Lift Enhancement Research Articles

Effect of Tip Vortices in Low-Reynolds-Number Poststall Flow Control

We numerically investigate the application of steady blowing to three-dimensional stalled flows around low-aspect-ratio rectangular flat-plate wings at a Reynolds number of 300. The objective of this study is to explore techniques to enhance lift by directly modifying the dynamics of the wake vortices. Out of various combinations of forcing location and direction considered, we identify two configurations that provide significant lift enhancement. In these cases, actuation appears to strengthen the tip vortices for increased downward induced velocity upon the leading-edge vortices. This in turn moves the low-pressure core directly above the top surface of the wing to greatly enhance lift.

Influence of Finite Span and Sweep on Active Flow Control Efficacy

Active flow control efficacy was investigated by means of leading-edge and flap-shoulder zero mass-flux blowing slots on a semispan wing model that was tested in unswept (standard) and swept configurations. On the standard configuration, stall commenced inboard, but with sweep the wing stalled initially near the tip. On both configurations, leading-edge perturbations increased CL,max and post stall lift, both with and without deflected flaps. Without sweep, the effect of control was approximately uniform across the wing span but remained effective to high angles of attack near the tip; when sweep was introduced a significant effect was noted inboard, but this effect degraded along the span and produced virtually no meaningful lift enhancement near the tip, irrespective of the tip configuration. In the former case, control strengthened the wingtip vortex; in the latter case, a simple semi-empirical model, based on the trajectory or streamline of the evolving perturbation, served to explain the observations. In the absence of sweep, control on finite-span flaps did not differ significantly from their nominally twodimensional counterpart. Control from the flap produced expected lift enhancement and CL,max improvements in the absence of sweep, but these improvements degraded with the introduction of sweep.

Experimental Investigation of Lift Enhancement and Roll Control Using Plasma Actuators

An experimental investigation into the use of trailing-edge-mounted plasma actuators for lift enhancement and roll control is described. Data are presented showing the effect that the actuators have on lift, pitch, and roll moment coefficients as a function of angle of attack and wind speed. The results indicate that the performance of the actuators is strongly influenced by Reynolds number effects. The implications of the findings on the utility of plasma actuators as lift and roll-control devices are discussed.

Numerical Simulation of Full-Span Delta Wing Buffet at High Angle of Attack

In this work, the buffet of a full-span, highly flexible, 50-deg-sweep delta wing at high angle of attack is studied using a computational aeroelastic solver. The aeroelastic solver couples a second-order finite difference solution of the Euler equations to a large-rotation nonlinear finite element structural solver. Particular attention is paid to the poststall region, in which previous experiments on highly flexible low-sweep delta wings have noted increased buffet response accompanied by lift enhancement and a delay in stall. It is thought that these phenomena are due to the reorganization of the flow and reformation of a leading-edge vortex structure. The nature of this enhanced lift is studied here for the flexible delta wing at an angle of attack of 25 deg and a flow velocity of 30 m/s. Using a prescribed wing motion, it is shown that it is possible to predict lift enhancement by solving only the inviscid Euler equations. The enhanced lift is due to a reorganization of the flow and the resulting region of increased suction near the apex of the wing. It is found that the mode of wing vibration has little influence on the enhanced lift phenomena, because both a prescribed symmetric first-mode motion and a prescribed antisymmetric third-mode motion led to lift enhancement. It is also insensitive to the wing vibration frequency for the range of frequencies tested, with the concession that below some minimum frequency, lift enhancement does not occur. The amplitude of wing vibration has some effect on the time-averaged lift coefficient. Fully coupled aeroelastic computations were also performed in this study for the wing. The fully aeroelastic computation did not predict the enhanced structural dynamic behavior and lift that was observed in the experiment. The dominant dynamic response of the wing was near the first mode, which has a frequency that is too low to initiate flow organization and the resulting enhanced lift due to increased suction.

Stochastic modeling of random roughness in shock scattering problems: Theory and simulations

Random roughness is omnipresent in engineering applications and may often affect performance in unexpected way. Here, we employ synergistically stochastic simulations and second-order stochastic perturbation analysis to study supersonic flow past a wedge with random rough surface. The roughness (of length d) starting at the wedge apex is modeled as stochastic process (with zero mean and correlation length A) obtained from a new stochastic differential equation. A multi-element probabilistic collocation method (ME-PCM) on sparse grids is employed to solve the stochastic Euler equations while a WENO scheme is used to discretize the equations in two spatial dimensions. The perturbation analysis is used to verify the stochastic simulations and to provide insight for small values of A, where stochastic simulations become prohibitively expensive. We show that the random roughness enhances the lift and drag forces on the wedge beyond the rough region, and this enhancement is proportional to ( d / A ) 2 . The effects become more pronounced as the Mach number increases. These results can be used in designing smart rough skins for airfoils for maximum lift enhancement at a minimum drag penalty.

Management of Irregular Astigmatism Following Rotationally Disoriented Free Cap After LASIK

To illustrate the challenges associated with a misaligned free flap (cap) and to report the outcome of applying wavefront-guided customized photorefractive keratectomy (PRK) followed by conventional PRK ablation to correct residual refractive error and aberrations after LASIK free cap complications. The clinical course and surgical interventions of two patients with free cap complications from LASIK surgery were reviewed. The first patient underwent a total of six interventions after the initial LASIK procedure, and the second patient underwent a total of five interventions. Interventions included cap lift, cap rotation, custom PRK, and conventional PRK enhancement with prophylactic topical mitomycin C (MMC). Customized PRK treatment and subsequent enhancements with prophylactic topical MMC led to the recovery of best spectacle-corrected visual acuity (BSCVA), neutralization of higher and lower order aberrations, and astigmatic neutrality. Symptoms related to higher order aberrations resolved in both patients. Wavefront-guided custom PRK for higher order aberrations followed by conventional PRK enhancement for residual lower order aberrations, both with topical MMC application, represents an efficacious strategy for treating patients with loss of BSCVA and visual symptoms due to LASIK free cap complications.

Lift Enhancement over Flexible Nonslender Delta Wings

Unsteady aerodynamics of flexible nonslender delta wings is investigated in an experimental study using various techniques. Dramatic fluid/structure interactions emerge with increasing wing flexibility and result in substantial lift enhancement in the poststall region. This recently discovered phenomenon appears to be a feature of nonslender wings. Self-excited antisymmetric vibrations of the wing promote reattachment of the shear layer, which results in the lift enhancement. These self-excited vibrations are not observed for a half-model. The Strouhal number of the dominant frequency of the structural vibration is on the order of unity for all nonslender wings, which also corresponds to the frequency of the shear-layer instabilities for the rigid wing. Velocity measurements demonstrate the striking difference between the flows over the flexible and rigid wings in the poststall region. The effect of flexibility is to promote the reattachment of the shear layer near or downstream of the apex, depending on the incidence. There are substantial effects on the vortical flow with increasing wing flexibility, which might lead to the axial flow developing within the reattached region. The time-averaged vorticity flux increases due to the oscillating leading edge, which leads to increased circulation.

Gurney flap—Lift enhancement, mechanisms and applications

Abstract Since its invention by a race car driver Dan Gurney in 1960s, the Gurney flap has been used to enhance the aerodynamics performance of subsonic and supercritical airfoils, high-lift devices and delta wings. In order to take stock of recent research and development of Gurney flap, we have carried out a review of the characteristics and mechanisms of lift enhancement by the Gurney flap and its applications. Optimum design of the Gurney flap is also summarized in this paper. For the Gurney flap to be effective, it should be mounted at the trailing edge perpendicular to the chord line of airfoil or wing. The flap height must be of the order of local boundary layer thickness. For subsonic airfoils, an additional Gurney flap increases the pressure on the upstream surface of the Gurney flap, which increases the total pressure of the lower surface. At the same time, a long wake downstream of the flap containing a pair of counter-rotating vortices can delay or eliminate the flow separation near the trailing edge on the upper surface. Correspondingly, the total suction on the airfoil is increased. For supercritical airfoils, the lift enhancement of the Gurney flap mainly comes from its ability to shift the shock on the upper surface in the downstream. Applications of the Gurney flap to modern aircraft design are also discussed in this review.

Lift Enhancement by Static Extended Trailing Edge

A static extended trailing edge attached to a NACA0012 airfoil section is studied for achieving lift enhancement at a small drag penalty. It is indicated that the thin extended trailing edge can enhance the lift, whereas the zero-lift drag is not significantly increased. Experiments and calculations are conducted to compare the aerodynamic characteristics of the extended trailing edge with those of the Gurney flap and the conventional flap. The extended trailing edge, as a simple mechanical device added on a wing without altering the basic configuration, has a good potential to improve the cruise flight efficiency.

Experimental Studies on Unsteady Lateral Blowing on NACA 0012

T HE initial study of steady lateral blowing was conducted by Dixon in 1969 [1]. The mechanism of this concept is analogous to the flow over slender delta wing platforms, which produces a leading-edge vortex and nonlinear lift curves. Steady lateral blowing might be thought of as providing the sweep effects such as those of the delta wings, for which the effective sweep is a function of the jet momentum. The lateral blowing jet can be described in terms of the chordwise position along the airfoil section, the nozzle height, and the nozzle size. Dixon et al. [2] andClarke [3] suggested that the best vertical nozzle positionmay be a function of the nozzle diameter and that the jet from a nozzle placed too near the airfoil surface may have a deleterious effect on the flow over the airfoil. Wong and Kontis [4] performed a comprehensive study on the steady spanwise blowing on a NACA 0012 airfoil section. The force measurements showed that the lift coefficient increasedwhen the steady lateral blowingwas applied, and the most effective blowing location was found to be at x=c 0:25 (x is the chordwise location and c is the airfoil chord), because the lift augmentation ratio Cl=C was always above one for all positive angles of attack [ Cl is the increment in the lift coefficient due to blowing, andC is the blowing momentum coefficient; the definition ofC is shown inEq. (1)].However, largemomentumwas required to generate the lift enhancement on the airfoil and this became a major issue in the effectiveness of blowing. As a consequence, Meyer and Seginer [5] performed some initial tests for generating the same lift increase using lower momentum through pulsed blowing. One of the main disadvantages of lateral blowing is due to the fact that air for blowing is bled from the compressor stage of the engine. This reduces the amount of air for combustion, hence reducing thrust. Unsteady blowing provides the beneficial effect of reducing the mass-flow requirement from the engine. Another adverse effect of the technology arises from the need to install plumbing, control valves, regulators, oscillators, etc., to supply and control the compressed air from the engine. This adds weight to the aircraft, and a higher lift is required. C thrust

Dual Location Separation Control on a Semispan Wing

Separation is controlled on a semispan wing via nominally two-dimensional zero mass-flux perturbations that are introduced at 1) the leading edge, 2) the shoulder of deflected flaps (20-40 deg deflection), and 3) both locations simultaneously. Leading-edge perturbations mimic the effect of a leading-edge device and flap-shoulder perturbations simulate additional flap deflection while simultaneously reducing drag. When perturbations are introduced simultaneously, at reduced frequencies of O(1) at poststall angles of attack, their combined effect on the wing's lift exceeds each individual contribution. When the physical frequencies are the same, lift is strongly dependent on the difference in phase between the two perturbations and an optimum is reached when the suction portion of flap-shoulder control coincides with the minimum shear layer proximity to the flap surface. At the optimum phase difference, increases in lift are observed over the entire semispan, resulting in an overall increase in the wing maximum lift coefficient between 0.24 and 0.44 depending on the flap deflection configuration and angle. Flap-shoulder control effectiveness does not diminish at the extremities of the finite inboard flap; neither inboard in the vicinity of the junction vortex, nor outboard in the vicinity of the flap-edge vortex. With a large flap deflection across the entire span, flap-shoulder control perturbations produce a powerful tip vortex that may be exploited for lift enhancement on low aspect ratio wings. The sensitivity to perturbation phase difference, however, diminished significantly toward the wing tip.

Thin-Airfoil Theoretical Interpretation for Gurney Flap Lift Enhancement

Thin-airfoil theory is applied to the lift problem of an airfoil with a Gurney flap. The lift and pitching moment coefficient increments are given as a square-root function of the relative Gurney flap height, and they are proportionally related. This model interprets the Gurney flap lift enhancement as a special camber effect. The theoretical relations are in good agreement with experimental and numerical data for several different wings. The theoretical method developed in this paper can be applied to similar trailing-edge devices for lift enhancement, and it is useful in the preliminary design of these flow control devices.

Two-Dimensional Wind Tunnel and Computational Investigation of a Microtab Modified Airfoil

*† ‡ A computational and wind tunnel investigation into the effectiveness of a microtab-based aerodynamic load control system is presented. The microtab-based load control concept consists of a small tab, with a deployment height on the order of 1% of chord, which emerges approximately perpendicular to a lifting surface in the vicinity of the trailing edge. Lift mitigation is achieved by deploying the tabs on the upper (suction) surface of a lifting surface. Similarly, lift enhancement can be attained by tab deployment on the lower (pressure) surface of a lifting surface. A sensitivity analysis using Reynolds-averaged NavierStokes methods was conducted to determine optimal sizing and positioning of the tabs for active load control at a chord Reynolds number of 1.0 million for the S809 baseline airfoil. These numerical simulations provide insight into the flow phenomena that govern this promising load control system and guided tab placement during the wind tunnel study of the S809 airfoil. The numerical and experimental results are largely in agreement and demonstrate that load control through microtabs is viable. Future efforts will include a study of the unsteady load variations that occur during tab deployment and retraction, and three-dimensional issues involving spanwise tab placement and tab gaps.

Synthetic Jet Control Effectiveness on Stationary and Pitching Airfoils

A computational study has been conducted to investigate the control effectiveness of a synthetic jet on a stationary and a pitching NACA 0015 airfoil at a Reynolds number of 360,000 based on the chord length. For the stationary airfoil, the optimum location of the synthetic jet was at 28% of the chord length. The nondimensional maximum forcing amplitude was 0.75 and the nondimensional forcing frequency was 18.67. The increase in lift was more pronounced at higher incidences, whereas the effectiveness of the synthetic jet reduced at lower incidences. Imparting movement of flow far away from separation point was useful for lift enhancement. The location of the jet close to separation point was suitable for drag reduction. The pitching airfoil was driven in a periodic cycle corresponding to α = 5 + 10 sin(0.15t). A synthetic jet with nondimensional maximum forcing amplitudes of 0.75 and 1.88, and nondimensional forcing frequencies of 18.67 and 31.13 was employed. The higher frequency was suitable for lift enhancement whereas the higher amplitude was suitable for drag reduction. A single synthetic jet was not able to suppress the vortex formation and shedding; however, the overall airfoil performance was enhanced. The k-w shear-stress transport model successfully predicted the flow events of both airfoil configurations.

Interaction of Synthetic Jet Propulsion with Airfoil Aerodynamics at Low Reynolds Numbers

The use of synthetic jets for propulsion of fixed-wing Micro Air Vehicles (MAVs) is investigated experimentally in the low Reynolds number range. Interaction of a synthetic jet located at the trailing-edge of the wing with wing aerodynamics is studied with flow visualization and velocity measurements at low Reynolds numbers. The effects of excitation frequency, momentum coefficient, angle of attack, and Reynolds number were studied in detail. The separated region in the wake becomes smaller as the momentum coefficient is increased gradually. Once the momentum coefficient reaches a critical value, there is zero net drag. There is also reattachment of the separated shear layer near the trailing-edge, with expected enhancement in lift. Above the critical value of the momentum coefficient, the jet produces net thrust. Even at relatively higher incidences, zero net drag can be obtained and the reattachment of the shear layer to the upper wing surface is possible. The reattachment of the separated shear layer becomes easier with increasing Reynolds number. The favorable effect of increasing Reynolds number is most apparent at low Reynolds numbers. There is a strong effect of pulsing frequency and the results suggest an optimum range of Strouhal number St = 2 to 5. Also, preliminary experiments for a generic airfoil with internal actuator were carried out. Although the drag of this design is somewhat larger, zero net drag can be achieved when the synthetic jet is activated.

High-Lift Propulsive Airfoil with Integrated Crossflow Fan

A concept for embedding a crossflow fan into a thick wing for lift enhancement and thrust production is proposed. The design places a crossflow fan propulsion system with raised inlet near the trailing edge of the wing. Flow is drawn in from the suction surface, energized, and expelled out the trailing edge. The integration of a crossflow fan within a modified Gottingen 570 airfoil section with 34% thickness to chord ratio is developed and simulated using the commercial CFD software Fluent. Unsteady sliding mesh calculations are used to visualize the flowfield and calculate fan performance and airfoil lift coefficient. The results of the CFD work show that the jet leaving the fan fills up the wake behind the airfoil, whereas the suction effect produced by the fan virtually eliminates flow separation at high angle of attack, yielding very high-lift coefficients. A system level analysis demonstrates the benefits of using an embedded crossflow fan for distributed aircraft propulsion. The system analysis yields tradeoffs between various design parameters and provides a basis for preliminary crossflow fan airfoil design.

Experimental investigations on the application of lift enhancement devices to forward-swept aircraft model

AbstractThe force measurements were conducted in low speed wind tunnel to investigate the effects of the scale, shape and the installation type of Gurney flap on a forward-swept aircraft model. The results indicated that both rectangular and triangular Gurney flaps can enhance the lift coefficient of the model tested, but with a little decrease of stall angle from 38° to 36°. The lift and drag coefficients increased with the Gurney flap scales. Meanwhile, the triangular Gurney flap can improve the aerodynamic performance more effectively when its high side is located near the wing root than the reverse installation with the low side near the wing root and the high side near the wing tip. Additionally, for the same Gurney flap, the model with smaller forward-swept angle can generate higher lift-enhancement in comparison with the larger forward-swept angle model.

Identification of new lift generation mechanism in flapping flight

In this paper the combined effect of two mechanisms for lift enhancement at low Reynolds numbers are considered, wing oscillations and wing flexibility. The force, deformation and flow fields of rigid and flexible low aspect ratio (AR=3) and high aspect ratio (AR=6) wings oscillating at a fixed post-stall angle of attack of 15° and amplitude of 15% of chord are measured. The force measurements show that flexibility can increase the time-averaged lift coefficient significantly. For low aspect ratio wings the maximum lift coefficient across all Strouhal numbers was Cl=1.38 for the rigid wing as opposed to Cl=2.77 for the flexible wing. Very similar trends were observed for the high aspect ratio wings. This increase is associated with significant deformation of the wing. The root is sinusoidally plunged with small amplitude but this motion is amplified along the span resulting in a larger tip motion but with a phase lag. The amount it is amplified strongly depends on Strouhal number. A Strouhal number of Src=1.5 was selected for detailed flow field measurements due to it being central to the high-lift region of the flexible wings, producing approximately double the lift of the rigid wing. For this Strouhal number the rigid wings exhibit a Leading Edge Vortex (LEV) ring. This is where the clockwise upper-surface LEV pairs with the counter-clockwise lower-surface LEV to form a vortex ring that self-advects upstream and away from the wing's upper surface. Conversely the deformation of the flexible wings inhibits the formation of the LEV ring. Instead a strong upper-surface LEV forms during the downward motion and convects close to the airfoil upper surface thus explaining the significantly higher lift. These measurements demonstrate the significant gains that can be achieved through the combination of unsteady aerodynamics with flexible structures.

The aerodynamic effects of wing–wing interaction in flapping insect wings

We employed a dynamically scaled mechanical model of the small fruit fly Drosophila melanogaster (Reynolds number 100-200) to investigate force enhancement due to contralateral wing interactions during stroke reversal (the ;clap-and-fling'). The results suggest that lift enhancement during clap-and-fling requires an angular separation between the two wings of no more than 10-12 degrees . Within the limitations of the robotic apparatus, the clap-and-fling augmented total lift production by up to 17%, but depended strongly on stroke kinematics. The time course of the interaction between the wings was quite complex. For example, wing interaction attenuated total force during the initial part of the wing clap, but slightly enhanced force at the end of the clap phase. We measured two temporally transient peaks of both lift and drag enhancement during the fling phase: a prominent peak during the initial phase of the fling motion, which accounts for most of the benefit in lift production, and a smaller peak of force enhancement at the end fling when the wings started to move apart. A detailed digital particle image velocimetry (DPIV) analysis during clap-and-fling showed that the most obvious effect of the bilateral ;image' wing on flow occurs during the early phase of the fling, due to a strong fluid influx between the wings as they separate. The DPIV analysis revealed, moreover, that circulation induced by a leading edge vortex (LEV) during the early fling phase was smaller than predicted by inviscid two-dimensional analytical models, whereas circulation of LEV nearly matched the predictions of Weis-Fogh's inviscid model at late fling phase. In addition, the presence of the image wing presumably causes subtle modifications in both the wake capture and viscous forces. Collectively, these effects explain some of the changes in total force and lift production during the fling. Quite surprisingly, the effect of clap-and-fling is not restricted to the dorsal part of the stroke cycle but extends to the beginning of upstroke, suggesting that the presence of the image wing distorts the gross wake structure throughout the stroke cycle.

A computational fluid dynamics of `clap and fling' in the smallest insects

In this paper, we have used the immersed boundary method to solve the two-dimensional Navier-Stokes equations for two immersed wings performing an idealized 'clap and fling' stroke and a 'fling' half-stroke. We calculated lift coefficients as functions of time per wing for a range of Reynolds numbers (Re) between 8 and 128. We also calculated the instantaneous streamlines around each wing throughout the stroke cycle and related the changes in lift to the relative strength and position of the leading and trailing edge vortices. Our results show that lift generation per wing during the 'clap and fling' of two wings when compared to the average lift produced by one wing with the same motion falls into two distinct patterns. For Re=64 and higher, lift is initially enhanced during the rotation of two wings when lift coefficients are compared to the case of one wing. Lift coefficients after fling and during the translational part of the stroke oscillate as the leading and trailing edge vortices are alternately shed. In addition, the lift coefficients are not substantially greater in the two-winged case than in the one-winged case. This differs from three-dimensional insect flight where the leading edge vortices remain attached to the wing throughout each half-stroke. For Re=32 and lower, lift coefficients per wing are also enhanced during wing rotation when compared to the case of one wing rotating with the same motion. Remarkably, lift coefficients following two-winged fling during the translational phase are also enhanced when compared to the one-winged case. Indeed, they begin about 70% higher than the one-winged case during pure translation. When averaged over the entire translational part of the stroke, lift coefficients per wing are 35% higher for the two-winged case during a 4.5 chord translation following fling. In addition, lift enhancement increases with decreasing Re. This result suggests that the Weis-Fogh mechanism of lift generation has greater benefit to insects flying at lower Re. Drag coefficients produced during fling are also substantially higher for the two-winged case than the one-winged case, particularly at lower Re.