The interaction of streamwise and axisymmetric vortices in an axisymmetric jet is studied by flow visualization and velocity measurement. The jet is excited by azimuthal and axial perturbations to enhance streamwise and axisymmetric vortices. The three-dimensional views of the jet-boundary surface and streamwise vortices are constructed by applying the Taylor hypothesis to the jet cross-sectional images, and the interaction model of streamwise and axisymmetric vortices is proposed. The interaction of spanwise and streamwise vortices in a plane jet is also studied by velocity measurement. The experiment is carried out under conditions similar to those of the axisymmetric jet. The vortical structure is discussed on the basis of the three-dimensional views of phase-averaged vorticities. It is confirmed that the interacting vortical structure is similar to that in an axisymmetric jet. The entrainment mechanism is also discussed in relation to the vortical structure.
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.
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 using 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., a vertically striped pattern is formed. On the basis of the result of the Karhunen-Loève (KL) expansion, a new interpretation of the coherent structure model in the self-preserving region of a turbulent plane jet has been given from the combination of “flapping” and “puffing”.
Flow visualization and measurements of mean and fluctuating velocities were performed on a 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 (2D and stereoscopic PIV) and laser-induced fluorescence (LIF). Axisymmetric and streamwise vortical structures were revealed in the near-field of the coaxial jet. The annular nozzle has six vortex generators in order to enhance the streamwise vortices generated in the mixing layer. Furthermore, the annular jet was excited by a shaker in order to enhance the axisymmetric vortices. For the tabbed coaxial jet, jet spreading downstream was greater than for the jet without tabs. The cause of the entrainment increment is the development of axisymmetric and streamwise vortex structures. In the case of excited jets, significant axisymmetric and streamwise vortical structures develop, and the jet width expands from the exit nozzle. Consequently, the flow rate of the excited jet with tabs is larger than that of the unexcited jet without tabs.
Flow characteristics of a two-dimensional jet with side walls have been studied experimentally. Three kinds of cylindrical walls and a flat wall were provided as the side walls, and they were combined and attached to a nozzle. Nine types of side wall conditions were investigated. Velocity was measured by a hot-wire probe and the separation point was measured by a Pitot tube. Mean velocity profiles, the growth of the jet half-width, the decay of jet maximum velocity, and the attachment distance were clarified. When cylindrical walls with different radii are installed, the flow pattern changes markedly depending on the velocity of the jet. A striking increase in the jet half-width is related to the separation of flow from the smaller cylindrical wall just behind the nozzle.
A Large Eddy Break-up (LEBU) device was applied to the flow management of a plane turbulent wall jet. The LEBU device was positioned at three different distances from the wall and the mean velocity, Reynolds shear stress profiles and wall shear stress were measured. The experimental data show that the LEBU device reduces the thickness of the shear layer and the magnitude of the Reynolds shear stress. The wall shear stress tends to be reduced even if the LEBU device is placed in the outer layer. These effects are most significant when the LEBU device is placed at the height at which the turbulent production term reaches its maximum. The length scale of the large eddies is reduced in manipulated flows, particularly in the spanwise direction.
The jet pump generally needs a long throat to mix the driving and induced fluids and transfer the momentum of driving fluid to induced fluid. Simultaneously, the energy loses when the fluids flow through the long throat because the friction loss occurs inside of the throat wall. Therefore, it is known that the throat length largely affects the jet pump efficiency. In this study, experimental studies are performed for a typical single nozzle jet pump using water at room temperature. It is revealed that surface roughness located nearer the throat inlet has a greatest effect on the jet pump efficiency because the local skin friction coefficient nearest the throat inlet is the largest. The best efficiency and its flow rate ratio decrease linearly as surface roughness increases. The frictional resistance coefficient in the throat for each roughness is made clear by fitting a one-dimensional theoretical prediction equation to the experimental results.
Jet pumps, driven by a Primary-Loop Recirculation (PLR) Pump, have been widely used in Boiling Water Reactor (BWR) plants to recirculate the reactor core coolant. A jet pump consists of a driving nozzle, a bell-mouth, a throat and a diffuser. The improvement of the jet pump efficiency for BWR plants brings an economic advantage because it reduces the operating power cost of the PLR pump. In order to improve the efficiency of the BWR jet pump, a 1/5 scale jet pump test loop for BWR plant was used and intensive tests were conducted focusing on the types of driving nozzles and shapes of the throat. These test data were used for CFD flow analysis code verification. The analytical data showed good agreement with the test results. After the analytical model verification, improvement of jet pump efficiency was conducted. It was shown by the CFD analysis that the peak efficiency of the improved jet pump will be 36% with the tapered throat.
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.
The wall temperature profile in the flow field of an impinging two-jet array has been controlled using a neural network. The jet excitation is achieved by injection and suction through fine slits co-located at the nozzle exits to obtain a desired wall temperature profile. The wall temperature profile is determined “uniquely” by the excitation pattern so that the flow field is essentially considered as the “function” with the excitation pattern as the input and the wall temperature profile as an output. A neural network learns the inverse function of the flow field via offline learning and online learning, and is then serves as the controller. As a result, the wall temperature distribution is controlled with high accuracy and this demonstrates the applicability of control on the convective heat transfer process.
The effect of cyclic perturbation on the mixing performance of millimeter-sized multiple air jets was investigated using flow visualization, hot-wire anemometry and laser Doppler anemometry techniques. Two types of external perturbation method were used, i.e., adding an acoustic sound and applying a pulsating flow to the jet. External acoustic sounds were found to be an additional dominant factor for the jet mixing, and in the multiple-jet case, the main jet located in the middle between the two same-sized jets was split into two, entrained into the neighboring jets, and eventually merged into one developed flow. For the pulsating-jet case, depending on the phase in one cycle, the jet flow condition alternately changed between laminar and turbulent. This affected the time-mean values of the jet flow profile. An adverse change in the mixing performance at the locations near the nozzle and far downstream was obtained between the pulsating- and steady-jet cases.
In order to develop an efficient jet mixing method, direct numerical simulations of combined jets are carried out. The Reynolds number defined with a nozzle diameter is Re=1500. Spatial discretization is performed by adopting a hybrid scheme of a sixth order compact scheme in the streamwise direction and Fourier series in the cross section. The distance between two jets is fixed at six times the jet diameter, and the inclination angle of the jets is changed from 45 to 70deg. The results reveal that the turbulence intensity increases with a decrease in the inclination angle and that the jet width increases viajet excitation. These findings suggest that the diverse requirements of jet mixing control can be satisfied by a flexible combination of jets.
Active control of the diffusion of a circular jet was attempted by application of a secondary film flow around the jet. Sinusoidal acoustic excitation of the film flow was carried out for VR values of 0.5 and 1.0, where VR is the ratio of the film flow velocity to the main jet velocity. For VR=0.5, the diffusion of the jet was suppressed compared to that of a single jet but it was somewhat enhanced by the acoustic excitation. The acoustic excitation shortened the potential core of the jet; however, the entrainment of the ambient flow was far less than in the case of a single jet, regardless of degree of excitation. For VR=1.0, diffusion was enhanced by acoustic excitation. Both turbulence intensity and ambient flow entrainment increased downstream of the jet. We conclude that film flow can control diffusion and that acoustic excitation can enhance the diffusion of jet flows. It is worth investigating this mechanism in detail in future studies.
Investigations of time-dependent laminar and transitional flows need accurate and reproducible generation of the flow as a function of time. For this purpose, an electronically controlled air valve, with which the mass flow rate is controlled, was designed and built. The working principle and lay-out of the valve are explained and the performance of the valve is demonstrated. It was found that with the present equipment one can study any non-periodic and periodic flow and the flow can be adjusted to be laminar, transitional and turbulent. The investigations on laminar and transitional time-dependent flows are reported. Laminar flow investigations showed that the generated sinusoidal mass flow rates agree well with that of analytical solution. Non-periodic transient and periodic sinusoidal pulsatile transitional flows were investigated. It was shown that elucidation of the transition in non-periodic transient flows may help in understanding the transition in periodic flows.
This paper describes an experimental study on the vortex formation of a pulsating jet issuing from a rectangular nozzle of an aspect ratio of 5 which has a short parallel section upstream of the jet exit. Visualization with hydrogen bubble method reveals three-dimensional vortex nature. It was confirmed that vortices which originate from short sides of the nozzle flow downstream faster than vortices from long sides and shift into the jet axis. On the other hand the vortices from the long sides move away from the jet axis and downstream bend at their middle with the edges of the vortices going ahead. The three-dimensional vortex deformation that the major and the minor axis of the vortex ring are switched was confirmed further downstream.
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 a 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.
The flow characteristics of upward gas-liquid two-phase flow through a vertical sudden contraction pipe is an area that has been less studied experimentally and numerically and that is still little known. The drag and its reduction for an upward bubbly two-phase flow in a sudden contraction pipe are investigated. Vortices are generated just prior to and after contraction and they significantly affect the flow characteristics. A simple and economical method to control and smoothen the flow is proposed by mounting a small obstacle (ring) upstream the contraction. The effects of ring position, Reynolds number and volumetric gas flow rate are examined and positive results were seen in all flow conditions. Moreover, the pressure fluctuation phenomenon and its control downstream the contraction was considered.
This paper proposes a two-dimensional vortex method, based on Vortex in Cell method, for gas-liquid two-phase free turbulent flow. The behavior of vortex element and the bubble motion are calculated through the Lagrangian approach, while the change in the vorticity due to the bubble is analyzed in the computational grids resolving the flow field. Therefore, the numerical procedure corresponds to the Lagrangian-Eulerian method. The present method is applied to simulate the air-water bubbly flow around a square-section cylinder. The simulation demonstrates that the bubble entrainment into the Karman vortex and the resultant reduction for the strength of vortex are successfully captured by the method. It is also confirmed that the vortex shedding frequency and the pressure distribution on the cylinder are favorably compared with the measured results.
A zigzag rising motion is a typical motion of bubbles in bubbly flows. Vortices shedding oscillatory behind a bubble and the wake of the bubble turning periodically similar to a sign of inequality are regarded as the most fundamental causes of the turbulence excited in bubbly flows, because these phenomena produce an alternating lift force on the basis of both conservation laws of circulation and momentum, respectively. In this paper, we discuss the lift production on a zigzag rising bubble trailing a hairpin vortex using a spinning sphere model, which is constructed by us to hold the aerodynamic universality independent of Reynolds number. We fluid-dynamically explicate the alternating lift production on a rising originally nonspin bubble and the zigzag turning of its path, and propose a chain of processes considered in this paper as a fluid-dynamic mechanism that produces the rising bubble zigzag motion.
The growth and three-dimensional deformation of a disturbance wave in the laminar-turbulent transition of a radial liquid sheet flow are studied. 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 induced by an unstable disturbance attributed to an inflectional velocity profile inside the liquid sheet. The detailed observation of a disturbance wave excited by a supersonic wave reveals the growth and three-dimensional deformation of the disturbance wave.
A plane air mixing layer loaded with spherical glass particles is simulated by the three-dimensional vortex method proposed for gas-particle two-phase free turbulent flow by the authors in a prior study. The vortex method computes simultaneously the behavior of vortex element and the particle motion by the Lagrangian approach. It is demonstrated that the simulated particle distribution, mean velocity and fluctuating velocity agree with the measurement and that the vortex method is indeed applicable to the analysis of particle-laden plane mixing layer. The change in the vortical structure due to the loaded particles is also discussed.
The aerodynamic mechanisms for the reduction of drag for a D-shape cylinder, wherein a front face of a circular cylinder is cut off, and an I-shape cylinder, wherein front and rear faces are cut off, are investigated. For the D-shape and I-shape cylinders with a cutting angle of 50-53° and for Reynolds number Re > 2.3× 104, the shear layer separated from the front edge reattaches on the circular arc of the cylinder, and a transition in the boundary layer as well as turbulent separation occur. As a result, the wake width decreases and the vortex formation region goes downstream. The Strouhal number increases beyond 0.28, the base pressure coefficient rises, and the drag coefficient of the cylinders decreases to half the value for a circular cylinder. The conditions of the above phenomena are clarified.
Wind tunnel experiments by the present authors have shown that a large crossflow oscillation can be induced on the upstream cylinder in cruciform cylinder system by two types of longitudinal vortices, i.e. trailing and necklace vortices. In this study, experiments on this phenomenon are carried out in a water tunnel over a Reynolds number range comparable with that of the wind tunnel experiments to investigate the effects of the mass ratio MR and the Scruton number Sc. The alternating lift coefficient CLR for Kármán vortex excitation in water flow agrees well with that in air flow in spite of the large differences in MR and Sc between them. However, the value of CLR for the trailing vortex excitation is much lower in water flow and no definite excitation due to the necklace vortex is observed.
The aerodynamic forces and moments of a flexible delta wing in pitching motion were experimentally studied in a low-speed wind tunnel. Three types of flexible delta wing were investigated, the flexible parts of which were 44, 70 and 99% of the delta wing. Aerodynamic characteristics were different among the three types of flexible and completely hard delta wing, and it was found that the winding-up of the leading edge of the delta wing is the key factor for determining the leading edge vortex on the upper side of the wing and the pressure distribution on the windward side. Lift, drag, and pitching moment formed a hysteresis loop with an angle of attack in pitching motion, particularly in a region with a large attack angle, accompanied by leading edge vortex breakdown. The flow visualization of leading edge vortices was also carried out to explain the dynamic characteristics of the delta wings.
In order to clarify vortex structures and vortex scales on an unsteady airfoil in a low Reynolds number region, we have visualized the flow around unsteady airfoils, such as a pitching airfoil and a heaving airfoil, at Re=4000 using two visualization techniques, dye flow and schlieren visualizations. In particular, we focus on dynamic behaviors of vortices shed from a leading edge, the number of vortex sheddings and their scales. At a low nondimensional pitching rate in a pitching airfoil, a number of discrete vortices were shed from the leading edge and their scale was markedly small. At a high nondimensional pitching rate in a pitching airfoil, on the other hand, the number of vortex sheddings from the leading edge in one cycle was low. However, their scale was approximately one-fourth the chord length.
Flow around a living tree was investigated as basic research of a windbreak forest. A type of conifer, which is named “goldcrest, ” was used as the test piece in a wind tunnel experiment. The drag coefficient of the living tree was measured in the range of a mean flow velocity of 5∼15m/s. The drag coefficient of the living tree was less than that of a two-dimensional circular cylinder. Because flow passes through the tree’s crown which has the permeability of branches and leaves, the drag coefficient was decreased as the flow velocity was increased. Moreover, the flexibility is that the bole of a living tree also plays an important role in drag reduction, bending itself so as to decrease the projected area. In the wake behind the living tree, reverse flow was found at further downstream region than the case of a circular cylinder.
Experimental results on the turbulent mixing of hot and cold airflows in a T-junction are reported, which simulates the HVAC unit used in an automobile air conditioning system. Experiments are conducted keeping Reynolds number and temperature of the main flow at 2.5× 104 and 12°C, respectively, and the velocity of the branch flow (60°C) is changed for three velocity ratios of 0.5, 1 and 2. The flow from the branch is separated at the edge of the T-junction and forms a large separation bubble. Longitudinal vortices are formed around this separation bubble, thus the flow field has a three-dimensional structure. In spite of such a complex flow field, the mean temperature in the thermal mixing layer shows quite uniform distributions in the spanwise direction, and the strong turbulence produced around the separation bubble does not work effectively to the thermal mixing of hot and cold airflows.
The 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 turbofan 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. Detailed mappings of the acoustic near-field provided more insight into the chevron noise suppression mechanisms by successfully identifying two primary chevron effects consistent with the results of the far-field measurements: chevrons penetration and shear velocity across them. Mean and turbulence data identified the physical flow mechanisms responsible for the effects documented in the far- and near-field studies.
The compressible Navier-Stokes equations are numerically solved to study the acoustic generation mechanism associated with the evolution of the structure in a compressible plane wake undergoing transition to turbulence. High-order compact finite difference schemes 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 vertical direction and periodic boundary conditions in the streamwise and spanwise directions. Three-dimensional structures of the wake are studied by means of temporally evolving plane wakes forced with a combination of unstable modes obtained from linear stability theory using a mapped Fourier method for the viscous compressible equations. Forcing with a pair of oblique subharmonic unstable modes yields streamwise/vertical counter- rotating vortices in the saddle region. As the streamwise/vertical vortices evolve outside, their self-induction causes inclined braidlike structures to form in the wake, which are similar to observations in the experimental supersonic flat wake transition. The oblique subharmonic unstable modes also cause the spanwise variations in the core that lead to roller distortion. Acoustic waves of plane wakes are generated when two-dimensional rollup structures appear and rotate in such wakes. Near-field sound wave pressure decreases downstream due to the three-dimensional evolution of the wake.
A passive control of aerodynamic noise generated by a laminar separating flow over a cavity is carried out in a wind tunnel test and by numerical simulation, using a thin plate inserted into the cavity as a passive flow-controlling device. In the experiment, a noise suppression effect up to 14dB is achieved by placing a plate in a proper location. It is shown that a plate of smaller vertical size is more effective. In the numerical simulation, it is shown that the location where the shear layer rolls up and the path of vortices are both very sensitive to plate position.
In this study, we investigate the active control of self-sustained oscillating flow over an open cavity using a moving bottom wall. The incompressible Navier-Stokes equations are solved using finite difference methods for the two-dimensional cavity with laminar boundary layer upstream. We move the cavity bottom wall tangentially with nondimensional velocities ranging from -0.2 to +0.2. The results show that wall velocity changes the characteristics of recirculating flow in the cavity and that the modification of recirculating flow plays an important role in changing the oscillation characteristics of the separated shear layer. When the wall velocity is less than -0.1, two recirculating vortices change to one clockwise recirculating vortex in the cavity, so that the self-excited shear layer oscillations are completely suppressed. When the wall velocity is more than +0.19, two stationary vortices exist on the upper side and lower side of the cavity and the self-excited shear layer oscillations are suppressed.
A method for controlling the position of an oscillatory cavity jet flow is demonstrated. The method involves secondary injection of a lower mass flow control jet into the cross-flow region of the primary jet. The primary jet in this case is a turbulent jet (Re=55000) which when injected into the rectangular cavity with no secondary control, attains a stable oscillation with a characteristic Strouhal number of St, W=0.013. The injection control method is investigated using a combined experimental and numerical approach with a water model test rig and a 2D and 3D computational fluid dynamics (CFD) model. Based on previous work, a baseline cavity, with a depth to width ratio of H/W=0.16 and entry nozzle submergence of S/W=0.38, is used to study the effect of secondary jet injection parameters on primary jet deflection angle (δ) as a function of momentum ratio (β ) and injection position (Yi). Results have shown primary jet deflection angles (δ) of up to 15° for a momentum ratio (β ) of 20% can be achieved for a secondary jet injection position of Y1/W=0.12.
A plane submerged turbulent water jet discharged parallel to the offset bottom wall in a channel with a finite water depth was investigated experimentally. Depending on the depth of the nozzle and its height from the bottom, the jet deflects towards either of two boundaries, the free surface or the solid wall. Detailed data on the velocity field measured by the Particle Image Velocimetry (PIV) method are obtained to examine the effect of the opposite boundary on the deflecting jet and the difference in the jet development between the deflection to the free boundary and that to the solid boundary. The mean velocity profiles and the spread rates of the jet are compared with those of a conventional plane free jet by applying a curved coordinate system along the jet centerline.
Aero-train is a new driving concept using aerodynamic technology under development by the Kohama Laboratory, Institute of Fluid Science, Tohoku University. It employs the wing-in-ground effect to enable travel at high speeds over land. Aero-train makes use of the ground effects of lift and side force between the wings and a U-shaped guideway for stability. The main wings have vertical wings at the tips, which are arranged in tandem to regulate the roll and yaw stability in the U-shaped guideway. However, the vertical wings deteriorate the lift-to-drag ratio of the Aero-train by aerodynamic interaction with the main wings. The present study was performed to improve the aerodynamic performance of the Aero-train by controlling wing-wing interaction. Installation of a single-slotted flap on the wings considerably improved the aerodynamic performance of the wings.
The large eddy simulation (LES) is applied to an unconfined swirling flow of an air surrounding a bluff-body having a central jet of air, and the complicated flow field that involves the recirculation and vortex breakdown is investigated. The Smagorinsky model is used as the sub-grid scale model. The results of the present numerical simulation are compared with the experimental data of the mean and stochastic root mean square (RMS) variations of two velocity components. Although the inflow conditions are specified in a simple manner, the obtained numerical results are in a reasonable agreement with the experiments, except for a part of RMS variation values behind the bluff body. The present numerical calculations can successfully reproduce the characteristics of the flow, i.e., an upstream recirculation region established just downstream of the burner plane. Additionally, the flow field is much different by the swirl number and axial velocity of the primary swirling air. Especially the additional recirculation region is established at the more downstream location in the lower swirl number and higher axial velocity of the primary swirling air.
In this paper, we present a digital holographic particle image velocimetry (DHPIV) technique with a spatio-temporal derivative method for velocity measurement in 3D space and the results of evaluating on its measurement accuracy. In this technique, hologram patterns are observed as digital images using an electronic camera, such as CCD or CMOS, and image reconstruction is carried out on a personal computer. Since an in-line observation system is utilized in conventional digital holography, a numerically reconstructed image is considerably enlarged in the depth direction and its depth resolution is extremely low; hence, the measurement accuracy in the depth direction is inaccurate in a digital holographic measurement. To overcome this difficulty, we apply a spatio-temporal derivative method to this technique for the detection of particle displacement along the z-axis. In a numerical simulation, measurement accuracy is evaluated for a multi particle model and a cubic cavity flow model. Furthermore, we examine the effect of noise on displacement measurement accuracy for numerically constructed noisy hologram patterns.
In this study, the virtual flux method (VFM) proposed by the authors was applied to the seamless calculation of heat fluid flow and heat conduction inside solid bodies. Flows both inside and around circular cylinders are calculated as examples. The estimated Nusselt numbers of the cylinder surfaces, which are calculated by the VFM, are compared with both numerical and experimental results obtained by other researchers. In order to show that the accuracy of the VFM is high, an exemplified seamless calculation of heat fluid flow and heat conduction inside a solid cylinder, which is an advantage of the VFM, are also performed.
A wavelet multi-resolution technique is applied to analyze the three vorticity components obtained simultaneously using an eight-wire probe in the far wake of a circular cylinder at a Reynolds number of 6000. Using this technique, the vorticity is decomposed into a number of orthogonal wavelet components based on different central frequencies, which correspond to the scales of turbulent structures. The vortical structure of each wavelet component is examined in terms of vorticity variance. The present result shows a relatively large contribution to the longitudinal vorticity variance from the large-scale structures than both to the transverse and to the spanwise vorticity components. The dominant contributions to the vorticity variances are from the intermediate and relatively small-scale structures.
Field measurement and numerical simulation were performed on the distortion of the compression wave generated by train entry and propagating through a slab track Shinkansen tunnel, which is the longest mountain tunnel in the world as of 2004. The compression wave was measured at twelve different locations. In the numerical simulation, the distortion of the compression waveform were calculated by one-dimensional compressible flow analysis, which takes account of steady and unsteady friction, combined with acoustic analysis on the effect of side branches in the tunnel. The results of numerical simulation are consistent with those of the field measurement. Furthermore, the results indicate that the compression wavefront steepens in the early stage and smoothes down in the later stage of propagation, and the maximum value of the pressure gradient of the compression wavefront reaches a peak under certain conditions of the initial compression wave and a tunnel length.
The single supersonic free jet discharging from a nozzle or an orifice has been often employed in various industrial processes. A number of studies have been done on the major features (Mach disk, barrel shock wave and the jet boundary configuration) of the supersonic jets. In the present study, numerical computations were performed to investigate the effect of initial degree of supersaturation of moist air at reservoir condition on under-expanded jet structures and the total pressure loss behind the Mach disk. From these studies, it was found that the position of Mach disk in the under-expanded moist air jets was almost the same as that in dry air jets. However, the diameter of Mach disk increased slightly with an increase in the initial degree of supersaturation of moist air. Furthermore, the total pressure loss behind the Mach disk of moist air jet changed largely in comparison to that of dry air jet.
An ultrasonic velocity profile monitor (UVP) measurement was performed to measure the average liquid velocity field around a large bubble rising in stagnant water in a round pipe of inner diameter D=54mm in order to obtain fundamental information for gas-liquid two-phase slug flows. Two ultrasonic transducers were set at different directions to obtain velocity vectors. The measured results are presented and compared with the results of some previous studies on the corresponding phenomena. In the liquid film near the bubble nose, the difference in bubble length does not affect the acceleration. The parameter z/D is more dominant than z itself, particularly when z/D is less than unity. In the wake region, a large ring vortex is recognized. The upward velocity at the pipe axis agrees well with previously predicted results. The effect of bubble length on the vortex length is also discussed.
Characteristics of the airflow over a train of waves moving at a constant speed are investigated using a direct numerical simulation. We simulate the flow for various wave ages (c/u*=0, 4, 8, 10, 12, 16 and 20), where c is the phase speed of the wind wave and u* is the friction velocity on the wave surface. The results show that both the mean flow profiles and the turbulent statistics depend strongly on the wave age. For example, the mean-flow speed in the log layer generally decreases at small wave ages, i.e., when the wave growth rate is positive. Vertical profiles of the flow change significantly at the critical height zc, where the mean flow speed and the wave speed c agree. Then, the wave-induced vertical flux of the horizontal momentum is positive below zc, decreases rapidly near zc, and becomes negative above zc.
In recent biological and chemical analyses, microchips have attracted attention because of advantages such as high efficiency, small heat capacity, and high-speed reaction. Biochemical reagents and samples flow into the chips with the wall surface biologically or chemically modified. The mechanisms of the complex flow are not well-known. In this paper, the mechanisms are investigated using pressure drop measurements of the flow of BSA-(bovine serum albumin, protein generally used in analytical fields) dispersed solutions in microtubes with three kinds of surfaces: glass, PEEK (polyetheretherketone) and Hirec-X1 (a highly water-repellent agent, NTT-AT Co.), which have different properties. In the cases in which BSA solution flows on the Hirec-X1 and on the PEEK surface, results show reductions in the friction factor. On the other hand, in the case in which non BSA solution flow on any surface, results agree well with the Hagen-Poiseuille equation. Furthermore, reduction ratio in the friction factor depends on the concentration of BSA. These results imply that the interaction between the wall and the bio-molecules influences the behavior of the flow in microtubes.
The purpose of this experimental research is to clarify both the aspect-ratio effect and the Reynolds-number effect, especially for the flow of cross-flow impellers with shorter axes. Particle-image-velocimetry (PIV) technique and a hot-wire anemometer are used for measurements of flow velocity. The impeller rotates without any casings. The authors study two kinds of the impellers, that is, one with forward-cambered blades and the other with radial-flat blades. As a result, observing eccentric-vortex revolution by using hot-wire measurements and flow visualisations, the flow can be classified into three modes. According to this classification, the authors show flow-regime maps for both impellers. Using PIV results, the authors define outflow rate Q from the impeller. Outflow-rate coefficient CQ is independent of the Reynolds number for both impellers. For the radial-flat-blade impeller, CQ is not affected by aspect ratio L/D2. But, for the forward-cambered-blade impeller, CQ increases with L/D2.
In automobile-exhaust systems, catalytic converters are main components and produce substantial pressure drops, which induce engine-power loss and fuel-consumption rise. On the other hand, catalytic converters are required for the uniformity of flow through a catalytic substrate, which causes uniformity of thermal distribution and high catalytic-conversion efficiency. The purpose of this study is to reduce the pressure losses and to improve the flow distribution simultaneously, under spatial constraints. The authors propose a new type of device and show its performance experimentally. Namely, the authors place a dome as a flow deflector inside the diffuser in front of a catalyst in order to suppress flow separation. The converter tested in the present study has a standard cylindrical ceramic substrate with a circular cross section. As a result, we present the optimum dome geometry that can reduce the pressure loss by 22% as compared to a no-dome converter.
The quality and power of an extracted beam from a Q-switched supersonic flow chemical oxygen-iodine laser have been investigated by numerical simulation. The flow system adopted in this study is the throat-mixing system proposed in a previous paper. Three-dimensional calculation for optics is coupled with one-dimensional calculation for gas flow including chemical reactions. Both geometric and wave optics are calculated and compared to assess the effects of diffraction. Wave optics is calculated with a time-dependent paraxial wave equation. The results indicate that wave and geometric optics are qualitatively similar in the time-dependent behavior of beam power. Quantitatively, it is found that diffraction reduces the extracted power by 9%. It is also found from the spreading half-angle of the beam for wave optics that the beam quality at a maximum power is equivalent to that of a plane wave; however, it is lower than that of continuous extraction.
In the present study, I investigate the effects of shaft vibration on asymmetric cavitation, which is one of the instability phenomena on cavitating 3-bladed inducers. Inducer tests under various unbalance conditions are performed to clarify the effects of shaft vibration. The following results are obtained. (1) The amplitude of shaft vibration has a strong effect on the occurrence of asymmetric cavitation, and in an inducer with a large vibration amplitude, asymmetric cavitation easily occurs. (2) The phase of the small-tip clearance caused by shaft vibration coincides with the position of the small cavity. In this study, it is shown that suppression of shaft vibration amplitude is extremely important in preventing harmful asymmetric cavitation.
As one kind of hygroscopic salts, bulk potash particle easily absorbs water vapor during storage and transportation process to form serious caking. The accumulation and movement of moisture is demonstrated to change with temperature. In this paper, a set of appropriate equations has been presented to formulate a mathematical model of coupled moisture and heat migration in thin pile beds of storage potash particle, assuming moisture diffusion and heat conduction as dominant transport mechanism. The numerical model is validated by comparison of numerical simulation results with experimental data from literature. Then the model is used as a reliable predictive tool to explore the moisture and heat migration for a packed potash bed with average particle size 3.66mm. The sensitivity analysis presents the influence of the variation of effective thermal conductivity and effective diffusion coefficient.
A vapor chamber is used as a novel heat spreader to cool high-performance MPUs (microprocessor units). The vapor chamber is placed between small heat sources and a large heat sink. This paper describes the effect of heat source size on the heat transfer characteristics of the vapor chamber. First, by the experiments, the effect of heat source size on the temperature distribution of the vapor chamber is investigated, and the validity of the mathematical model of the vapor chamber is confirmed. Secondly, by the numerical analyses, the effect of heat source size on the thermal resistances inside the vapor chamber is discussed. It is found that the heat source size greatly affects the thermal resistance of the evaporator section inside the vapor chamber. Although the thermal resistance is hardly affected by the heat generation rate and the heat flux of the heat source, it increases as the heat source becomes smaller.
Five different kinds of domestic-size renewable energy system configurations for very cold climate regions were investigated. From detailed numerical modeling and system simulations, it was found that the consumption of fuel oil for the auxiliary boiler in residential-type households can almost be eliminated with a renewable energy system that incorporates photovoltaic panel arrays for electricity generation and two storage tanks: a well-insulated electric water storage tank that services the hot water loads, and a compact boiler/geothermal heat pump tank for room heating during very cold seasons. A reduction of Greenhouse Gas Emissions (GHG) of about 28% was achieved for this system compared to an equivalent conventional system. The near elimination of the use of fuel oil in this system makes it very promising for very cold climate regions in terms of energy savings because the running cost is not so dependent on the unstable nature of global oil prices.
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