The Aeolian tone generated from two-dimensional objects has been widely studied by many researchers. In this study, the Aeolian tone and the related flow fields of three types of tapered cylinders are measured and are compared with those of straight cylinders. It is found that the level of the Aeolian tone decreases with the taper ratio. For the cylinder of smaller taper ratio, different peaks in the velocity fluctuation appear in blocks along spanwise location, corresponding to different peaks in Aeolian tone spectrum. The wake for higher taper ratio cylinder does not show the Karman vortex shedding and peaks of the Aeolian tone is not found.

View full abstract

In order to clarify the effect of turbulence on aerodynamic noise generation, aerodynamic sound and noise source distribution were measured. In the case of weak turbulence, the sound source was clearly observed at the formation region of Karman vortex. Except the fundamental frequency of Karmna vortex, sound source distribution depends on the scattering effect of the cylinder surface. On the other hand, the strength of sound source became large in the formation domain of Karman vortex. In the strong turbulence field, the sound source was disappeared. The result showed that the level of sound source around a circular cylinder strongly depended on a velocity fluctuation of mean flow direction.

View full abstract

In the present paper, the relationship between Aeolian Tone and Karman vortex in the wake of a finned tube was experimentally investigated. We measured the intensity of velocity fluctuation, spectrum of velocity fluctuation, coherence of Karman vortex in the spanwise direction, and SPL of the aerodynamic noise generated from finned tubes with various fin pitches. An Aeolian tone induced by Karman vortex shedding was observed in the case of a finned tube. When the pitch of the fins decreased, the peak level of the sound pressure spectrum decreased. A correlation analysis between the flow field and Aeolian tone was carried out.

View full abstract

This paper presents the prediction of the aerodynamic sound sources and the far-field sound on the wing-tip flow. Analysis object is NACA0012 of the lifting condition and Reynolds number based on chord length and uniform velocity is 2×10^5. The aerodynamic sound sources were calculated using incompressible LES (Large-Eddy Simulation) with 40 million grid points around the wing. Distribution of ∂T_<ij>/∂X_i∂X_j indicated that the sound source exist on the wing-tip. The far-field sound predicted by lihgthill tensor showed that the contribution into the airfoil noise by the sound generated from wing-tip flow is small at the Strouhal number 6 to 11.25 Hz.

View full abstract

Flow-induced acoustic resonance in a piping system containing closed coaxial side branches was investigated experimentally. Dominant frequency and arbitrary scale power of the frequency spectrum as functions of the flow rate in the main pipe was obtained by a microphone. The results reveal that the dominant frequency was locked to five different values with relatively high acoustic powers. These fixed frequencies indicate that five different acoustic modes are generated when the flow rate in the main pipe increased gradually. The lock-in frequencies were in good agreement with the results calculated from an empirical equation. The time-averaged velocity fields corresponding to the second hydrodynamic oscillation mode were also demonstrated at various phase of the acoustic cycle.

View full abstract

In this study, we investigated the relationship between the turbulent flows around and fluid noise from the blade of a centrifugal blower by the visualization experiment using smoke and PIV analysis. The visualization on the different cross-sections in axial direction of the impeller found that in the cross-section near the main plate of the impeller, the flow around a blade mainly flows to the radial direction, whereas in the middle cross-section between the main plate and side plate, the flow following the trailing edge of the blade occurs and the flow generates the vortices and the turbulence which causes the fluid noise.

View full abstract

Three-dimensional structure and unsteady nature of vortical flow fields in a half-ducted propeller fan have been investigated by a detached eddy simulation (DES) based on k-ω two-equation turbulence model. To elucidate the complex vortical flow phenomena, vortex structures were identified by the critical point theory, and limiting streamlines visualized by a line integral convolution (LIC). Sound pressure levels induced by each flow structure were predicted by Lighthill's acoustic analogy using the pressure data on the solid surfaces. It is found that the high pressure fluctuation on the shroud surface, which is caused by the rotating interaction of the blade tip loading and tip vortex with the shroud, play a major role in characteristics of the half-ducted propeller fan noise.

View full abstract

Aerodynamic sound generated from a propeller fan is simulated by decoupling approach of flow and sound fields. The incompressible Large Eddy Simulation (LES) is performed by finite element method with overset grids. In the acoustic computation, the Helmholtz equation is solved by boundary element method with a point dipole source, which is obtained by calculating the fluid force acting on the blades and struts of the fan. The predicted sound pressure spectrum shows good agreement with the experimental result.

View full abstract

Numerical simulations were conducted by using LES(Large Eddy Simulation) with DSM(Dynamic Smagorinsky Model) about the axial flow small fan for cooling electronic devices. In this study, the tested fan has an obstacle in the suction side. Distance to the obstacle is 20mm and 40mm. The numerical simulations of the flow around the fan blades were compared. In all conditions, fan has low pressure areas in center of suction side surface in blade tip. In addition, the low pressure areas in center of suction side surface in blade tip spread as the obstacle approached. The pressure level of low pressure areas fluctuates by fluctuation of tip vortex. This fluctuation of tip is remarkable as the obstacle approached.

View full abstract

Numerical simulations were performed by using LES(Large Eddy Simulation) with DSM(Dynamic Smagorinsky Model) about the small axial flow fan for cooling electronics devices. 3 different models with different rotating shaft (non-eccentric shaft, 1mm offset shaft of casing, actual eccentric shaft) were prepared to research effect of eccentricity of rotation axis. The Numerical simulations of those models were compared. Therefore, At no load condition, each model has surface pressure fluctuation area due to fluctuation of tip vortex and interference of motor stay at blade tip. And there is little difference in surface pressure fluctuation distribution on blade between these models.

View full abstract

The purpose of this research is to clarify the generation condition of aeroacoustic resonance noise radiated from a cascade of flat blades. Resonance frequencies and the sound pressure levels were measured in a low noise wind tunnel. The results showed the strong tonal noise generated when the shed frequency of wake vortex was equal to the acoustic resonance frequency of the flat-plate. Moreover, an empirical model for the estimation of the open-end correction of the resonance frequency was suggested. Also, we clarified the effects of the offset length between the blades on the range of velocity for the occurrence of the 2nd peak of aeroacoustic resonance noise and the sound pressure level of the 2nd peak.

View full abstract

This paper is concerned with the hole-tone feedback cycle, where an air jet issuing from a circular nozzle impinges upon a plate with a similar hole located a little downstream, generating self-sustained flow oscillations. Focus is on the influence of (i) a closed cavity and (ii) a tail pipe. The system can be viewed as a simplified model of an automotive muffler with a tailpipe.

View full abstract

Direct computations and experiments of a hole-tone feedback system are conducted. The mean velocities of an air-jet are 8 and 10 m/s in the computations, 6-13 m/s in the experiments. The diameters of a nozzle and an end plate hole are both 50 mm, and an impingement length between the nozzle and the end plate is 50 mm. The computational results agree well with the experimental data in terms of a relationship between the most dominant hole-tone frequency and a jet speed. Based on the computational results of the air-jet speed of 10 m/s, the propagation of pressure waves and associated vortical structures are discussed.

View full abstract

In the present paper the attention is focused on relationship between acoustic resonance and vortex shedding from closely arranged In-line tube banks which occurred in the two-dimensional model of boiler. We have examined the characteristics of vortex shedding, acoustic damping and acoustic resonance generated from in-line tube banks with tube pitch ratio of 1.33, 1.44, 1.56, 1.67, 1.76 and 2.0 in the flow direction. As the tube pitch ratio in the flow direction decreased, the acoustic damping increased and the vortex shedding frequency became broad-band. The resonance mode of low acoustic damping was generated in the broad-band vortex shedding frequency. It was easy to generate acoustic resonance of low order transverse mode as the tube pitch ratio in the flow direction increased.

View full abstract

There was a possibility that hole-tone was generated in a main steam stop valve (MSV) and caused the vibration and sound noise in the main steam line of the thermal power plant. To reduce the sound noise and vibration, scale air tests were conducted in the present study. Hole-tone broke out in Mach number more than 0.18. Low height slant tubs installed in the MSV seat completely suppressed the sound noise due to hole-tone.

View full abstract

Main steam stop valves in thermal or nuclear power plants are liable to do an excitation of sound, as called hole-tones. This phenomenon is undesirable because die induced sound causes loud noises and vibrations of mechanical structures. In this study, we investigated the hole-tone in the main steam stop valve using the finite difference lattice Boltzmann method (FDLBM). In order to improve accuracy of the FDLBM, an explicit four-stage second order Runge-Kutta time integration method and a sixth order central difference scheme were applied to discretization of the governing equation. Dynamic Smagorinsky model was also introduced into the governing equation as a turbulence model. Numerical results showed that expansion waves were generated by collision of vortices with the downstream edge. A pressure spectrum obtained by the numerical simulation agreed well with that of the experimental results.

View full abstract

There is a check valve which is widely used as one of the directional control valve. The valve is simple structures that push the valve body by the spring. But the vibration may cause the internal state of flow and channel shape and the valve body. It is important to assess the mechanism of this phenomenon and valve behavior in terms of longevity and reliability of fluid machinery. Therefore, in this research, we reveal the internal flow condition and visualization system that the model's valve.

View full abstract

In check valves used in fluid equipments, the pendulum oscillation-like vibration motions of the plug are observed in a visualization experiment using a high speed camera, when heavy noises generate around the valves. The same kind of motion was re-created by a numerical simulation which is based on the coupled analysis of the plug motion with the fluid flow motion analyzed by a CFD with Moving-Dynamic-Mesh method. Changes of the simulated plug vibration motion with one of the valve operation or geometrical parameters show the same tendency as the actual changes. Therefore, this proposed analytical method used here is expected to contribute to the valve-designs aiming at preventing the above-mentioned vibrations.

View full abstract

Butterfly's flight attracts us in terms of flapping-flight mechanism, in which separation vortices generated by flapping wing play an essential role. In this paper, mathematical analysis of such flapping flight are presented. Simplified flapping models of free-flight are numerically analyzed in terms of the bifurcation structure of the flight states, which provides us with an insight of the nature of vortex- using flight. Theoretical analysis of vortex-using flapping flight are also introduced. The theory is constructed based on the generalized Blasius formulae, which enable us to calculate the force acting on insects only by the far-field information.

View full abstract

The purpose of this study is to compare characteristics of flight motion between straight flapping motion and natural one. In order to understand the basis of lift and thrust generation, the finite difference method based on the ALE method is employed. Computation are performed for various flapping and feathering phase angles between the move like drawing figure-eight and straight translation motions. Time history of the unsteady force compared each other. Moreover, we survey the effect of Re, flapping frequency, feathering angle on fluid force in two flapping type.

View full abstract

In result years, Micro-Air-Vehicles (MAVs) mimic the flight mechanisms of insects have been attracting significant attention. However, these robots were not practical. One of the reasons for this is that the flying mechanism of insects has not yet been clarified sufficiently. The purpose of the present study is to clarify the dynamic behavior and the detailed structure of the vortices of the flapping butterfly wing. The vortex ring is formed over the wings regardless of the butterflies, but its scale varies among butterflies. When the wings flap upward to top dead position, the vortex ring passes through the butterfly wings completely and grows up to the wake.

View full abstract

Micro-Air-Vehicle (MAV) using insect and bird flight mechanisms have been researching all over the world in recent years since the micro-electro-mechanical systems (MEMS) have been developed. Many researchers have developed MAV with various actuators and devices however their MAV have not been put to practical use yet. The purpose is clarifying the flight trajectory of micro flapping robot at different flapping frequencies. The micro flapping robot moves upward in high flapping frequency flight and moves downward in low flapping frequency flight. The flapping frequency of micro flapping robot on a flight is one of important parameter for the flight attitude control.

View full abstract

Protoplasmic streaming is a phenomenon of particle movement in living cell. The particles were contained the cellular organelles and other particles called mitochondria and chloroplasts. We want to know the role of protoplasmic streaming in living plant cell. In this study we observed two types of protoplasmic streaming of Nitella and Tradescantia. The vesicle of moving speed and size were measured by microscopic observation. The cell division time was estimated from the volume of vesicle and cell, the moving speed and distribution of vesicles. As a result, the tradescantia cell division time was about 3.47×10^<17> hours. It was found that the protoplasmic streaming of Tradescantia was contributed 1.64×10^<-15>% for cell volume expansion

View full abstract

It aims to establish a method of measuring friction drag of bio-material under unsteady moving condition by using a motion capture system. The model covered with several materials was fallen in a water tank by gravity, and measured each speed by using a high speed video camera. We carried out the estimation of friction drag of the bio-material surface by an unsteady motion equation. In this study, the materials we evaluated were a hydrophobic cloth, artificial skins and a sea snake skin. As a result, the hairless artificial skin showed the smallest drag-coefficient and the hydrophobic cloth the largest.

View full abstract

Velocity distribution in the anterior chamber of porcine eyes under a simulated cataract surgery has been measured by using stereoscopic PIV. Surface location of the cornea was determined based on the images of laser-induced fluorescent light emitted from the fluorescent dye solution introduced in a posterior chamber. Cataract surgery was performed with a coaxial phacoemulsification system. Viscous shear stress on the corneal endothelium was estimated based on the measured mean velocity distribution. The maximum shear stress for most cases reached 3.5dyne/cm^2. This value of shear stress is large enough to detach endothelial cells.

View full abstract

This paper describes observations of thrombus formation process on orifice flows by laser sheet and velocity measurements by PIV method in the same circuit. In the observation of thrombus formation process, the brightness of CCD image corresponds to protein or thrombus concentration. In this paper, thrombus formation rate was estimated by the gradient of brightness history near the wall. In the PIV measurements, shear stress distribution on each orifice was obtained. Comparing the thrombus rate with the shear stress distribution on orifice of AB and CB, it was found that high shear stress region affects the thrombus rate in the orifice flow.

View full abstract

The purpose of this paper is to develop a computational model of microcirculation for heat transfer analysis. It is desirable for human heat transfer analysis to be able to consider macro and micro circulation. In a computational model, it was difficult to consider macro and micro circulation simultaneously. Therefore, in our model, we regard cell tissue and capillary vessel on microcirculation area as porous media to take a macroscopic view of microcirculation. The volume of fluid method and the finite volume method are adapted for its strategy. Blood flow and heat exchange among cell tissue, blood and vessel are considered simultaneously.

View full abstract

In this paper, assuming that the blood flow is multiphase flow composed of blood plasma and activated fibrinogen, the effects of surface tension near the wall are included in the modified lattice Boltzmann Method (LBM). To predict thrombus formation in every shear stress flied of flow, effects of threshold level of physical parameters such as shear rate, adhesion force (effective distance from the wall) is used for estimation thrombus formation rate. In this model, the most important point is to include the surface tension and adhesion force to the wall by introducing simple parameters such as the material properties of the wall. The new model for predicting thrombus formation by adding surface tension near the wall was proposed and the trend of thrombus's attaching to the wall could be simulated more adequately than in case of previous model

View full abstract

Understanding and utilization of unsteadiness associated with the flow field in turbomachines is considered very important to develop breakthrough-technologies for more efficient and environment-friendly turbomachines. This report mainly describes what unsteady flows in turbomachines are and how those unsteadinesses affects the aerodynamic performance of the turbomachines in favorable and adverse manners, followed by the description on recent progresses of computational and experimental studies to control and exploit the unsteady flows.

View full abstract

This paper details experimental studies on the flow field around a low-pressure linear turbine cascade. Highly loaded low-pressure turbine (LPT) blades are one of the key paths to successful future aero-engines, however those blades are usually accompanied with separation bubble, especially at cruise condition, eventually leading to the increase in aerodynamic loss. The purpose of this study is to clarify the effect of incoming wakes upon the aerodynamic loss of the rotor blade cascade, through the measurements of wake-affected boundary layers upon including separation bubble for low Reynolds number conditions. Two types of cylindrical bars on the timing belts work as wake generator to emulate upstream stator wakes that impact the rotor blade. Hot-wire probe measurement is conducted over the blade suction surface to understand to what extent and how the incoming wakes affect the boundary layers containing separation bubble. This paper focuses on difference of bar diameter in the characteristics of the wake affected separated boundary layer.

View full abstract

The effects of secondary flow in a turbine rotor on its downstream flow field are presented in this paper. We performed a RANS simulation to investigate complex flow phenomena in a turbine rotor. The simulation showed that the passage vortex, which was generated by the secondary flow inside the rotor, drastically developed on the hub side while the horseshoe vortex was not clearly identified at the leading edge of the rotor because of the skewed boundary layer. The passage vortex governed the flow field still at the downstream of the rotor, which could result in a negative impact on the flow field in the next stage.

View full abstract

The control of surge for centrifugal compressors was performed by using a nozzle injection system. A part of the flow at the scroll outlet of compressor was re-circulated to an injection nozzle installed on the inner wall of the suction pipe of compressor through the bypass pipe and injected to the impeller inlet. Two types of compressors were tested at the rotational speeds of 50,000rpm and 60,000rpm with the parameter of the circumferential position of the injection nozzle. The experimental results suggested that the optimum circumferential position, which effectively deduced the flow rate for the onset of surge, did not depend on the compressor system.

View full abstract

The rotating instability has been investigated at a near-stall condition in an axial flow compressor rotor with large tip clearance by EFD and CFD. This flow phenomenon has disturbances that move relative to the rotor blades and cause high amplitude rotor blade vibrations and tip clearance noise. Furthermore, it has closely related to rotating stall inception. In the present study, it has been found that rotating instability disturbances propagate in cyclic process of strong breakdown and weak breakdown of the tip leakage vortex.

View full abstract

Effects of the rotor tip clearance on the rear stator at near-stall conditions in an axial flow compressor stage have been investigated experimentally. For two different tip clearances, the internal flow structure and performance was analyzed by measuring the three-dimensional velocity field upstream and downstream of the rear stator. For a small rotor tip clearance, the flow blockage at the rear stator hub side suppresses the rotor stall inception. For a large rotor tip clearance, however, rear stator's wake is thin near the hub and thick near the shroud, and so the flow blockage at the stator hub side has small effect on the rotor stall inception.

View full abstract

The mechanism of the favorable stall-prevention effect by an optimum air separator in an axial flow fan has been clarified in terms of changes in the internal flow distributions caused by the air separator in the previous study [3]. In order to study further the effect, flow turbulence behaviors related with the improvement were surveyed with a hot-wire anemometer. Although rotating stalls were found in stalling in the solid wall condition, no such phenomena were detected in the air separator condition. Furthermore, the behaviors of flow unsteadiness were also seen to have been altered significantly by the air separator. The changes suggest improved internal flow situations achieved by the air separator from the aspect of flow turbulences also.

View full abstract

Small-sized axial fan is used as air cooler for electric equipments. But there is demand for higher power of fan according to the increase of quantity of heat from electric device. Therefore, higher rotational speed design is necessary but it causes the deterioration of efficiency and increases the noise. Then the adoption of contra-rotating rotor for small-sized fan was proposed for the improvement of performance. In the present paper, experiment was conducted to investigate the effect on performance by changing the axial distance between front and rear rotors of contra-rotating small-sized axial fan. And the relationship between the performance and axial distance between front and rear rotors was clarified with the experimental results.

View full abstract

The aerodynamic noise with broadband low frequency generated from a multi-blade fan has been investigated by experimental and numerical analyses. In the experiment analysis, static pressure fluctuations on the scroll side-wall of the multi-blade fan were measured by high response pressure transducers, and their frequency characteristics were analyzed by fast Fourier transform. In the numerical analysis, internal flow phenomena in the scroll were investigated by Reynolds-averaged Navier-Stokes (RANS) simulations. It is found that the noise with broadband low frequency is generated by the interference between flows with high turbulent kinetic energy and the scroll side-wall.

View full abstract

We developed a high performance blower stages to meet the need for reducing total energy-consumption in sewage treatment plants. Each blower stage has a two-dimensional impeller, low solidity vaned diffuser, and return channel. In impeller design, a shape-optimization utilizing an one-dimensional performance prediction tool was conducted. In design of the low solidity vaned diffuser and return channel, sensitivity analyses for the loss in the stage were conducted. Then new diffuser and return channel were designed using results of the sensitivity analyses. Orthogonal arrays were used in the sensitivity analysis. Model tests of both the conventional and optimized blower stages were carried out to compare the stage efficiency and operating range. The optimized blower stage improved in stage efficiency by 3% and in operating range by 5% compared with the conventional blower stage.

View full abstract

This paper presents the result of design optimization for three-bladed pump inducer using a three-dimensional inverse design approach, Computational Fluid Dynamics (CFD) and DoE (Design of Experiments) taking suction performance and cavitation instability into consideration. The parameters to control streamwise blade loading distribution and spanwise work for inducer were chosen as design optimization variables for the inverse design approach. The volume of the inducer cavity region with a void ratio above 50% was selected as the objective function for inducer suction performance. In order to evaluate cavitation instability by steady CFD, the dispersion of the blade surface pressure distribution on each blade was selected as the evaluation parameter. The effective design parameters on suction performance and cavitation instability were confirmed by sensitivity analysis during the design optimization process. Inducers with specific characteristics (stable, unstable) designed using the effective parameters were evaluated through experiments.

View full abstract

The degree of thermodynamic effect on inducer cavitation was discussed based on the non-dimensional thermodynamic parameter τ/τ_T. We attempted to relate the depression of cavitation number Δσ to the thermodynamic parameter τ/τ_T through the previous experiment. It was found that the depression of cavitation number Δσ increases as increasing the thermodynamic parameter τ/τ_T with rotational speed and temperature.

View full abstract

Visualization of tip leakage vortex cavitation was conducted to investigate thermodynamic effects on cavitation in liquid nitrogen. The photographs denoted that the tip leakage vortex cavitation in liquid nitrogen was consisted of tinier bubbles than that in water and collapsed near the trailing edge. These can be considered because the development of cavity was suppressed by thermodynamic effects and the tip leakage vortex was modified by the tiny bubbles. Moreover, the image analyses focusing on the evolution of cavity radius denoted that the radius was smaller and the peak shifted downstream as thermodynamic parameter Σ^* was larger. This can be considered because the mechanism of thermodynamic effect on tip leakage vortex cavitation is different between a large cylindrical bubble in water and a cluster of tiny bubbles in liquid nitrogen.

View full abstract

In order to realize the effective micro-chemical plant, which is expected to be one of the next generations of chemical plant with high chemical reaction efficiency, the development of suitable pump which must be able to deliver working fluids precisely with keeping the flow being laminar is necessary. We have been studying on the multiple-disc pump as a candidate pump for the micro-chemical plants. In the present study, the effects of the gap between discs and the rotational velocity on pump performances are investigated using a large scale model of multiple disc pumps. The effect of casing geometry is also examined. Performance similarity is investigated toward the development of real-size multiple-disc pumps.

View full abstract

Mixed convection heat transfer from a horizontally placed upward-facing semi-infinite plate to a uniform downward forced flow is numerically analyzed. The Reynolds and the modified Grashof numbers are 500 and 5×10^7, respectively. Characteristic longitudinal vortical structure, which was observed in the recent experimental study, is successfully simulated. The typical structure appears near the plate as a pair of counter-rotating vortices, which is elongated over the plate. Special attention is focused on the relationship between this structure and the heat transfer. After the emergence of the vortices, overall Nusselt number, which shows irregular unsteadiness, rises up by more than 30 percent on average. Time series of the overall Nusselt numbers and the average heights of the vortices have a significant negative correlation, which means that the longitudinal vortices near the plate enhance the overall heat transfer at that moment, and vice versa.

View full abstract

We studied the influence of gas-liquid interface on the drag reduction effect by numeric simulation. Level set method was used for an analysis of gas-liquid interface. The liquid penetrates in the microstructure when the surface is hydrophilic. But for hydrophobic surface, it was clarified that a gas layer keeps the shape in the microstructure in the flow condition. And the position of gas-liquid interface can move due to velocity of main channel. In addition, from the analytical result using the model that alternately applied slip and no-slip boundary condition to the wall surface, the drag reduction effect is dependent on not only Reynolds number but also the position of gas-liquid interface.

View full abstract

We investigated splash produced by spherical object plunging to water surface. We focused on the effect of swelling of object. We compared the splash versus of swelling of gel object surface. The splash form we observed was crown-type that is formed by water jets with many droplets. The height of splash became higher as swelling increased, arid the speed of gel object sinking down in the water also became higher. The gel surface of object was effective for the reduction of friction of an object moving in liquid.

View full abstract

Drag reduction phenomenon of a pulsating turbulent pipe flow has been experimentally examined when the cycle-averaged friction Reynolds number based on the friction velocity, the radius and the kinematic viscosity was set to be 110. The rotating speed of a vortex pump was periodically changed for generating the pulsating flow. The turbulence statistics were measured by Particle Image Velocimetry (PIV). The cycle-averaged Reynolds shear stress was deteriorated in the near-wall region at y^+<60, resulting in the efficient drag-reduction effect. The phase-averaged Reynolds shear stress aty^<60 was decreased in almost all the phases.

View full abstract

Experimental study of a new three-dimensional (3-D) blade riblet is carried out. The lateral spacing of our 3-D riblet surface is periodically changed in the streamwise direction. Obtained drag reduction rate is about 6% at Re_τ= 150. The flow structure over the 3-D riblet was also analyzed in instantaneous velocity fields by using 2-D particle image velocimetry on a horizontal near-wall plane and compared with the corresponding flow over the flat surface in an attempt to identify the physical mechanisms that cause drag reduction. Moreover, the spanwise two-point correlation R_<u'u>(Δz^+) length increases on the riblet surface, indicating that the local friction velocity u_τ is decreased. It is also found that the streamwise persistence of the streaky structures becomes shorter on the drag reducing surface.

View full abstract