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 statistics of turbulence. Two alternative definitions of wake width, lS = 2yS and lF = 2yF, are proposed in terms of skewness and flatness factors, respectively, which are considered as thirdand fourth-order statistics of turbulence in this study. Between these two new definitions of wake width, lF can represent the actual sectional range of shear turbulence in a wake.

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The present study is devoted to PIV investigation of vortex breakdown (VB) in swirling jets and lean premixed flames. A set of PIV experiments was carried out for submerged water and air jet flows, and also for propane/air and methane/air flames. Time-averaged and spectral characteristics of the velocity fluctuations were estimated, and properties of coherent structures (CS) were analyzed on the basis of decomposition routines. VB for the low- and high-swirl non-reacting flows is specified as spiral-type and bubble-type, respectively. In the former case, no stable RZ was present, and the vortices in the outer shear layer and near the jet axis were presumably associated with local helical instabilities. In the latter case a stable bubble-type RZ was formed, and the flow dynamics was governed by strong precession of the vortex core. The measurements for propane/air lean flame demonstrated that combustion can divert flow dynamics from these limit-cycle oscillations.

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Previous studies have shown that a modification of the boundary layer of a bluff body can change the wake properties like the vortex shedding frequency and the base drag. In the present study the boundary layer has been modified by applying suction over the top and bottom surfaces of a bluff body. A level of suction above a certain threshold has been shown to give an asymptotic suction boundary layer. For the asymptotic suction boundary layer the boundary layer properties become independent of the free stream velocity and hence the boundary layer Reynolds number. The bluff body studied can be described as a rectangular forebody with a smooth leading edge and a blunt trailing edge. Hot-wire anemometry measurements have been carried out in the boundary layer of the forebody. For four different Reynolds numbers, all within the laminar regime, the suction coefficient has been varied in the range from the neutral case to the case for which the asymptotic suction boundary layer is reached. It is observed that as the suction coefficient approaches the asymptotic suction boundary layer threshold, the Strouhal number based on the shedding frequency approaches a constant value. The base pressure decreases as the boundary layer thickness decreases. Furthermore it is shown that the Strouhal number does not always decrease for an increase of the displacement thickness.

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Over the last decade, synthetic jets adequate for micro machinery have received attention as substitutes for continuous jets. The development of synthetic jet actuators with, for example, a diaphragm, a piston, or a speaker cone, instead of mechanical drivers, is required for the downsizing of flow control systems in fluid machines. In this study, an experimental prototype of a synthetic jet actuator using the nonlinear oscillation of bubbles produced by iterative electric discharge is proposed. Numerical simulations clarify the fundamental flow characteristics of the synthetic jets produced by ideal bubble oscillation. The discharge conditions include the influence of the metal particle concentration in liquid on bubble generation. The onset condition based on the non-dimensional stroke of the synthetic jet and the relation between the bubble motion and the change of the jet velocity with time are discussed. Especially, the influence of T* (the ratio of driving time of a bubble to the down time) on the unsteady flow pattern and the time-averaged jet structure is investigated. In addition, the flow characteristics of a synthetic jet with down time are compared with that of a normal synthetic jet with equivalent velocity.

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A dielectric barrier discharge plasma actuator (DBD-PA) was applied to diffusion control of jet flows issued at Reynolds numbers (based on nozzle outlet diameter) of Re = 1.0X103 and 2.0X103 from a nozzle having an outlet with a diameter of 10 mm. The detailed flow structure was documented using particle image velocimetry (PIV) and laser flow visualization. In the experiments in the present study, the jet flow was controlled using a coaxial DBD-PA, which generates an induced flow in the same direction as the jet direction. The influence of the flow induced by the DBD-PA on the main jet flow was investigated as a function of the voltage, frequency, and intermittency. Plasma was generated by applying an alternating voltage of 2-6 kV with a frequency of 4-15 kHz and intermittency frequencies of 80, 160, 240, 320 Hz. The results indicate that flow induction becomes stronger as the frequency and voltage increase. Moreover, diffusion of the kinetic momentum in the jet was promoted. The potential core length was shortened dramatically by the DBD-PA, and jet stream diffusive mixing was promoted by the collapse vortex ring at an early stage. Since the Kelvin-Helmholtz (K-H) instability was enhanced by the DBD-PA, vortex rings formed near the nozzle and underwent early collapse.

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The flow structure generation by multiple jets impinging a supersonic crossflow in the divergent section of a Convergent-Divergent (C-D) duct is investigated using compressible Large Eddy Simulations (LES). The supersonic flow-field in the C-D duct is mainly characterized by the evolving shock-structure. The effect of increasing the compressible jet to crossflow velocity ratio R to the generation of flow structures and the ability to modify the shock-pattern in the duct was studied. Traversing R, the shock-pattern can be significantly altered. This paper demonstrates that for close located jets in crossflow the vortical structures generated by the jets can interact and give rise to vortical structures in the interspace plane between the jets. The spectra for different probes are shown illustrating the characteristic flow frequencies. For all simulated cases the spectra show peaks for a defined Strouhal-number of 0.5. The jets choke in the crossflow above an R of about 0.65, which results in a faster disruption of the coherent flow structures induced by the jets. The flow field is analyzed using Proper Orthogonal Decomposition (POD).

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Unsteady measurements of convective heat transfer was performed to a water flow in a horizontal circular pipe with inner diameter of D = 20.4 mm for Reynolds number from 1000 to 30000. The test surface for the heat transfer measurement was fabricated from a titanium foil of 22 μm thick coated with black paint, which was heated electrically under conditions of constant heat flux. The time-spatial distribution of the heat transfer coefficient was evaluated from the temperature fluctuation on the test surface measured using a high-speed infrared thermograph (〜800Hz). The time-averaged heat transfer coefficient measured here agreed well to the conventional empirical correlation. As a result of the present measurement, the dynamic feature of the thermal streaks in a circular pipe was revealed, which elongated along the streamwise direction and flows downstream with some meanderings. The statistic values of the spatio-temporal characteristics, such as the mean spanwise wavelength of thermal streaks, was investigated and compared with those of the previously published literature.

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Direct numerical simulation is performed for a two-dimensional turbulent jet dispersed by sphere particles in the jet core region. The temporary evolving jet in a periodic domain is solved by the Fourier Spectral method. The particle is treated by the immersed boundary to consider the fluid disturbance from the sphere particles. The Reynolds number based on the initial jet-half width is 1500. The simulation is made for six cases including the pure-jet case. In cases of the dispersed jet, the particle diameter is changed at 0.5 and 0.7 and the density ratio from 2.0 to 10.0. The 1923 grids are tried to be used to check the grid dependency. The results from 1923 grids are not essentially different from 1443 grids. The 1443 grids are used as a standard mesh. These calculations showed that the dispersion of the fluid and the development were hasty by particles.

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In this study, flow and acoustic fields of a transitional supersonic free jet are investigated. Compressible Navier-Stokes equations are solved by a high-order compact scheme for investigating the effects of inflow shear layer characteristics. The Mach and Reynolds numbers are set to 2.1 and 70,000, respectively. Three different jets with different shear layer thickness are analysed, and the shear layer thickness effects are discussed. With increasing the shear layer, the turbulent fluctuation along the shear layer becomes larger and resulting Mach wave radiation becomes stronger.

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Flapping characteristics of jets from fluidic nozzles are investigated. These fluidic devices which produce oscillating flows consist of a slit and a rectangular duct. By means of flow visualization and velocity measurement, the optimal configuration for causing flapping jet is clarified. It is found that the flapping jet has a large spreading angle, and shows remarkable decay rate of mean velocity. It is also noticed that the frequency of the flapping jet depends on the configuration of the duct and the mean velocity at the slit. In addition, the deflection of flapping jet is revealed to be caused by the vortex in the rectangular duct.

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With the aim of optimizing performance of a hydrofoil bubble introducer for drag reduction of ship, the flows and free surface deformation around a hydrofoil were investigated experimentally in a wide range of Reynolds number, Froude number, gap ratio (normalized depth), and angle of attack. Image processing for the obtained images of flows and free surface elucidated the key factor of the free surface deformation and the conditions of flow separation on hydrofoils. Flow fields around the hydrofoil determined by PTV represent influence of the free surface with large deformation on the flow field quantitatively. These results provide fruitful information for the optimization of this hydrofoil facility.

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Conditional statistics, conditioned on mixture fraction, are experimentally investigated in a planar liquid jet with a second-order chemical reaction A + B → R. Here, reactants A and B are separately premixed into the jet and the ambient flows, respectively. In this study, instantaneous concentrations of reactive species are simultaneously measured by an optical fiber probe based on the light absorption spectrometry to investigate the conditional statistics, conditioned on mixture fraction. The results show that the conditional mean concentration of reactive species and the conditional mean reaction rate are independent of the lateral position of the jet flow in the downstream region, whereas they change with the lateral position in the upstream region near the jet centerline.

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Turbulent structure in a boundary layer developed over two different types of three-dimensional rough walls, namely, cavity and protrusion types have been investigated to understand effect of the geometrical feature on the turbulent statistics. Over the protrusion type of rough wall, the local skin friction coefficient is much larger than those of two-dimensional and the cavity type rough wall flows. As momentum thickness Reynolds number increases, the local skin friction coefficient tends to almost the same value over the two-dimensional and cavity type rough walls. Deficit in mean velocity profiles is the largest over the protrusion type of rough wall and takes almost the same value over the two-dimensional and cavity type rough walls. With higher-order moments of fluctuating velocity components, coherent vortical motions near the wall such as hairpin or necklace vortices are discussed to describe turbulent production and diffusion mechanisms in the near wall for the three-dimensional roughness.

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Vertical axis wind turbine is widely noticed as wind turbine of wind power generation system because it can be easily miniaturized, generates low noise and rotates regardless of wind direction. Vertical axis wind turbine has, however, low rotation performance as compared with that of horizontal axis wind turbine. Therefore, we need to improve rotation performance of vertical axis wind turbine. This study proposes new blade shape of vertical axis wind turbine effective for wide range of wind speed. Proposed new blade profile has largely rounded leading and trailing edges. Results obtained by numerical simulation and wind tunnel experiments suggested proposed new blade profile has higher performance as compared with conventional blade profile.

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A feedback control is attempted against an unsteady flow separation artificially generated on the plate surface by periodically moving the curved ceiling up and down at a constant frequency. The controlling system consists of a turbulence ejecting apparatus on the upstream side and several tufts on the downstream side. The movement of the tufts is monitored by a movie camera. When a separation is detected by processing the images of the tufts, a jet of turbulent fluid is ejected into the boundary layer through a slot in the plate. It is shown that the separation region shifts downstream and becomes smaller by the control.

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Three-dimensional deformation and merging of instability waves in a radial liquid sheet was clarified. A radial liquid sheet forms when a radially flowing liquid film on a disk emerges from the disk and destabilizes just outside the disk. Such instability is caused by the inflection point in the flow velocity profile, and is therefore referred to as "inflectional instability." The instability initially generates two-dimensional waves that grow and deform to cause turbulent transition. The three-dimensional deformation of instability waves is similar to that of of vortices formed by inflectional instability in a single-phase free shear flow. This suggests that the deformation of the waves is caused by the enhancement of perturbed vortex filaments. However, the waves scarcely merge, whereas vortices in the free shear flow merge frequently. This implies that merging of instability waves in liquid sheets is prevented by surface tension.

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This paper presents a computational study on the effect of the variable pitch angle and the tip speed ratio on the performance of a small vertical-axis wind turbine. By conducting two-dimensional unsteady computational fluid dynamics simulations using RNG k-ε, Realizable k-ε, and SST k-ω models, the power and torque of the vertical-axis wind turbine and the flow around the straight blades were analyzed. The numerical simulation results were validated using wind tunnel experimental data. The results of both of both the numerical simulations and experiments showed that the vertical-axis wind turbine with variable-pitch blades had better performance than one with fixed-pitch blades. The numerical simulation performance using the RNG k-ε turbulence model had good qualitative agreement with the experiment. The numerical simulation using the SST k-ω model could capture the presence of a vortex on a blade when dynamic stall occurred at a low tip speed ratio.

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In this work, the oscillatory control of a two-dimensional bi-stable jet discharged from a flip-flop nozzle is investigated. The flip-flop nozzle, which has several engineering applications, is known as a fluidic device that can produce an oscillating jet without any moving parts. Its oscillatory frequency strongly depends on the nozzle configuration. The aim of this work is to estimate the external force required to oscillate the jet at an arbitrary frequency using vibrating diaphragms on both control ports of the nozzle. A relationship between the operating amplitude and frequency of the diaphragms was derived from an approximated momentum equation. In order to verify this relation, numerical simulations of the oscillating jet were conducted at Re = 100. It was found that the jet oscillation can be synchronized with the diaphragm's vibration when the operating velocity of diaphragm satisfies the proposed relation.

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The Preston's method is considered as one of the many ways to measure the wall shear stress. However, it is only possible to determine the wall shear stress from measured pressure of the Preston tube and undisturbed static pressure, combined with calibration curves, which depend on the Preston tube diameter, fluid density, and viscosity. Since its invention, no significant advancement in theory has been made, and calibration curves proposed by Preston (1954),Patel (1964) ,Bechert (1995) are still in use. In the present study, a need to measure surface shear stress over a circular cylinder prompted us to develop our original Preston tube system. The developed system has been calibrated by measuring the wall shear stress in the fully developed turbulent flow regime in a circular pipe. The present results confirm the previously reported calibration curves.

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A flat plate wing was applied to a flow management of stronger wall jet with constant velocity ratio of Um/Ue=4 in a self-preserving development. Reynolds number based on jet height and jet flow velocity was kept to be constant of 12,000. The normal straight flat plate and three-dimensional wing were employed and located at the centre of the outer layer in the jet with constant angle of attach of -4 degree. The three-dimensional wing has notched trailing edge which is of the same order of half width of the jet, 2b2. The fluctuating velocity measurement and momentum flux evaluation were made with constant temperature anemometers. The two wings increase the maximum velocity and also velocity in the outer layer. Streamwise momentum flux integrated in the shear layer is increased by the wing manipulation. The three-dimensional wing gives larger increasing rate of the momentum flux. Longitudinal vortical structures are produced behind the threedimensional wing.

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Vortex rings have been historically the most fundamental target for studying fluid mechanics and its relation to turbulence [1][2]. We have measured influence of background rotation on the spatial transition of vortex rings, which arises during impacting on a solid wall. The combination of the background rotation and presence of the wall provides unique three dimensional reaction of vortex ring due to rapid spiral distortion of fluid that is induced by the interaction between stretching of vortex cores and Coriolis force [3][4]. The phenomenon associates with a variety of fluid behaviours from geo-physics to wall turbulence. Our PIV measurement, which is mounted on the rotating frame, has shown two characteristic flow structures. One is cyclonic structure for angular momentum conservation during the impact, which erases azimuthal waves away. Another is generation of a spiral column in the wake region, which emits fluid like a high-speed jet in opposite direction to the wall.

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Simultaneous measurements of instantaneous axial velocity and concentration are performed in axisymmetric jet diffusion fields. The CO2 jets are issued into the nearly homogeneous and isotropic, freely decaying region of regular and fractal grid turbulence to investigate the influence of mainstream turbulence on the jet diffusion field. The composite probe consisting of two concentration-sensitive I-type hot-wire probes is used. For both flows, the mesh Reynolds number based on the grid mesh size and the mainstream velocity is 6,000 and the jet Reynolds number based on the nozzle diameter (3 mm) and the relative velocity between mainstream and jet exit velocity is 5,000. The results show that the growth of half widths of the mean velocity and mean concentration, axial turbulent mass flux, and eddy diffusivity are significantly increased under the influence of strong mainstream turbulence generated by the fractal grid.

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A novel technique to measure temperature and velocity fields in liquid metal layer was proposed to understand localized fluid motions accompanied by heat transfer. A thin fluid layer of liquid gallium was adopted to simplify the fluid motion, and a sheet of thermo-chromic liquid crystal paint and ultrasonic velocity profiling provided visualization of temperature and velocity fields. Flows due to injection of small blobs of liquid gallium into the liquid gallium layer were investigated to demonstrate and evaluate the measurement technique and the setup.

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Production and transport of turbulent kinetic energy in multiscale/fractal-generated turbulence is investigated in a wind tunnel. A fractal grid, which is composed of fractal elements with square shapes, is placed at the inlet of the test section. The Reynolds numbers (Re0) based on the thickness of the biggest grid bar t0 and the inflow velocity U∞ are set to 5,900. The spatial developments of cross-sectional profiles of turbulent kinetic energy K, dissipation rate of K, ε, advection A*, production ?*, triple-correlation transport Γ*, pressure transport Π*, and viscous diffusion D* terms in the transport equation of K are measured by using a hot wire anemometer with I and X type probes. The results show that in the upstream region, ? = ?*/ε at the region downstream of the interior of the biggest grid bar has a value larger than that around the centerline, and turbulent kinetic energy at this region is transported to the central and outward regions by Γ = Γ*/ε . In the decay region, ? at the aforementioned region has a low value and turbulent kinetic energy at this region is mainly transported outward rather than to the central region by Γ.

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The present paper serves as an accompanying document to the two keynote lectures of the present authors at the 4th International Conference on Jets, Wakes and Separated Flows (4th ICJWSF) held at Nagoya, Aichi, Japan from September 17th to 21st, 2013. Namely "Turbulent boundary layers in pipes and on plates: Tripping, suction, structures, and pressure gradients" by Philipp Schlatter and "High Reynolds number scaling in wallbounded flows: A Stockholm perspective" by Ramis Orlu. In particular it is intended as a summary of the authors own publications.

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Effects of freestream turbulence on a zero-pressure-gradient turbulent boundary layer are investigated by means of laboratory experiments using a wind tunnel. The freestream turbulence is generated by grids installed upstream of the turbulent boundary layer, which generate nearly isotropic turbulence. In this experiment, two grids with the same blockage of 36% and with different mesh sizes (i.e., M = 30 and 10 mm) are used. The turbulence intensities in the freestream are approximately 1.9% and 1.1% at the measurement point (x = 1,880 mm) for M = 30 and 10 mm grids, respectively. Here, x is the streamwise distance from the entrance of the wind tunnel where the turbulent boundary layer starts to develop. For both cases, the integral length scale of freestream turbulence, L∞, is much smaller than the boundary layer thickness δ, i.e., L∞/δ<<1. The Reynolds numbers Reθ based on the momentum thickness is 3,000. Instantaneous velocities U and V are measured by hot-wire anemometry with an X-probe. The results show that the log-low profile for the time-averaged mean velocity holds for turbulent boundary layers even with freestream turbulence. However, turbulence intensities normalized by the friction velocity uτ are increased by the freestream turbulence. On the other hand, Reynolds shear stress normalized by uτ is decreased in the inner region ( y+ < 600 ) and increased in the outer region of y+ > 700 by the freestream turbulence.

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The effects of compression on turbulent swirling flows are studied using Large-Eddy Simulations (LES). In this study, the geometry investigated is a cylinder with swirling motions of different strengths with superimposed isotropic turbulence. During compression the evolution of turbulence and vorticity is investigated. During early compression, rapid diffusion of turbulence is found. In the later part of the compression an increase of turbulence and vorticity is observed and linked to vorticitydilatation interaction. It is shown that the swirling motion suppresses turbulence and turbulent anisotropy. The longitudinal integral length scale of the tangential fluctuations is found to be approximately twice the transverse length scale. The longitudinal length scale is largely unaffected by compression whereas an effect on the transverse length scale is observed.

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The effect of ice accretion on the wake of an airfoil section is studied using Large Eddy Simulations (LES) for the flow computations and Lagrangian Particle Tracking (LPT) to track water droplets impacting on the surface. The flow around a clean airfoil is computed together with two cases with the same amount of accreted ice, but different smoothness of the ice layer. The computations revealed that ice accretion influences less the average velocity field and more the velocity fluctuations.

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The aim of this article is to study the flow and heat transfer characteristics of row of jet impingements with simulated cross-flow. Elongated orifices with an aspect ratio of orifice length to orifice width at AR=4 was studied and compared with the case of circular orifices (AR=1) under the same cross-section area. Four of impinging jets with inline arrangement impinge on inner surface of wind tunnel with rectangular cross-section having height of two times of equivalent diameter of orifice. Two differences of cross-flow velocities corresponding to velocity ratios (jet velocity/cross-flow velocity), VR=3 and 7, and attacking angle defined as angle of major axis of elongated orifice to cross-flow direction at θ=0°, 15° and 45° were examined. In order to measure the heat transfer on the impinged surface, temperature distributions were monitored using Thermochromic Liquid Crystal sheet (TLCs), and heat transfer coefficient distributions were evaluated using an image processing method. The flow characteristics on the impingement surface were visualized using the oil film technique. The numerical simulation was also employed to gain insight into the fluid flow. Results show that the θ=0°can minimize the cross-flow effect, and the area of high heat transfer is extended in upstream region of jet impingement.

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Pressure drops and frictional coefficients were in-vestigated for water and several types of aqueous solutions of surfactants, which have spherical micelles and wormlike micelles, in the flows through capillaries ranged from 0.430 mm to 2.87 mm. Good agreement for water flow was obtained between the experimental results and the predictions of laminar flows and Blasius expressions. For the spherical micelle surfactant solutions, pseudo-laminarization was slightly suggested until 2.7 X 103 of Reynolds number. Furthermore, the wormlike micelle surfactant solutions exhibited the huge drag reduction, and pseudo-laminarization was strongly suggested. In the explaining their drag reduction, non-Newtonian properties were discussed. Their estimated properties of the wormlike micelle surfactant solutions was supported to the discussion.

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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, it is confirmed that the jet widely spreads perpendicular to the rotating axis. Further in order to quantify the mixing efficiency of the dynamic control, as the mixing measure, a statistical entropy is examined. Compared to the uncontrolled jet, the mixing efficiency is markedly improved, thus it turns out that the dynamic control can be expected to be useful for the improvement of mixing performance.

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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 nonrotating 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.

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In order to improve the performance of heat transfer with multiple impinging jet(MIJ), we investigate the DNS (direct numerical simulation) of four round impinging jet arranged at an inflow of flow field. As a control parameters, a separation between each jet is varied. From view of instantaneous vortical structures and time-averaged velocity distribution, it reveals that the generation of vortical structures are enhanced due to an interaction between each jet, compared to that of a single impinging jet, and that various type of upward flow which does not exist in the single one, appears according to the control parameter. In addition the heat transfer performance of MIJ is numerically turn out.

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Both CFD and experiment were conducted to make clear the relationship between aerodynamic broadband noise and flow fluctuations in a sirocco fan. Velocity and pressure fluctuations near the trailing edge of the rotating blades were measured experimentally, while the unsteady flow field in the sirocco fan was analyzed using CFD. In the experiment, SPL (Sound Pressure Level) was also measured at some locations outside the shroud. As a result, it was found that flow fluctuations become large near the tongue that is the bifurcation between two flow passages. These results suggest that those flow fluctuations are associated with generation of the aerodynamic broadband noise.

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This work is aimed at experimental investigation of the single rib effect on the recirculation region behind the backfacing step. Measurements were carried out by the PIV method. The experiments were performed at Reynolds number Re = 15000 in the rectangular working channel of 21 X 150 mm and length of 1 m. At the distance of 600 mm from the channel inlet there was the back-facing step of the constant height H=9 mm. Rib height d and distance to the back-facing step S were varied (d from 3 to 6 mm and S from 0 to 77 mm). According to measurements, in this case the following interaction mechanism between separated flows occurs: if there is a rib in the channel in front of the step, the boundary layer separates. Depending on the distance between the rib and the step in the channel the flow will attach in front of the step or behind it. In our experiments the flow attached at S/D >7.5 and Δ=3mm and S/Δ > 10 at Δ= 6mm. In the case when the flow attaches in front of the step, the recirculation zone behind the step decreases to 35%. This relates to the fact that the flow separated behind a rib influences the mixing layer behind the step and intensifies the mixing processes, what finally leads to reduction in the length of recirculation zone. When S/Δ < 7.5, the recirculation zone behind the back-facing step increases, and in the limit case of S/Δ = 0 (the rib is at the step edge) becomes 60% larger for Δ= 3mm. We should note the general tendency of interaction between the separated flows of different scales. A rib changes the typical profile of longitudinal average velocities and mean-root velocity pulsations behind the back-facing step. For the studied configurations it was not enough time for velocity profiles and profiles of velocity pulsations to be restored to the values, corresponding to the separated flow behind the step without a rib. Maximal perturbations within the initial region of separated flow development behind the step were observed at the distance from the wall, corresponding to the coordinate of rib top. In general, the experimental data agree with experimental results of Miau et al., 1991 for S/Δ=12 and calculation results of Neumann, Wengle, 2004. The mechanism of interference for the separated flows of different scales is discussed in detail in the current study.

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The purpose of this study is to know differences between steady and unsteady drag coefficients of a sphere. We propose a way based on the equation of motion to obtain the unsteady drag coefficient considering an added math. To confirm validity of the way, we measured experimentally the motion of the falling sphere in water by using a high-speed camera and a motion capture method. The drag coefficients as a function of time were obtained by substituting measured values of velocity and acceleration into the equation of motion. Comparing with the values obtained by the other previous studies, our result is reasonable.

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An experimental study was made on the promotion and control of turbulent mixing of hot and cold airflows in a T-junction with rectangular cross sections. A delta-wing row was attached on the bottom wall of the main channel before the flow merging 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. The development of the thermal mixing layer could be promoted effectively by the delta wings and the thermal mixing could be controlled by changing the angle of attack of wings. Longitudinal vortices produced by the delta wings disappeared just after the merging of two flows. High turbulence generated by the interaction of those vortices and branch flow was, however, maintained to further downstream cross sections and contributed to the promotion of the thermal mixing.

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Large eddy simulations are used to investigate the emergence of flow asymmetry behind the symmetric geometry of a simplified notchback vehicle body. Two different angles of the backlight were simulated, resulting in asymmetric and symmetric flows in agreement with previous experimental observations. Simulations were made at Reynolds numbers 104, 3×104 and 5 × 104 based on the incoming velocity and the height of the body. The Reynolds number dependency of the asymmetry phenomenon was observed when the Reynolds number was changed from 104 to 3×104. The LES results are used to present a new picture of the time-averaged flow around the notchback geometry. The instantaneous flow was found to contain regular shedding of the horse-shoe vortices from the deck at the nondimensional frequency of St = 0.42, in agreement with previous experimental observations.

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The flow field around a vertical axis wind turbine (VAWT) is much complicated because the blades pass in non-uniform flow field by the blade wake during rotation. This paper presents a mean 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.

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The effect of geometrical parameters on the flow field and heat transfer at jet impingement cooling of an obstacle in the form of a spherical cavity, whose edge conjugates with a flat wall, is analyzed on the basis of experimental data. It is shown that the flow around cavity is accompanied by formation of large-scale vortices in depression, nonstationary separation of the flow at the outlet, and low heat transfer. The structure of the flow field and heat transfer intensity can be changed by variation of geometrical parameters.

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The critical phenomenon of wake structure occurs at 30 degrees of the rear slant angle of Ahmed's bluff body. Originally, such the characteristic phenomenon was pointed out by L.J. Janssen and W.H. Hucho in 1975. In 1984 S.R. Ahmed was conducted systematic measurement by changing the slant angle of the rectangular bluff body, namely Ahmed's body, to find the critical phenomena. After that, in 2001, D.B. Sims-Williams found that Ahmed's body had the vortex structure which had specific frequency. However, the reason why the phenomenon was occurred at the angle has not been clarified yet. Then this study is performed to analyze the fluctuation of the wake structure by observing the frequency due to the typical vortex shedding with the hot-wire anemometer and dynamic PIV system. As a result, the behavior of the vortex shedding which has specific frequency was confirmed.

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Simultaneous measurement of fluctuating velocity and pressure is conducted in a wake of circular cylinder with the Reynolds number of 3900. and the measured velocity-pressure correlation is compared with the computational reference data obtained by a large-eddy simulation in order to validate the measurement results. It is shown that the experimental and computational resuits of correlation between the streamwise velocity component and pressure, up, are in good agreement, while there was certain disagreement between the profiles of up, the correlation between the transverse velocity component and pressure. It is found in the further investigation that up is extremely sensitive to a small time lag between the velocity and pressure signals whist up is insensitive, and the experimental results with correction of the time lag is in acceptable agreement with the computational result.

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In this study, simultaneous measurement of the temperature at two points with cold-wire sensors is performed at three different downstream locations of a heated plane jet. The measurement apparatus consists of a wind tunnel, a skimmer, a side-wall, and a heater set at the blower intake of the wind tunnel. The temperature of the jet is controlled to become 5 K higher than the ambient fluid. A thin tungsten wire whose diameter is 3.0 m is used as a sensor and is connected to the self-made constant current circuit. The output of the cold-wire sensor is compensated theoretically to increase the temporal resolution of the measurement. By applying the Karhunen-Loeve (KL) expansion in space and time, the structural development of the heated plane jet is investigated from the viewpoint of both space and frequency. Results obtained in the selfpreserving region show that the energy contribution of the 1st mode of the temperature is approximately 20%. It was also found that the dominant mode of the fluctuating temperature field in a plane jet expresses and relates to a flapping motion, which is the coherent structure observed in the plane jet.

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The effects of a turbulence-generating grid on fluid mixing and chemical reaction were experimentally investigated in a liquid sheared mixing layer. The grid was installed at various streamwise positions to find out the optimal position as a promoter for mixing and reaction. Instantaneous velocities and concentration were simultaneously measured by a laser-Doppler velocimeter and a laser-induced fluorescence technique, respectively. The results show that the grid generates disturbance at small scales and enhances fluid mixing. However, as the mixing proceeds, turbulence intensity becomes smaller than those in the flow without the grid. The latter negative effect remains almost entire region downstream of the grid whereas the former positive effect appears only in the near region of the grid. As a result, fluid mixing and chemical reaction are promoted most effectively when the grid is installed in the downstream region where the mixing layer has developed.

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This study investigates the interaction between the bubble and a swirling flow around the central axis (vertical axis) of a bubble plume. The experiment is conducted in a cylindrical water tank. Small air bubbles are successively released from the bottom of the tank to generate a bubble plume, and a stirring disc on the tank bottom is made to rotate to impose a swirling water flow around the bubble plume. The bobble motion and the characteristics of the swirling water flow are explored to clarify the effects of the bubble volume flow rate and the rotational speed of thie stirring disc.

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The mixing phenomena of a density stratified fluid by a jet flow in a tank are experimentally investigated. The jet issues vertically upward from a circular nozzle mounted at the bottom of the tank into the density stratified fluid composed of water and salt water. The jet Reynolds number Re varies from 80 to 4200, and the salt density ranges from 2 to 8 percent. This study classifies the jet behavior into three patterns according to Re and the density difference between the two fluids. The patterns are discussed in relation with the mixing phenomena caused by the jet.

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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.

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Bridging methods are intended to provide the best possible physical fidelity on any given numerical grid while varying seamlessly between the Reynolds-averaged Navier-Stokes (RANS) model and direct numerical simulation (DNS). Two of the most developed bridging methods are the partially integrated transport model (PITM) and partially averaged Navier?Stokes (PANS) model. We choose here the PANS model as the basic approach for the further theoretical extension but the conclusions derived in this work are equally applicable to other bridging methods. In already well-established approach for the PANS, the main model resolution parameter is obtained from the grid spacing and the integral length scale of turbulence. This paper proposes the new approach for the calculation of the integral scale of turbulence by deriving an additional equation for the scale supplying variable which in physical sense can be understood as the modelled resolved kinetic energy.

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Behavior 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 ? are changed, while the mean ejection velocity is fixed. When vortex rings were generated at a short time interval, independent of LO/DO and ?, 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.

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Effect of ambient pressure on spray jet flame behavior is studied. Three-dimensional large-eddy simulation (LES) is applied to spray jet flames at ambient pressures of 0.1, 0.5 and 1.0 MPa. Jet-A is used as liquid fuel, and the evaporating droplets' motions are tracked by a Lagrangian method. As turbulent combustion model, a flamelet/progress-variable approach which considers 274 chemical species and 1537 elementary reactions is used. The results show that as the ambient pressure increases, increase in gas temperature and evaporation of droplets become remarkable in the upstream region.

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