Showing papers on "Vortex published in 1989"

Oblique and Parallel Modes of Vortex Shedding in the Wake of a Circular Cylinder at Low Reynolds Numbers

Abstract: Two fundamental characteristics of the low-Reynolds-number cylinder wake, which have involved considerable debate, are first the existence of discontinuities in the Strouhal-Reynolds number relationship, and secondly the phenomenon of oblique vortex shedding. The present paper shows that both of these characteristics of the wake are directly related to each other, and that both are influenced by the boundary conditions at the ends of the cylinder, even for spans of hundreds of diameters in length. It is found that a Strouhal discontinuity exists, which is not due to any of the previously proposed mechanisms, but instead is caused by a transition from one oblique shedding mode to another oblique mode. This transition is explained by a change from one mode where the central flow over the span matches the end boundary conditions to one where the central flow is unable to match the end conditions. In the latter case, quasi-periodic spectra of the velocity fluctuations appear; these are due to the presence of spanwise cells of different frequency. During periods when vortices in neighbouring cells move out of phase with each other, ‘vortex dislocations’ are observed, and are associated with rather complex vortex linking between the cells. However, by manipulating the end boundary conditions, parallel shedding can be induced, which then results in a completely continuous Strouhal curve. It is also universal in the sense that the oblique-shedding Strouhal data (S_θ) can be collapsed onto the parallel-shedding Strouhal curve (S_0) by the transformation, S_0 = S_θ/cosθ, where θ is the angle of oblique shedding. Close agreement between measurements in two distinctly different facilities confirms the continuous and universal nature of this Strouhal curve. It is believed that the case of parallel shedding represents truly two-dimensional shedding, and a comparison of Strouhal frequency data is made with several two-dimensional numerical simulations, yielding a large disparity which is not clearly understood. The oblique and parallel modes of vortex shedding are both intrinsic to the flow over a cylinder, and are simply solutions to different problems, because the boundary conditions are different in each case.

Airfoil self-noise and prediction

Abstract: A prediction method is developed for the self-generated noise of an airfoil blade encountering smooth flow. The prediction methods for the individual self-noise mechanisms are semiempirical and are based on previous theoretical studies and data obtained from tests of two- and three-dimensional airfoil blade sections. The self-noise mechanisms are due to specific boundary-layer phenomena, that is, the boundary-layer turbulence passing the trailing edge, separated-boundary-layer and stalled flow over an airfoil, vortex shedding due to laminar boundary layer instabilities, vortex shedding from blunt trailing edges, and the turbulent vortex flow existing near the tip of lifting blades. The predictions are compared successfully with published data from three self-noise studies of different airfoil shapes. An application of the prediction method is reported for a large scale-model helicopter rotor, and the predictions compared well with experimental broadband noise measurements. A computer code of the method is given.

Anisotropic critical currents in Ba2YCu3O7 analyzed using an extended Bean model

Abstract: We have extended Bean’s critical state model to explicitly include anisotropic critical currents. Measurements at 30 K of the critical currents parallel to the Cu‐O planes but with vortex motion either parallel or across twin boundaries show twin boundaries are probably not an important cause of vortex pinning. In the critical state, current flow perpendicular to the Cu‐O planes is about 30 times smaller than flow parallel to these planes.

Optical and acoustic investigations of the dynamics of laser-produced cavitation bubbles near a solid boundary

Abstract: The dynamics of laser-produced cavitation bubbles near a solid boundary and its dependence on the distance between bubble and wall are investigated experimentally. It is shown by means of high-speed photography with up to 1 million frames/s that jet and counterjet formation and the development of a ring vortex resulting from the jet flow are general features of the bubble dynamics near solid boundaries. The fluid velocity field in the vicinity of the cavitation bubble is determined with time-resolved particle image velocimetry. A comparison of path lines deduced from successive measurements shows good agreement with the results of numerical calculations by Kucera & Blake (1988). The pressure amplitude, the profile and the energy of the acoustic transients emitted during spherical bubble collapse and the collapse near a rigid boundary are measured with a hydrophone and an optical detection technique. Sound emission is the main damping mechanism in spherical bubble collapse, whereas it plays a minor part in the damping of aspherical collapse. The duration of the acoustic transients is 20-30 ns. The highest pressure amplitudes at the solid boundary have been found for bubbles attached to the boundary. The pressure inside the bubble and at the boundary reaches about 2.5 kbar when the maximum bubble radius is 3.5 mm. The results are discussed with respect to the mechanism of cavitation erosion.

Low Froude Number Flow Past Three-Dimensional Obstacles. Part I: Baroclinically Generated Lee Vortices

Abstract: We study the flow of a density-stratified fluid past a three-dimensional obstacle, using a numerical model. Our special concern is the response of the fluid when the Froude number is near or less than unity. Linear theory is inapplicable in this range of Froude number, and the present numerical solutions show the rich variety of phenomena that emerge in this essentially nonlinear flow regime. Two such phenomena, which occupy Parts I and II of this study, are the formation of a pair of vertically oriented vortices on the lee side and a zone of flow reversal on the windward side of the obstacle. The Ice vortices have been explained as a consequence of the separation of the viscous boundary layer from the obstacle however, this boundary layer is absent (by design) in the present experiments and lee vortices still occur. We argue that a vertical component of vorticity develops on the lee side owing to the tilting of horizontally oriented vorticity produced baroclinically as the isentropes deform in r...

Some Aspects of Vortex Structure Related to Tropical Cyclone Motion

Abstract: Some effect of tropical cyclone structure on the vortex motion are examined in a nondivergent, barotropic numerical model with no basic current. As suggested earlier by DeMaria, the initial maximum wind speed has little effect on the track. Vortex translation associated with the beta effect depends sensitively on the strength of the flow between 300 and 1000 km from the center. If the flow in this annulus is made more cyclonic, the track will turn cyclonically and move more toward the west in the Northern Hemisphere. The dynamics of this beta-drift is studied via a decomposition into symmetric and asymmetric circulations. The symmetric flow experiences a slight weakening of the maximum wind speed and an anticyclonic circulation is induced beyond 600 km. The asymmetric circulation is dominated by an azimuthal wavenumber one circulation with an anticyclonic gyre east of the center, a cyclonic gyre to the west and a nearly uniform, broad-scale ventilation flow between the gyres. The vortex translati...

Numerical models of Plinian eruption columns and pyroclastic flows

Abstract: Numerical simulations of physical processes governing the large-scale dynamics of Plinian eruption columns reveal conditions contributing to column collapse and emplacement of pyroclastic flows. The simulations are based on numerical solution of the time-dependent, two-phase, compressible Navier-Stokes equations for jets in a gravitational field. This modeling effort is directed toward studying the steady discharge phase of eruptions in contrast to our previous models of the initial, unsteady blast phase. Analysis of 51 eruption models covers a wide range of vent exit pressures, inertial and buoyancy driving forces, and coupling of energy and momentum between gas and pyroclasts. Consideration of three dimensionless groups (Richardson and Rouse numbers and thermogravitational parameter) facilitates this analysis and defines conditions leading to column collapse. For eruptions with similar particle size characteristics, exit pressure ratios are also very important in determining column behavior; column behavior is much more sensitive to exit pressure ratio than to the density ratio between the column and the atmosphere. Model eruption columns with exit pressures exceeding atmospheric pressure have diamond-shaped patterns at their bases with internal dynamics that correspond closely to observations of overpressured jets in laboratory experiments. Collapsing fountains form pyroclastic flows that consist of low-concentration fronts, relatively thick heads, vortex development along the top surfaces, and rising clouds of buoyant ash. The presence of coarse-grained proximal deposits primarily reflects tephra size sorting within the eruption column before collapse, as opposed to that which occurs during lateral transport of the material in pyroclastic flows. The dynamics and particle behavior in the proximal zone around collapsing eruption columns is examined; the modeling indicates that flow within a few kilometers of a vent will be at its highest particle concentration relative to other parts of the flow field.

Vortex splitting and its consequences in the vortex street wake of cylinders at low Reynolds number

Abstract: Based on the observation of vortex splitting in the laminar wake of thin flat plates placed parallel to the flow, an investigation on the consequences of such events for the von Karman vortex street in the wake of circular cylinders was carried out. It was found that a ‘‘design break line’’ of vortex axes can lead to the decoupling of a wake flow from the always present disturbances deriving from the ends. The decoupling gives rise to parallel vortex shedding of a slightly higher frequency, instead of the oblique or slanted vortex shedding at a lower frequency usually observed.

Ozone destruction by chlorine radicals within the Antarctic vortex: The spatial and temporal evolution of ClO‐O3 anticorrelation based on in situ ER‐2 data

Abstract: The chemical evolution of the Antarctic vortex region was studied during August 23-September 22, 1987 on the basis of in situ O3 and ClO data collected by the ER-2 aircraft. Particular attention is given to the evolution of the ClO-O3 anticorrelation from the first flight on August 23, 1987, which penetrated well into the vortex, through the course of 10 flights culminating on September 22, 1987. It is concluded that the disappearance of ozone within the Antarctic vortex results from halogen-catalyzed recombination of O3 to molecular oxygen.

Synoptic and chemical evolution of the Antarctic vortex in late winter and early spring, 1987

Abstract: Evidence from the ER-2 and DC-8 aircraft flights is considered, together with analyses of temperature, winds, potential vorticity and trajectories, satellite data, and ozonesonde observations, to come to a view of whether the air in the lower stratosphere over Antarctica during winter and early spring of 1987 was a fixed slug of air, or if there was significant mass flow through the system It is concluded that synoptic-scale forcing, via “sudden coolings,” produced polar stratospheric clouds, which intervened to alter the homogeneous gas phase chemical balance As a result, there was a source of the ClO molecule and sinks for H2O, NOy (equal to the sum of reactive gas phase nitrogen compounds), and O3 operating within the region of high potential vorticity gradients on isentropic surfaces, that is, inside the vortex It is further concluded that as a result of horizontal mixing, downward diabatic motion, and, at potential temperatures below 400 K, advective transfer, that the effects of these polar sinks and chemical reactions can be transmitted to middle latitudes

Pulsatile flow through constricted tubes: an experimental investigation using photochromic tracer methods

Abstract: A photochromic tracer method has been used to record pulsatile flow velocity profiles simultaneously at three axial locations along a flow channel. Two major advantages of this multiple-trace method are that it enables velocity data to be acquired in an efficient non-invasive manner and that it provides a detailed description of the spatial relationship of the flow field. The latter is found to be particularly useful in the investigation of transitional type flows; for example, in describing coherent flow structures. Studies of the flow patterns in tubes with mild to moderate degrees of vessel constriction were performed using a 2.9 Hz sinusoidal flow superimposed on a steady flow (frequency parameter of 7.5; mean and modulation Reynolds numbers of 575 and 360, respectively). With mild constrictions (< 50% area reduction), isolated regions of vortical and helical structures were observed primarily during the deceleration phase of the flow cycle and in the vicinity of the reattachment point. As expected, these effects were accentuated when the constriction was asymmetric. For moderate constrictions (50%–80%), transition to turbulence was triggered just before peak flow through the breakdown of waves and streamwise vortices that were shed in the high-shear layer. During this vortex generation phase of the flow cycle, the wall shear stress fluctuated quite intensely, especially in the vicinity of the reattachment point, and its instantaneous value increased by at least a factor of eight. Such detailed descriptions of the transition to turbulence and of the spatial and temporal variation of the wall shear stress, particularly near the reattachment point, have not been previously reported for pulsatile flow through constricted tubes. The observed wall shear stress variations support a proposal by Mao & Hanratty (1986) of an interaction of the imposed flow oscillation with the turbulent fluctuations within the viscous sublayer.

Structure of Tip Clearance Flow in an Isolated Axial Compressor Rotor

Abstract: Ensemble-averaged and phase-locked flow patterns in various tip clearances of two axial compressor rotors were obtained by aperiodic multisampling technique with a hot wire in the clearance and with a high-response pressure sensor on the casing wall. A leakage flow region distinct from a throughflow region exists at every clearance. In the case of a small tip clearance, the leakage jet flow interacts violently with the throughflow near the leading edge, and a rolling-up leakage vortex decays downstream. As the clearance increases, a stronger leakage vortex comes into existence at a more downstream location, and a reverse flow due to the vortex grows noticeably. A scraping vortex is recognized at the pressure side near the trailing edge only for the small clearance. A horseshoe vortex appears in the upstream half of the through flow region for every tip clearance. The solidity does not affect the flow pattern substantially except for the interaction of the leakage vortex with the adjacent blade and wake.

The dynamics of the near field of strong jets in crossflows

Abstract: An inviscid three-dimensional vortex-sheet model for the near field of a strong jet issuing from a pipe into a crossflow is derived. The solution for this model shows that the essential mechanisms governing this idealized flow are the distortion of the main transverse vorticity by the generation of additional axial and transverse vorticity within the pipe owing to the pressure gradients induced by the external flow, and the convection of both components of vorticity from the upwind side of the jet to its downwind side. The deformation of the cross-section of the jet which is predicted by this model is compared with the deformation predicted by the commonly used time-dependent two-dimensional vortex-sheet model. Differences arise because the latter model does not take into account the effects of the transport of the transverse component of vorticity. The complete three-dimensional vortex-sheet model leads to a symmetrical deformation of the jet cross-section and no overall deflection of the jet in the direction of the stream. To account for viscous effects, the initial region of a strong jet issuing into a uniform crossflow is modelled as an entraining three-dimensional vortex sheet, which acts like a sheet of vortices and sinks, redistributing the vorticity in the bounding shear layer and inducing non-symmetrical deformations of the croas-section of the jet. This leads to a deflection of the jet in the direction of the stream, and the loci of the centroids of the cross-sections of the jet describe a quadratic cur+e. Deformations predicted by each of the three models are compared with measurements obtained from photographs of the cross-sections of a jet of air emerging into a uniform crossflow in a wind tunnel. Mean velocity measurements around the jet made with a hot-wire anemometer agree with the theory ; they clearly invalidate models of jets based on 'pressure drag'.

An experimental study of global instabilities due to the tidal (elliptical) distortion of a rotating elastic cylinder

Abstract: Broad band secondary instability of elliptical vortex motion has been proposed as a principal source of shear-flow turbulence. Here experiments on such instability in an elliptical flow with no shear boundary layer are described. This is made possible by the mechanical distortion in the laboratory frame of a rotating fluid-filled elastic cylinder. One percent ellipticity of a 10 cm diameter cylinder rotating once each second can give rise to an exponentially-growing mode stationary in the laboratory frame. In first order this mode is a sub-harmonic parametric Faraday instability. The finite-amplitude equations represent angular momentum transfer on an inertial time scale due to Reynolds stresses. The growth of this mode is not limited by boundary friction but by detuning and centrifugal stabilization. On average, a generalized Richardson number achieves a marginal value through much of the evolved flow. However, the characteristic flow is intermittent with the cycle: rapid growth, stabilizing mom...

Jupiter's Great Red Spot as a Shallow Water System

Abstract: Most current models of Jupiter's Great Red Spot (GRS) are cast in terms of a two-layer model, where a thin upper weather layer, which contains the vortex, overlies a much deeper layer, which is meant to represent the neutrally stratified deep atmosphere. Any motions in the deep layer are assumed to be zonal and steady. This two-layer model is dynamically equivalent to a one-layer model with meridionally varying solid bottom topography, called the reduced-gravity model. Specifying the motions, or lack thereof, in the lower layer of the two-layer model is equivalent to specifying the bottom topography, and hence the far-field potential vorticity, in the reduced-gravity model. Current models of the GRS start by guessing the deep motions and then proceed to study vortices using the implied bottom topography. Here, using the GRS cloud-top velocity data, we derive the bottom topography, up to a constant that depends on the unknown radius of deformation (or equivalently, the product of the reduced gravity and the mean thickness of the upper layer). The bottom topography is inferred from three quantities derived from the velocity data—Bernoulli streamfunction, kinetic energy per unit mass, and absolute vorticity—all of which are functions only of horizontal position in the reference frame of the vortex. Far from the vortex, potential vorticity versus latitude is calculated from the observed cloud-top zonal velocity and the derived bottom topography. The results show that the deep atmosphere is in differential motion and that the far-field potential vorticity gradient changes sign at several latitudes. Numerical shallow water experiments are performed, using both the derived bottom topography and the bottom topographies prescribed by current models. The results of three published studies are reproduced in our numerical experiments. Each of these models is successful in maintaining a long-lived, isolated vortex, but only the present model yields absolute vorticity profiles along streamlines that agree with those observed for the GRS by Dowling and Ingersoll. In all the models, large vortices form by merging with smaller vortices. In the present, observationally based model, and in one other published model, the smaller vortices arise spontaneously because the observed cloud-top zonal velocity profile is unstable. These two models require an additional momentum source to maintain the upper-layer zonal velocity profile. In the other two models, the bottom topography stabilizes the zonal velocity profile. If dissipation is present, the latter two models require an additional source of smaller vortices to maintain the larger one. A crucial unanswered question for the present model, and for Jupiter itself, is how the cloud-top zonal velocity profile is maintained in its present unstable state.

Tripolar vortices in a rotating fluid

Abstract: THE emergence of coherent flow structures is a well-known feature of quasi-geostrophic or two-dimensional turbulence, and because of their relevance to large-scale geophysical flows the dynamics of these structures have been studied increasingly over the past decade1–5 The most common coherent structures are the axisymmetric (monopolar) vortex, with circular streamlines, and the vortex dipole, both of which have been found to arise in a variety of situations under different (sometimes nondescript) forcing conditions. The emergence of vortex dipoles, for example, has been observed in turbulence in soap films3, in electrically forced magnetohydrodynamic flows5 and in collapsing turbulence in a continuously stratified fluid6. There is growing evidence4,7–9 that, in addition to the monopolar and dipolar vortices, a tripolar coherent flow structure exists. Here we describe a laboratory experiment in which a tripolar structure is found to emerge from an unstable cyclonic vortex in a homogeneous rotating fluid. The tripole seems to be a very stable structure, even persisting in a highly sheared fluid environment, and might therefore be found in natural geophysical flows.

Singular solutions and ill-posedness for the evolution of vortex sheets

Abstract: The evolution of a planar vortex sheet is described by the Birkhoff–Rott equation. Duchon and Robert [C.R. Acad. Sci. Paris, 302 (1986), pp. 183–186], [Comm. Partial Differential Equations, 13 (198...

Direct numerical simulation of compressible free shear flows

Abstract: Direct numerical simulations of compressible free shear layers in open domains are conducted. Compact finite-difference schemes of spectral-like accuracy are used for the simulations. Both temporally-growing and spatially-growing mixing layers are studied. The effect of intrinsic compressibility on the evolution of vortices is studied. The use of convective Mach number is validated. Details of vortex roll up and pairing are studied. A simple explanation of the stabilizing effect of compressibility is offered. Acoustic radiation from vortex roll up, pairing and shape oscillations is studied and quantified.

The generation of vortices in high‐resolution, two‐dimensional decaying turbulence and the influence of initial conditions on the breaking of self‐similarity

Abstract: The results of a very‐high‐resolution numerical integration of a two‐dimensional turbulent flow is presented. The very long integration time allows a discussion of both the initial and the ultimate stage of the decay. It is established that the k−3 enstrophy inertial range is only obtained as a transient state of the system. For very long time the emergence of coherent vortices destroys the scale invariance and produces a rather steep spectrum. In addition, the long term behavior strongly depends on the initial conditions. For steep enough initial spectrum, a large‐scale energy containing a range in which vortices are directly linked to the initial conditions separates from a small‐scale range in which many coherent structures originate from inviscid instabilities. The results are compared with previous studies and the origin of self‐similarity breaking in turbulent decay is discussed.

Buoyant diffusion flames

Abstract: Planar visualization was employed to study flame structure and low frequency flame oscillation. Two distinct vortices were visualized in the flames studies: large toroidal vortices outside the luminous flame and small roll-up vortices inside the luminous flame. The flame oscillation frequency and the convective velocity of the toroidal vortices were measured for ethylene, methane, and propane diffusion flames over a wide range of test conditions. The frequency was typically in the range 10 to 20 Hz and the convective velocity was approximately, 0.8 m/s. The frequency of the toroidal vortices was found to correlate with the flame oscillation frequency. The potential effects of the toroidal vortices on the flame dynamics at low fuel flow rates are discussed, for example, low frequency flame oscillation, non-linear flame bulge motion, and quenching of the luminous flame surface.

Dipole formation and collisions in a stratified fluid

Abstract: BECAUSE of its relevance to large-scale geophysical flows, two-dimensional turbulence has attracted increasing attention during the past two decades. An important feature of such flows is the so-called inverse energy cascade—the spectral flux of kinetic energy from small to larger scales of motion1,2. Numerical simulations of two-dimensional turbulence have demonstrated the emergence of coherent vortices from a randomly generated initial flow field3,4. These calculations revealed the occurrence of two categories of coherent structures, specifically, the monopolar axisymmetric vortex, characterized by a non-zero angular momentum, and the dipolar vortex, characterized by a non-zero linear momentum and a steady translation; recent experiments5,6 demonstrated the existence of a tripolar vortex structure, characterized by a non-zero angular momentum and a steady rotation of its axis. Here we report on the formation of two-dimensional dipole structures by the gravitational collapse of a compact region of three-dimensionally turbulent, mixed fluid in a quiescent stratified environment. The dipole thus produced seems to be very stable, its robustness being demonstrated by experiments on colliding dipoles.

Numerical simulation of boundary-layer transition and transition control

Abstract: The laminar-turbulent transition process in a parallel boundary-layer with Blasius profile is simulated by numerical integration of the three-dimensional incompressible Navier-Stokes equations using a spectral method. The model of spatially periodic disturbances developing in time is used. Both the classical Klebanoff-type and the subharmonic type of transition are simulated. Maps of the three-dimensional velocity and vorticity fields and visualizations by integrated fluid markers are obtained. The numerical results are compared with experimental measurements and flow visualizations by other authors. Good qualitative and quantitative agreement is found at corresponding stages of development up to the one-spike stage. After the appearance of two-dimensional Tollmien-Schlichting waves of sufficiently large amplitude an increasing three-dimensionality is observed. In particular, a peak-valley structure of the velocity fluctuations, mean longitudinal vortices and sharp spike-like instantaneous velocity signals are formed. The flow field is dominated by a three-dimensional horseshoe vortex system connected with free high-shear layers. Visualizations by time-lines show the formation of A-structures. Our numerical results connect various observations obtained with different experimental techniques. The initial three-dimensional steps of the transition process are consistent with the linear theory of secondary instability. In the later stages nonlinear interactions of the disturbance modes and the production of higher harmonics are essential.We also study the control of transition by local two-dimensional suction and blowing at the wall. It is shown that transition can be delayed or accelerated by superposing disturbances which are out of phase or in phase with oncoming Tollmien-Schlichting instability waves, respectively. Control is only effective if applied at an early, two-dimensional stage of transition. Mean longitudinal vortices remain even after successful control of the fluctuations.

The Generation of Tripoles from Unstable Axisymmetric Isolated Vortex Structures

Abstract: Tripolar coherent vortices are shown to emerge from the unstable evolution of perturbed two-dimensional axisymmetric flows. They are obtained from the nonlinear equilibration of barotropically linearly unstable normal modes, as well as from more general initial perturbations. This instability is proposed as an important mechanism for the generation of both dipolar and tripolar coherent vortex structures.

Cross‐linking of two antiparallel vortex tubes

Abstract: The detailed mechanisms in vortex cross-linking are unveiled by adequately resolved, direct numerical simulation of two viscous vortex tubes. There are three characteristic phases: (1) inviscid induction followed by core flattening and stretching; (2) bridging of the two vortices by accumulation of annihilated and then cross-linked vortex lines; and (3) threading of the remnants of the initial vortex pair in between the two bridges as they pull apart. These phases and the role of threading (along with bridging) in the mixing and the enstrophy cascade are explained, and it is shown that the mechanism is insensitive to asymmetries.

Effect of initial swirl distribution on the evolution of a turbulent jet

Abstract: An existing cold-jet facility at NASA Lewis Research Center was modified to produce swirling flows with controllable initial tangential velocity distribution. Distinctly different swirl velocity profiles were produced, and their effects on jet mixing characteristics were measured downstream of an 11.43 cm (4.5 in.) diameter convergent nozzle. It was experimentally shown that in the near field (i.e., x/D Q.6. This remarkable result leads to the conclusion that the integrated swirl effect, reflected in the swirl number, is inadequate in describing the mean swirling jet behavior in the near field. The relative size (i.e., diameter) of the vortex core emerging from the nozzle and the corresponding tangential velocity distribution are the controlling parameters influencing the swirling turbulent free-jet evolution.

The role of the recirculation vortex in improving fuel-air mixing within swirling flames

Abstract: An important example that shows how a flame interacts with a vortex is the case of a swirlstabilized flame. Such a flame essentially is a toroidal vortex into which a jet of fuel is injected along the torus centerline. Because of the central toroidal vortex, the overall fuel-air mixing rate within a swirl-stabilized flame is found to be a factor of five times greater than that of a simple jet flame, as evidenced by a fivefold shortening of flame length. Swirl flames are unique in that the fuel-air mixing rate can be varied by controlling the amount of swirl. Flow visualization shows how internal recirculation helps to enhance the mixing. The englufment mechanisms is similar to that of a simple jet flame but is enhanced because of two factors that increase the fuel-air contact area. First, the recirculation zone acts like a large eddy with a characteristic velocity and length scale that are much larger than those associated with eddies in a simple jet. The active role of the recirculation zone contradicts a previouslyheld concept that the recirculation zone is a passive obstacle and that its internal velocity is not important. Air is entrained into the toroidal vortex primarily in the downstream region of the vortex. A second reason why mixing is more intense within swirling flames than within jet flames is that there is impingement of opposed jets at the forward stagnation point. Thus pressure gradients, which are not present in simple jets, enhance the mixing rates. Flame length, which is a measure of overall fuel-air mixing, was found to have a different scaling in swirl flames than for simple jet flames. The physical reasons are explained by a scaling which was developed and which explains four trends in the flame length data. It is proposed to use a general, nondimensional circulation parameter that allows for general comparison of mixing efficiencies of swirl, bluff-body, and dump-combustor flames.