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1.
D. G. Akhmetov 《Journal of Applied Mechanics and Technical Physics》2008,49(1):18-22
Experimental estimates of energy and energy dissipation of a vortex ring are presented. The energy losses during the motion
of a vortex ring and a streamlined solid are compared.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 1, pp. 24–30, January–February, 2008. 相似文献
2.
Characteristics of high Mach number compressible vortex ring generated at the open end of a short driver section shock tube
is studied experimentally using high-speed laser sheet-based flow visualization. The formation mechanism and the evolution
of counter rotating vortex ring (CRVR) formed ahead of the primary vortex ring are studied in details for shock Mach number
(M) 1.7, with different driver section lengths. It has been observed that the strength of the embedded shock, which appears
at high M, increases with time due to the flow expansion in the generating jet. Strength of the embedded shock also varies with radius;
it is strong at smaller radii and weak at larger radii; hence, it creates a velocity gradient ahead of the embedded shock.
At critical Mach number (M
c ≥ 1.6), this shear layer rolls up and forms a counter rotating vortex ring due to Biot-Savart induction of the vortex sheet.
For larger driver section lengths, the embedded shock and the resultant shear layer persists for a longer time, resulting
in the formation of multiple CRVRs due to Kelvin–Helmholtz type instability of the vortex sheet. CRVRs roll over the periphery
of the primary vortex ring; they move upstream due to their self-induced velocity and induced velocity imparted by primary
ring, and interact with the trailing jet. Formation of these vortices depends strongly upon the embedded shock strength and
the length of the generating jet. Primary ring diameter increases rapidly during the formation and the evolution of CRVR due
to induced velocity imparted on the primary ring by CRVR. Induced velocity of CRVR also affects the translational velocity
of the primary ring considerably. 相似文献
3.
The development of a buoyant vortex ring in the near field was examined experimentally, and the findings were compared with
those of a non-buoyant ring with a similar Reynolds number. The experiments were performed in a water tank, and the vortices
were produced by a cylindrical tube of aspect ratio 2. Laser sheet flow visualization and PIV measurements were carried out.
In the near field, the initial column of the buoyant fluid breaks down due to the presence of Rayleigh–Taylor instability
at the buoyant fluid interface. Subsequently, a large diameter vortex ring with a large spreading rate, compared with the
non-buoyant ring, emerges. The celerity of buoyant vortex continued to decrease throughout the range examined, in contrast
to the constant celerity of the non-buoyant ring. The vorticity in the core of buoyant and non-buoyant vortex rings is symmetric
and has a Gaussian distribution. However, the buoyant vortex ring evolves into a thin core ring, whereas the non-buoyant ring
becomes a thick core ring shortly after the ring formation. This difference is brought on by the rapid entrainment and the
significant growth of the buoyant ring following the breakup of the initial formation. 相似文献
4.
Vortex–ring interactions with oblique boundaries were studied experimentally to determine the effects of plate angle on the
generation of secondary vorticity, the evolution of the primary vorticity and secondary vorticity as they interact near the
boundary, and the associated energy dissipation. Vortex rings were generated using a mechanical piston-cylinder vortex ring
generator at jet Reynolds numbers 2,000–4,000 and stroke length to piston diameter ratios (L/D) in the range 0.75–2.0. The plate angle relative to the initial axis of the vortex ring ranged from 3 to 60°. Flow analysis
was performed using planar laser-induced fluorescence (PLIF), digital particle image velocimetry (DPIV), and defocusing digital
particle tracking velocimetry (DDPTV). Results showed the generation of secondary vorticity at the plate and its subsequent
ejection into the fluid. The trajectories of the centers of circulation showed a maximum ejection angle of the secondary vorticity
occurring for an angle of incidence of 10°. At lower incidence angles (<20°), the lower portion of the ring, which interacted
with the plate first, played an important role in generation of the secondary vorticity and is a key reason for the maximum
ejection angle for the secondary vorticity occurring at an incidence angle of 10°. Higher Reynolds number vortex rings resulted
in more rapid destabilization of the flow. The three-dimensional DDPTV results showed an arc of secondary vorticity and secondary
flow along the sides of the primary vortex ring as it collided with the boundary. Computation of the moments and products
of kinetic energy and vorticity magnitude about the centroid of each vortex ring showed increasing asymmetry in the flow as
the vortex interaction with the boundary evolved and more rapid dissipation of kinetic energy for higher incidence angles. 相似文献
5.
The periodic formation of vortex rings in the developing region of a round jet subjected to high-amplitude acoustic forcing
is investigated with High-Speed Particle Image Velocimetry. Harmonic velocity oscillations ranging from 20 to 120% of the
mean exit velocity of the jet was achieved at several forcing frequencies determined by the acoustic response of the system.
The time-resolved history of the formation process and circulation of the vortex rings are evaluated as a function of the
forcing conditions. Overall, high-amplitude forcing causes the shear layers of the jet to breakup into a train of large-scale
vortex rings, which share many of the features of starting jets. Features of the jet breakup such as the roll-up location
and vortex size were found to be both amplitude and frequency dependent. A limiting time-scale of t/T ≈ 0.33 based on the normalized forcing period was found to restrict the growth of a vortex ring in terms of its circulation
for any given arrangement of jet forcing conditions. In sinusoidally forced jets, this time-scale corresponds to a kinematic
constraint where the translational velocity of the vortex ring exceeds the shear layer velocity that imposes pinch-off. This
kinematic constraint results from the change in sign in the jet acceleration between t = 0 and t = 0.33T. However, some vortex rings were observed to pinch-off before t = 0.33T suggesting that they had acquired their maximum circulation. By invoking the slug model approximations and defining the slug
parameters based on the experimentally obtained time- and length-scales, an analytical model based on the slug and ring energies
revealed that the formation number for a sinusoidally forced jet is L/D ≈ 4 in agreement with the results of Gharib et al. (J Fluid Mech 360:121–140, 1998). 相似文献
6.
The evolution of total circulation and entrainment of ambient fluid during laminar vortex ring formation has been addressed
in a number of previous investigations. Motivated by applications involving propulsion and fluid transport, the present interest
is in the momentum evolution of entrained and ejected fluid and momentum exchange among the ejected, entrained fluid and added
mass during vortex ring formation. To this end, vortex rings are generated numerically by transient jet ejection for fluid
slug length-to-diameter (L/D) ratios of 0.5–3.0 using three different velocity programs [trapezoidal, triangular negative slope (NS), and positive slope
(PS)] at a jet Reynolds number of 1,000. Lagrangian coherent structures (LCS) were utilized to identify ejected and entrained
fluid boundaries, and a Runge-Kutta fourth order scheme was used for advecting these boundaries with the numerical velocity
data. By monitoring the center of mass of these fluid boundaries, momentum of each component was calculated and related to
the total impulse provided by the vortex ring generator. The results demonstrate that ejected fluid exchanges its momentum
mostly with added mass during jet ejection and that the momentum of the entrained fluid at jet termination was < 11% of the
total ring impulse in all cases except for the triangular NS case. Following jet termination, momentum exchange was observed
between ejected and entrained fluid yielding significant increase in entrained fluid’s momentum. A performance metric was
defined relating the impulse from over-pressure developed at the nozzle exit plane during jet ejection to the flow evolution,
which increased preferentially with L/D over the range considered. An additional benefit of this study was the identification of the initial (i.e., before jet initiation)
location of the fluid to be entrained into the vortex ring. 相似文献
7.
Formation and evolution of a compressible vortex ring generated at the open end of a short driver section shock tube has been
simulated numerically for pressure ratios (PR) of 3 and 7 in the present study. Numerical study of compressible vortex rings
is essential to understand the complicated flow structure and acoustic characteristics of many high Mach number impulsive
jets where simultaneously velocity, density and pressure fields are needed. The flow development, incident shock formation,
shock diffraction, vortex ring formation and its evolution are simulated using the AUSM+ scheme. The main focus of the present
study is to evaluate the time resolved vorticity field of the vortex ring and the shock/expansion waves in the starting jet
for short driver section shock tubes—a scenario where little data are available in existing literature. An embedded shock
and a vortex induced shock are observed for PR = 7. However the vortex ring remains shock free, compact and unaffected by
the trailing jet for PR = 3. Numerical shadowgraph shows the evolution of embedded shock and shock/expansion waves along
with their interactions. The velocity and vorticity fields obtained from simulation are validated with the particle image
velocimetry results and these data match closely. The translational velocity of the vortex ring, velocity across the vortex
and the centre line velocity of the jet obtained from simulation also agree well with the experimental results. 相似文献
8.
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
9.
G. V. Kovalenko V. I. Terekhov A. A. Khalatov 《Journal of Applied Mechanics and Technical Physics》2010,51(6):839-848
The boundaries of the domains of existence of flow regimes past single dimples made as spherical segments on a flat plate
are determined with the use of available experimental results. Regimes of a diffuser-confuser flow, a horseshoe vortex, and
a tornado-like vortex in the dimple are considered. Neither a horseshoe vortex nor a tornado-like vortex is observed in dimples
with the relative depth smaller than 0.1. Transformations from the diffuser–confuser flow regime to the horseshoe vortex regime
and from the horseshoe vortex flow to the tornado-like vortex flow are found to depend not only on the Reynolds number, but
also on the relative depth of the spherical segment. Dependences for determining the boundaries of the regime existence domains
are proposed, and parameters at which the experimental results can be generalized are given. 相似文献
10.
The effects of exit-wedge angle on evolution, formation, pinch-off, propagation and diffusive mass entrainment of vortex rings
in air were studied using digital particle image velocimetry. Vortex rings were generated by passing a solenoid-valve-controlled
air jet through a cylindrical nozzle. Experiments were performed over a wide range of exit-wedge angles (10° ≤ α ≤ 90°) of the cylindrical nozzle, initial Reynolds numbers (450 ≤ Re ≤ 4,580) and length-to-diameter ratios (0.9 ≤ L/D ≤ 11) of the air jet. For sharp edges (α ≤ 10°), a secondary ring may emerge at high Reynolds numbers, which tended to distort the vortex ring if ingested into it.
For blunt edges (α ≥ 45°), by contrast, stable vortex rings were produced. The formation phase of a vortex ring was found to be closely related
to its evolution pattern. An exit-wedge angle of 45° was found to be optimal for rapid pinch-off and faster propagation and
better stability of a vortex ring. Diffusive mass entrainment was found to be between 35% and 40% in the early stages of a
vortex ring propagation and it gradually increased throughout the course of vortex ring propagation. Entrainment fraction
was found to be sensitive to the L/D ratio of the initial jet and decreases when the L/D ratio is increased. 相似文献
11.
The axisymmetric vortex sheet model developed by Nitsche & Krasny (1994) has been extended to study the formation of vortex
rings (pairs) at the edge of circular (2D) tube and opening. Computations based on this model are in good agreement with the
experiments (Didden (1979) for circular tube and Auerbach (1987) for 2D tube and opening). Using this new model, evidences
are provided to show that the main failure of the similarity theory (the false prediction of axial trajectory of vortex ring)
is due to its ignorance of the self-induced ring velocity (mutual induction for vortex pair). We further reason why the similarity
theory succeeds in its prediction of radial movement of vortex ring. The effects of various parameters such as turning angle
α and piston speedU
p
(t) on the formation of vortex ring are investigated. Numerical result shows that turning angle α has no effect on circulation
shed τ. We also discuss Glezer (1988)'s summary on the influence ofU
p
upon the shedding circulation, and finally give the variation of core distribution of vortex ring with α andU
p
(t).
The project is supported by National Natural Science Foundation of China and Doctoral Program of Institution of Higher Education 相似文献
12.
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
13.
Exploratory measurements of the effect of a stable continuous vertical stratification on horizontally propagating vortex rings
show that the rings are subject to a stratification induced instability and subsequent collapse, which forms a well mixed
intrusion. For initial ring Froude numbersF
0=U
0/Nd
0=1.0–2.0, instability and collapse occurs when the ring Froude number has decreased to a value in the range 0.6–1.0. 相似文献
14.
To obtain practical schemes of vortex–flame interactions, a series of organized eddies formed in the plane premixed shear layer is investigated, instead of a single vortex ring or a single vortex tube. The plane premixed shear layer is first formed between two parallel uniform propane–air mixture streams. For getting clear qualitative pictures of vortex–flame interactions in the plane premixed shear layer, two extreme ignition points are assigned; one is assigned at the center of an organized eddy where the vortex motion plays an important role, the other at the midpoint between two adjacent organized eddies where the rolling-up motion prevails. A premixed flame is initiated by an electric discharge at one of the two assigned points and propagates either in the large scale organized eddy or along the interface between two uniform mixture streams. Propagation and deformation processes of the flame are observed using the simultaneously two-directional and high-speed Schlieren photography. The tangential velocity of organized eddy and the equivalence ratio of premixed shear flow are varied as two main parameters. The outline of propagating flame after the midpoint ignition is numerically analyzed by superposing the flame propagation having a constant burning velocity on the vortex flow field simulated with the discrete vortex method. The results obtained show that there exists another type of vortex–flame interaction in the plane shear layer in addition to the vortex bursting, and that it is caused by the rolling-up motion particular to the coherent structure in the plane shear layer and is simply named the vortex boosting. It is qualitatively concluded therefore that, in the ordinary turbulent premixed flames formed in the plane premixed shear layer, these two fundamental vortex-flame interactions get tangled with each other to augment the propagation velocity. An empirical expression which qualitatively takes into account of the effects of both vortex and chemical properties is finally proposed. 相似文献
15.
The flow in a planar-radial vortex chamber. 1. An experimental study of the velocity field in transient and steady flows 总被引:1,自引:0,他引:1
F. A. Bykovskii E. F. Vedernikov 《Journal of Applied Mechanics and Technical Physics》1999,40(6):1097-1105
Two methods for determining the flow velocity in a vortex chamber of planar-radial geometry under transient and steady-state
conditions are proposed. Local flow velocities throughout the entire volume of the chamber are measured, and the flow is found
to be rotational. The effect of accumulation of particles heavier than air in the butt-end boundary layer is revealed.
Lavrent'ev Institute of Hydrodynamics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from
Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 6, pp. 112–121, November–December, 1999. 相似文献
16.
B. M. Cetegen 《Experiments in fluids》2006,40(6):967-976
An experimental study of scalar mixing in a laminar vortex is presented for vortices generated between two gas streams flowing parallel to each other in a rectangular flow channel. An isolated line vortex is initiated on demand by momentarily increasing one stream velocity in relation to the other using an electromagnetically actuated piston. The temporal piston motion profile is tailored to generate vortices of different strengths corresponding to vortex Reynolds numbers, Re≡Γ/2πν=130–210. Evolution of mixing is monitored by laser-induced fluorescence of acetone vapor premixed into one of the gas streams as the vortex structure evolves with increasing downstream distance from its point of origin. Vortex is generated by pulsing either of the gas streams (seeded or unseeded stream). Vortex initiation process affects the abundance of the gas in the vortex core from the pulsed stream. Spatial mixing statistics are obtained by determining scalar concentration probability density functions (pdf) and the mean mixed fluid concentrations obtained from these pdfs. It is found that the interfacial area generation as a result of vortex kinematics and molecular diffusion along this interface are principally responsible for mixing. The mean mixed fluid concentration in the vortex interaction region scales with the product of vortex circulation and the elapsed time of interaction. These results are similar to those found in liquid mixing experiments, but the rate of mixing is significantly higher due to higher diffusivity of gases. 相似文献
17.
The relationship between the maneuvering kinematics of a Giant Danio (Danio aequipinnatus) and the resulting vortical wake is investigated for a rapid, ‘C’-start maneuver using fully time-resolved (500 Hz) particle
image velocimetry (PIV). PIV illuminates the two distinct vortices formed during the turn. The fish body rotation is facilitated
by the initial, or “maneuvering” vortex formation, and the final fish velocity is augmented by the strength of the second,
“propulsive” vortex. Results confirm that the axisymmetric vortex ring model is reasonable to use in calculating the hydrodynamic
impulse acting on the fish. The total linear momentum change of the fish from its initial swimming trajectory to its final
swimming trajectory is balanced by the vector sum of the impulses of both vortex rings. The timing of vortex formation is
uniquely synchronized with the fish motion, and the choreography of the maneuver is addressed in the context of the resulting
hydrodynamic forces. 相似文献
18.
Results are presented from an experimental investigation into the interaction of a planar shock wave with a vortex ring. A
free-falling spherical soap bubble is traversed by the incident shock wave and develops into a vortex ring as a result of
baroclinically deposited vorticity (?r×?p 1 0{\nabla\rho\times\nabla p \neq 0}). The vortex ring translates with a velocity relative to the particle velocity behind the shock wave due to circulation.
After the shock wave reflects from the tube end wall, it traverses the vortex ring (this process is called “reshock”) and
deposits additional vorticity. Planar Mie scattering is used to visualize the atomized soap film at high frame rates (up to
10,000 fps). Particle image velocimetry (PIV) was performed for an argon bubble in nitrogen accelerated by a M = 1.35 shock wave. Circulation was determined from the PIV velocity field and found to agree well with Kelvin’s vortex ring
model. 相似文献
19.
Yu. D. Shmyglevskii 《Fluid Dynamics》1998,33(2):298-300
Chains of coaxial vortex formations of the “vortex breakdown“ type in axisymmetric swirling incompressible viscous and ideal
fluid flows are represented in analytic form.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, pp. 174–176, March–April, 1998. 相似文献
20.
We consider a combined experimental (based on flow visualization, direct force measurement and phase-averaged 2D particle
image velocimetry in a water tunnel), computational (2D Reynolds-averaged Navier–Stokes) and theoretical (Theodorsen’s formula)
approach to study the fluid physics of rigid-airfoil pitch–plunge in nominally two-dimensional conditions. Shallow-stall (combined
pitch–plunge) and deep-stall (pure-plunge) are compared at a reduced frequency commensurate with flapping-flight in cruise
in nature. Objectives include assessment of how well attached-flow theory can predict lift coefficient even in the presence
of significant separation, and how well 2D velocimetry and 2D computation can mutually validate one another. The shallow-stall
case shows promising agreement between computation and experiment, while in the deep-stall case, the computation’s prediction
of flow separation lags that of the experiment, but eventually evinces qualitatively similar leading edge vortex size. Dye
injection was found to give good qualitative match with particle image velocimetry in describing leading edge vortex formation
and return to flow reattachment, and also gave evidence of strong spanwise growth of flow separation after leading-edge vortex
formation. Reynolds number effects, in the range of 10,000–60,000, were found to influence the size of laminar separation
in those phases of motion where instantaneous angle of attack was well below stall, but have limited effect on post-stall
flowfield behavior. Discrepancy in lift coefficient time history between experiment, theory and computation was mutually comparable,
with no clear failure of Theodorsen’s formula. This is surprising and encouraging, especially for the deep-stall case, because
the theory’s assumptions are clearly violated, while its prediction of lift coefficient remains useful for capturing general
trends. 相似文献