Vortex Burst Research Articles

Tail Buffet of F/A-18 at High Incidence with Sideslip and Roll (Part 1)

The unsteady pressure was measured simultaneously at 24 locations on both sides of a vertical e n of a rigid, 6% scale model of the F/A-18 aircraft in a wind tunnel. This pressure was integrated over the entire e n surface and over time to provide the mean and rms normal force, bending moment, and torsional moment on the e n in order to determine its buffet loading. Results are available for Mach numbers M =0.25, 0.60, and 0.80; angles of attack ®=25, 30, and 32.5 deg; and a sideslip angle i 15 deg < ¯ < 15 deg or a roll angle i 30 deg < ‐ ’ < 30 deg. At zero roll and sideslip the mean force and moment coefe cients generally increased with increasing angle of attack; but as the sideslip or roll angles increased some coefe cients changed sign, and their relationships to ® became more complex. A measurable trend of the mean force coefe cient to decrease with increasing Mach number was also observed. The rms normal force coefe cient increased signie cantly as ® increased from 25 to 32.5 deg, but showed no appreciable trends as the Mach number increased from 0.25 to 0.80. The wind-tunnel tests are complemented by a e ow-visualization study of the leading-edge extensions vortices of a 1:48 scale model of the F/A-18 in a water tunnel, showing the vortex burst locations at different aircraft orientations. Part 2 of this study presents statistical results of the forces and moments.

Aerodynamic Characteristics of Spanwise Cambered Delta Wings

A wind-tunnel investigation was undertaken to determine the effect of various spanwise camber distributions on the aerodynamic characteristics of a 75-deg delta wing. Data presented include force balance, seven-hole probe survey, as well as surface e ow visualization and vortex burst trajectories. The experimental results indicate that some of the tested dihedral and anhedral geometric variations can augment lift signie cantly, requiring only minor modie cations to the planar wing geometry to implement. It was found that anhedral generally promotes lift augmentation compared to the basic wing, whereas dihedral has the opposite effect. Anhedral and dihedral have the greatest effect when applied near the wing tips, with anhedral enhancing lift through an increase in strength of both the primary and secondary wing leading-edge vortices. Nonplanarity had a small effect on the vortex burst characteristics.

Effects of Anhedral and Dihedral on a 75-deg Sweep Delta Wing

An investigation into the effects of spanwise camber in the form of anhedral and dihedral on a 75-deg sweep delta wing is detailed. Data are presented encompassing force balance, surface pressure measurement, seven-hole probe surveys, and vortex burst trajectories. The results show that the net effect of this form of nonplanarity is an increase in lift for anhedral and a decrease in lift for dihedral compared to the planar wing. Small anhedral angles aremosteffectivein augmenting lift. Anhedral doesnot appearto greatly augmentthestrength of theleading-edge vortex. The major benee t from anhedral would appear to be due to its displacing effect on the vortex trajectory: drawing the vortex closer to the wing surface and inboard compared to the planar wing. As the vortex is drawn inboard, its induced surface loading acts on a greater area of the wing. Dihedral also draws the vortex closer to the wing surface (to a greater extent then anhedral ) while moving the vortex toward the wing leading edge. In addition, anhedral doesnotappearto introduceany detrimental effectson longitudinal stability and doesnot incur any penalties in terms of vortex burst characteristics.

Wave nature in vortex-bursting initiation

A new mechanism responsible for rapid flame propagation in a flammable vortex tube (vortex bursting) has been previously proposed by one of the authors. (1) The vortex tube is radially expanded due to thermal expansion at the flame surface. (2) For the conservation of angular momentum, vortex filaments which constitute the original vortex tube are deformed into spiral form, thus producing a torus of azimuthal-vorticity distribution of the shoulder part of the parabola-like flame surface. (3) By Biot-Savart's law, the azimuthal-vorticity distribution induces an axial velocity at the flame-tip location, so that the flame is convected forth with a speed corresponding to the maximum rotational speed of the vortex tube. In the present study, this theory is extended to examine the transient behavior of a flame propagating in a vortex tube. The most important findings are a wavelike nature of the azimuthal-vorticity distribution, which is produced by the spiraling of axial filaments, and its effect on flame extinction. The azimuthal-vorticity distribution propagates as a wave with its own speed, which is determined from its circulation and form. When the rotational speed of vortex tube is too high relative to the flame heat-release rate, the azimuthal-vorticity torus passes the flame tip, and the burned gas region is immersed in a straining flow which is produced by the circulating flow around the azimuthal-vorticity torus. Thus, the elongated burned gas region is rapidly cooled by the surrounding cold gas, and the tapered flame front suffers from the danger of extinction. The steady flame propagation state is established when the propagation speed of the azimuthal-vorticity wave coincides with the flame-tip propagation speed, which is consistent with the previously proposed expression for steady flame propagation speed.

Pneumatic Lift and Control Surface Technology for High Speed Civil Transports

Experimental evaluations have been conducted of blown high-lift devices and control surfaces applied to improve the low-speed terminal-area performance of generic High Speed Civil Transport (HSCT) aircraft. Plain blown flaps and advanced pneumatic high-lift devices have been integrated into highly swept vortex-dominated wings. These produced large lift increases, as well as significant drag reductions greater than full jet thrust recovery. Because conventional horizontal tails were inadequate to trim these configurations, blown canards were employed. In addition to providing positive lift increments for pitch trim, the canards were found to favorably influence the higher angle-of-attack vortex-lift characteristics of the wings. The downwash from these canards resulted in a delay of wing vortex burst. Details of these investigations and test results that confirm the effectiveness of combined pneumatic high-lift devices and control surfaces on these HSCT aircraft configurations are presented

Intermittency in the closed flow between coaxial corotating disks

We report an experimental study of the swirling flow generated in the gap between two coaxial corotating disks in an enclosed geometry, i.e. when the flow is enclosed in a cylindrical vessel. In this situation the angular momentum generated by the disk rotation tends to concentrate in a strong axial vortex. In the range of high Reynolds numbers explored, we show that two regimes exist, depending on the disk angular velocities. When they rotate at quite different speeds, we observe very intermittent fluctuations of the axial vorticity associated with quasi-periodic vortex bursting, whereas at comparable rotation rates a more stable vorticity structure is observed. We describe these regimes using global measurements; we also show, using local velocimetry, how the structure of the flow affects the small scale turbulence.

Flame Speeds in Combustible Vortex Rings

Flame speeds in vortex rings of a stoichiometric methane/air mixture have been investigated. The flame speeds have been measured by a high-speed video camera, and the maximum tangential velocities in the vortex have been measured by a hot-wire anemometer. Results show that the flame velocity is approximately proportional to the maximum tangential velocity and its proportionality factor is about unity. By considering conservation of mass, linear momentum, and angular momentum across the flame, a simple theory has been proposed. A good agreement has been obtained between the theory and the experiment. The vortex bursting mechanism and the baroclinic torque mechanism have been discussed.

Automatic vortex core detection

This case study presents applications of the eigenvector method to recent aeronautical studies at NASA Ames Research Center and Rensselaer Polytechnic Institute. We highlight its usefulness for detecting flow features such as vortex cores, vortex bursts, spiral vortex breakdowns, and vortex diffusion. The eigenvector method has also been used as a data analysis tool to guide an automatic mesh refinement program and improve the accuracy of a rotorcraft simulation.

Control of High-Incidence Vortical Flow on Double-Delta Wings Undergoing Sideslip

An experimental study of the vortical flow over a baseline double-delta wing model and a diamond-fillet double-delta wing model, both with sharp leading edges, was conducted in a water tunnel using the dye-injection technique at a nominal flow Reynolds number of 1.875 x 10 4 . The diamond fillets at the strake/wing junction increased the wing area of the baseline model by 1%. The main focus of this study was to evaluate the effect of juncture fillets on the vortex core trajectory and the vortex burst location at high angles of attack with sideslip angle. Comparison of the test results between the baseline and the diamond-fillet models indicates a clear delay for the latter model in terms of both the vortex core trajectory, and the breakdown location at high angle of attack, with sideslip angles. The vortex breakdown data for the diamond-fillet model implies lift augmentation during sideslip motion, thus supporting the concept of flow control using fillets. From the maneuvering viewpoint, the data suggest that the diamond-fillet model has a better operational envelope.

Experimental investigation of vortex dynamics on a 65° delta wing in sideslip

Abstract The flow structure on the upper side of a delta wing is extremely complex. At moderate angle of attack the flowfield is dominated by organised vortical flow structures emanating from the leading edge. The pressure distribution created on the wing surface by these leading edge vortices causes an increment of lift which may be a relevant percentage of the total wing lift, depending on sweep angle.Delta wing performance is conditioned, however, by a phenomenon known as vortex breakdown or vortex bursting which appears at high angle of attack. This leads to a drastic change in the flowfield which influences the trend of the aerodynamic coefficients.With the aim of giving a contribution to the understanding of the phenomenon of vortex breakdown, a 65° delta wing has been extensively tested in the low speed windtunnel of Politecnico di Torino (TPI) both in static and dynamic conditions, under forced oscillatory motions.Many experiments have been carried out in an effort to understand the factors which can affect the breakdown by varying angle of attack, sideslip angle and Reynolds number.Flow visualizations have been performed using helium bubble tracers. This technique showed a very good capability for visualizing vortical flows and breakdown, both in static and dynamic conditions.For the static case, pressure distributions are presented, correlated to force measurements and flow visualizations, at different angles of attack and sideslip for different flow conditions.For the dynamic case force measurements are shown, compared with flow visualizations, for different angles of attack and for different oscillation frequencies.

Delta wing vortex manipulation using pulsed and steady blowing during ramp-pitching

The effectiveness of steady and pulsed blowing as a method of controlling delta wing vortices during ramp pitching has been investigated in flow visualization experiments conducted in a water tunnel. The recessed angled spanwise blowing technique was utilized for vortex manipulation. This technique was implemented on a beveled 60 delta wing using a pair of blowing ports located beneath the vortex core at 40% chord. The flow was injected primarily in the spanwise direction but was also composed of a component normal to the wing surface. The location of vortex burst was measured as a function of blowing intensity and pulsing frequency under static conditions, and the optimum blowing case was applied at three different wing pitching rates. Experimental results have shown that, when the burst location is upstream of the blowing port, pulsed blowing delays vortex breakdown in static and dynamic cases. Dynamic tests verified the existence of a hysteresis effect and demonstrated the improvements offered by pulsed blowing over both steady blowing and no-blowing scenarios. The application of blowing, at the optimum pulsing frequency, made the vortex breakdown location comparable in static and ramp pitch-up conditions.

Pressure measurements on a half delta wing oscillating in pitch

AbstractA rigid, 55° sweep, half delta wing has been oscillated in pitch at subsonic speeds, and the unsteady pressures on both the upper and lower surfaces recorded for pre-stalled conditions. The oscillations were of low amplitude and at frequencies equivalent to a typical wing first bending mode for full scale applications.When compared to a quasi-steady approximation, the unsteady pressures on the upper surface of the wing lag the steady case along the line of the primary attachment. The lag represents a constant convective time from the leading-edge with increasing frequency of oscillation. A further localised area of lagging flow is observed beneath the vortex burst point, the location of which is a function of mean angle of attack.The magnitude of the unsteady pressure variations was seen to increase linearly with the amplitude of the pitching oscillation while the phase lag was seen to increase linearly with frequency parameter.

Kill Probability in Antiaircraft Firing Theory

The control parameter Kf in Eq. (9) is the only variable needed to fine-tune the vortex-burst predictions. It was adjusted for the 60-deg delta wing to achieve onset of vortex burst at the wing trailing edge at a = 13 deg. A value of 2.6 for Kf gave this result. When this same value of Kf was used for VBM/VLM computations over a range of angles of attack for each of the wing shapes tested, the vortex burst locations shown in Fig. 1 resulted. Wind-tunnel values for burst locations on similar models from Refs. 8 and 9 are plotted in Fig. 2 for comparison. Note the good agreement between the model and wind-tunnel data for all shapes tested.

Pitch rate and pitch-axis location effects on vortex breakdown onset

Computational results are presented for the onset of vortex breakdown above a 75-deg sweep delta wing subject to a ramp-type pitch maneuver to high angle of attack. The flows are simulated by solving the full threedimensional unsteady NavierStokes equations on a moving grid using the implicit Beam-Warming algorithm. An assessment of the effects of numerical resolution, and comparison with experiment are employed to validate the computational approach. The effects of pitch rate and pitch-axis location are examined. For a range of these parameters, it is found that either increasing pitch rate or moving the axis downstream results in a larger angular delay of vortex bursting. These effects are correlated with the motion-induced effective incidence along the wing. For fixed pitch rate, the change in pivot location is equivalent to a shift in time without significant alteration of the flow structure.

Numerical analysis of the vortical flow around a delta wing-canard configuration

In this artical a numerical study of the vortical flows around a delta-shaped wing, wiht and without canard, is undertaken by solving the Euler equations. The analysis elucidates the essential physics that are involved in establishing the complex vortical flows around the configurations, especially with the required radial pressure gradients that provide the accelerating forces for spiraling. Further emphasis is put in studying the flow gradients arising over the wing surface and along the vortex axis. An interesting physical feature is the occurrence of two saddle points on the vortex axis at high angles of incidence, one causing flow reversal and the other the vortex bursting. The beneficial effect of the canard vortex on the wing vortex has been clearly demonstrated, it leads to a significant retardation of the vortex bursting. All essential experimental findings known until now have been closely reproduced, and thus, confirmed and.

Delta wing vortex control via recessed angled spanwise blowing

A novel vortex control technique, known as recessed angled span wise blowing, has been investigated in a water tunnel on a beveled 60-deg delta wing. Three pairs of blowing ports, located on the suction side of the wing and beneath the vortex core, were canted upward in the spanwise direction such that the injected flow is parallel to the beveled edge. Flow visualization experiments were performed using dye injected into the leadingedge vortices near the wing apex with the vortex burst location visually observed. Blowing coefficient values up to 0.1 were used in the experiments, although the effectiveness of blowing at a single chord location remained the same above blowing coefficients of 0.05. Maximum improvement in vortex breakdown location of 15% chord was obtained at an angle of attack of 22 deg and blowing coefficient of 0.05. Blowing from a port downstream of the natural burst location delayed vortex breakdown, whereas blowing upstream of the natural burst prompted vortex breakdown at the blowing port. Blowing ports at 41 % chord had the greatest effect in delaying vortex breakdown. These observations are explained via Rayleigh's stability criterion for circular flow.

Compressible inviscid vortex flow of a sharp edge delta wing

An analysis is presented on the compressible inviscid vortex flow over three delta wings with sharp leading edges with leading edge sweep angles φ=60, 70, and 76 deg using numerical solutions of the Euler equations. Presentations of results are given for Mach numbers ranging from M∞=0.1 to 0.8 and for angles of attack up to the onset of vortex breakdown. The paper focuses the attention on the effects of the vortex flow on the flowfield of a delta wing. The occurrence of shock waves of two types, crossflow and terminating (or rear) shocks in the vortical flowfields, are investigated; the possibility of shock-induced vortex breakdown and the effects of compressibility on the rapid performance degradation of delta wings after vortex bursting are studied in detail. It is shown that the onset of the vortex bursting at the wing trailing edge is only weakly influenced by the freestream Mach number. The upstream progression of the vortex bursting, however, is faster for supersonic vortex cores. For transonic flows, terminating shocks are observed downstream of vortex bursting. Crossflow shocks, however, may already develop for low-subsonic freestream Mach numbers. Also, it is found that the core flow is already compressible for freestream Mach numbers M∞≃0.1 and that significant amounts of entropy and circulation are generated within the core

Analysis of vortex bursting utilizing three-dimensional laser measurements

From the analysis of experimental three-dimensional laser velocimeter data of a vortex burst condition on a generic fighter configuration, a criterion for vortex breakdown has been evaluated. The measured velocity vectors were rotated from the tunnel coordinate system to the vortex core trajectory to examine the relevant parameters for vortex instability. The Rossby parameters that govern stability were formulated in terms of the local vortex flow variables. The ratio of axial-to-crossflow energy evaluated from the data encompassing a region bounded by the vorticity provides for a parameter that is consistent with Ludwieg's spiral instability analysis for conical flow. Benjamin's analysis for supercritical flow transition is correlated with a parameter for the mean flow vortex variables. The magnitudes of the critical parameters decrease downstream from the adverse pressure gradient along the vortex trajectory providing a threshold that correlates with the vortex breakdown position.

Numerical simulation of steady and unsteady, vorticity-dominated aerodynamic interference

The problem of modeling steady and unsteady aerodynamic interference is discussed. A configuration resembling the X-29 is used as an example. The general unsteady vortex-lattice method is used to model the flowfield. By considering the components operating alone and as members of the complete configuration, we demonstrate the importance of accurately simulating the wakes of the upstream components. The wakes of the canards as well as the canards themselves have a strong negative influence on the lift generated by the main wing; the main wing has a positive influence on the lift generated by the canards. The forward sweep of the main wing tends to focus the generated upwash in the vicinity of the canards. It is shown that the maximum influence of the vorticity shed from the canards does not develop until the shed vorticity convects downstream directly over the main wing. The general unsteady vortex-lattice method appears to be a reasonably accurate model of closely coupled, vorticity-dominated flowfields as long as the lines of the separation are known and vortex bursting does not occur near the wings.

Forebody tangential slot blowing on an aircraft geometry

The effect of forebody tangential slot blowing on the flowfield about an F/A-18 aircraft is investigated numerically using solutions of the Navier-Stokes equations. Computed solutions are obtained for an aircraft geometry which includes the fuselage, a wing with deflected leading-edge flap, empennage, and a faired-over engine inlet. The computational slot geometry corresponds to that used in full-scale wind-tunnel tests. Solutions are computed using flight test conditions and jet mass flow ratios equivalent to wind-tunnel test conditions. The effect of slot location is analyzed by computing two nontime-accurate solutions with a 16-in. slot located 3 in. and 11 in. aft of the nose of the aircraft. These computations resolve the trends observed in the full-scale wind-tunnel test data. The flow aft of the leading-edge extension vortex burst is unsteady. A time-accurate solution is obtained to investigate the flow characteristics aft of the vortex burst, including the effect of blowing on tail buffet.