, and compensated for the temperature dependence of PSP using temperature distribution map obtained by using TSP. Moreover, we clarified the change of irradiation light intensity distribution, which is caused by deformation of propeller fan rotor surface, produce significant source of error in pressure measurement. We propose the method to compensate for the change of irradiation light intensity distribution in addition to the nonuniform temperature distribution, and verify the compensation effect.

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Water thin film is created on stationary blades as wet stream flow expands through the last few stages of Low Pressure (LP) steam turbine. It is torn off by high-speed steam flow and broken up into various size droplets. Some large-size and accordingly low-velocity droplets strike leading edges of rotating blades with almost circumferential velocity of them. This fact causes severe erosion damage and has been recognized as a limiting factor on the overall design of the turbine. Sequential motions of water film flow driven by a turbulent air flow from wave patterns on a wall to deformation and break-up patterns on a wall edge are recorded with a high-speed camera. It indicates the wave patterns in film flows dependence on an air stream velocity and also a strong correlation between the upstream wave patterns and the downstream deformation and break-up patterns.

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Aimed at improving efficiency of painting operation in shipbuilding, we investigated behavior of particles in airless spray by using of high speed video camera. An actual airless sprayer was employed for the experiment and silicone oil was used as test fluid. The spray particles with diameter of about over 50 micrometer were visualized by means of backlight method. Two-dimensional velocity vectors of the particle were measured by particle tracking method. We obtained temporal changes in number, direction of motion and velocity of the spray particles, which may affect the painting efficiency.

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The onset of transient turbulence in plane Couette flow is identified as a homoclinic tangency with respect to a simple edge state for the Navier-Stokes equation, i.e., the gentle periodic orbit found by Kawahara & Kida (J. Fluid Mech., vol. 449, 2001, pp. 291-300). The tangency of a pair of distinct homoclinic orbits is numerically discovered at the critical Reynolds number Re ≡ Uh/ν = 241.3 (U, h, and ν being half the difference of the two wall velocities, half the wall separation, and the kinematic viscosity of fluid, respectively) above which transversal homoclinic points appear on the Poincar´e section to generate a transient chaos through a Smale horseshoe.

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An outline of the state space of planar Couette flow at high Reynolds numbers (Re < 10^4) is investigated via a variety of efficient numerical techniques. It is verified from nonlinear analysis that the lower branch of Hairpin Vortex State (HVS) asymptotically approaches the primary (laminar) state with increasing Re. It is also predicted that the lower branch of HVS at high Re belongs to the stability boundary that initiates transition to turbulence. These facts suggest HVS may provide a criterion to estimate a minimum perturbation arising transition to turbulent states at the infinite Re limit.

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In order to understand dynamics of low-frequency fluctuation of centerline velocity in plane turbulent Couette flow, Proper Orthogonal Decomposition (POD) is applied to the streamwise velocity profile in the wall-normal direction to obtain the empirical orthogonal function. Instantaneous velocity profile was obtained by rake of 9ch hot wire anemometry. The 1st and 3rd modes are responsible to velocity fluctuation at the centre of the channel. Dynamical model is formulated using the POD eigenfunctions to project a simplified Navier-Stokes equation onto POD modes, since only single component of velocity was measured. We will report the performance of low-dimensional POD-based dynamical model in the meeting.

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Although the governing equations of flow are well known,we find no analytical expressions that exactly describe the complexity of turbulent motion,due to the complexity with the turbulence. But recenty periodic solutions in the spatially localized turbulence are discovered by Avilla et al.(2013). In this study,edge-tracking,a kind of bisection,is conducted in order to find the boundary of turbulent flow and laminar flow. We exhibit the features of spatially localized structures in the boundary,and their differences from those in a well-known turbulent puff.

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The VITA method is one of the statistical methods to investigate the coherent motion and the bursting phenomena in the turbulent boundary layer. Some researchers pointed out that the results analyzed by the VITA method contain the invalid detection of the bursting events and the VITA method can hardly capture the dynamic structures in the turbulent boundary layer. However, the reason of the incorrect detection has not been discussed in detail. The present paper focused on the turbulent coherent structure detected by the VITA method. We performed the DNS of channel flow with Re_τ=180 and investigates the characteristics of stochastic technique to capture the bursting phenomena. The numerical results of the present analysis showed that the coherent motion in the turbulent boundary layer was smeared out by the VITA method and those features are not easy to distinguish.

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