The 4th International Conference on Jets, Wakes and Separated Flows (ICJWSF2013) was held in Nagoya, Japan, in September 2013. Advanced papers were presented at the conference, contributing to the progress of the research. This report is a summary of the published papers, focusing on turbulent flows. The papers are classified to seven fields: generation of turbulence fields, free shear flows, separated flows, wall shear flows, multi-physics flows, fluid machinery, and numerical methods/bio-fluid mechanics. The research works in each field are reviewed, noting the contribution to the understanding and control of turbulent flows. For generation of turbulent fields, the unique features of fractal-grid turbulence are discussed by the results of DNS and experiments. For free shear flows, the fundamental features are investigated, and various flow-control methods are demonstrated in engineering applications. For separated flows, many studies on separation control are presented, especially demonstrating the effectiveness of plasma actuators. For wall shear flows, the latest works are presented, revisiting the characteristics of statics and turbulent structures by the recent results of DNS and experiments. For multi-physics flows, the studies for engineering applications are presented in the fields of multiphase flows, reacting flows related to combustion, and aeroacoustics. The results suggest useful ideas to develop innovative technologies in each field. For fluid machinery, interesting studies of wind turbines are presented to increase efficiency. In conclusions, the contribution of the conference is summarized and the future issues on the research of turbulent flows are mentioned.
This paper 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 an 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 when changing from steam-diluted to dry conditions, and a swirl-stabilized flame subjected to strong axial forcing mimicking thermo-acoustic oscillations.
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 phase synchronization in order to relate velocity and vortices to the rotating blades. The tip vortices are investigated in successive azimuthal positions of the rotor. A specially developed algorithm based on the circulation maximum detects the positions of the vortex cores and permits to use conditional averaging technique. The analysis of obtained velocity fields enables to determine the vortex core diameter, the swirl velocity distribution and the vortex diffusion as functions of the vortex age. The quality of obtained results permits to use them as reference for the validation of numerical computations.
This paper considers flow-induced pressure pulsations in engineering systems that involve a cavity mounted in a pipeline. These self-sustained oscillations pose substantial risk, as they can cause fatigue damage to the system and the associated acoustic pressure levels can be harmful to humans. This review focuses on experimental and semi-empirical methods of prediction of the acoustic response of pipeline-cavity systems, with specific emphasis on the description of the acoustic noise source and the interaction between the separated flow that exists in the vicinity of the cavity opening and three types of resonant acoustic modes that can be excited by the flow: longitudinal modes of the main pipeline, transverse modes of a side branch (deep cavity) and azimuthal modes of an axisymmetric cavity. Fundamental insight into the physical mechanism of excitation of the resonant acoustic modes provided by quantitative flow imaging is discussed. In particular, application of particle image velocimetry (PIV) for identifying and quantifying the features of the acoustic source is described for each type of the acoustic modes, along with the associated challenges and the limitations of applicability. Moreover, effects of the modifications of the system geometry on the acoustic response and the structure of the noise source are presented. The geometrical features considered in this paper include asymmetry of the main pipeline, splitter plates located in the vicinity of the opening of the cavity, and chamfers of the leading and the trailing edges of the cavity.
In order to elucidate the heat and flow characteristics of multiple impinging jet (MIJ), we conduct the DNS (direct numerical simulation) of four round impinging jet arranged at an inflow of flow field. To realize the high accurate computation, the discretization in space is performed with hybrid scheme in which sine or cosine series and 6th order compact scheme are used. For the surrounding boundary condition, in order to reduce the artificial noise due to the occurrence of vortex at the outflow boundary, a fringe technique is newly developed and introduced at the surroundings of computational volume. As a control parameters, a separation between each jet, i.e., jet-to-jet spacing is varied. From view of instantaneous and time-averaged flow field, it reveals that the generation of vortical structures are enhanced due to interactions between each jet, and that a large-scale upward flow, generally called as fountain is formed between each jet according to the jet-to-jet spacing. In addition it turns out that the both flow and heat transfer characteristic of each jet comes close to that of single impinging jet (SIJ) beyond a specified jet-to-jet spacing. Further the heat transfer performance in the interaction region between jets is evaluated compared to that of SIJ and the contribution of recirculation flow to the heat transfer is clarified.
In this study, the direct numerical simulation (DNS) for homogeneous shear turbulence in the system rotating along the streamwise direction is fulfilled. Due to the rotation effect, the turbulence energy becomes small during the initial short time and the Reynolds shear stress are suppressed more strongly with increasing the system angular velocity. Since the redistribution related to the rotation and pressure-strain terms of the Reynolds normal stress is weakened by the streamwise rotation, the anisotropy of the normal stresses is strengthened. In the strong rotation case the anisotropy of the velocity spectra is strong in the whole wavenumber region in contrast with the isotropy of the non-rotating case in the large one. From the viewpoints of the probability density function (PDF) for the vorticity vector angle and visualization for the vortex structure, we find that the vortex structures become large and stand in a line by the streamwise rotation axis. Moreover, we suggest that the rapid distortion theory (RDT) simulation reproduces the rotation effect on the mean quantities of the DNS results in only short time immediately after a calculation start.
In this study, a new patch-type hot-film sensor is devised to measure wall shear stress, and its measurement accuracy is validated. The sensor consists of heaters on a thin polyimide film and a flexible printed circuit. The film is supported by silicone rubber with a hole in it, which forms a cavity under the heating element to improve the response of the sensor. Static and dynamic response tests show that the sensor properly responds to the wall shear stress and that its roll-off frequency is 10 Hz. Results also show that the sensor can measure the time-averaged wall shear stress in a boundary layer of the wall jet and can detect wall shear stress caused by the velocity gradient of the cross-streamwise profile of streamwise velocity in a boundary layer.
In this study, the Schmidt number dependence of the scalar statistics and velocity-scalar joint statistics is investigated experimentally in a liquid axisymmetric jet. In the experiments, the axial velocity, radial velocity, concentration of the diffusing dyes, and temperature are measured by an X-type hot-film, optical fiber probes, and an I-type cold-film, respectively. In addition simultaneous measurements of velocities and concentration, and velocities and temperature are also conducted. As diffusing dyes, C.I.Direct Blue 86 and Rhodamine 6G are chosen whose Schmidt numbers are 3,800 and 1,000, respectively. The Prandtl number of temperature is about 7. Experimental results show that the Schmidt number dependence of the spatial derivative statistics is observed. The skewness of concentration spatial derivative is smaller than that of temperature spatial derivative. Further, the radial velocity-concentration cospectrum and radial velocity-temperature cospectrum show differences each other. The slope of radial velocity-temperature cospectrum is -7/3, while that of radial velocity-concentration cospectrum is about-2.
The lattice Boltzmann method 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. In order to examine the validity of the red blood cell (RBC) model, the method is applied to the motion of a biconcave disk-shaped body in a pressure-driven circular pipe flow. The validation is demonstrated by investigating the relation between the deformation index and terminal axial velocity of the RBC in the pipe flow. In this study, the behavior of a biconcave disk-shaped body in constricted pipe flows is simulated under various geometrical conditions. The square and circular pipes with various lengths and sizes of the constriction are considered, and the flow is induced by the pressure difference between the inlet and outlet. From the results, it is found that as the length of the constriction becomes smaller, the body is deformed larger and accelerated at the entrance and exit in the constriction, although the speed of the body is reduced while passing through the constriction. Also, it is found that as the size of the constriction becomes larger, the deformation index linearly decreases and the axial velocity exponentially increases. These results indicate that the present method has applicability to simulation of the motion of RBCs in microscale capillary blood vessels.
We investigated the mechanism for the removal of fine particles from a solid wall using a high-speed impinging air jet. In general, it is difficult to remove fine particles of the order of micrometers by the impingement of simple air flow because they strongly adhere to the surface by van der Waals forces and remain immersed in the viscous sublayer. To overcome this, we developed high-speed air jet nozzles with triangular cavities that add strong velocity and pressure fluctuations to the high-speed air flow. The experimental results showed that the cavity nozzle enhances the removal performance for particles larger than 1 μm. The effect of the pressure fluctuation induced in the jet flow on the removal performance is discussed from the experimental results. First, the adhesive force was measured experimentally from the centrifugal force acting on particles with 5-25 μm diameters set on a rotating disk. Based on a simple theoretical consideration regarding the balance of moments acting on a particle, we estimated the effects of hydrodynamic removal forces such as drag, lift, and pressure gradient fluctuation against measured adhesive forces. The theoretical estimation showed that drag plays a major role, and the force of the pressure gradient could be effective for the removal of large particles. The proposed model is able to explain the experimental results indicating that the removal rates for 3-μm-sized particles are improved by the air flow velocity fluctuations generated by the cavity nozzle.
Over the last decade, synthetic jets suitable for micro-machinery have received attention for their potential to replace continuous jets. The development of synthetic jet actuators with, for example, a diaphragm, a piston, a piezoelectric element, or a speaker cone instead of mechanical drivers, is required for the downsizing and weight reduction of flow control systems in fluid machines. In this study, an experimental prototype for a synthetic jet actuator that uses the nonlinear oscillation of bubbles produced by repetitive electric discharge is proposed. Numerical simulations are performed to clarify the fundamental flow behavior of the synthetic jets produced by bubble motion. The behavior of a bubble induced by a single discharge and the estimated change in nozzle exit velocity with time are shown, and typical flow patterns for synthetic jets produced by periodic electric discharge are discussed. The influence of the ratio of the bubble driving cycle period to the electric discharge cycle period (T*) on the unsteady flow pattern and the time-averaged jet structure is investigated in detail. In addition, the flow characteristics of a synthetic jet with downtime are compared with those of a normal synthetic jet produced by linear oscillation under the condition that the strokes of both jets are equivalent.
Array of vortex rings in circular synthetic jets was investigated experimentally by instantaneous velocity measurements using a hot-wire anemometer. The ensemble averaged velocities were derived by referencing the oscillatory flow in a circular orifice. The contours of the ensemble averaged velocity showed that the elliptical shaped high-velocity regions were generated per a cycle of the oscillatory flow. These high-velocity regions were caused by vortex rings, consequently their travelling and disappearance in the downstream indicated the convection and collapse of the vortex rings. The structures of vortex array were examined for seven dimensionless strokes of the oscillatory flow range of L/d = 0.712-7.10, where the stroke L was the length of the fluid ejected in an oscillatory cycle and d is the diameter of the orifice. It was found that vortex rings with nearly equal intervals along the jet axis travelled downstream, and collapsed in a further downstream location. In the process of vortex collapse, no direct interactions with the neighbouring vortices were observed. For the dimensionless strokes larger than 3.6, it was observed the vortex ring accompanied weak and small trailing vortex rings. It was also found that dimensionless convection velocities of vortex ring were independent of the stroke, but the spatial intervals of vortex rings increased as the stroke was increased. As the stroke was increased, the location of the vortex collapse moved downstream. The correlation between the location of the vortex collapse and the starting point of jet width growth was good. The effect of the stroke on the jet evolution was attributed to the change in the location of the vortex collapse.
The free turbulent shear flow behind a long axisymmetric cylinder, i.e., a wake, is two-dimensional in nature and is composed of inner shear turbulence and outer potential, irrotational flow (free stream) regions. The conventional definitions of wake width, lu, are based on the mean velocity field, namely, the first order statistic of turbulence. Two alternative definitions of wake width, lS = 2yS and lF = 2yF, are proposed in terms of skewnes and flatness factors, respectively, which are considered as third- and fourth-order statistics of turbulence in this study. Between these two new definitions of wake width, lF can represent a more proper sectional range of shea turbulence in a wake.
In the piping system such as power plants, the pipe wall thinning caused by flow accelerated corrosion (FAC), liquid droplet impingement (LDI) erosion, and cavitation erosion (C/E), are very serious problems because they lead to serious damage and destruction of the piping system. In this study, the pipe wall thinning caused by FAC in the downstream of an orifice (flow meter) was examined. Experimental and numerical analyses were carried out to clarify the characteristics of FAC, generation mechanism, and to propose of a new governing parameter for the prediction of the thinning and a reduction method. The corrosion pattern on the pipe wall was also examined by an experimental analysis. This clarified that the occurrence of thinning mainly depends on the amount of the pressure fluctuation p' on the pipe wall. It was also found that the wall thinning rate TR can be estimated by p' and that the suppression of p' or TR can be realized by replacing the orifice with a tapered one having an angle to the upstream or by using a downstream pipe with a smaller diameter than that of the upstream of the orifice.
Jet flow phenomenon is one of the most important basic flows in the field of fluid dynamics because it includes the free and wall bounded turbulent shear flows and large-scale vortex structures. In addition, it is used widely in many industrial fields for heating, cooling, mixing, diffusion, fire extinguishing, propulsion, and others. By the way, in order to improve the mixing and diffusion characteristics between the jet and surroundings there are many trials, for example, by using of various non-circular nozzles. In this study, the flow characteristics, mixing and diffusion properties, of the submerged free jet from the petal-shaped special nozzle with 6 petals are examined experimentally. It is expected that 6 petal-shaped free jet will have a good mixing and diffusion characteristics because of a large wetting perimeter and forming of large vortex structures.
The laminar-turbulent transition of a mixing layer excited by oscillating flat plates at an exit of a two-dimensional nozzle was experimentally investigated. The mixing layer was formed between the jet issued from the nozzle and the surrounding quiescent fluid. The plates oscillated vertically in relation to the mean flow. Upper and lower flat plates oscillated anti-symmetrically. The oscillation frequency, 5 Hz, was two orders of magnitude smaller than the fundamental frequency of the velocity fluctuation in the natural transition process. Mean and fluctuating velocity components in the streamwise and normal directions were measured by hot-wire anemometers. The results were compared with the previous case in which the plates oscillated symmetrically. Anti-symmetrical oscillation promoted the expansion of the mixing layer and promoted the disappearance of the potential core more than symmetrical oscillation. The periodic motion of the flow promotes the fluctuation from the time-averaged velocity. The fluctuations from time and phase averages in the anti-symmetrical oscillation were larger than those in the symmetrical oscillation. The contribution of periodic fluctuation disappeared downstream in the symmetrical oscillation but persisted longer in the anti-symmetrical oscillation. In the linear region of the transition, the irregular fluctuation grew exponentially. There were not any differences in the spatial growth rate between the streamwise and normal directions and among three oscillating states. In the nonlinear region, factors other than the temporal fluctuation energy convection and production rates may contribute to the increase in velocity temporal fluctuation in the streamwise component. On the other hand, the convection and production rates may contribute to the increase in velocity fluctuation in the normal component there.
The heat transfer characteristics for the process tubes of a cracking furnace and the combustion chamber wall of a liquid oxygen/gas hydrogen rocket engine, both of which use multiple nozzles, are very important in terms of performance and safety. In order to determine the heat transfer characteristics of the combustion chamber wall, the present study investigates both experimentally and numerically the combustion characteristics of H2/air annular jet flames using multiple shear coaxial nozzles in a small combustion chamber under normal conditions. Three-dimensional simulations are performed in order to clarify the flame-flame interaction. The standard k-ε model is used as the turbulence model, and so the evaluation of the model for multiple nozzles is also an objective of the present study. Each flame appears as an independent flame until amalgamation, at which point the temperature increases. Further downstream, the high-temperature regions once again merge and form a large flame. At this point, the flame becomes squeezed and the temperature distribution spreads rapidly in the radial direction downstream. The wall heat flux is strongly influenced by the flow characteristics. Heat transport is weak in the near field. The turbulent heat transport downstream is dominant where turbulence is developed, and thus the wall heat flux is increased. An increase in Reynolds number based on the airflow Reair shifts the peak position of the wall heat flux upstream because the turbulent heat transport is enhanced. The increase in the recess shifts the amalgamation position upstream and shortens the flame length. Spreading of the flame is also suppressed. The temperature decreases downstream. The increase in the recess leads to a reduction in EINOx. Under the present experimental conditions, the numerical method reproduces the combustion characteristics with a high degree of certainty.
The addition of steam to the combustion process is a promising avenue, in order to reduce harmful emissions and simultaneously increase the efficiency. In contrast to traditional flames where the heat release exhibits a thin flame front, operating under high levels of steam dilution leads to further distributed flames with less steep temperature gradients. It is shown that a non-diluted flame suppresses the occurrence of a helical instability present at isothermal conditions. Injecting high amounts of steam alters the flow field and the helical structure is re-established. This report is dedicated to the simulation of methane flames with high steam contents applying Large Eddy Simulations and detailed chemistry in a model gas turbine combustor. A suitable reaction scheme is identified and is then employed for the simulations of the reacting flow. For the validation of the simulations OH* chemiluminescence recordings and PIV measurements serve as a reference.
We improve a spatial resolution of concentration measurement system based on the light absorption spectrometric method, which is used to measure instantaneous concentrations of multiple absorptive species. Light-emitting diodes (LEDs), which have a higher light intensity than a halogen lamp used in the former concentration measurement system, are used as light sources of the new concentration measurement system. We also develop a new optical fiber probe with a higher spatial resolution than the former one. The diameter of the optical fiber bundle and the length of the sampling volume of the new optical fiber probe are 0.1 mm and 0.5 mm, respectively. The sampling volume of the new optical fiber probe is 35 times smaller than that of the former one. In a turbulent planar liquid jet with a second-order chemical reaction A ＋ B → R, concentrations of all reactive species are measured by using the new concentration measurement system, and we validate the new concentration measurement system by comparing the measurement results obtained by the new system with those obtained by the former system. The results show that the mean concentrations of reactive species measured by the new system are consistent with those measured by the former system. The power spectra of concentration fluctuations of reactive species show that the new system is able to measure the concentration fluctuations up to higher frequency ranges than the former system.
An experimental study was conducted on the promotion and control of turbulent mixing of hot and cold airflows in a rectangular channel with a T-junction, which simulated an HVAC unit for automobile air-conditioning system. A delta-wing row was installed on the bottom wall of the main channel before the T-junction to promote and control the turbulent thermal mixing. The mean temperature and velocity distributions were measured in several cross sections after the flow merging by thermocouples and PIV, respectively, and the flow in the mixing region was visually observed as well. The development of the thermal mixing layer was promoted effectively by delta wings and the degree of thermal mixing could be controlled by changing the angle of attack of wings. Longitudinal vortices produced by delta wings disappeared just after the merging of two flows, but high turbulence generated by the interaction of those vortices and the branch flow was maintained to further downstream cross sections. The proper orthogonal decomposition analysis was applied to the fluctuating concentration and the fluctuating velocity fields to extract the dominant spatial structures in the T-junction. It was suggested that longitudinal vortices with clockwise and counter-clockwise rotations were shed alternately just behind each delta wing.
In order to develop a new mixing procedure, we conduct DNS (direct numerical simulation) of dynamic controlled free jets. As the dynamic control, it is assumed that the inflow of jet rotates around the streamwise axis. To realize the high accurate computation, the discretization in space is performed with hybrid scheme in which sine or cosine series and 6th order compact scheme are used. From view of instantaneous vortex structures, it is found that the flow pattern considerably changes according to the rotating frequency, i.e., according to the increasing the frequency, helical and entangled mode appear in turn. From the ensemble averaged flow properties such as half jet width and entrainment, it is confirmed that the jet widely spreads perpendicular to the rotating axis and that the mixing of upstream jet is markedly enhanced due to the dynamic control. Further in order to quantify the mixing efficiency of the dynamic control, as another mixing measure, a statistical entropy is examined. Compared to the uncontrolled jet, it is confirmed that the mixing efficiency is markedly improved, suggesting that the dynamic control can be expected to be useful for the improvement of mixing performance.
Jet powered aircraft radiate a substantial amount of noise around airports. Among several noise sources of the aircraft, high-speed jet behind the nozzles of jet engines generates strong sound through mixing process of jet with the surrounding flow. One of the effective measures against the jet mixing noise is to control the flow field so as to weaken shear in the jet plume. The authors have proposed a claw mixer as a retractable mixing control device. The claw, composed of half-edged nails, is placed at the nozzle end. In the preliminary noise test using a jet engine, the nails with a configuration of 60° inclination relative to jet and a 30% penetration provided sufficient noise reduction in the downstream of jet. However, this configuration of nails resulted in increase of additional noise at higher frequencies. To improve this acoustic property, this study attempted to redesign the nail configurations with a help of large eddy simulation. The revised nail configurations, e.g., shallower inclination and less penetration of the nails, suggested less turbulent kinetic energy at higher frequency and thus less additional noise increase to the lateral of jet. The subsequent scale-model test on the revised claw mixer confirmed suppression in both broadband peak noise and the higher-frequency noise.
T-junction pipes are frequently used in industrial facilities to split one flow into two flows or to merge two flows into one flow. In the counter-flow type of confluence in this type of pipe, separated vortex flow regions are formed near the junction corners and these regions cause a large flow resistance. The corners of a T-junction pipe are generally made round to avoid flow separation and to reduce the flow resistance or loss, but this method requires a junction corner removal process. In this study, we mount small weir-shaped obstacles on the walls of pipes upstream of the junction corner to reduce the loss. We examine the effect of the flow rate ratio on the loss and clarify the obstacle height and position conditions that reduce the loss for a wide range of flow rate ratio.
An attempt to control an unsteady flow separation from a flat plate surface using a jet ejection is carried out. The unsteady flow is artificially generated by periodically moving a curved ceiling up and down at a constant frequency. The controlling system consists of a jet ejecting device on the upstream side and tufts attached to the surface on the downstream side. The movements of the tufts are monitored by a USB camera to watch for a flow separation. Once a separation is detected, a jet is ejected into the boundary layer through a slot in the plate. The jet ejection causes the separation region to shift downstream and become smaller. In addition, it is revealed that the control effect can be enhanced by stirring the fluid, generating turbulence, before it is ejected into the boundary layer.
This study presents the detailed structure of separated flow downstream of a backward facing step affected by a non-uniform periodic disturbance along spanwise direction induced by a synthetic jet array. The laminar channel geometry over a backward-facing step with an expansion ratio of 1.67 and with a step height of 4.0 mm was used in this study. The Reynolds number based on the step height ranged from 300 to 900. The frequency of the synthetic jet actuation was selected within the acceptance frequency range of separating shear layer. Periodic transverse vortices were generated changing their sizes and shapes corresponding to the strength of the disturbance by the synthetic jet array. The effect of different injection velocities in the jet array from those of adjacent jets on the transverse vortex structure and resulting reattachment process is discussed based on the wall shear stress measured by the Micro Flow Sensor (MFS) and flow visualization. Near wall behavior of the transverse vortex above the MFS was related to the sensor output. The results show that non-uniform injection velocity manipulated in the jet array induces remarkable difference in the distorted vortex structure and reattachment distribution in spanwise direction, which strongly depend on the Reynolds number and injection velocities of the synthetic jet array.
The flow control performance of the pulse-actuated plasma actuator and their mechanism was investigated. To clarify the flow mechanism of the separation control on a circular cylinder when pulsed plasma actuator is applied, the characteristics of the induced jet and their influence on the separated flow were separately examined. The characteristics of the induced jet from the pulsed plasma actuator were 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 examined in the viewpoint if aerodynamic force and the flow field measured with time-resolved PIV at the low speed wind tunnel test. For the induced jet analysis, the strength of the induced jet is found to be widely varied with the modulation frequency. When the modulation frequency is high, over 250Hz, the thrust of the induced jet increases with the modulation frequency increased. For the separation control test, the drag is significantly dropped with increasing the modulation frequency. From these results, it turns out that the enhancement of the induced jet generated from plasma actuator in high modulation frequency regime is the main cause of the enhancement of drag reduction performance.
This study deals with the separation control of an airfoil when a dielectric barrier discharge (DBD) plasma actuator is mounted on its leading edge. The experiments were performed at Reynolds number, Re ≈ 67,000 in an external airflow of 10 m/s. The DBD plasma actuator was installed at x/c = 0.025 of a NACA0015 airfoil with a 100-mm chord and 150-mm width. Flow visualization, lift force and velocity measurements were conducted with on and off modes of the actuator (pulse-modulated drive). We found that the application of pulse modulation condition was able to delay a stall up to angle of attack α =17° compared to the Duty = 100% (α =15°). At α =17°, the maximum lift coefficient Cl was obtained for a non-dimensional pulse modulation frequency St of 4.0. With an increase in α, the lift coefficient Cl decreases sharply after reaching a maximum, which is entirely different from the case of St = 0.6, where the value of Cl falls gradually. For a high angle of attack (α =18°) that exceeds the maximum Cl, St = 0.6 produces a better lift improvement than when St = 4.0. Our results showed that the separation when St = 4.0 is larger than that when St = 0.6. In addition, we also found that there is an optimum actuation time of about 0.25 ms.
A streamwise vortex pair induced by a counter-type plasma actuator interacts with the separated shear layer from a circular cylinder. The spanwise array of the plasma actuator placed on the surface of the circular cylinder works in place of the vortex generator. Induced jets from the adjacent actuators interact with each other and produce streamwise vortex pairs. Flow visualizations in quiescent air and in the wake region of the cylinder were performed using a high speed camera and PIV analysis. The spanwise flow structure in the cylinder wake was investigated through hot-wire measurements. Flow visualization revealed that vortex pairs generated by the counter-type plasma actuator were effectively introduced into the separated shear layer. Hot-wire measurements showed that the interaction between the streamwise vortex pair with the shear layer enhanced the turbulent mixing, improving the velocity defect in the wake.
The flow field around a vertical axis wind turbine (VAWT) is very complicated because the blades pass in non-uniform flow field by the blade wake during rotation. There is some difficulty in measuring the aerodynamics acting on VAWT at a low tip speed ratio during rotation, because the relative wind speed and the angle of attack for the blade are periodically changing. So few experimental data are available for the low tip speed ratio compared with the high tip speed ratio. This paper presents a method for wind velocity distribution around VAWT at low tip speed ratios. According to this research, the flow field around a straight-bladed VAWT which has three blades is analyzed. The wind velocity in flow field is measured by using the Laser Doppler Velocimeter (LDV) in wind tunnel, and the wind velocity distribution is obtained under different azimuth angles. Flow field characteristics are also investigated for several values of tip speed ratio. Firstly, a wide low wind velocity field appears from the wind turbine internal region to downstream region. Secondly, from upstream to downstream region, the velocity deficit has become greater in the mainstream direction, at the same time, the reverse flow is only generated in the back of the cover of the rotation axis at optimum tip speed ratio. Finally, large turbulence intensity is generated at the inside of wind turbine.
The effect of 3-dimensional blade on the turbine characteristics has been investigated experimentally by model testing under steady flow conditions and simulated numerically by quasi-steady analysis under sinusoidal flow conditions, in order to improve the stall characteristics of Wells turbine for wave energy conversion in the study. Aim of the use of 3-dimentional blade for Wells turbine is to prevent flow separation on the suction surface near tip. The chord length is constant with radius and the blade thickness increases gradually from hub to tip. The blade profiles are NACA0015 at hub, NACA0020 at mean radius and NACA0025 at tip. The performance of Wells turbine with 3-dimensional blades has been compared with those of original Wells turbine, i.e., the turbine with 2-dimensional blades. As a result, it has been concluded that both the efficiency and stall characteristics of Wells turbine can be improved by the use of 3-dimensional blade.
This study focuses on the characteristics of the Aeolian tone and Karman vortex shedding from a circular cylinder with a solid spiral fin. The fin pitch ratios range from 0.11 to 0.80. We measured the turbulence intensity, the spectrum of velocity fluctuation, the coherence of the Karman vortex in the spanwise direction, and the sound pressure level (SPL) of aerodynamic sound. The Aeolian tone induced by Karman vortex shedding was observed in the case of a solid spiral finned tube. For a small fin pitch, the peak level of the SPL spectrum, periodicity of vortex shedding, and coherent spanwise scale of Karman vortex all increased. However, the locations of the maximum turbulence intensity in the wake of a finned tube was found not move downstream when the fin pitch was decreased. The vortex shedding frequency could be estimated by the equivalent diameter of finned tube, as well as the bare tube, in a manner similar to the Strouhal number.
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 et al. (1996) 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.(14) 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 features of the Mach wave generated in supersonic round jets, which lead to extinction of the Mach wave by introducing disturbance condition. Upstream disturbance conditions play an important role for the emission of the Mach wave in supersonic jets. The pressure fluctuations generated by the growth of the opposite helical mode are shown to be linearly superimposed into the jet near sound field. The numerical results indicate that the jet forced with a pair of first helical modes can indicate the elimination of Mach waves at restricted emission azimuthal angles due to the interference of these modes. The partial elimination of the Mach wave also appears in a turbulent jet at the frequency of the artificially forced an optimal combination of the helical modes at the inlet region. This forcing technique will be extended to the Mach wave reduction in the distinct azimuthal direction.
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 1.0 × 105 (available only for steady components), 3.2 × 105 and 5.0 × 105. Results showed that distributions of fluctuating normalized pressures obtained here and those made by Shiraishi et al. (2006) for the Reynolds number of 3.25 × 10 6 coincided within 0.04 of the dynamic pressure. These distributions had the same tendency: The strong fluctuating region existed in the curvature inside and had concave/convex shapes at the upstream/downstream ends, respectively. Power spectral density functions of fluctuating pressures mostly exhibited the slope of the minus seven-third law, which is seen in the inertial range of turbulence, 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 slope at large Strouhal numbers became negatively steep near the region where the peak spectrum was seen. The peak intensity having the Strouhal number of 0.5 was quantitatively in accordance with that of the data obtained in the experimental setup that Shiraishi et al. used (Yamano et al., 2011), suggesting that the law of dynamical similarity could be applied with regard to this oscillation. Cross correlations of pressure fluctuations showed that the pressure fluctuation having the Strouhal number of 0.5 propagated as a planar wave with the bulk velocity.
In the case of an elastically mounted rectangular prism with a side ratio of less than D/H = 0.5 (where D: depth of a bluff body in the flow direction, H: height of a bluff body normal to the flow direction), it is well known that low-speed galloping appears at a lower reduced velocity than the resonant reduced velocity. In order to investigate the effects of the geometric shape of the bluff body's cross section on the flow-induced transverse vibration, we performed free-vibration tests of cantilevered prisms with two kinds of cross sections, i.e., rectangular and D-section prisms. The effects of the side ratio D/H and the turbulence intensity of the free stream on the flow-induced vibration were also investigated. The response amplitude of the transverse vibration increases with a decrease in the side ratio. The response amplitude of a D-section prism with D/H = 0.23 vibrates at a lower reduced velocity than a rectangular prism with D/H = 0.2, and the increment rate of the response amplitude for a D-section prism with D/H = 0.23 is higher than that of the other prisms. It is found that the free-vibration characteristics of rectangular and D-section prisms are influenced by the high turbulence of the free stream.
In order to understand the aspect of the mutual interference flow from two circular cylinders, the visual observation experiment was performed. The cylinder setting conditions were three kinds of distance ratios (L/d=1.5, 2.5 and 5.5), and seven kinds of arrangement angles (α=0, 15, 30, 45, 60, 75 and 90 degrees). The oscillating conditions were four kinds of amplitude ratios (2a/d=0.25, 0.5, 0.75 and 1.0), and the oscillation frequency ratio f/fK in 24 steps. The Reynolds number was about 640. As the result of experiment, even if the distance ratio was the same, the vortex shedding characteristics changed with arrangement angles. The mutual interference will become remarkable if the distance ratio is small. In the arrangement angle, 30 degrees and 45 degrees are carrying out mutual interference most. Even when a forced in-line oscillation was performed under the conditions in which two circular cylinders are carrying out mutual interference, it was found that a lock-in phenomenon occurs. The vortex shedding features were obtained and flow pattern distributions were shown. The lock-in characteristics were investigated and the lock-in ranges have been presented in each distance ratio. Four kinds of typical flow patterns at the time of the lock-in of staggered arrangement oscillating two circular cylinders were shown.
Flying animals and insects flap their wings using a combination of pitching and heaving oscillations. Unsteady fluid forces presumably play an important role in the flight mechanism, and also in the propulsive force of swimmers performing the front crawl. In particular, the crawl stroke invokes a unique S-shaped pull motion, in which the direction of circulation around the hand changes during a single stroke cycle. We intend to elucidate the relationship between unsteady fluid forces and the three-dimensional vortex structure in the case of high reduced frequency. In the present study, pitch-oscillating motion was carried out as a basic unsteady motion. The vortex structure and behavior of a discoid airfoil (simulating a swimmer's hand) were investigated during pitch-oscillating motion. Vortical flow fields were measured by a particle image velocimetry (PIV) technique. Vortical field was evaluated in a wind tunnel test, which allows the easy alteration of the complex parameters affecting unsteady phenomena. Throughout one cycle of the pitch-oscillating motion, vortices grew to a large size and were eventually released into the wake close to the airfoil. Fluid force increased as the vortex grew near the airfoil, peaking twice during the pitching cycle. Thereafter, the fluid force decreased as the vortex was shed and traveled in the downstream direction.
The present study is an experimental investigation on the effect of axial and transverse acoustic forcing on a generic swirl flow. The aim is to provide a qualitative understanding of typical swirl flow response to transverse acoustic forcing, for a better understanding of the response of swirl-stabilized flames to the same configuration of acoustic forcing. The latter is critical for the ongoing research on thermoacoustic instability in annular gas turbine combustors. A single burner test-rig with transverse extensions to facilitate transverse acoustic modes is employed in this experimental study. The swirl flow, established using a generic radial swirl generator, features vortex breakdown. Two transverse forcing configurations are studied: a) symmetric forcing which leads to a pressure antinode at the burner, and, b) antisymmetric forcing which results in a velocity antinode at the burner location. The study is based on results from planar streamwise and crosswise flow field measurements. We find that while the symmetric forcing configuration causes a flow response similar to axial forcing, antisymmetric forcing results in a helical response.
Behavior of the series of laminar vortex rings with circumferential flow, so-called swirl, were investigated using flow visualization, to evaluate the transport efficiency of the ejected fluid as vortex rings. In this study, the interval time of the vortex ring ejection, the formation number of vortex ring LO/DO (the normalized length of the ejected slug of fluid), and the angular velocity of the ejected fluid ω are changed, while the mean ejection velocity is fixed. When vortex rings were generated at a short time interval, independent of LO/DO and ω, they were broken, and most of the fluid included in them was diffused near the orifice. When the vortex rings with little mutual interference were generated at an appropriate interval time, the breakdown of vortex ring structure is suppressed with moderate swirling flow. In those cases, each vortex ring moves separately for a long distance and the distribution area becomes wider as LO/DO increases.
In this paper, direct numerical simulation (DNS) is carried out to investigate the flat plate boundary layer with heat transfer affected by a wake of a square bar. The bar is installed parallel to the plate and normal to the flow direction at the beginning of the boundary layer. The gap between the bar and the plate is varied in two cases. The fractional step method based on the Runge-Kutta and central difference schemes is used in the simulation. The instantaneous structures along with the statistical characteristics of the flow and thermal fields are presented and discussed. The results show that, in the large-gap case (C/d＝3.0), Karman vortices are shed from the bar and they perturb the boundary layer by advecting the fluid wallward and outward in turn. In the small-gap case (C/d＝0.25), however, the positive vortices shed from the lower side of the bar are small and the large negative vortices shed from the upper side of the bar dominate over the boundary layer, resulting in the strong backward sweep motions of fluid near the plate. Such motions contribute to the reduction in skin friction in comparison with the large-gap case. On the other hand, the thermal boundary layer in the small-gap case is strongly suppressed by the large negative vortices. This results in the larger temperature gradient and more active heat transfer in comparison with the large-gap case. These indicate that careful design is required to enhance heat transfer through a flat plate by utilizing a wake of a square bar.
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 a blockage ratio of 3.0% and a Reynolds number of 8000. Both end walls supporting the cylinder were flat plates with different tip conditions on which transitional boundary layers were formed. The ratio of the boundary layer thickness to cylinder diameter at the cylinder position was 0.60 and 0.83, and the aspect ratio was varied from 8 to 32. Regarding the thick wall boundary layer, the base pressure coefficient along the cylinder axis was mostly constant for all aspect ratios, with the exclusion of the end wall vicinity, but rapidly rose as the aspect ratio decreased. In the case of the thin wall boundary layer, the base pressure coefficient was very little constant along the cylinder axis, but the difference for various aspect ratios was relatively small. The local pressure drag coefficient at the mid-span increased in the case of the high aspect ratio and the thin boundary layer.
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