In order to develop a new mixing procedure, we conduct DNS (direct numerical simulation) of vector controlled free jets. The inflow velocity of jet is periodically oscillated perpendicular to the jet axis. To realize the high accurate computation, the discretization in space is performed with hybrid scheme in which Fourier spectral and 6th order compact scheme are adopted. From view of instantaneous vortex structures, it is found that the flow pattern considerably changes according to the oscillating frequency, i.e., according to the increasing the frequency, wave like mode, bifurcation mode and flapping mode appear in turn. In contours of ensemble averaged streamwise velocity and turbulence kinetic energy, the jet diffuses largely in the oscillating direction. On the other hand, the jet width in the perpendicular to the oscillating direction is similar to that of uncontrolled jet. Further in order to quantify the mixing efficiency under the vector control, as the mixing measure, the statistical entropy is investigated. Compared to the uncontrolled jet, the mixing efficiency is improved in order of the wave- , the flapping- and the bifurcating mode. Thus the vector control can be expected for the improvement of mixing efficiency.
In this paper, experimental study was undertaken to investigate the effect of excitation frequency of a synthetic jet on the generation of a longitudinal vortex produced by the interaction between a jet and a crossflow in a wind tunnel. A synthetic jet device is a tool for generating a jet, and a synthetic jet actuator is a useful tool for active flow control. For separation control, it is necessary to clarify the periodic behavior of a synthetic jet. A smoke flow visualization was used to reveal the characteristics of the vortical structures produced by the synthetic jet, and averaged vortical fields were also measured by using hot-wire anemometry during the entire cycle of issuing the synthetic jet. For f=60 Hz, the vortex rings were advected away from an orifice and was deformed. The position where the vortex ring broke down agreed with the position where the half-width of the jet velocity significantly increased. For f=200 Hz, the vortex rings showed wavy movements in the azimuthal direction, and therefore jet velocity profiles did not have a sharp peak in the near field of the orifice exit. A significant difference was observed in the flow field for the synthetic jet issuing into the crossflow for f=60 and 200 Hz. Longitudinal vortices were observed for f=60 Hz but not for f=200 Hz. The generation of longitudinal vortices was affected by the excitation frequency of the synthetic jet.
Over the last decade, several studies have investigated synthetic jets. However, there are still many clarifications needed, including details of the structure and Coanda effect of synthetic jets. The present study clarifies some fundamental flow characteristics of free synthetic jets and synthetic jets near a rigid boundary by conducting an experiment and numerical simulations. As the main results, it is found that the velocity distribution of free synthetic jets depends on K = Re/S2 (the ratio of the Reynolds number to the square of the Stokes number) and can be identified by the maximum velocity at the centerline and the jet half-width. In addition, it is confirmed that the flow characteristics of the synthetic jet near a rigid boundary and the re-attachment length of the synthetic jet are determined not only by H1/b0 (normalized step height) but also K.
The flow structures in a coaxial jet with axisymmetric and helical instability modes for a comparatively low Reynolds number were investigated. The flow visualization and the measurements of velocities were carried out in an open water tank. In addition, three-dimensional numerical simulation of a coaxial jet was also performed using the commercial CFD software FLUENT 6.3. It was confirmed that the helical vortex was shed for the range of ratios of velocity of the inner to that of the outer jet from 0.5 to 1.0. Two characteristic flow regimes, i.e., axisymmetric and helical instability modes, were simulated in the flow field. For the coaxial jet with helical instability, the axial velocity along the centerline of the jet decreased more than that of the coaxial jet with axisymmetric instability. The axial velocity fluctuation at the centerline of the jet was small near the nozzle exit. However, the radial velocity fluctuation at that location increased. The convection velocity of vortices in the inner shear layer was larger than that for the outer shear layer. The convection velocity of vortices with the helical instability was slightly larger than that of the vortices with the axisymmetric instability. Consequently, the variation of velocity fluctuations and the convection velocity was associated with the arrangement of the vortex street in the shear layer.
The centerline velocity of an orifice jet increases in the downstream direction from the nozzle exit and reaches a maximum of 1.2uce (where uce is the nozzle exit maximum velocity) at x/do≈2 (where do is the nozzle exit diameter) due to the vena contracta effect; this may be effective for enhancing the heat transfer performance of an impinging jet. However, the sudden contraction at the nozzle exit creates a large flow resistance. In this study, we considered the use of notches to reduce the flow resistance and increase turbulence for the sake of good heat transfer performance from the vena contracta effect. Specifically, the effects of using a notched-orifice nozzle with taper angle α on heat transfer characteristics were examined experimentally. Hot-wire measurements were also conducted to demonstrate the spreading or mixing performance of the notched-orifice nozzle. The small notches reduced the nozzle resistance or operating power as well as increased the turbulence at the nozzle exit. The heat transfer characteristics of the notched orifice jets with tapering increased significantly because of the high turbulence intensity introduced by the notches.
The mixing and combustion performances of a baffle-plate-type millimeter-scale confined multi-jet were investigated experimentally by measuring the blow-off limit of the flames, exhaust gas components and two-dimensional unburned flow velocity profiles in a cylindrical chamber. Two types of multi-jet nozzle, parallel- and swirl-type nozzles, were examined in this study. In the parallel-type case, a small change in the distance between the fuel and air nozzles showed significant effects on the blow-off limit. When the air nozzles were closely located around the center of the chamber, the flame was easily extinguished and a pair of elongated flow recirculation was observed near the chamber sidewalls. When the distance was increased, large-scale reverse flows were observed in the central region that led to a stable and efficient combustion. In the swirl-type case, swirl flows made the flame more stable. Some cases of excessive swirl, however, deteriorate the fuel-and-air mixing performance and increase the CO concentration.
We analytically examine nonlinear instabilities and breakup phenomena of a viscous compound liquid jet which consists of a core and a surrounding annular phase. Applying the long wave approximation to both phases, a set of reduced nonlinear equations is derived for large deformations of the jet. Breakup phenomena are numerically examined when sinusoidal disturbances are fed at the end of the semi-infinite jet. Typical breakup profiles are shown for surface tension ratios and Reynolds numbers. In particular, for sufficiently small Reynolds numbers, the core phase is found to be choked at a bottle neck when the jet is pinching, which is followed by the ballooning of the annular phase in the upstream. It is shown that there exit the most unstable frequencies of input disturbances for each parameters of the surface tension, viscosity and amplitudes of disturbances. From variations of the breakup time and distance for such frequencies, it is expected that there exist critical Weber numbers, above which the jet becomes convectively unstable and below which absolutely unstable.
A theoretical and experimental study is conducted to investigate the bubble separation from an air jet injected into a liquid turbulent boundary layer. The following three patterns of bubble separation are dependent on the jet velocity: (a) a single bubble, (b) a coalescent bubble, and (c) a continuous jet. The critical jet velocity from (a) to (b) is theoretically estimated from the condition that the jet from the nozzle overtakes the rear end of the separated bubble, which shrinks due to surface tension. The calculated results generally reproduce the experimental results observed by a high-speed video camera. In addition, the process of bubble separation from the continuous jet (c) is classified into two patterns: (i) the bubble separates from the swell at the front end of the jet, or (ii) the jet breaks due to the instability at the liquid-gas interface. The separated bubble diameter of pattern (i) is theoretically determined by considering the force balance at the front end of the jet between the surface tension, the drag from the free stream, and the virtual mass force. The bubble diameter of pattern (ii) is calculated from the most unstable wavelength of Rayleigh instability. Both of these theoretical results also agree well with those obtained experimentally.
Concerned on the numerical simulation of high-speed water jet accompanied with intensive cavitation, a practical compressible mixture flow method is developed by coupling a simplified estimation of bubble cavitation and a flexible compressible flow computation procedure. Two-phase fluid media of cavitating flow are treated as a mixture of liquid and bubbles, and the mean flow of the mixture is computed by applying Reynolds Averaged Navier-Stokes equations for compressible fluids considering the effect of bubble expansion a/o contraction. The intensity of cavitation is evaluated by gas volume fraction which is governed by the compressibility of bubble-liquid mixture at the current status of mean flow field. High-speed submerged water jets issuing from a convergent-divergent nozzle are treated under different cavitation conditions. The results demonstrate that pressure decreases in the convergent section and cavitation occurs initially in the low-pressure region behind the throat near the wall. The gas volume fraction of cavitation bubbles reaches to 0.5 locally when the cavitation number is decreased to 0.1, and cavitation bubbles generated in the shear layer near the throat flow downstream along the jet periphery. Under the effect of cavitation bubbles the mass discharge coefficient of water jet decreases obviously compared to the case of no-cavitating one although the maximum velocity at the throat increases.
A cavitating jet is a useful tool for the practical application of cavitation. Cavitation impact arises when bubbles collapse and can be utilized to modify the surfaces of materials as an alternative to shot peening. Peening methods using cavitation impact are called “cavitation peening” or “cavitation shotless peening”, as shot is not required. In cavitation peening, cavitation is generated by injecting a high-speed water jet into water, i.e., a cavitating jet. In practical applications, it is very important to maximize the aggressive strength of the jet. In the present paper, in order to do this, the outlet geometry of the nozzle of a cavitating jet was optimized. High-speed observations were carried out to investigate any instability in the jet. The introduction of compressive residual stress into stainless steel was demonstrated to show the effect of cavitation peening. Scaling- and velocity-effects, i.e., the effect of the injection pressure, at the nozzle throat was also examined. It was shown that a large cavitating jet at low injection pressure was more aggressive than a small jet at high injection pressure, for the same power.
Linear Stochastic estimation (LSE) is employed to compare the large-scale structure of a jet exhausted through a coaxial nozzle configuration with and without mixing devices on the coaxial stream. Particle Image Velocimetry (PIV) is used to measure the streamwise and radial velocity components on a 2D streamwise plane over the first 11 equivalent jet diameters. A comparison of the turbulent kinetic energy (TKE) for these two cases show that the streamwise vortices generated by the mixing devices result in high turbulence levels near the nozzle and reduced turbulence levels downstream. Reconstruction of the TKE profiles using LSE also captured these trends. An in-depth-analysis into number and placement of sensors was performed by correlating the raw data set to the data set formed by reconstructing the field using LSE. LSE was most successful when the reference signals were at the peak amplitude of the TKE radial profiles, and the correlation levels increased as the number of sensors was increased. It was shown that a jet with mixing devices lowered the correlation levels at the downstream positions by breaking up the large-scale structures.
In the present study, we investigate a self-excited oscillatory phenomenon of a two-dimensional confined jet with a rectangular-cross-section cylinder as a downstream target. More specifically, in order to reveal the effects of a kinematic parameter the Reynolds number Re and various geometric parameters upon the jet-oscillation frequency fD, we conduct experiments at Re < 5000 in water and in air by an ultrasonic velocity profiler and a hot-wire anemometer, respectively. As a result, we specify the effects of Re and four non-dimensional geometric parameters upon fD, which is non-dimensionalised as the Strouhal number St. As the four geometric parameters, we consider (1) the streamwise target size, (2) the channel breath, (3) the cross-streamwise target size and (4) the target distance in non-dimensional forms with a length scale of the jet's breadth. It is found that the Re effect is negligible. This guarantees a wide-range workability as flowmeters. All the results can be summarised in an empirical formula describing the relation of St with the four geometric parameters.
This study is concerned with an investigation into the sound produced when a jet, issued from a circular nozzle or hole in a plate, goes through a similar hole in a second plate. The sound, known as a hole-tone, is encountered in many practical engineering situation. Direct computations and experiments of a hole-tone feedback system are conducted. The mean velocities of the air-jet are 8 and 10 m/s in the computations, and 6-13 m/s in the experiments. The diameters of the nozzle and the end plate hole are both 50 mm, and the impingement length between the nozzle and the end plate is 50 mm. The computational results agree well with the experimental data in terms of the qualitative vortical structures and the relationship between the most dominant hole-tone peak frequency and the jet speed. Based on the computational results of the air-jet speed of 10 m/s, the shear-layer impingement on the hole edge, the resultant propagation of pressure waves, the associated vortical structures and a newly proposed feedback mechanism is discussed. As far as the authors know this is the first direct computation of the hole-tone feedback system that predicts its dominant frequency successfully. Non-axisymmetry observed in the present computation is also shown.
The authors have previously extensively investigated the flow field around an unsteady airfoil both experimentally and a numerically. In particular, we have performed a number of studies on wake structures and have reported the characteristics of dynamic thrust of an unsteady airfoil. However, the behaviors of vortices that form wake structures and processes from generation to development have not yet been clarified. The authors measured the flow field in the vicinity of a wall including the boundary layers by particle image velocimetry measurements and clarified the growth and development of vortices generated in the vicinity of the wall and the process of wake structure formation quantitatively. We clarified that the vortex flow in the vicinity of the wall and the process of wake structure formation are different between the rigid NACA0010 and the elastic NACA0010, whereas the wake structures for which flow fields are averaged for one heaving cycle become almost the same in the wake of the rigid NACA0010 and the elastic NACA0010.
A delta wing with a sawtooth leading edge has an ability to improve its aerodynamic performance, which was suggested and confirmed by the authors' previous study . It should not be practical, economical and easily controllable, because the sawtooth configuration part extends from the apex of the delta wing to the trailing edge. In this research a few petit delta parts beside the whole sawtooth leading edge was suggested on the basis of computational fluid dynamics (CFD) results for its optimization by changing their location on the leading edge, the number of petit deltas, and the configuration of the petit part. The aerodynamic characteristics of the suggested wing were investigated by experimental as well as numerical studies with the comparison to both the base delta wing without a sawtooth leading edge and the whole sawtooth delta wing. It was found that a series of three petit deltas with the modified rear configuration, which are located at the middle of the leading edge, gives the best performances at the attack angle for its taking off and landing. The reason for the best lift and such the complicated flow field as the primary leading edge separation vortex interacting with the flow from the petit deltas was examined in detail.
An experimental investigation was made on a turbulent boundary layer near the trailing edge on a long flat plate with a blunt trailing edge. The flow was controlled by an additional splitter plate attached to the trailing edge along the wake center line. The length of the splitter plate, l, was varied from 0.3 to 3 times the trailing edge thickness, h. Measurement of fluctuating velocity was made under the freestream zero pressure gradient. Almost independent of the installation of the splitter plate, the turbulent intensity and the Reynolds shear stress in the inner layer decreased when approaching the trailing edge of the flat plate (x / h=0), compared with that in the fully-developed turbulent boundary layer under the zero pressure gradient. The only exception was the distribution at x / h=0 near the wall in the case without the splitter plate (l / h=0), where the degree of the decrease in urms / Ue was relatively small, and vrms / Ue and -uv / Ue2 were larger than those of the zero pressure gradient. The effect of l / h on turbulent quantities was shown just upstream of the trailing edge upon close comparison, and was classified into two groups: l / h<1 and l / h≥1. The tendency of the turbulent energy production terms approaching x / h=0 coincided with the decrease in the streamwise turbulent intensity and in the Reynolds shear stress.
The flow and temperature/concentration fields in the mixing channel have complex three-dimensional and unsteady natures that are accompanied by flow separations and reattachments, longitudinal vortices, and large-scale velocity fluctuations. In this paper, detailed experimental results are presented on turbulent flow and mixing characteristics in a counter-flow type T-junction. The test fluid is water, and a fluorescence tracer is dissolved into the main-channel flow. The velocity and concentration fields are measured by PIV and PLIF with high temporal resolutions. At first, the three-dimensional structures of the velocity and concentration fields and the distributions of the turbulent mass fluxes that are dominant in the mixing process of two fluids with different concentrations are addressed. Then, the POD analyses are applied to the fluctuating velocity and concentration fields to extract their dominant structures. Based on the results of the POD, the turbulent mass transport process between two flows is examined.
Flows in the T-junction of a counter-flow pipe are run counter to each other and they usually flow out vertically together. A flow separated from the junction corner forms separated vortex regions and they reduce the effective cross-sectional areas of the pipe, and this increases flow resistance, i.e., drag (pressure loss). The corner of the junction is generally rounded to prevent the flow from separating and to reduce drag. This method can reduce drag by 30% with a rounded radius of 0.1D (D: pipe inner diameter), but some process is needed to remove the corners. We propose a simple method of reducing drag in the flows of T-junction pipes by mounting two small weir-shaped obstacles on the upstream of the walls of the two pipes beside the junction corners. This method is a simple way of reducing drag without having to use a removal process. The pressure distribution along the pipes was measured and the drag in a T-junction pipe was derived. The flow pattern was visualized with a tracer method and this was evaluated to confirm the separation of flow from the corners. As a result, we clarified that drag in a T-junction could be reduced by a maximum of about 30% by mounting small obstacles at heights of 0.30D and 0.47D from the upstream of the corners.
To reduce aerodynamic drag, reduction of skin friction is required. Vibrant flexible wall has been proposed to reduce skin friction drag for a long time. However, there are many problems for the flexible wall to get practical durability, easy maintenance and reasonable cost. On the other hand, vibrant solid wall, which is much adequate for a car body, has been rarely proposed. In this research, mechanism of drag reduction by a vibrating solid wall is considered from the velocity distribution near the wall surface. Reduction of skin friction drag on a vibrant wall is mainly caused by decrease of Reynolds shear stress, and the effect of shear stress from velocity gradient near the wall is small. The optimum condition of vibration for drag reduction is also discussed in this paper.
In this study, two kinds of circular cylinders which cut grooves to the cylinder surface for the purpose of cylinder drag reduction were produced. One of which is the circular cylinder which set 2 grooves in the cylinder upstream side, and other is the circular cylinder which provided 6 grooves every 60 degrees. Experiments were performed using a wind tunnel varying an attack angel α = 0 to 60 degrees in the range of Reynolds number Re= 1×104 to 1.2×105. The relationship between a Strouhal number and Reynolds number, the pressure distribution on the surface of cylinder, the flow feature around the cylinder, and drag reduction effect were investigated. As the results of experiments, in the case of 2 groove cylinder, the Strouhal number of the cylinder with grooves increases from 0.20 to 0.28-0.30, the base pressure coefficient rises from -1.4 to -0.8, then the drag coefficient decreases from 1.3 to 0.55 at α + θf < 80 degrees, in the range of Re > 4×104. However, it became clear that the drag reduction effect was lost as an attack angle α is attached. In order to compensate this weak point, the 6 groove cylinder was proposed, and the characteristic test was performed. It was obtained that the cylinder with the deep depth of groove is not influenced by the direction of wind. The value of drag coefficient was about 56% of value of the drag coefficient of smooth cylinder. It was shown that the wake width of the proposed cylinders narrows from the wake width of the smooth cylinder.
An attempt is made to clarify the flow instabilities downstream of inlet guide vanes by conducting experiments and performing a numerical simulation. The onset condition including the cell number and oscillating characteristics of the unsteady flow is discussed based on the measured pressure fluctuation. The propagating angular velocity ratio of the instability for various values of r3/r2 (ratio of the radius of the trailing edge of the vanes to the radius of the outlet of the device) is presented as a function of vane angle β2. The phenomenon taking place downstream of the inlet guide vanes is demonstrated to be of a different type from the rotating stall of the blades, where the amplitude of the flow oscillation depends on the number of cells and the chord length. In addition, applying the slope angle on the lower disc is found to be effective for controlling the flow instabilities.
An incompressible turbulent planar mixing layer is composed of two different flow types in its flow field, namely a shear layer in the central region and two free streams in each outer high- and low-speed sides. Shear layer is formed right after the trailing edge of the splitter plate and develops stream-wisely through successively distinct regions, namely the near field region and the self-preserving region. A new definition of the mixing length (lω) is proposed on the basis of an effectively pure shear-induced vorticity component (ΩSH) by means of a triple decomposition method, that is, lω = yH - yL where yH and yL are the two transverse positions, at which |ΩSH| normalized with the maximum ∂U/∂y at the virtual origin is equal to 0.05, in the high- and low-speed free stream sides, respectively. It is shown that the linear growth rate of lω along stream-wise distance can be, then, used as one of the necessary and sufficient conditions for identifying the achievement of the self-preserving state in turbulent mixing layer.
The present experimental study is an attempt to measure the concentration of jet diffusion. A hot-wire concentration sensor is used to measure the concentration of the components in a binary mixture of gases. King's equation, which gives the heat transfer from a fine wire heated by Joule heating in a gas flow with a low Reynolds number, includes two main parameters, velocity and thermal conductivity. The effect of velocity is suppressed by the passage of gases through a sonic nozzle. Thus, in the hot wire, the influence of thermal conductivity exceeds that of velocity. The concentration probe is calibrated using a helium jet and a carbon dioxide jet. The calibration curve is prepared using a helium-air mixture and a carbon dioxide-air mixture. The time constant of the probe is measured to be 0.74 ms for helium and 3.48 ms for carbon dioxide. The contours of velocity and concentration of helium and carbon dioxide gases are measured.
The flow field of a free square jet generated from an unsteady wind tunnel with a step-like axial velocity variation was measured using an ultrasonic anemometer. Three-dimensional velocity vector fields were synthesized by computational modeling from the measured time-series velocity data. The vorticity vector field was calculated after appropriate interpolation and extrapolation. A three-dimensional geographical representation of the isosurface of the vorticity vector norm showed that an isolated vortex ring separated from the shear layer and changed its shape as it moved downstream. In order to elucidate the cause of the overshoot peak observed in time-dependent variations of the axial velocity for the step-like jet, the velocities induced at an observation point on the jet axis, first by the isolated vortex ring and then by the other unsteady portions, were calculated using Biot-Savart's Law. The contribution to the overshoot peak from the other portions, including the re-developing shear layer, was larger than that from the isolated vortex ring.
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