Showing papers on "Starting vortex published in 1993"

Large-scale isentropic mixing properties of the Antarctic polar vortex from analyzed winds

Abstract: Winds derived from analyzed geopotential height fields are used to study quasi-horizontal mixing by the large-scale flow in the lower stratosphere during austral spring. This is the period when the Antarctic ozone hole appears and disappears. Trajectories are computed for large ensembles of particles initially inside and outside the main polar vortex. Mixing and transport are diagnosed through estimates of finite time Lyapunov exponents and Lagrangian dispersion statistics of the tracer trajectories. At 450 K and above prior to the vortex breakdown: Lyapunov exponents are a factor of 2 smaller inside the vortex than outside; diffusion coefficients are an order of magnitude smaller inside than outside the vortex; and the trajectories reveal little exchange of air across the vortex boundary. At lower levels (425 and 400 K) mixing is greater, and there is substantial exchange of air across the vortex boundary. In some years there are large wave events that expel small amounts of vortex air into the mid-latitudes. At the end of the spring season during the vortex breakdown there is rapid mixing of air across the vortex boundary, which is evident in the mixing diagnostics and the tracer trajectories.

Calculation of Vortex Shedding Past a Square Cylinder with Various Turbulence Models

Abstract: The vortex-shedding flow past a square cylinder at Re = 22.000 was calculated with various turbulence models. The 2D periodic shedding motion was resolved in an unsteady calculation, and the superimposed stochastic turbulent fluctuations were simulated both with the k — e eddy-viscocity model and with a Reynolds-stress equation model. For both models, the viscosity-affected near-wall region was either bridged by wall functions or was resolved with a simpler one-equation model using a prescribed length-scale distribution. The k — e model with wall functions does not yield unsteady vortex motion while the other model variants do. The two-layer k —e model underpredicts severely the periodic fluctuations and also the Strouhal number and drag coefficient. The Reynoldsstress-equation models yield considerably better agreement with experiments, but tend to overpredict the periodic fluctuating motion and also miss some other details of the flow behaviour.

Control of vortices on a delta wing by leading-edge injection

Abstract: This experimental investigation addresses the control of flow past a half-delta wing at high angle of attack. Application of steady blowing, steady suction, or alternate suction-blowing in the tangential direction along the leading edge of the wing can retard substantially the onset of vortex breakdown and stall. The most effective period of the alternate suction-blowing is on the order of one convective time scale of the flow past the wing. As a result of this type of control, the vortex structure in the crossflow plane is modified from a fully stalled condition to a highly coherent leading-edge vortex. This transformation to a restabilized vortex is represented by instantaneous velocity fields, streamline patterns, and vorticity contours

Experimental investigation of vortex-fin interaction

Abstract: An experimental investigation has been conducted to examine the mechanisms of vortex-fin interaction on a twin-fin configuration The investigation included a parametric study of the effect of tail location The vortices were generated by a 76 deg sharp-edged delta wing with vertical tails mounted behind the wing The model included both a dynamically-scaled flexible tail and a pressure instrumented rigid tail Surface oil-flow patterns, off-body laser light sheet visualizations, aerodynamic load measurements, mean and unsteady flexible tail response, and unsteady tail surface pressure measurements were obtained The results show that the tail location did not affect the upstream trajectory of the delta wing vortex The tail location did affect the location of vortex breakdown, the global structure of the flow field, the aerodynamic loads, and the fin buffeting levels The buffeting levels were reduced as the fins were moved laterally toward the vortex core trajectory Two distinct peaks were observed in the pressure excitation spectra in the post-breakdown flow Finally, the presence of the flexible tail opposite the rigid pressure tail altered the pressure measurements at one angle of attack

Experimental Study on the Three-Dimensional Flow Within a Compressor Cascade With Tip Clearance: Part I—Velocity and Pressure Fields

Abstract: Experimental results from a study of the three-dimensional flow in a linear compressor cascade with stationary endwall at design conditions are presented for tip clearance levels of 1.0, 2.0, and 3.3 percent of chord, compared with the no-clearance case. In addition to five-hole probe measurements, extensive surface flow visualizations are conducted. It is observed that for the smaller clearance cases a weak horseshoe vortex forms in the front of the blade leading edge. At all the tip gap cases, a multiple tip vortex structure with three discrete vortices around the midchord is found. The tip leakage vortex core is well defined after the midchord but does not cover a significant area in traverse planes. The presence of the tip leakage vortex results in the passage vortex moving close to the endwall and the suction side.

Near-field behavior of a tip vortex

Abstract: The near-field behavior of a tip vortex trailing behind a low aspect ratio wing, attached to an axisymmetric body, is investigated in this paper This study was performed in a tow tank and involved the use of particle displacement velocimetry Evolution of the tip vortex was studied by mapping its instantaneous lateral velocity at several consecutive axial locations The axial velocity distribution was also measured Experiments were repeated at various Reynolds numbers and incidence angles Repeatability was also examined at the same conditions The results indicate that roll-up is almost complete at the trailing edge and that less than 66% of the root circulation is entrained into the vortex At Rec = 22 x 10s, the results are steady, and the spatially averaged tangential velocity agrees well with the existing models At Rec

Experiments on liquid mixing and reaction in a vortex

Abstract: A series of experiments were conducted in water to study mixing in the field of a single, two-dimensional vortex. The experimental configuration is that of a laminar line vortex initiated along a diffusion layer between two streams of different scalar concentrations. Measurements of passive scalars in inert and chemically reactive environments were made using a planar laser-induced fluorescence technique. A fast acid/base isothermal reaction was utilized to highlight the molecular mixing. The experimental results show that the mixing enhancement in the presence of a vortex is linearly dependent on the vortex strength and the time elapsed since vortex initiation. In particular, the mixedness, defined as the spatially integrated second moment of concentration field in the vortex, and the spatially averaged scalar dissipation are found to follow this dependence. This variation is mainly attributed to the contact area generation along the diffusion layers between the two streams as a result of inviscid deformations in the vortical flow field. The results presented pertain to mixing in liquids and in the limit of high Schmidt numbers.

Experimental Study on the Three-Dimensional Flow Within a Compressor Cascade With Tip Clearance: Part II—The Tip Leakage Vortex

Abstract: An analysis of the experimental data of a linear compressor cascade with tip clearance is presented with special attention to the development of the tip leakage vortex. A method for determining the tip vortex core size, center position, and vorticity or circulation from the measured data is proposed, based on the assumption of a circular tip vortex core. It is observed that the axial velocity profile passing through the tip vortex center is wakelike. The vorticity of the tip vortex increases rapidly near the leading edge and reaches its highest values at a short distance downstream, from which it gradually decreases. In the whole evolution, its size is growing and its center is moving away from both the suction surface and the endwall, approximately in a linear way

An experimental investigation of vortex motions near surfaces

Abstract: Laminar vortex rings approaching a solid planar surface and a free surface have been investigated experimentally over the range of Reynolds numbers based on the circulation, ReΓ, from 900 to 2350. The emphasis is on the process of vortex stretching, induction of boundary layer on the surface, formation of secondary vortices, and rebounding of the primary vortex ring. Detailed measurements were made using laser‐induced photochemical anemometry (LIPA), a nonintrusive visualization technique which enables multipoint simultaneous measurement of the unsteady velocity field. Results show that enstrophy, rather than circulation, clearly signifies three stages of the behavior of the primary vortex ring—free‐traveling stage, vortex stretching stage, and vortex rebounding stage. Although the flow phenomena are very similar when a vortex ring approaches a solid surface or a slightly contaminated free surface, the difference between the induced boundary layers under two surface conditions is obvious in terms of the circulation and the enstrophy. In addition, the surface deformation and subsequent surface waves resulting from the impacting vortex ring on the contaminated free surface are also discussed. The present study provides a basis for validation and comparison with numerical simulations.

Propeller Tip Vortex Cavitation Suppression Using Selective Polymer Injection

Abstract: This paper presents results of experiments where selective injection of a drag-reducing polymer solution into the tip vortex region of the blades of an 11.5 in. diameter propeller was effective insignificantly delaying tip vortex cavitation. The most critical phase of the investigation was the selection of the position of the injection ports. For well-positioned injection ports, at a fixed water channel speed the propeller cavitation number had to be decreased by as much as 35% in order to reestablish cavitation inception. Injections of water and a viscous mixture of water and glycerin for the same conditions did not affect the inception characteristics of the modified blades. Preliminary analysis of the results indicates that the viscoelastic properties of the Polyox solution injected in the vortex core played a significant role in thickening the viscous core of the tip vortex and thus reducing the pressure drop at the vortex centre without affecting circulation or lift.

The collapse of an axi-symmetric, swirling vortex sheet

Abstract: An axi-symmetric and swirling vortex sheet is investigated as the simplest flow in which there is non-trivial vortex stretching and as a possible setting for studying vortex cancellation and singularity formation. Rayleigh's criterion indicates linear stability of a single sheet but instability for other configurations of sheets. Due to the simplicity of vortex sheet problems, the linear modes and growth rates (or frequencies) can be explicitly expressed. Subsequent nonlinear evolution is numerically simulated using a vortex method. The numerical results for an axi-symmetric swirling sheet with a vortex line along the axis of symmetry show detachment of a vortex ring from the sheet into the outer fluid, and collapse of the sheet onto the vortex line at some points. Vortex cancellation, which in the presence of viscosity would likely lead to vortex line reconnection, seems to occur in both of these phenomena. The evolution of two co-axial, axi-symmetric, swirling vortex sheets is similar.

Schlieren studies of compressibility effects on dynamic stall of transiently pitching airfoils

Abstract: : Compressibility effects on the flowfield of an airfoil executing rapid transient pitching motion from 0-60 deg over a wide range of Mach numbers and pitching rates were studied using a stroboscopic schlieren flow visualization technique. The studies have led to the first direct experimental documentation of multiple shocks on the airfoil upper surface flow for certain conditions. Also, at low Mach numbers, additional coherent vortical structures were found to be present along with the dynamic stall vortex, whereas at higher Mach numbers the flow was dominated by a single vortex. The delineating Mach number for significant compressibility effects was 0.3 and the dynamic stall process was accelerated by increasing the Mach number above that value. Increasing the pitch rate monotonically delayed stall to angles of attack as large as 27 deg.

Response of superfluid vortex filaments to concentrated normal-fluid vorticity

Abstract: We simulate the motion of quantized vortex filaments moving under the influence of concentrated vorticity in the normal fluid. These simulations show an exponential growth of ordered superfluid vortex filaments in the core of the normal-fluid vortex. We explain the cause of this growth and develop formulas for the growth time scales and the minimum normal-fluid vortex circulation that causes this growth. Finally, we compare these values to the lifetime and circulation of the vortex tubes, which should be present in the classical turbulence of the normal fluid.

Instabilities of two-dimensional inviscid compressible vortices

Abstract: An investigation of the linear stability and subsequent nonlinear evolution and acoustic radiation of a planar inviscid compressible vortex is presented. The effects of the entropy gradient are investigated, and for the particular entropy profile chosen, the positive average entropy in the vortex core is destabilizing, while the opposite is true for the negative average entropy gradient. Finite-difference methods are used to study the linear initial value problem. These methods are found to be capable of accurately computing the frequencies and weak growth rates of the normal modes. When the initial condition consists of random perturbations, the long-time behavior is found to correspond to the most unstable normal mode in all cases. The numerical solution of the Euler equations is used to study the nonlinear development of an elliptic-mode perturbation.

Transonic flow around the leading edge of a thin airfoil with a parabolic nose

Abstract: Transonic potential flow around the leading edge of a thin two-dimensional general airfoil with a parabolic nose is analysed. Asymptotic expansions of the velocity potential function are constructed at a fixed transonic similarity parameter (K) in terms of the thickness ratio of the airfoil in an outer region around the airfoil and in an inner region near the nose. These expansions are matched asymptotically. The outer expansion consists of the transonic small-disturbance theory and it second-order problem, where the leading-edge singularity appears. The inner expansion accounts for the flow around the nose, where a stagnation point exists. Analytical expressions are given for the first terms of the inner and outer asymptotic expansions. A boundary value problem is formulated in the inner region for the solution of a uniform sonic flow about an infinite two-dimensional parabola at zero angle of attack, with a symmetric far-field approximation, and with no circulation around it. The numerical solution of the flow in the inner region results in the symmetric pressure distribution on the parabolic nose. Using the outer small-disturbance solution and the nose solution a uniformly valid pressure distribution on the entire airfoil surface can be derived. In the leading terms, the flow around the nose is symmetric and the stagnation point is located at the leading edge for every transonic Mach number of the oncoming flow and shape and small angle of attack of the airfoil. The pressure distribution on the upper and lower surfaces of the airfoil is symmetric near the edge point, and asymmetric deviations increase and become significant only when the distance from the leading edge of the airfoil increases beyond the inner region. Good agreement is found in the leading-edge region between the present solution and numerical solutions of the full potential-flow equations and the Euler equations.

Oblique collision of two vortex rings and its acoustic emission

Abstract: A theory of vortex sound is formulated in the form of multipole expansions and an explicit formula is presented for the wave pressure excited by a time-dependent vorticity field localized in space. This is applied to the case of the oblique collision of two vortex rings at right angles, in which higher-order terms are important to represent asymmetric emission. The vortex motion and the generated waves are also studied experimentally and numerically. The initial setup of the two vortices is arranged so that they come into contact by their own motions and perform a reconnection of the vortex lines

Turbulent structure of a wingtip vortex in the near field

Abstract: The turbulent rollup of a vortex generated by a rectangular wing has been investigated. Extensive mean and turbulence measurements of the flowfield on a wingtip and in the near field have been completed. Velocity fluctuation measurements show that the near-field core is not laminar. A large axial velocity excess was found to exist in the core of the vortex. A momentum balance in the near-field of the wingtip showed that the magnitude of the core Reynolds-stress gradient terms are the same order as the largest terms in the governing equations. Navier-Stokes computations of the identical configuration, including wind tunnel walls and using measured inflow and outflow boundary conditions, reproduced many of the features of the experiment. Inherent limitations of the Baldwin-Barth turbulence model combined with limited grid resolution caused the computed vortex core to be more diffuse than desired. The momentum balance also demonstrated that the level of numerically generated false diffusion in the vortex core is relatively high.

Vortex capture by a two-dimensional airfoil with a small oscillating leading-edge flap

Abstract: In order to exploro the possibility of capturing a vortex an the upper surface of an airfoil at high angle of attack by unsteady excitations, a sharp-leading~edge airfoil with ascil- lating leading edge flap was testcd in a low speed wind tunncl and simulated by numerical compulation. The most important experimental result is that a 60%-70% of lift increment was achicved with little or even no penalty on drag at post stall condition, when the excitation changed the massively sepa- rated flow to a closed separation bubble in the mean sense, and thus held a strong vortex on the upper surface of the airfoil. As a complement to this experimentally obtained time-averagcd picture, preliminary numerical compulation revealed tlic cor- responding instantaneous process. The result shows that, with excitation, vortices are still shedding, but the probability of a wcll-organized lifting vortex staying above the airfoil is much larger than that without excitation. This finding suggests a potential of designing innovative higli~lift airfoils and wings.

Formation of a tripolar vortex in a stratified fluid

Abstract: This paper reports on an experimental study of vortices in a stratified fluid. The vortices were generated by two different stirring devices, viz. a rotatting sphere and a rotating bent rod. It was found that the vortices created with the rotating sphere are mostly axisymmetric and stable, whereas the vortices produced with the bent rod generally show instabilities, under certain conditions leading to the formation of a tripolar vortex. This report concentrates on this tripolar structure and presents quantitative information about the flow obtained through streak photography of tracer particles.

Ignition and structure of a laminar diffusion flame in the field of a vortex

Abstract: The distortion of flames in flows with vortical motion is examined via asymptotic analysis and numerical simulation. The model consists of a constant density, one step, irreversible Arrhenius reaction between initially unmixed species occupying adjacent half-planes which are then allowed to mix and react in the presence of a vortex. The evolution in time of the temperature and mass fraction fields is followed. Emphasis is placed on the ignition time and location as a function of vortex Reynolds number and initial temperature differences of the reacting species. The study brings out the influence of the vortex on the chemical reaction. In all phases, good agreement is observed between asymptotic analysis and the full numerical solution of the model equations.

Motion of an elliptical vortex under applied periodic strain

Abstract: The effect of subjecting a uniform elliptical vortex to a periodically varying plane straining field is considered. For plane steady straining fields, it is known that a Rankine‐type vortex core of uniform vorticity and elliptical shape can exist in a state in which it (a) is steady and stationary, (b) rotates about its axis or nutates about a fixed axis, or (c) elongates indefinitely, smearing the core into a thin layer. In state (a), for sufficiently weak straining fields, a vortex of small enough aspect ratio of the ellipse persists under such a plane strain, being robust to small two‐dimensional disturbances present in the flow field. It is shown that if, however, a small periodically varying component is added to the basic straining field, then at frequencies corresponding to the natural frequency of vibration of the vortex and their harmonics, a resonance phenomenon takes place which destabilizes the apparently stable stationary vortex in finite time, causing it to flip from state (a) into states (b) or (c), in which action of instabilities associated with a higher, nonelliptical, mode of deformation of the vortex boundary by disturbances in the flow field would lead to disintegration of the vortex structure. In fact it is found that the vortex also flips to states (b) and (c) for a range of non‐natural frequencies of oscillation of the straining field. The effect of the periodic plane straining on the three‐dimensional Crow instability is also discussed.

Euler Calculations of Unsteady Interaction of Advancing Rotor with a Line Vortex

Abstract: The unsteady, three-dimensional flowfield of a helicopter rotor blade in forward flight encountering a concentrated line vortex is calculated using an implicit, finite difference numerical procedure for the solution of Euler equations. A prescribed vortex method is adopted to preserve the structure of the interacting vortex. The test cases considered for computation correspond to the two-bladed model rotor experimental conditions of Caradonna et al. and consist of parallel and oblique interactions