All first-order spatial derivatives of the turbulent velocity fluctuations were measured using a pair of X hotwire probes. Measurements were performed in the self-preserving region of a turbulent plane wake downstream of a cylinder and in an axisymmetric wake behind the sphere. Good spatial resolution of the measurements was ensured by choosing small values for the cylinder/sphere diameter and a low flow speed. Errors due to the finite hot-wire length and the wire and probe separation were analysed using Wyngaard's correction method. The derived corrections were verified experimentally. The measuring technique and the experimental results were systematically checked and compared with the results available in the literature. The budgets of the turbulent kinetic energy were computed from the measured data. In contrast to the results obtained in the plane wake, where the pressure transport is nearly negligible, in the axisymmetric wake it was found to play an important role and closely follows the estimate made by Lumley, <u_ip>^^^-/p&ap;-0.2<q^2u_i>^^^-.

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Active control of flow separation from a bluff object embodies the flow of curved jets and wakes both steady and unsteady and their effect on the forces experienced by the object. The efficacy of separation control by steady blowing, suction, or by periodic excitation is being extensively reinvestigated. The efficacy of steady blowing was measured by the increase in lift or the reduction in drag relative to the momentum added to the flow, while the efficacy of suction was assessed by the mass flow removed from the flow field. Recent experiments suggest that a combination of momentum and mass coefficients represents a better characterization of both methods of control. In the absence of mass flux, periodic excitation makes separation and circulation control much more effective than steady blowing provided the momentum input is small, but its advantage is smaller with respect to suction. When the momentum input is large blowing becomes advantageous because most of the input is recovered as thrust. Various experiments combining blowing suction and periodic excitation will be described.

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The paper presents a perspective of combining large-eddy simulations (LES) and Reynoldsaveraged Navier-Stokes (RANS) strategies for computation of high Reynolds (Re) and Rayleigh (Ra) number turbulent flows and heat transfer. Limitations and constraints in LES are revisited and illustrated by examples of impinging jets and thermal convection. Whilst it is recognized that RANS will further play an important role, especially in industrial and environmental computations, a new niche for RANS is identified in hybrid combinations with LES. Two routes, the zonal and seamless hybrid RANS/LES are discussed with an attempt to identify advantages and shortcomings of each approach. The potential of very large eddy simulations (VLES) of flows dominated by (pseudo)-deterministic eddy structures, based on transient RANS (T-RANS) and similar approaches is also discussed and illustrated in an example of "ultra-hard" (very high Ra) thermal convection.

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This paper reviews research performed to advance the understanding of state-of-the-art technologies capable of reducing coaxial jet noise simulating the exhaust flow of turbo fan engines. The review focuses on an emerging jet noise passive control technology known as chevron nozzles. The fundamental physical mechanisms responsible for the acoustic benefits provided by these nozzles are discussed. Additionally, the relationship between these physical mechanisms and some of the primary chevron geometric parameters are highlighted. Far-field acoustic measurements over a wide range of nozzle operating conditions illustrated the ability of the chevron nozzles to provide acoustic benefits. Reductions in EPNL as high as 2.6 EPNdB were documented. Spectral and directivity results showed the chevrons to be most effective at angles close to the exhaust axis and at lower frequencies in the range of the peak jet noise frequency. The acoustic benefit diminished at more forward angles and at higher frequencies. The far-field measurements successfully identified trends in the acoustic benefit with respect to the chevron geometry and the nozzle operating condition. Chevron design with a higher level of penetration into the flow provided greater low frequency reduction but provided an inferior high frequency benefit as compared to a lower penetration design. Each of the chevron nozzles tested also showed a strong dependence on the nozzle shear velocity, which is defined as the difference in the velocity of the core stream and that of the surrounding air. Detailed mappings of the acoustic near-field provided more insight into the chevron source mechanisms by successfully identifying two primary chevron effects consistent with the results of the far-field measurements. First, the chevrons provided very effective suppression of low frequency noise, in the range of the peak jet noise frequency. Reductions as high as 10dB were observed at axial distance in excess of 7 equivalent nozzle diameters. This low frequency effectiveness is consistent with the low frequency, aft angle benefits seen in the far-field results. The second effect was an increase in mid and high frequency noise at axial distances between 1 and 4 equivalent diameters. This increase was shown to be directly dependent on the chevron geometry and the nozzle operating condition, with higher levels of chevron penetration and nozzle shear velocity independently producing more mid and high frequency noise. These noise increases did not produce corresponding increases in the farfield due to the effects of atmospheric absorption and attenuation. However, this increased mid to high frequency noise is consistent with the diminished acoustic benefit seen at high frequencies in the far-field. The overall impact of these effects on the acoustic near-field was to draw the peak noise generating region closer to the nozzle exit plane and reduce its spatial extent. Mean and turbulence data identified the physical flow mechanisms responsible for the effects documented in the far- and near-field studies. These measurements were successful in linking the two effects identified by the near-field mappings with two fundamental flow mechanisms. Mean flow results showed the chevron to create a radial lobe structure which distributes energy outward from the high velocity core jet to the lower velocity fan stream. The result of this redistribution of energy is a more rapid decay of the peak velocities in the jet plume and a consequent shortening of the jet potential core. Under certain operating conditions, the chevrons reduced the peak jet velocity by nearly 10% at an axial distance of 5 equivalent diameters. This effect is consistent with the reduced low frequency jet noise seen in the far- and near-field acoustics results. However, the increased radial velocity associated with the lobe structure produces increases in turbulent kinetic energy (TKE) as

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Recent progress in active control of jet mixing and combustion is introduced. Miniature electromagnetic flap actuators are mounted on the periphery of an axisymmetric nozzle exit. It is demonstrated that even weak disturbances introduced into the initial shear layer by these actuators can significantly modify the large-scale vortical structures. This control technique is extended to the control of methane/air mixing and diffusion combustion by using a coaxial jet nozzle with the same flap actuators. As a result, the flame characteristics can be much improved in terms of stability and emission. Direct numerical simulation of a confined coaxial jet control has also been carried out. Although the distributed actuators are modeled somewhat ideally, DNS clearly demonstrates enormous effects of the present control scheme on the initial shear layer dynamics and concentration mixing.

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This paper reviews the works presented at the meetings of the JSME research group "Fundamentals and Advanced Applications of Jet Flows". The outlines of thirty five topics are described, focusing on the contributions to the development of research in the field of jet flows.

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Large Eddy Simulation (LES) of shear flows driven by Kelvin-Helmholtz instabilities such as mixing layers, wakes, and jets is of great interest because of their crucial role in many practical applications. The Monotone Integrated LES (MILES) approach is motivated here for these studies, and the basic components involved in a typical MILES jet model are described. Examples from MILES jet studies are used to address major aspects of transition to turbulence from laminar conditions at the nozzle exit including, the occurrence of global instabilities, complex three-dimensional vorticity geometries, and their impact on jet entrainment. Quantitative analysis of the small-scale features of the transitioning simulated jets is presented, and convergence issues are addressed in this context.

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Flow characteristics of two-dimensional jet with side walls has been studied experimentally. Three kinds of cylindrical walls and a flat wall are provided for the side wall and they are combined and attached to the nozzle. Nine types of side wall conditions are investigated here. Velocity has been measured by a hot wire probe and separation point has been measured by a Pitot tube. Mean velocity profiles, growth of jet half width, decay of jet maximum velocity, and attachment distance are clarified. It is found that when cylindrical walls with different radii are installed, flow pattern changes remarkably depending on the jet velocity. Striking increase in jet half width is found to be related to the flow separation from the smaller cylindrical wall just behind the nozzle by the flow visualization.

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This paper deals with statistical properties and dynamical structures of turbulence in a plane wall jet (PWJ) and a three-dimensional wall jet (3DWJ.) Measurements have been made in the flow fields created from jets impinging tangentially on a plate from a rectangular slit and circular nozzle, respectively. Basic quantities of these flows measured with a hot-wire anemometer are compared with each other. The nozzle-exit Reynolds numbers for the PWJ and 3DWJ were 15,000 and 100,000, respectively. The velocity profiles of U and W in the cross section of the fully-developed 3DWJ are nearly similar when they are nondimensionalized with the local maximum velocities and nozzle diameter, which imply that the length scale in the y-direction varies little in the z-direction. Multi-point velocity measurements were made on both flows with hot-wire rake probes. Coherent structures in the y-direction were analyzed by the POD in spatial domain.

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Large Eddy Break-up Device (LEBU) was applied to flow management of a plane turbulent wall jet. LEBU was located at three different distances from the wall and the mean velocity, Reynolds shear stress profiles and the wall shear stress were measured. The experimental data show that LEBU reduces development of thickness of the shear layer and magnitude of Reynolds shear stress. The wall shear stress tends to be reduced even if LEBU is placed in the outer layer. These effects are largest when LEBU is placed at the height where the turbulent production term reaches its maximum. Length scale of the large-eddies is reduced in manipulated flows, especially, in the spanwise direction.

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In the present paper, drag reduction and heat transfer enhancement were studied in a fully developed two-dimensional water channel flow. Surfactant solutions at different concentrations were used to examine the influence of surfactant additives on the skin friction drag and heat transfer coefficient. The present results show that there is no heat transfer reduction when 30 ppm of surfactant is added into the flow. With the increase of surfactant concentration to 90ppm, heat transfer was reduced by about 60%. A row of low-profile vortex generators were installed in the spanwise direction to enhance heat transfer rate over the heating surface. The effect of the vortex generators on heat transfer was examined for surfactant concentration of 90 ppm at Reynolds numbers of 7000 and 12000. The results show that the local Nusselt number is enhanced by about 200% as compared with that obtained in the surfactant solution without the use of vortex generators.

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A simple relation between the profiles of the hydrodynamic parameters in a boundary layer on an impermeable plate and those in an asymptotic suction boundary layer is found for high Reynolds numbers. (The asymptotic suction boundary layer is a one-dimensional flow in which all mean parameters are functions of only the transverse coordinate.) The profiles of the mean velocity in the outer region (beyond the viscous sublayer) of two flows are related to each other in terms of quadratures. The distributions of shear stress are determined from the velocity profile of one of the flows. Turbulent normal stresses are calculated from the corresponding component of the Reynolds tensor and velocity profile of the other flow. The rms transverse velocity fluctuation in the intermediate wall region of the boundary layer with suction is shown to be proportional to the logarithm of the distance from the wall, whereas the rms longitudinal velocity fluctuations, to this logarithm in a power of 3/2. The results are obtained only from the equations of motion and analysis of dimensions.

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An integrated approach has been employed to characterize the flow field and heat transfer configuration, in witch air jets undergoes to regime transition. Heat/mass transfer analogy has been used to infer the local heat exchange. A naphthalene film is ablated from the substrate disk, due to jet exposure, and from the local naphthalene loss rate the local heat transfer is then inferred. Under natural conditions the investigated impinging jet is substantial laminar and coherent structures are created upon excitation with acoustical perturbations and need to be investigated in the jet vicinity. Two laser techniques are employed: Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV). Both techniques have been used in order to investigate the influences and coherent structures generated in the jet by acoustic perturbation forced in the flow. With the acoustical perturbation a substantial increase of the local heat transfer coefficient was observed.

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Heat transfer characteristics of a circular turbulent impinging jet with a swirl were experimentally examined using air as a working fluid. A swirl was produced by inserting air from two exits on the side surface of a circular nozzle. The flow was visualized by smoke-inducing method. Impingement surface temperature was measured using thermosensitive liquid crystal by transforming from color to temperature. Local heat transfer has two peaks due to the inserting angle. The swirl enhanced the impingement heat transfer, which was arranged by the ratio of circumferential momentum and axial momentum, within the present conditions.

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The objective of this study is to improve the two-equation heat-transfer turbulence model so as to accurately predict impinging jets with heat transfer. First, we have assessed existing modeled expressions of Reynolds stress and turbulent heat-flux with the aid of a direct numerical simulation (DNS) database. Then, in order to confirm the performance of existing two-equation heat-transfer models for impinging jets, we have carried out numerical simulation in a plane impinging jet with heat transfer. It is found that none of the existing models can adequately predict the correct behavior of statistical turbulence quantities obtained in the DNS. Based on these assessments, we have reconstructed the improved Reynolds stress expression, and obtained the improvements necessary for the two-equation heat-transfer model.

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When the underexpanded supersonic jet impinges on the obstacle, it is well known that the self-induced flow oscillation occurs at the specific condition, namely the pressure ratio in the flow field, the position of an obstacle and so on. This oscillation is related with the noise problems of aeronautical and other industrial engineering so that the characteristic and the mechanism of self-induced flow oscillation have to be cleared to control the various noise problems. But, it seems that the characteristics of the oscillated flow field and the mechanism of oscillation have to be more cleared to control the oscillation. This paper aims to clear the characteristic of oscillation frequency, pressure fluctuation on the surface of cylindrical body and the control of self-induced flow oscillation during the impingement with the cylindrical body. The numerical and experimental investigations were carried out using the TVD method and discussed.

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The liquid film thickness of a plug bubble in a micropipe is of interest in micro heat pipe, microfluidics, and two-phase flows. A novel optical technique was further extended to measure the liquid film thickness of a plug like bubble in a capillary pipe utilizing spatial fringe scattering method. The scattered fringes were measured by CCD camera and the calculated spatial frequencies were used to determine the film thickness between the plug and the wall. To demonstrate the capability of the newly developed technique, a validation experiment was conducted with water/air and water-honey-mixture/gas-plug flows. The velocity dependent film thickness can also be observed using this measurement technique. The interfacial film dynamics of oscillating plug/slug flows in a capillary channel was investigated. The Ripple effect of film thickness was observed when the oscillating frequency reached 2Hz. This newly developed method is easy to be implemented and it will be a very useful technique for micro heat pipe research.

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In order to predict the water-vapor two-phase flow behavior in a fuel bundle of an advanced light-water reactor, large-scale numerical simulations were carried out using a newly developed two-phase flow analysis method. Conventional analysis methods such as subchannel codes need composition equations based on many experimental data. Therefore, it is difficult to obtain highly prediction accuracy on the thermal design of the advanced light-water reactor core if the experimental data are insufficient. Then, a new two-phase flow analysis method using a modified interface tracking method was proposed. The coalescence and fragmentation of small bubbles were analyzed numerically and the bubbly flow dynamics in narrow fuel channels were clarified. Moreover, the liquid film flow in a tight-lattice fuel bundle which is used to the advanced light-water reactor core was analyzed and the three-dimensional water and vapor distributions around fuel rods with spacers were estimated quantitatively.

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Wave generation on the surface of a liquid due to the instability of free-surface shear layer is investigated experimentally and theoretically. A laser beam refraction technique is employed for measurement of periodic two-dimensional waves arising in the jet initial region. It is shown that the linear perturbation equation has temporally-neutral, spatially-unstable solutions for local velocity profiles predicted for an initially-laminar shear layer. The frequency of the most-unstable solution shows fair agreement with the measured dominant frequency of the wave. The linear stability analysis can be simplified drastically by introducing linear or piecewise-linear approximation of the velocity profile. Such simplification does not result in large discrepancy from the detailed-model predictions, or experimental data, except in the initial region of the jet where there is arbitrariness in approximation with simplified velocity profiles.

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Growth process and three-dimensional deformation process of the disturbance wave which appears in the laminar-turbulent transition process of a radial liquid sheet flow are clarified. The radial liquid sheet is formed by the release of a radial liquid film flowing on a disk from the edge of the disk to still air. When the Reynolds number is large, a concentric disturbance wave appears and grows downstream on the free surfaces of the liquid sheet. The disturbance wave is caused by the unstable disturbance which is attribute to a inflectional velocity profile inside the liquid sheet. The detailed observation of the disturbance wave, which is excited by supersonic wave, reveals that the growth process and the three-dimensional deformation process of the disturbance wave.

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Nonlinear behavior of an annular viscous liquid jet is analytically examined. By means of the membrane approximation, a set of axisymmetric nonlinear equations is derived for the annular sheet with finite thickness when the surroundings are ignored. Preliminary linear analysis shows that the temporal growth rate of disturbances decreases as the viscosity increases, while the wavenumber region for the instability is left unchanged. It is found in the numerical analysis that the jet collapses through the sealing-off by closing of the annular part rather than the disintegration of the part when the viscosity increases. It is also found that the viscosity of the liquid not only delays the breakup, but also strongly affects the jet profiles in the vicinity of the collapse, for which some similarity is found in the renornialized annular radii.

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In order to clarify the large-scale coherent structure in a turbulent plane jet, the simultaneous measurement of the main streamwise and the cross-streamwise velocity at 9 points in the self-preserving region of a turbulent plane jet has been performed by an array of X-type hot-wire probes. From the time variation of the main streamwise fluctuating velocity field, it is found that there exists a pair of fluid lumps with the positive and negative fluctuating velocity on opposite sides of the jet centerline. On the other hand, the instantaneous cross-streamwise fluctuating velocity shows the same sign over the cross-section, i.e., the vertically-striped pattern is formed. On the basis of the result of the KL (Karhunen-Loeve) expansion, a new interpretation of coherent structure model in the self-preserving region of a turbulent plane jet has been given from the combination of the "flapping" and the "puffing".

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The interaction of vortex rings and streamwise vortices in an axisymmetric jet is studied by flow visualization and velocity measurement. The jet is excited by axial and azimuthal perturbations to stabilize and enhance axisymmetric and stream-wise vortices. A laser fluorescent dye and a laser light sheet are used to visualize the jet. The three-dimensional views of jet-boundary surface and streamwise vortices are constructed by applying the Taylor hypothesis to the jet cross-sectional images, and the interaction model of axisymmetric and stream-wise vortices is proposed. The characteristics of the flow field and the entrainment of the jet are also discussed in relation to the vortical structure. Moreover, the relation of the vortical structure and the entrainment in the initial region of a plane jet was also investigated with four types of vortex generators by vorticity measurements. The vortices were enhanced and stabilized by streamwise and spanwise disturbances. The three-dimensional views of the vortices were constructed by applying the Taylor hypothesis to the phase-average vorticities, and the entrainment was discussed in relation to the vortical structures. The result reveals the enhancement of entrainment by the interaction of streamwise and spanwise vortices.

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Three dimensional deformations of roll vortices in a parabolic two dimensional jet were controlled by an acoustic bimodal excitation and tabs installed around the jet exhaust. The results confirm the possibility of the present technique to lock up the process of vortex pairing to chase the transition process of the two dimensional jet in detail through flow visualization and hotwire measurement. When the initial phase difference between the fundamental and its first subharmonic components was given to be 0 degree, vortex pairing occurred regularly. The vortex pairing occurs between a pair of three-dimensionally deformed roll vortices connected by the rib structures. The vortex pairing occurred only occasionally in the transition region when the initial phase difference was 120 degree. When the pairing didn't occur, the symmetric arrangement was maintained until far downstream of the nonlinear interaction region.

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The present study investigates experimentally the effect of the nozzle contraction profile on the downstream development of a plane turbulent jet. The variation of the contraction profile was made by using various orifice plates with different radii. It is found that the decay and spread rates of the jet's mean velocity increase as the radius decreases. A decrease in the radius also results in a higher formation rate of the primary vortices. Moreover, the turbulence intensity is found to depend on the contraction profile.

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The three-dimensional time dependent compressible Navier-Stokes equations are numerically solved to study acoustic emission mechanism in a supersonic plane jet at high convective Mach numbers using high-order compact upwind schemes. High-order compact schemes of 5th order developed by Deng and Maekawa (1996, 1997) [1] [2] are used for spatial derivatives and a 4th order Runge-Kutta scheme is employed for time advancement. Navier-Stokes characteristic boundary conditions are used in the streamwise and vertical directions and periodic boundary conditions in the spanwise direction. Numerical results for the convective Mach number Mc=1.17 are presented (Mc is denned by eq. (16) in Section 2). Two cases were investigated. The first case is the jet flow forced randomly. The second case is the jet forced by random disturbamces and linear unstable oblique modes. The numerical results provide new physical insights into three-dimensional structures and acoustic wave generation mechanisms in a plane turbulent jet. Upstream disturbance conditions play an important role for the evolution of the downstream structure, such as development of shear layers in a jet. Growth of the oblique mode is responsible for the A structure in a plane jet, which yields sooner decay of the centerline velocity. The result indicates that the jet forced with the oblique modes can mix faster with the surounding fluid medium than the jet forced randomly. The intense sound radiation observed in the randomly forced jet can be reduced by forcing with a pair of oblique modes.

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Flow visualization and measurements of mean and fluctuating velocities were performed on the coaxial jet with a velocity ratio of 0.6 at a Reynolds number of 3000 in an open water tank using hot-film anemometry, particle image velocimetry (PIV) and laser-induced fluorescence (LIF). The axisymmetric and streamwise vortical structures were revealed in the nearfield of coaxial jet. The annular nozzle has six vortex generators in order to enhance the streamwise vortices generating in the mixing layer. Furthermore, the annular jet was excited by a shaker in order to enhanced the axisymmetric vortices. For the tabbed coaxial jet, the jet spreading was increased than that of the jet without tabs in the downstream. While the axisymmetric and the streamwise vortex structures independently increased jet spreading, a combination of the two diminished the effect. As a result, the turbulent intensity of the tabbed coaxial jet in the inner mixing region is not higher than that of the coaxial jet without tabs.

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Flow structure in the near field of an annular jet is investigated experimentally by means of LDV for three ratios of the inner to outer diameter of an annular nozzle. The jet leaves the nozzle exit with an inward velocity component; a recirculating vortex region is formed between the nozzle exit and the stagnation point located on jet center line at the distance ranging from 0.6D to 1.2D, depending on the exit velocity and the nozzle diameter ratio. The dependence on the exit velocity larger than 10m/s is very slight and the stagnation point does not go beyond 1.2D from the exit with approaching the exit velocity to zero. The existence of a wall at the inner nozzle body has a negligible effect on the flow structure except for the static pressure in the recirculating vortex region, which tends to become proportional to the dynamic pressure of the issuing jet.

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This paper presents the results of an experimental study on starting square jets at three Reynolds numbers (R_e=2358, 3528, 4716) utilizing Planar Laser Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV). Vortex leapfrogging and coalescence were found to appear at the lowest Reynolds number investigated while vortex pinch-off tended to appear at higher Reynolds numbers. The 'imperfect' leapfrogging between the third and the fourth vortex rings appears to promote growth of instability which leads to the transition to turbulence. The maximum circulation that a vortex ring can attain during its formation is reached at the non-dimensional time (t*=tW_0/D_e) within the range of 1.0〜2.33 which is lower than the universal time scale obtained by Gharib et al. for the circular jets with uniform initial velocity distribution. Axis switching was found to play an important role for the vortex development in square jets.

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The experimental results of the jet flow issuing from the circular pipe with the pliable fluttering fins have been discussed in this paper. In order to investigate the effects of these fins on the jet diffusion, the flow visualization by the LIF and the image processing by the PIV method have been carried out. As a result, it has been found that the jet width becomes large in the case of the jet issuing from the pipe with the fluttering fin. Moreover, the wavelet multi-resolution analysis has been also conducted, by using the image obtained from the flow visualization. As a result, it has been clarified that the fluttering fin affects the flow structure largely.

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An investigation of the effects of pipe length on flooding during adiabatic air-water counter-current two-phase flow in inclined pipes of 16mm ID. was conducted experimentally. A constant electric current method and visual observation were utilized to elucidate the flow mechanisms at the onset of flooding. It was found that the pipe length influences the phenomena at the onset of flooding. Upper flooding moves to lower flooding as the pipe length decreases. In the range of pipe length tested, the upper flooding disappeared when the pipe length was 0.5m. The onset of flooding decreases as the pipe length increases.

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A new type of machine that uses a two-phase jet flow has been developed. This machine is known as an aerator, and operates not only an aerating function using the normal two-phase jet flow, but also employs a simple method to carry out an anaerobic operation as well. The economic efficiency of such machines is a key issue, and this along with a high effectiveness is a desirable quality of any aeration machine. To respond to this challenge, it is necessary to gain hydrodynamic understanding of the flow phenomena in the tank, and this knowledge should be applied to an effective simulation method. Even now however, a complete understanding of gas-liquid dynamics in the tank has yet to be gained. Therefore, this study focuses on the flow properties inside the tank induced by the two-phase jet flow during the aeration process. As an underlying process of the flow phenomena whereby the two-phase jet flow falls to the free surface and produces a circulating flow in the tank, we chose to examine the phenomena just after a droplet falls into the gas-liquid interface. This was done in practice by observations using high speed video, experimental modeling, and also by numerical analysis. Subsequently, we could gain insights into the influence of a free-drop onto the gas-liquid surface. This basic data can clarify the mechanism of the flow in the tank, which is induced by the two-phase jet flow, and is detailed in this study.

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The bubbly flow around a square-section cylinder is simulated by the vortex method proposed by the authors in a prior study. The cylinder is located in vertical upward bubbly flow. The Reynolds number based on the width of the cylinder and the water volumetric velocity is 15000. Air bubbles with diameter 1mm are loaded uniformly upstream of the cylinder, and the volumetric fraction α_<g0> ranges from 0 to 0.02. The air volumetric fraction is higher in the wake of the cylinder and reaches its maximum value at the center of the vortices. The vortex shedding frequency, the pressure distribution on the cylinder and the pressure difference between the front and back stagnation points of the cylinder change as function of ago in accordance with the experimental results.

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This study describes experimental results on the wake structure generated by a bubble swarm using LDV. The authors have considered the individual bubble motion of a bubble swarm and the liquid-phase motion driven by the buoyancy. The bubbles were simultaneously released from 19 needles under precise control; two bubbles were released from each needle at a short interval; as a result a total of 38 bubbles composed the bubble swarm. The 38 bubbles were almost uniform and 2.6mm in equivalent diameter. The center-gravity motion of the each bubble was obtained from high-speed visualization. The liquid-phase motion induced by the dispersed bubble buoyancy was measured via LDV. The following results were obtained: in the frontal region of the bubble swarm, the liquid-phase motion induced by the individual bubbles of the swarm did not interact with each other; in the rear region of the bubble swarm, the liquid-phase motion intensively interacts with each other through the wakes; inside of the bubble swarm, the liquid-phase motion was affected by the center-gravity motion of each bubble.

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