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1.
A system of two stratified layers at a free surface, consisting of distilled water above a layer of salty water separated
by an interface, is studied under laboratory conditions involving uniform temperature heating from below. Shadowgraph and
particle images have been used with temperature and salt concentration measurements to investigate the interface instability
induced by convection when it is developing in the upper and lower layer. It is found that the interface is governed by local
shear flow that induces a Kelvin–Helmholtz instability. Moreover, the entrainment interface is subject to a combination of
two closely related effects: (1) double diffusion and convective motion and (2) double diffusion and Kelvin–Helmholtz instability.
Received: 22 December 1999/Accepted: 31 October 2000 相似文献
2.
This study utilizes a U-shape platform device to generate a single cavitation bubble for a detailed analysis of the flow field
characteristics and the cause of the counter jet during the process of bubble collapse caused by sending a pressure wave.
A high speed camera is used to record the flow field of the bubble collapse at different distances from a solid boundary.
It is found that a Kelvin–Helmholtz vortex is formed when a liquid jet penetrates the bubble surface after the bubble is compressed
and deformed. If the bubble center to the solid boundary is within one to three times the bubble’s radius, a stagnation ring
will form on the boundary when impinged by the liquid jet. The fluid inside the stagnation ring will be squeezed toward the
center of the ring to form a counter jet after the bubble collapses. At the critical position, where the bubble center from
the solid boundary is about three times the bubble’s radius, the bubble collapse flow will vary. Depending on the strengths
of the pressure waves applied, the collapse can produce a Kelvin–Helmholtz vortex, the Richtmyer–Meshkov instability, or the
generation of a counter jet flow. If the bubble surface is in contact with the solid boundary, the liquid jet can only move
inside-out without producing the stagnation ring and the counter jet; thus, the bubble collapses along the radial direction.
The complex phenomenon of cavitation bubble collapse flows is clearly manifested in this study. 相似文献
3.
In this work we examine first the flow field of a confined jet produced by a turbulent flow in a long cylindrical pipe issuing
in an abrupt angle diffuser. Second, we examine the dispersion of inertial micro-particles entrained by the turbulent flow.
Specifically, we examine how the particle dispersion field evolves in the multiscale flow generated by the interactions between
the large-scale structures, which are geometry dependent, with the smaller turbulent scales issued by the pipe which are advected
downstream. We use Large-Eddy-Simulation (LES) for the flow field and Lagrangian tracking for particle dispersion. The complex
shape of the domain is modelled using the immersed-boundaries method. Fully developed turbulence inlet conditions are derived
from an independent LES of a spatially periodic cylindrical pipe flow. The flow field is analyzed in terms of local velocity
signals to determine spatial coherence and decay rate of the coherent K–H vortices and to make quantitative comparisons with
experimental data on free jets. Particle dispersion is analyzed in terms of statistical quantities and also with reference
to the dynamics of the coherent structures. Results show that the particle dynamics is initially dominated by the Kelvin–Helmholtz
(K–H) rolls which form at the expansion and only eventually by the advected smaller turbulence scales. 相似文献
4.
Lobe-forced mixer is one typical example of the passive flow controllers owing to its corrugated trailing edge. Besides the
spanwise Kelvin–Helmholtz vortex shedding, streamwise vortices are also generated within its mixing layer. The geometrical
configuration of the lobe significantly affects these two types of vortices, which in turn affects the mixing performance
of the mixer. In the present investigation, characteristics of mixers with five different configurations were examined and
evaluated for two velocity ratios (r = 1, 0.4). The mixers have only one lobe in order to eliminate any possible interactions between neighboring vortices generated
by the adjacent lobes. Hot-wire anemometer was used to examine the Kelvin–Helmholtz vortices via the spectrum analysis while
laser Doppler anemometer was employed to examine the streamwise vortices. It was found that there were two main frequencies
for the Kelvin–Helmholtz vortices in the wake of the mixer; and the Strouhal numbers approached their respective maximum values
at high Reynolds number. The rectangular mixer had similar mixing performance with the semicircular one; and both of them
were better than the triangular mixer. The scalloping modification enhanced mixing by generating additional streamwise vortices
while the scarfing modification could not improve the mixing performance. 相似文献
5.
Particle image velocimetry measurements and time-resolved visualization are used for the reconstruction of the Kelvin–Helmholtz
vortex passing in the near field of a round jet and of a lobed jet. For the round jet, the entrainment is produced in the
braid region, where streamwise structures develop. In the Kelvin–Helmholtz ring, entrainment is dramatically affected by the
attenuation of the streamwise structures. As for the lobed jet, the special geometry introduces a transverse shear leading
to a breakdown of the Kelvin–Helmholtz structures into “ring segments.” Streamwise structures continuously develop at the
resulting discontinuity regions and control the lobed jet self-induction. In this case, the entrainment rate is less affected
by the primary structures dynamics. 相似文献
6.
A thin liquid sheet present in the shear layer of a compressible gas jet is investigated using an Eulerian approach with mixed-fluid
treatment for the governing equations describing the gas–liquid two-phase flow system, where the gas is treated as fully compressible
and the liquid as incompressible. The effects of different topological configurations, surface tension, gas pressure and liquid
sheet thickness on the flow development of the gas–liquid two-phase flow system have been examined by direct solution of the
compressible Navier–Stokes equations using highly accurate numerical schemes. The interface dynamics are captured using volume
of fluid and continuum surface force models. The simulations show that the dispersion of the liquid sheet is dominated by
vortical structures formed at the jet shear layer due to the Kelvin–Helmholtz instability. The axisymmetric case is less vortical
than its planar counterpart that exhibits formation of larger vortical structures and larger liquid dispersion. It has been
identified that the vorticity development and the liquid dispersion in a planar configuration are increased at the absence
of surface tension, which when present, tends to oppose the development of the Kelvin–Helmholtz instability. An opposite trend
was observed for an axisymmetric configuration where surface tension tends to promote the development of vorticity. An increase
in vorticity development and liquid dispersion was observed for increased liquid sheet thickness, while a decreasing trend
was observed for higher gas pressure. Therefore surface tension, liquid sheet thickness and gas pressure factors all affect
the flow vorticity which consequently affects the dispersion of the liquid.
相似文献
7.
Fundamentals of rotating detonations 总被引:17,自引:0,他引:17
A rotating detonation propagating at nearly Chapman–Jouguet velocity is numerically stabilized on a two-dimensional simple
chemistry flow model. Under purely axial injection of a combustible mixture from the head end of a toroidal section of coaxial
cylinders, the rotating detonation is proven to give no average angular momentum at any cross section, giving an axial flow.
The detonation wavelet connected with an oblique shock wave ensuing to the downstream has a feature of unconfined detonation,
causing a deficit in its propagation velocity. Due to Kelvin–Helmholtz instability existing on the interface of an injected
combustible, unburnt gas pockets are formed to enter the junction between the detonation and oblique shock waves, generating
strong explosions propagating to both directions. Calculated specific impulse is as high as 4,700 s.
相似文献
8.
M. F. El-Sayed 《Archive of Applied Mechanics (Ingenieur Archiv)》2001,71(11):717-732
Summary The electrohydrodynamic Kelvin–Helmholtz instability of the interface between two uniform superposed viscoelastic (B′ model)
dielectric fluids streaming through a porous medium is investigated. The considered system is influenced by applied electric
fields acting normally to the interface between the two media, at which there are no surface charges present. In the absence
of surface tension, perturbations transverse to the direction of streaming are found to be unaffected by either streaming
and applied electric fields for the potentially unstable configuration, or streaming only for the potentially stable configuration,
as long as perturbations in the direction of streaming are ignored. For perturbations in all other directions, there exists
instability for a certain wavenumber range. The instability of this system can be enhanced (increased) by normal electric
fields. In the presence of surface tension, it is found also that the normal electric fields have destabilizing effects, and
that the surface tension is able to suppress the Kelvin–Helmholtz instability for small wavelength perturbations, and the
medium porosity reduces the stability range given in terms of the velocities difference and the electric fields effect. Finally,
it is shown that the presence of surface tension enhances the stabilizing effect played by the fluid velocities, and that
the kinematic viscoelasticity has a stabilizing as well as a destabilizing effect on the considered system under certain conditions.
Graphics have been plotted by giving numerical values to the parameters, to depict the stability characteristics.
Received 27 March 2000; accepted for publication 3 May 2001 相似文献
9.
In this experimental study, a thin annular moving water sheet is placed between two annular co-flowing air streams. The shear
at the interface gives rise to Kelvin–Helmholtz type instabilities and promotes development of a sinuous surface wave at the
gas–liquid interface. The amplitude of the surface wave is amplified as it travels downstream of the nozzle exit until it
ruptures forming spanwise and streamwise ligaments. The liquid sheet is illuminated with high-powered halogen lamps. High-speed
imaging is used in this study to qualitatively visualise the structure of the spray—of particular interest is the evolution
of the spray into a ligament structure during the primary break-up and the role the outer air stream plays in this process.
Sequences of images with high temporal resolution (∼2,000 fps) are recorded for image processing and analysis of the surface
waves and ligament formation. A preliminary analysis of the waveform of the outer gas–liquid interface of the annular liquid
sheet over a range of conditions shows the sheet Strouhal number to increase with increasing gas to liquid momentum ratio.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
10.
A. Mahalov M. Moustaoui B. Nicolaenko K. L. Tse 《Theoretical and Computational Fluid Dynamics》2007,21(6):399-422
The generation and physical characteristics of inertia-gravity waves radiated from an unstable forced jet at the tropopause
are investigated through high-resolution numerical simulations of the three-dimensional Navier–Stokes anelastic equations.
Such waves are induced by Kelvin–Helmholtz instabilities on the flanks of the inhomogeneously stratified jet. From the evolution
of the averaged momentum flux above the jet, it is found that gravity waves are continuously radiated after the shear-stratified
flow reaches a quasi-equilibrium state. The time–vertical coordinate cross-sections of potential temperature show phase patterns
indicating upward energy propagation. The sign of the momentum flux above and below the jet further confirms this, indicating
that the group velocity of the generated waves is pointing away from the jet core region. Space–time spectral analysis at
the upper flank level of the jet shows a broad spectral band, with different phase speeds. The spectra obtained in the stratosphere
above the jet show a shift toward lower frequencies and larger spatial scales compared to the spectra found in the jet region.
The three-dimensional character of the generated waves is confirmed by analysis of the co-spectra of the spanwise and vertical
velocities. Imposing the background rotation modifies the polarization relation between the horizontal wind components. This
out-of-phase relation is evidenced by the hodograph of the horizontal wind vector, further confirming the upward energy propagation.
The background rotation also causes the co-spectra of the waves high above the jet core to be asymmetric in the spanwise modes,
with contributions from modes with negative wavenumbers dominating the co-spectra.
Dedicated to the memory of our colleague Dr. Binson Joseph 相似文献
11.
A novel particle tracking system working with a high particle concentration for the measurement of flow fields beneath water
waves is described. It features a 1–4 cm thick light sheet parallel to the main wave propagation direction so that the seeding
particles stay long enough in the illuminated area to enable tracking over several wave periods. An area of up to 14.0×10.0 cm2 is observed by a CCD camera with up to 200 fields/s. The polychromatic scattering theory of small particles in a light sheet
illumination is investigated, enabling the segmentation of individual particles at high particle concentration.
Received: 12 July 1995/Accepted: 18 April 1997 相似文献
12.
High-resolution two-dimensional (2D) measurements on a large plane mixing layer provide new quantitative information of its
spatial and temporal evolution to turbulence. Periodic acoustic excitation with three frequencies was used to stabilize the
fundamental instability of the mixing layer (roll-up) and its first and second subharmonics (vortex pairings). Phase-locked
velocity measurements of the time evolution in 2D space (x, y, t) reveal accurate spatially resolved primary (2D) instabilities of the mixing layer and turbulence transition. The measurements
unveil new quantitative details of the initial Kelvin–Helmholtz waves and their spatial and temporal evolution into vortex
shedding and the effect of the second subharmonic on the first vortex pairing. The second-subharmonic effect hastens alternate
first pairings of the rollers, with the result that pairing is completed at two downstream locations. The pairings that occur
closer to the knife-edge are more organized (laminar) than those occurring farther downstream (transitional). This effect
is corroborated using Taylor’s hypothesis to compute the vorticity distributions from the measured velocity field and a pseudo-spectral
simulation of the temporal evolution of the mixing layer.
Received: 26 March 1998/Accepted: 2 March 1999 相似文献
13.
14.
A novel method for combined particle image velocimetry and laser induced fluorescence is described and results from an experiment
in a stratified flow are presented. A standard two-dimensional, one camera particle image velocimetry configuration is used,
acquiring images of the seeding particles and the dye marking the current simultaneously and separating the two fields digitally.
The implementation of the postprocessing method, its capabilities and the necessary conditions for its use are discussed in
detail. The proposed method is applied to an arrested density current front. The front is made stationary by opposing a uniform
velocity profile, obtained from the combination of a moving floor and the recirculation of fresh water in the channel. To
improve the quality of the images, the current is made optically homogeneous by matching the refractivity index throughout
the domain. Instantaneous and time averaged fields are obtained for both velocity and density. Simultaneous measurements of
these fields provide insight in the mixing processes at the front of the density current. In particular persistent billow
generation, similar to that found in shear layers and associated with Kelvin–Helmholtz instabilities, is observed.
Support for this work was partially provided by the US Office of Naval Research through the Coastal Geosciences Program and
by Exxon Mobil Exploration Company. The authors would like to thank Dr. Wing Lai of TSI, Inc, for loaning part of the PIV
equipment used in this study. 相似文献
15.
A single-camera coupled particle tracking velocimetry–laser-induced fluorescence (PTV–LIF) technique and validation results
from an experiment in a neutrally buoyant turbulent round jet are presented. The single-camera implementation allows the use
of a 12-bit 60 frame-per-second 1024 × 1024 pixel digital CCD camera capable of streaming images in real time to hard disk
resulting in very accurate PTV and LIF with excellent spatial and temporal resolution. The technique is capable of determining
the turbulent scalar flux, as well as the Reynolds stress and mean and fluctuating velocity and concentration fields. Details of dye choice, corrections
for attenuation due to dye, particles and water, photobleaching, vignetting, CCD calibration, and illumination power and geometry
corrections are presented. Detailed results from the validation experiment confirm the accuracy and resolution of the technique,
and in particular, the ability to measure . Bootstrap 95% uncertainty intervals are presented for the calculated statistics.
Received: 28 July 2000/Accepted: 8 November 2000 相似文献
16.
To comprehensively understand the effects of Kelvin–Helmholtz instabilities on a transitional separation bubble on the suction
side of an airfoil regarding as to flapping of the bubble and its impact on the airfoil performance, the temporal and spatial
structure of the vortices occurring at the downstream end of the separation bubble is investigated. Since the bubble variation
leads to a change of the pressure distribution, the investigation of the instantaneous velocity field is essential to understand
the details of the overall airfoil performance. This vortex formation in the reattachment region on the upper surface of an
SD7003 airfoil is analyzed in detail at different angles of attack. At a Reynolds number Re
c < 100,000 the laminar boundary layer separates at angles of attack >4°. Due to transition processes, turbulent reattachment
of the separated shear layer occurs enclosing a locally confined recirculation region. To identify the location of the separation
bubble and to describe the dynamics of the reattachment, a time-resolved PIV measurement in a single light-sheet is performed.
To elucidate the spatial structure of the flow patterns in the reattachment region in time and space, a stereo scanning PIV
set-up is applied. The flow field is recorded in at least ten successive light-sheet planes with two high-speed cameras enclosing
a viewing angle of 65° to detect all three velocity components within a light-sheet leading to a time-resolved volumetric
measurement due to a high scanning speed. The measurements evidence the development of quasi-periodic vortex structures. The
temporal dynamics of the vortex roll-up, initialized by the Kelvin–Helmholtz (KH) instability, is shown as well as the spatial
development of the vortex roll-up process. Based on these measurements a model for the evolving vortex structure consisting
of the formation of c-shape vortices and their transformation into screwdriver vortices is introduced. 相似文献
17.
Optical distortions have previously prevented non-intrusive measurements of dissolved oxygen concentration profiles by Laser
induced fluorescence (LIF) to within 200 μm of the air–water interface. It is shown that by careful experimental design, reliable
measurements can be obtained within 28 μm of moving air–water interfaces. Consideration of previously unidentified optical
distortions in LIF imagery due to non-linear effects is presented that is critical for robust LIF data processing and experimental
design. Phase resolved gas flux measurements have now been accomplished along wind forced microscale waves and indicate that
the highest mean gas fluxes are located in the wave troughs. The local mean oxygen fluxes as determined by LIF techniques
can be reconciled to within 40% of those obtained by bulk measurement in the water. These data provide a new perspective on
wind-wave enhancement of low solubility gas transfer across the air–water interface. 相似文献
18.
H. Ó. Haraldsson H. X. Li Z. L. Yang T. N. Dinh B. R. Sehgal 《Heat and Mass Transfer》2001,37(4-5):417-426
This paper presents results of experimental and analytical investigation on molten alloy drop fragmentation in water pool.
Emphasis is directed towards delineating the roles which melt to coolant heat transfer and melt solidification play in the
fragmentation process. The strong impact of coolant temperature upon fragmentation process is addressed. A set of 23 drop
fragmentation experiments were performed, in which 8 experiments employed a low melting point alloy, cerrobend-70 and 15 experiments
using Pb–Bi eutectic alloy as drop fluid. The results show strong impact of coolant temperature on particle size distribution
of the fragmented drops. A linear stability analysis of the interface between the two liquid fluids with thin crust growing
between them, is performed. A modified dimensionless Aeroelastic number, for Kelvin–Helmholtz instability, is obtained and
used as a criteria for fragmentation of molten drops penetrating into another liquid coolant media with lower temperature.
The nondimensionalized mean diameter of the fragmented particles is correlated with the Aeroelastic number.
Received on 26 March 2000 相似文献
19.
A Direct Numerical Simulation (DNS) of flow in the V103 Low-Pressure (LP) compressor cascade with incoming wakes was performed.
The computational geometry was chosen largely in accordance with the setup of the experiments performed by Hilgenfeld and
Pfitzner (J Turbomach 126:493–500, 2004) at the University of the Armed Forces in Munich. The computations were carried out on the NEC-SX8 in Stuttgart using 64
processors and 85 million grid points. The incoming wakes stemmed from a separate DNS of incompressible flow around a circular
cylinder with a Reynolds number of Re
d
= 3300 (based on mean inflow velocity and cylinder diameter). The boundary layer along the suction surface of the blade was
found to separate and roll up due to a Kelvin–Helmholtz instability triggered by the periodically passing wakes. Inside the
rolls further transition to turbulence was found to occur. The boundary-layer flow along the pressure surface did not separate,
instead it underwent by-pass transition. 相似文献
20.
Large-scale horizontal vortices associated with deformations of the aluminum-electrolyte interface have been observed in operating
aluminum reduction cells as well as in physical and numerical models. To expose their importance, we analyze a particular
class of magnetohydrodynamic (MHD) interfacial instabilities which are induced by rotation. As we focus on a single vortex,
a cylindrical geometry is preferred. Two analytical models are proposed. In a first model based on the MHD shallow-water approximation,
we consider a vortex that has a solid rotation profile to obtain a wave equation and a dispersion relation. A more realistic
second model includes a viscous rotation profile and the treatment of the base-state interface deformation. Energetics of
the flow gives further insight on how an initial perturbation evolves as an oscillatory or a non-oscillatory instability,
depending on the direction of rotation. We find that the mechanism at the very origin of these instabilities is neither due
to a shear between the two layers—and are therefore not Kelvin–Helmholtz instabilities—nor simply due to magnetic force alone,
but rather to the indirect action of the centripetal pressure due to the rotation induced by magnetic force.
相似文献