Showing papers on "Starting vortex published in 1992"

Rossby-wave breaking, microbreaking, filamentation, and secondary vortex formation-The dynamics of a perturbed vortex

Abstract: The behavior of an isolated vortex perturbed by topographically forced Rossby waves is studied using the method of Contour Dynamics. For a single-contour vortex a distinct forcing threshold exists above which the wave breaks in a dynamically significant way, leading to a disruption of the vortex. This breaking is distinguished from the process of weak filamentary breaking described by Dritschel and classified here as microbreaking; the latter occurs in nondivergent flow even at very small forcing amplitudes but does not affect the vortex in a substantial manner. In cases with finite Rossby deformation radius (comparable with the vortex radius) neither breaking nor microbreaking occurs below the forcing threshold. In common with previous studies using high-resolution spectral models, the vortex is not diluted by intrusion of outside air, except during remerger with a secondary vortex shed previously from the main vortex during a breaking event. The kinematics of the breaking process and of the vor...

Unsteady flow past an airfoil pitching at a constant rate

Abstract: The unsteady flow past a NACA 0012 airfoil that is undertaking a constant-rate pitching up motion is investigated experimentally by the PIDV technique in a water towing tank. The Reynolds number is 5000, based upon the airfoil's chord and the free-stream velocity. The airfoil is pitching impulsively from 0 to 30 deg. with a dimensionless pitch rate alpha of 0.131. Instantaneous velocity and associated vorticity data have been acquired over the entire flow field. The primary vortex dominates the flow behavior after it separates from the leading edge of the airfoil. Complete stall emerges after this vortex detaches from the airfoil and triggers the shedding of a counter-rotating vortex near the trailing edge. A parallel computational study using the discrete vortex, random walk approximation has also been conducted. In general, the computational results agree very well with the experiment.

The efficiency of symmetric vortex merger

Abstract: The coalescence of two identical vortices with uniform vorticity is investigated using the numerical method of ‘‘contour surgery,’’ for two‐dimensional inviscid, incompressible vortex dynamics (2VD), and quasigeostrophic shallow‐water dynamics (QGSW), to quantify the differences and similarities between the two models. High‐resolution calculations show that the evolution of the vortices may fall into three different regimes depending on the initial intercentroid separation. The efficiency of the merger of two vortices into a single elliptical‐like vortex is determined by calculating the loss of globally conserved quantities to filaments and small‐scale structures. In the 2VD model, it is found that the resultant central vortex always has at least 56% more circulation than either of the initial vortices, showing that the merger process indeed forms larger scales. From energetics, the initial separation for an inviscid transition from two elliptical vortices to a single elliptical vortex is determined, and ...

Effects of ambient turbulence on the decay of a trailing vortex wake

Abstract: Experiments were conducted to investigate the effects of ambient turbulence on the evolution of a trailing vortex wake. The wake was generated by towing an NACA 0012 wing, with a span of 10.2 cm and a chord of 5.1 cm, at an incidence of —10 deg and a speed of 40 cm/s in a tow tank. The chord Reynolds number was 20,400. The ambient turbulence was generated by towing three grids, with square meshes of 1.45, 10.2, and 20.3 cm, upstream of the wing. Turbulence parameters were measured with crossed hot-film probes. The trailing vortex wake, tagged with a fluorescent dye, was visualized from different perspectives and its decay was derived from 16-mm movie records. For weak turbulence with large integral scales compared with the vortex separation, vortex linking is the dominant mode of instability. The dominant wavelength of the linking decreases with increasing turbulence intensity or dissipation rate. As the turbulence intensity increases, vortex bursting appears and eventually replaces linking as the dominant mode of instability. For turbulence with a small integral scale as compared with the vortex separation, vortex instability is predominantly of the bursting type.

Dynamics of singular vortices on a beta-plane

Abstract: A new singular-vortex theory is presented for geostrophic, beta-plane dynamics The stream function of each vortex is proportional to the modified Bessel function Ko(pr), where p can be an arbitrary positive constant If p−1 is equal to the Rossby deformation scale Rd, then the vortex is a point vortex; for p−1 ≠ Rd the relative vorticity of the vortex contains an additional logarithmic singularity Owing to the β-effect, the redistribution of the background potential vorticity produced by the vortices generates a regular field in addition to the velocity field induced by the vortices themselves Equations governing the joint evolution of singular vortices and the regular field are derived A new invariant of the motion is found for this system If the vortex amplitudes and coordinates are set in a particular way then the regular field is zero, and the vortices form a system moving along latitude circles at a constant speed lying outside the range of the phase velocity of linear Rossby waves Each of the systems is a discrete two-dimensional Rossby soliton and, vice versa, any distributed Rossby soliton is a superposition of the singular vortices concentrated in the interior region of the soliton An individual singular vortex is studied for times when Rossby wave radiation can be neglected Such a vortex produces a complicated spiral-form regular flow which consists of two dipoles with mutually perpendicular axes The dipoles push the vortex westward and along the meridian (cyclones move northward, and anticyclones move southward) The vortex velocity and trajectory are calculated and applications to oceanic and atmospheric eddies are given

Linear Motion of a Shallow-Water Barotropic Vortex as an Initial-Value Problem

Abstract: This paper revisits calculation of motion for a shallow-water barotropic vortex with fixed mean axisymmetric structure. The algorithm marches the linear primitive equations for the wavenumber 1 asymmetry forward intime using a vortex motion extrapolated from previous calculations. Periodically, it examines the calculated asymmetry for the apparent asymmetry due to mispositioning of the vortex center, repositions the vortex to remove the apparent asymmetry, and passes the corrected vortex motion on to the next cycle. This approach differs from the author's earlier variational determination of the steady-state motion after initial transients had died away. The steady-state approach demonstrated that the vortex had normal modes at zero frequency and, when an annulus of weak anticyclonic flow encircled the cyclonic inner vortex, at the most anticyclonic rotation frequency of the mean flow. Forcing of the former model led to too rapid steady-state poleward motion on a beta plane. At least for the line...

Numerical investigation of vortex breakdown on a delta wing

Abstract: A numerical investigation of leading-edge vortex breakdown on a delta wing at high angles of attack is presented. The analysis has been restricted to low-speed flows on a flat-plate wing with sharp leading edges. Both Euler and Navier-Stokes (assuming fully laminar and turbulent flows) equations have been used in this study and the results are compared against experimental data. Predictions of vortex breakdown progression with angle of attack with both Euler and Navier-Stokes equations are shown to be consistent with the experimental data. However, the Navier-Stokes predictions show significant improvements in breakdown location at angles of attack where the vortex breakdown approaches the wing apex. The location of the primary vortex and the level of vorticity in the prebreakdown regions are affected very little by the viscous effects. In the postbreakdown regions, however, the levels of vorticity in the primary vortex have increased differences between the Euler and Navier-Stokes solutions. Navier-Stokes solutions indicate the presence of a secondary vortex even after the primary vortex is burst. The predicted trajectories of the primary vortex are in very good agreement with the test data with the laminar solutions providing the overall best comparison.

Control of leading-edge vortices on a delta wing

Abstract: The unsteady flow structure of leading-edge vortices on a delta wing has been investigated using new types of experimental techniques, in order to provide insight into the consequences of various forms of active control. These investigations involve global control of the entire wing and local control applied at crucial locations on or adjacent to the wing. Transient control having long and short time-scales, relative to the convective time-scale C/U(sub infinity), allows substantial modification of the unsteady and time-mean flow structure. Global control at long time-scale involves pitching the wing at rates an order of magnitude lower than the convective time-scale C/U(sub infinity), but at large amplitudes. The functional form of the pitching maneuver exerts a predominant influence on the trajectory of the feeding sheet, the instantaneous vorticity distribution, and the instantaneous location of vortex breakdown. Global control at short time-scales of the order of the inherent frequency of the shear layer separating from the leading-edge and the natural frequency of vortex breakdown shows that 'resonant' response of the excited shear layer-vortex breakdown system is attainable. The spectral content of the induced disturbance is preserved not only across the entire core of the vortex, but also along the axis of the vortex into the region of vortex breakdown. This unsteady modification results in time-mean alteration of the axial and swirl velocity fields and the location of vortex breakdown. Localized control at long and short time-scales involves application of various transient forms of suction and blowing using small probes upstream and downstream of the location of vortex breakdown, as well as distributed suction and blowing along the leading-edge of the wing applied in a direction tangential to the feeding sheet. These local control techniques can result in substantial alteration of the location of vortex breakdown; in some cases, it is possible to accomplish this without net mass addition to the flow field.

Advection of a vortex pair atmosphere in a velocity field of point vortices

Abstract: Two‐dimensional inviscid flows governed by a vortex pair in the presence of another point vortex or vortex pair on an unbounded plane are considered analytically and via numerical simulations. For some integrable cases of vortex motion, the stirring process of a fixed closed volume of surrounding fluid (‘‘atmosphere’’) initially trapped by vortex pair is investigated. Using the full classification of vortex movement types, it is shown that for all cases of vortex pair direct and exchange scattering the stirring process is regular. Some internal atmosphere regions conserve their existence and form after vortex interaction, resulting a ‘‘solitonlike’’ behavior. For general cases of vortex pair mutual trapping, the stirring process is chaotic. For limiting cases of vortex motions, the fluid particles reveal a regular behavior. A simple model for the qualitative description of recent experiments on vortex dipoles interaction is discussed. Although clearly an extreme idealization, the model appears to shed some light on what to expect in laboratory experiments.

The Development of a Turbulent Junction Vortex System (Data Bank Contribution)

Abstract: The growth and development of a horseshoe vortex system in an incompressible, three-dimensional turbulent junction flow were investigated experimentally. A streamlined cylinder mounted with its axis normal to a flat surface was used to generate the junction vortex flow. The flow environment was characterized by a body Reynolds number of 183,000, based on the leading edge diameter of the streamlined cylinder. The study included surface flow visualizations, surface pressure measurements, and mean flow measurements of total pressure, static pressure, and velocity distributions in three planes around the base of the streamlined cylinder, and in two planes in the wake flow. Some characterizations of vortex properties based on the measured mean cross-flow velocity components are presented. The results show the presence of a single large, dominant vortex, with strong evidence of a very small corner vortex in the junction between the cylinder and the flat surface. The center of the dominant vortex drifts away from both the body and the flat surface as the flow develops along and downstream of the body. The growth and development of the core of the large, dominant vortex are documented.

Vortex modeling for rotor aerodynamics - The 1991 Alexander A. Nikolsky Lecture

Abstract: The efforts toward realistic vortex modeling for rotary wings which began under the guidance of professor A. A. Nikolsky of Princeton University in 1955-1956 are discussed. Attention is given to Nikolsky's flow-visualization studies and major theoretical considerations for vortex modeling. More recent efforts by other researchers have led to models of increasing complexity. The neglect of compressibility and viscous effects in the classical approach is noted to be a major limiting factor in full-scale rotor applications of the classical vortex theory; it has nevertheless been valuable for the delineation of problem areas and the guiding of both experimental and theoretical investigations.

Determination of vortex burst location on delta wings from surface pressure measurements

Abstract: A shape parameter for the vortex-induced upper surface pressure distribution on a delta wing has been derived from a simple two-dimensional potential flow model For this model the half-width of the suction peak is a function solely of the vortex height above the wing surface Published experimental data are used to show that this result holds for real delta wing flows at low angles of attack, thus allowing a good estimate of vortex trajectory and strength to be made from surface pressure measurements alone At higher angles of attack, where the vortex burst region is over the wing, the simple model breaks down; however, the variation in half-width with both chordwise location and angle of attack appears to correlate well with the condition of the adjacent vortex In particular, the burst point corresponds to an abrupt, well-defined change in half-width This observation offers an alternative to flow visualization techniques for experimental determination of burst location

Two-fence concept for efficient trapping of vortices on airfoils

Abstract: Previous work on the use of a vortex trapped above a wing in order to produce high lift at low angles of attack is extended here. It is first postulated that the optimum way to trap a vortex is to design the airfoil section and wing so that the flow along the vortex core is minimized. It is then shown that a vertical fence both in front of and behind the separation bubble generated by the trapped vortex is an effective way to reduce the mass flow removal and its associated drag to a negligible amount. In order to show that vertical surfaces upstream and downstream of the vortex separation bubble have an opposite effect on the source requirements for vortex trapping, conformal mapping methods are used to obtain the solutions for a variety of simple two-dimensional, inviscid, incompressible flow configurations. Trapped-vortex flowfield solutions for the flow over flat plate and Clark-Y airfoils are then used to demonstrate that the heights of the fences can be tailored to make the required mass withdrawal (and therefore, the drag due to trapping) to be vanishingly small.

Navier-Stokes simulation of a close-coupled canard-wing-body configuration

Abstract: The thin-layer Navier-Stokes equations are solved for the flow about a coplanar close-coupled canard-wing-body configuration at a transonic Mach number of 0.90 and at angles of attack ranging from 0 to 12 degrees. The influence of the canard on the wing flowfield, including canard-wing vortex interaction and wing vortex breakdown, is investigated. A study of canard downwash and canard leading-edge vortex effects, which are the primary mechanisms of the canard-wing interaction, is emphasized. Comparisons between the computations and experimental measurements of surface pressure coefficients, lift, drag and pitching moment data are favorable. A grid refinement study for configurations with and without canard shows that accurate results are obtained using a refined grid for angles of attack where vortex burst is present. At an angle of attack of approximately 12 deg, favorable canard-wing interaction which delays wing vortex breakdown is indicated by the computations and is in good agreement with experimental findings.

Head‐on collision of a large vortex ring with a free surface

Abstract: The generation of free surface waves due to the head‐on collision of a vortex ring with the free surface is studied experimentally and numerically. The interaction process is investigated by looking at the free surface evolution with the aid of a shadowgraph image, by measurement of the free surface elevation, and by visualizing the vortex ring with hydrogen bubbles. Three different free surface patterns are observed, depending on the strength of the vortex ring: (1) A single circular depression of the free surface for weak vortex rings and at very early times for strong vortex rings, (2) axisymmetric radially propagating waves in the early stage of the interaction of strong vortex rings with the surface, and (3) fully three‐dimensional waves that depend on the local structure of the vortex core. Numerical simulations are used to address the observed axisymmetric phenomena.

Experimental Studies of Vortex Flaps and Vortex Plates. Part 1. 0.53m 60 deg Delta Wing

Abstract: Low-speed wind tunnel tests have been made to investigate the flow around a leading-edge vortex flap at the maximum L/D condition. Tests were also made to measure the performance of the inverted vortex flap and the vortex plate. The force measurements and flow visualization tests were conducted on a 60deg delta wing model. Results indicate that the lift to drag ratio is a maximum for any given flap deflection angle when the flow comes smoothly onto the deflected vortex flap without forming a large leading-edge separation vortex on the flap surface. The benefit of the vortex plate is seen in the drag results which are smaller than those for the datum wing. This benefit is due to some leading-edge suction acting on the forward facing region between the delta wing and the vortex plate. Starting in 1946 as the College of Aeronautics, the Cranfield Institute of Technology was granted university status in 1969. In 1993 it changed its name to Cranfield University.

Enhancement of laminar boundary layer heat transfer by a vortex generator

Abstract: The enhancement of heat transfer caused by the presence of a single vortex generator in a laminar boundary layer was experimentally investigated. The local heat transfer coefficients just down stream of the vortex generator and the mean and fluctuation components of velocity were measured. The influence of the height, angle of attack and geometry of the vortex generator on heat transfer was investigated

Close interaction between a vortex filament and a rigid sphere

Abstract: The evolution of a linear vortex filament close to a rigid sphere is investigated at high Reynolds number. The limiting evolution in an ideal flow, is analysed using a cutoff method and the results are compared with those of a singular vortex approach able to account for a viscous effect on the vortex structure evolution. The computed results show the creation of a closed vortex structure in ideal flow and also, at low Reynolds number, an unrealistic reattachment of the vortex to the surface of the body. The nature of the boundary-layer development, when the no-slip condition is satisfied, is calculated near the symmetry plane. The solutions show the development of an unsteady, vortex-driven, separating boundary layer with the three-dimensional separations dependent on the initial distance of the filament from the wall. All the solutions ultimately show a rapid growth of the secondary vorticity field near the surface and suggest an ejection from the boundary layer, followed by a strong viscous-inviscid interaction.

Ring type vortex flowmeter and method for measuring flow speed and flow rate using said ring type vortex flowmeter

Abstract: A vortex flowmeter which is capable of measuring flow speed and flow rate both rapidly and precisely. The vortex flowmeter is composed of a ring-type bluff body situated in a pipe as a vortex shedder. The ring is bevelled at an acute angle at the inner and outer edges of the ring and the vortex shedding frequencies generated by the ring can be transformed to flow speeds and flow rates. The geometrical parameters G/W and D/W of the ring are G/W=0.50 to 0.53 and D/W=5.03 to 10.63, where G denotes the gap width between the outer edge of the ring and the pipe wall, W denotes the width of the ring, and D denotes the mean diameter of the ring. After these vortex shedding frequencies are nondimensionalized, they show a relationship with the Reynolds Numbers. The optimal streamwise location for installation of a pressure sensor is in the region of X/W=2.25 to 2.84, where X denotes the axial distance from the leading surface of the ring frontal surface. The vortex shedding frequencies of the ring can be obtained from the wall pressure measurement. In addition, the present invention also provides a method that uses the vortex flowmeter to measure flow speed and flow rate.

Three-dimensional vortex formation from an oscillating, non-uniform cylinder

Abstract: A cylinder having mild variations in diameter along its span is subjected to controlled excitation at frequencies above and below the inherent shedding frequency from the corresponding two-dimensional cylinder. The response of the near wake is characterised in terms of timeline visualisation and velocity traces, spectra, and phase plane representations. It is possible to generate several types of vortex formation, depending upon the excitation frequency. Globally locked-in, three-dimensional vortex formation can occur along the entire span of the flow. Regions of locally locked-in and period-doubled vortex formation can exist along different portions of the span provided the excitation frequency is properly tuned. Unlike the classical subharmonic instability in free shear flows, the occurrence of period-doubled vortex formation does not involve vortex coalescence; instead, the flow structure alternates between two different states.

From vortex layers to vortex sheets

Abstract: This paper shows that the solution of the Birkhof–Rott equation for the vortex sheet can be approximated, for short times, by the solutions of the Euler equation for a thin vortex layer of vorticity, when its thickness vanishes and its vorticity intensity diverges suitably. The result is obtained in an analytical setup, and an example seems to indicate that this is indeed necessary.

Viscous effects on a vortex wake in ground effect

Abstract: Wake vortex trajectories and strengths are altered radically by interactions with the ground plane. Prediction of vortex strength and location is especially important in the vicinity of airports. Simple potential flow methods have been found to yield reasonable estimates of vortex descent rates in an otherwise quiescent ambient background, but those techniques cannot be adjusted for more realistic ambient conditions and they fail to provide satisfactory estimates of ground-coupled behavior. The authors have been involved in a systematic study concerned with including viscous effects in a wake-vortex system which is near the ground plane. The study has employed numerical solutions to the Navier-Stokes equations, as well as perturbation techniques to study ground coupling with a descending vortex pair. Results of a two-dimensional, unsteady numerical-theoretical study are presented in this paper. A time-based perturbation procedure has been developed which permits the use of analytical solutions to an inner and outer flow domain for the initial flow field. Predictions have been compared with previously reported laminar experimental results. In addition, the influence of stratification and turbulence on vortex behavior near the ground plane has been studied.

Physical Modeling of Ground Effects on Vortex Wakes

Abstract: Towing-tank experiments were conducted to investigate ground effects on vortex wakes. Two methods were used to generate the vortex wakes: 1) a towed NACA 0012 wing and 2) a slotted-jet vortex generator. Trajectories derived from trailing vortex wakes, tagged with a fluorescent dye and released near the simulated ground surface, have confirmed the inadequacy of the two-dimensional inviscid solution. The generation of a relatively weak secondary vortex causes the primary vortex to rebound from the surface as a result of unsteady separation. There is no significant difference between the trajectories, whether the ground is simulated by a rigid surface or a free surface. The introduction of a rigid surface close to the exit of the slotted-jet vortex generator results in the formation of secondary vortices at the ground outboard of the primary vortices. The secondary vortices generated by the slotted-jet vortex pair are more coherent and persistent than those generated by the trailing vortex pair. A simple modification of the two-dimensional inviscid theory—namely, adding a secondary vortex to the system—recreates the rebound phenomenon, which agrees qualitatively with the trend seen in the experiments.

Experimental investigation of the perpendicular rotor blade-vortex interaction at transonic speeds

Abstract: Transonic perpendicular rotor blade-vortex interaction (BVI) tests at Mach numbers ranging from 0.68 to 0.9 and Reynolds numbers (based on the airfoil chord) of 3.8-5.5 million were conducted in the UTA highReynolds number, transonic Ludwieg-tube wind tunnel. The scheme involved positioning a lifting wing (vortex generator) upstream of an instrumented NACA 0012 airfoil so that the trailing vortex interacted with the downstream airfoil. Tests were performed at several vortex strengths as well as several vortex core heights above the downstream airfoil. The results obtained from these experiments indicate that a substantial change in the pressure distribution of the downstream airfoil occurs, but most of the effects were confined to the leading 30% of the airfoil chord. A spanwise drift of the vortex core as it passes over the trailing airfoil, similar to results observed previously in low-speed wind tunnel tests, as well as a high degree of unsteadiness in the vicinity of the vortex center were observed.

Experimental investigation of the flowfield of an oscillating airfoil

Abstract: The flowfield of an airfoil oscillated periodically over a wide range of reduced frequencies, 0 less than or = k less than or = 1.6 is studied experimentally at chord Reynolds numbers of R sub c = 22,000 and 44,000. The NACA0012 airfoil is pitched sinusoidally about one quarter chord between angles of attack (alpha) of 5 and 25 degrees. Detailed flow visualization and phase averaged vorticity measurements are carried out for k = 0.2 to document the evolution and the shedding of the dynamic stall vortex (DSV). In addition to the DSV, an intense vortex of opposite sign originates from the trailing edge just when the DSV is shed. After being shed into the wake, the two together take the shape of a large 'mushroom' while being convected away from the airfoil. The unsteady circulation around the airfoil and, therefore, the time varying component of the lift is estimated in a novel way from the shed vorticity flux and is found to be in good agreement with the lift variation reported by others. The delay in the shedding of the DSV with increasing k, as observed by previous researchers, is documented for the full range of k. The DSV, for example, is shed nearly at the maximum alpha of 25 degrees at k = 0.2, but is shed at the minimum alpha of 5 degrees at k = 0.8. At low k, the flowfield appears quasi-steady and the bluff body shedding corresponding to the maximum alpha (25 degrees) dominates the unsteady fluctuations in the wake.

Coherence of intense localized vorticity in decaying two‐dimensional Navier–Stokes turbulence

Abstract: The coupled evolution of an intense localized vortex and decaying Navier–Stokes turbulence in two dimensions is examined in order to identify and describe the physical process allowing regions of intense vorticity to decouple from the eddy cascade and survive as coherent structures. It is found that the shear straining of turbulent eddies by the differential rotation of the intense vortex gives rise to vortex coherence. Dimensional analysis and analytic solution of a closure representation in the asymptotic limit of large shear are used to determine a nonuniform vortex diffusivity produced by turbulent straining, and the spatial structure of turbulence in the vortex. When the shear straining rate exceeds the appropriate measure of the ambient turbulent straining rate, turbulent fluctuations in the vortex interior localize to a narrow layer in the vortex edge. Likewise, vortex diffusion becomes weak, the vortex lifetime becomes large, and greatly exceeds the eddy lifetimes of ambient turbulence.

Underwing compression vortex attenuation device

Abstract: A vortex attenuation device is presented which dissipates a lift-induced vortex generated by a lifting aircraft wing. The device consists of a compression panel attached to the lower surface of the wing and facing perpendicular to the airflow across the wing. The panel is located between the midpoint of the local wing cord and the trailing edge in the chord-wise direction and at a point which is approximately 55% of the wing span as measured form the fuselage center line in the spanwise direction. When deployed in flight, this panel produces a positive pressure gradient aligned with the final roll-up of the total vortex system which interrupts the axial flow in the vortex core and causes the vortex to collapse.