Lift-induced Drag Research Articles

Surface roughness effects on transonic aircraft performance: Experimental/numerical comparisons

This work highlights the necessity of taking into account surface roughness when conducting experimental tests, and when using numerical simulations to precisely calculate the turbulent lift and drag of wind-tunnel models or real aircraft in transonic conditions. The present article is a continuation of “Turbulent drag induced by low surface roughness at transonic speeds: Experimental/numerical comparisons,” Physics of Fluids, Vol. 32, 045108 (2020) by Hue and Molton. The outcomes of this former study, which was focused on flat plate samples, are here applied to a three-dimensional aircraft configuration: the Common Research Model used as a reference in the recent international Drag Prediction Workshops. Experimental campaigns have been performed in the largest ONERA wind tunnels S1MA and S2MA involving models with average surface roughness heights Ra close to 0.5 micrometers, wingspans up to 3.5 meters, Mach and Reynolds numbers up to 0.95 and 5 million respectively. Reynolds-averaged Navier–Stokes computations based on the wind-tunnel tests have then been carried out, using the equivalent sand-grain roughness height approach as well as a Musker-type correlation to determine relevant ks values. The results of both the experimental and numerical campaigns have demonstrated that the aerodynamic coefficients of the aircraft can be significantly affected by the surface roughness, even with roughness Reynolds numbers ks+ potentially below the usual threshold values sometimes considered in engineering applications (i.e. in the order of 3.5 to 5). In particular, the surface roughness effects on lift and drag have been studied using far-field analyses to evaluate the responses of friction, viscous pressure, wave and lift-induced drag components. Finally, the numerical studies have been extended to the full-scale geometry in flight conditions in order to assess the roughness effects and potential gains in realistic aircraft operating conditions.

Aerothermal and aerodynamic characteristics of reusable hypersonic vehicles with heat transfer minimised sweepback

AbstractEstablishing aerothermal criteria for swept leading-edge hypersonic vehicle design is the predominant purpose of this work. This study is focused on two different configurations of vehicles based on the swept-back angle ( $\varLambda$ ) viz. minimum drag ( $\varLambda$ Drag-min), and minimum heat transfer to vehicle ( $\varLambda$ HT-min). Maximum wall temperatures obtained from the simulation performed in ANSYS 2020 with the k-epsilon turbulence model are 1,013 and 970K for $\varLambda$ Drag-min and $\varLambda$ HT-min, respectively. These temperatures are used to obtain the corresponding thicknesses of thermal protection systems to maintain inner wall temperature at 323K. Further study is divided into two cases depending on the direction of thickness of thermal protection system with respect to vehicle body. For constant payload capacity, the direction of thickness is outside; whereas for constant overall volume case, direction of thickness is inside. For constant payload volume case, the percentage weight reduction of thermal protection system is 4.8%. For constant overall volume case, the percentage payload capacity increases with design at $\varLambda$ HT-min by 4.04% in addition to thermal protection system weight reduction. The lift-induced drag on vehicles with design at $\varLambda$ HT-min is significantly reduced for both cases, by 47.68% (for constant payload volume) and 45.27% (for constant overall volume).

A deep learning framework for reconstructing experimental missing flow field of hydrofoil

Hydrofoils play a crucial role in enhancing the efficiency of fluid machinery designed for ocean environments, reducing lift-induced drag and contributing to improved overall performance. To optimize hydrofoil design, a profound comprehension of the complex fluid flows around the hydrofoil structure is essential. In fluid mechanics, a precise and continuous representation of flow dynamics is essential for analysis and control purposes. Thus, obtaining a complete flow field, either through computational fluid dynamics (CFD) or experimental testing is of great significance. Nevertheless, the rigorous requirements of flow field tests render it impractical to directly measure complete flows around the airfoil using current instrumentation, especially for those with complex physical geometries. To tackle this issue, a novel deep learning framework is proposed to reconstruct the complete flow field by leveraging incomplete complementary flow data. As the representative benchmark problems, the flows around the Clark-Y hydrofoil at Re = 7 × 105 and the experimental NACA0012 2D hydrofoil at Re = 1800 have been investigated under different missing-flow scenarios of varying proportions, locations and orientations. Results demonstrate a remarkable agreement between the reconstructed flow field and the ground truth data, indicating the excellent performance of the proposed deep-learning model for missing flow reconstruction. A sensitivity analysis assesses the impact of the snapshot number and the latent space, revealing the method's robustness in selecting these parameters and simplifying its implementation in practical applications. This deep learning method offers the advantage of being implemented using paired incomplete flow fields, without the need for pre-known ground truth results as labels, holding the potential for addressing more complex full-field reconstruction problems in the future.

Effect of Tip-Mounted Propeller and Trailing-Edge Flap on Wing's Lift and Drag Coefficients and Vortical Wake

Abstract The lift and drag forces and vortical wake of a rectangular semiwing equipped with a tip-mounted propeller and a plain trailing-edge flap both outside and in ground effect were investigated experimentally at Re = 2.49 × 105. Outside the ground effect, the slipstream of the inboard-up propeller rotation attenuated the wingtip vortex, significantly decreasing the lift-induced drag. The propeller rotation also led to an increased maximum lift and stall angle. At a fixed lift condition, the propeller rotation led to a reduced drag. The deployment of the flap further increased the lift curve slope and maximum lift but at the expense of an increased drag and earlier stall as compared to the unflapped wing. In ground effect, the lift augmentation of the unflapped wing increased pronouncedly with propeller rotation as the ground was approached. For the flapped wing, the propeller rotation, however, produced a smaller lift increment than the unflapped wing due to the recirculation of the deflected slipstream into the propeller disk. The extent of this recirculation increased with flap deflection and propeller rotation, contributing to the inferior lift production of the flapped wing with a tip-mounted propeller in ground effect.

Lift and Tip Vortices Generated by Tapered Backward-Swept and Forward-Swept Wings under Stationary Ground Proximity

An experimental investigation of the aerodynamics and near-field tip-vortex flow field behind tapered backward- and forward-swept wings with a stationary ground effect was conducted at Reynolds number (Re)=1.81×105. The results showed a large lift increase of 26.7% and 12.3% for the backward-swept wing (BSW) and forward-swept wing (FSW), respectively, with reduced ground clearance, along with a significant drag reduction of 45% and 30% for the BSW and FSW. For the BSW, a multiple-vortex system appeared in close ground proximity, consisting of a tip vortex, a corotating ground vortex, and a counterrotating secondary vortex. The ground vortex strengthens the tip vortex, whereas the secondary vortex negates its vorticity. For the FSW, the multiple-vortex system was not readily identifiable due to its unique geometry, which always keeps the inboard region of the wing at a close ground effect while leaving the tip region less affected by the ground effect. The root stall of the FSW also produced a continuously strengthening tip vortex with the increasing angle of attack. In contrast, the tip stall of the BSW led to a monotonically increasing vortex strength only up to the static-stall angle. Regardless of the wing model, the weak tip vortex also translates into a small lift-induced drag compared with the total drag. Finally, the lift force computed through the integration of the spanwise circulation distribution, inferred from the cross-flow measurements, at selected ground distances was also found to be in good agreement with the direct wind-tunnel force-balance data, with 101% and 98% consistency for BSW and FSW, respectively. The aerodynamics and tip-vortex measurements of both wing models outside the ground effect were also acquired to serve as a comparison.

기동모델이 변침기동 탄도탄 추적에 미치는 영향에 대한 고찰

This paper addresses the maneuvering ballistic target tracking problem for advanced missile defense systems. Based on the fact that the performance degradation of the existing tracking filters is mainly caused by the uncertainties in motion model of a ballistic target, an explicit maneuver model is introduced by utilizing a prior knowledge on the lift-induced drag characteristics. This model exploits the constraints among the ballistic coefficients, which enables us to design a tracking filter with more reliable target tracking performance in practice. Moreover, since the mismatch between our explicit ballistic target maneuver model and the real target motion leads to the bias of the residual sequence of the tracking filter, this property can also be used for target classification or recognition. Through simulation using a typical ballistic target tracking scenario, it is demonstrated that the proposed filter provides superior and reliable maneuvering ballistic target tracking performance.

Lift Computation Through Crossflow Measurement Behind a Rectangular Semiwing in Ground Effect

Abstract The lift force of a rectangular semiwing in stationary ground effect was computed based on the spanwise circulation distribution inferred from wake crossflow measurement and was compared with the direct force balance data at Re = 2.48 × 105. The lift calculated was found to be in good agreement with the force-balance measurement for ground distances h larger than 10% of the airfoil chord c. In close ground proximity (h/c ≤ 10%), a large discrepancy however existed due to the appearance of a multiple vortex system consisting of a tip vortex, a corotating ground vortex, and a counter-rotating secondary vortex. The counter-rotating secondary vortex offset the vorticity of the tip vortex. By considering its contribution additive, the discrepancy observed between the lift forces computed and measured with the force balance in close ground proximity became marginal. The impact of ground effect on lift-induced drag was also discussed.

Review of experimental investigations of wings in ground effect at low Reynolds numbers

The ground effect-induced large lift increase and lift-induced drag reduction have long been recognized and utilized in the design and construction of wing-in-ground effect (WIG) craft. Various wing planforms have been employed in WIG craft. In this study, the experimental investigations of rectangular wings and delta wings of reverse and regular configurations at low Reynolds numbers are reviewed. For rectangular wings, both chord-dominated and span-dominated ground effects on the aerodynamics, tip vortex, and lift-induced drag are reviewed. For reverse delta wings, in addition to the experimental measurements of the aerodynamics and tip vortex flow at different ground distances, passive flow control utilizing Gurney flap, cropping, and anhedral are reviewed. The impact of ground effect on delta wings is also discussed. Suggestions for future investigations applicable to each wing planform in-ground effect are provided.

Investigation of power generation from river surface runoff with a novel tilted axis hydrokinetic turbine for off-grid sites

In this study, a novel tilted axis hydrokinetic turbine system is presented and tested to generate electricity from surface water streams in off-grid locations. This system consists of a turbine, differential, reducer, and alternator, and resembles a Pelton turbine. It is used in shallow waters near river banks with the free flow velocity. This study reports significant hydrodynamic efficiencies with field tests consistent with theoretical estimates obtained at the design and fabrication stage, for different flow velocities (0.5–3 m/s in 0.5 m/s internal) and blade numbers (7, 9, and 11 blades) on the Namnam Stream in the Muğla province/Turkey. On the same turbine system, 7, 9, and 11 blades can be disassembled in the center of the turbine. In order to facilitate testing at different water flow velocities, the flow velocity is adjusted by excavation operations in the stream bed. The results of the study indicate that as the water speeds increase, the turbine power, speed, and efficiency of all blade turbines also increase. In terms of hydrokinetic energy potential, the water flow velocity and turbine speed are 2 m/s and 3 r/min, respectively. Because of parasitic drag and lift-induced drag, 7- and 11-blade turbines have higher mechanical losses, lower turbine efficiency, and increased cost due to weight. A 9-blade turbine is preferred for such a prototype due to torque, load power (800 W), and efficiency (15.9%) compared to the others. As a result, this study reports that such a prototype can be used in unpowered riverbanks or free surface currents.

Performance comparison of rectangular nozzle with circular nozzle of military aircraft engine

Aircraft performance depends on its propulsion system, and the type of exhaust nozzle used in the aircraft plays a major role in jet propulsion. The exhaust nozzle of an aircraft is also the prominent source of infrared (IR) signature from the rear-view, especially in short-wave (1.9–2.9 μm) and mid-wave (3–5 μm) IR bands. Aircraft IR signatures (passive) are used for the detection and tracking by the IR-guided (heat-seeking) missiles. Alterations in engine exhaust geometry are important to reduce the IR emissions from the aircraft’s rear-view. Nozzle shape (e.g., two-dimensional exit, lobed nozzle, S-shaped nozzle) has an important influence on the aircraft and plume IR signature, especially from the rear-view. Rectangular nozzle exit with a high aspect ratio (AR > 5) has an advantage in terms of reduced IR signature, convenience in thrust vectoring, and ease in maneuverability. The wetted perimeter increases with an increase in AR of the rectangular nozzle exit, which results in an increase in the mixing of plume (hot exhaust gases) with the surrounding. This analytical study compares the aero-engine performance and visibility of a rectangular nozzle exit of different AR with that of the circular nozzle. The rectangular exit nozzles of different AR are designed using Solid Edge and their visibility is compared with a circular nozzle of same cross-sectional area, from the direct rear-view. This study shows that the visibility of hot engine parts decreases from the rear and side views with the increase in AR, which is important in reducing aircraft IR-signature. But results in increase in visibility from the bottom view, which is undesirable concerning for the surface to air missiles. The weights of rectangular exit nozzle of different AR are compared with that of the circular nozzle. It is found that the nozzle weight increases with an increase in AR, which increases the aircraft’s lift-induced drag. For performance analysis, the effect on specific thrust is obtained for different nozzle isentropic efficiency. For isentropic flow through nozzle, the results for specific thrust are compared with the values obtained from GasTurb software.

Invariant Vortex-Force Theory Extending Classical Aerodynamic Theories to Transonic Flows

Recent studies about the Lamb vector have led to the development of the vortex-force theory: a formulation able to predict the aerodynamic force in compressible flows and decompose it into lift, lift-induced drag, and profile drag. Here, a revised formulation of the vortex-force theory developed at ONERA in collaboration with the University of Naples is presented and applied to steady transonic flows. In the mathematical developments, special care is given to the presence of shock wave discontinuities within the flowfield. The equivalence between the new definition, the Kutta–Joukowski lift theorem, Maskell’s lift-induced drag formula (“Progress Towards a Method for the Measurement of the Components of the Drag of a Wing of Finite Span,” Procurement Executive, Ministry of Defence, Royal Aircraft Establishment TR 72232, Farnborough, England, U.K., 1972) and Betz’s profile drag formula (“A Method for the Direct Determination of Wing-Section Drag,” NACA TM 337, 1925) are emphasized. The revised formulation also presents several practical advantages: the decomposition is naturally invariant to the reference point chosen for the calculation of moment transformations, and the computation can be performed in a fine part of the grid, close to the body skin. The formulation is finally tested on the NASA Common Research Model wing–fuselage configuration in cruise flight conditions.

EFFECT OF VARIABLE CANT ANGLE ON SWEEPBACK WING

Winglets are commonly used drag-reduction and fuel-saving technologies in today’s aviation. The primary purpose of the winglets is to reduce the lift-induced drag, therefore improving fuel efficiency and aircraft performance. Traditional winglets are designed as fixed devices attached at the tips of the wings. However, because they are fixed surfaces, they give their best lift-induced drag reduction at a single design point. In this work, we propose the use of variable cant angle winglets which could potentially allow aircraft to get the best all-around performance (in terms of lift-induced drag reduction), at different angle-of-attack values.This paper describes a XFLR 3-dimensional winglets analysis that was performed on a wing of NACA2412 cross sectional airfoil. The wing has span 2.40 m, root chord 0.180 m, tip chord 0.110 m, sweep angle 5 degree and taper ratio 0.556 and for a winglet NACA 0024 cross-sectional airfoil was considered of max thickness 24 percent at 30 percent chord, max camber 0 percent at 0 percent chord. The present study shows effects of wing without winglet, wing with winglet at cant angle 30, 60, 90.The results obtained from the analysis demonstrate that by carefully adjusting the cant angle, the aerodynamic performance can be improved.

CFD analysis of variable geometric angle winglets

PurposeWinglets play a major role in saving fuel costs because they reduce the lift-induced drag formed at the wingtips. The purpose of this paper is to obtain the best orientation of the winglet for the Office National d’Etudes et de Recherches Aérospatiales (ONERA) M6 wing at Mach number 0.84 in terms of lift to drag ratio.Design/methodology/approachA computational fluid dynamics analysis of the wing-winglet configuration based on the ONERA M6 airfoil on drag reduction for different attack angles at Mach 0.84 was performed using analysis of systems Fluent. First, the best values of cant and sweep angles in terms of aerodynamic performance were selected by performing simulations. The analysis included cant angle values of 30°, 40°, 45°, 55°, 60°, 70° and 75°, while for the sweep angles 35°, 45°, 55°, 65° and 75° angles were used. The aerodynamic performance was measured in terms of the obtained lift to drag ratios.FindingsThe results showed that slight alternations in the winglet configuration can improve aerodynamic performance for various attack angles. The best lift to drag ratio for the winglet was achieved at a cant angle of 30° and a sweep angle of 65°, which caused a 5.33% increase in the lift to drag ratio. The toe-out angle winglets as compared to the toe-in angles caused the lift to drag ratio to increase because of more attached flow at its surface. The maximum value of the lift to drag ratio was obtained with a toe-out angle (−5°) at an angle of attack 3° which was 2.53% greater than the zero-toed angle winglet.Originality/valueThis work is relatively unique because the cant, sweep and toe angles were analyzed altogether and led to a significant reduction in drag as compared to wing without winglet. The wing model was compared with the results provided by National Aeronautics and Space Administration so this validated the simulation for different wing-winglet configurations.

Vorticity Confinement Applied to Accurate Prediction of Convection of Wing Tip Vortices and Induced Drag

The vorticity confinement (VC) method was applied to tip vortices shed by edges of wings in order to predict induced drag using far-field integration. The optimal VC parameter was determined by its application to tip vortices shed by 3-D stationary and rotating wings. The VC was used with a total variation diminishing (TVD) approach to reduce the needed confinement. The TVD minmod and differentiable flux limiters were tested to evaluate their effect on the VC parameter. The 3-D inviscid simulations were post-processed using the wake-integral technique to determine lift-induced drag force. Dependence of the VC parameter on the flight Mach number and the angle of attack was evaluated. Grid convergence studies were conducted for induced drag generated by 3-D wing. The VC approach was combined with the Reynolds stress equation turbulence model, and the results were compared to experimental data of tip vortex evolution.

Effect of Ground Boundary Condition on Near-Field Wingtip Vortex Flow and Lift-Induced Drag

Abstract The ground proximity is known to induce an outboard movement and suppression of the wingtip vortices, leading to a reduced lift-induced drag. Depending on the ground boundary condition, a large scatter exists in the published lift-induced drag and vortex trajectory. In this experiment, the ground boundary condition-produced disparity in the vortex strength and induced drag were evaluated. No significant discrepancy appeared for a ground distance or clearance larger than 30% chord. As the stationary ground was further approached, there was the appearance of a corotating ground vortex (GV), originated from the downstream progression of a spanwise ground vortex filament, which added vorticity to the tip vortex, leading to a stronger tip vortex and a larger lift-induced drag compared to the moving ground. For the moving ground, the ground vortex was absent. In close ground proximity, the rollup of the high-pressure fluid flow escaped from the wing's tip always caused the formation of a counter-rotating secondary vortex, which dramatically weakened the tip vortex strength and produced a large induced-drag reduction. The moving ground effect, however, induced a stronger secondary vortex, leading to a smaller lift-induced drag and a larger outboard movement of the tip vortex as compared to the stationary ground effect.

Definition of an Invariant Lamb-Vector-Based Aerodynamic Force Breakdown Using Far-Field Flow Symmetries

Some recent developments in the Lamb-vector-based aerodynamic force breakdown used the concept of vortex force to define the lift and to decompose the drag into lift-induced drag and profile drag. However, the Lamb vector formulation involves moment transformations, and the associated force breakdown may depend on the reference point adopted for their computation. Yet, the force acting on an airplane cannot be dependent on this point. Thus, a systematic method based on the far-field flow symmetries is here proposed to eliminate this dependence. Those symmetries also allow for a better understanding of the progressive invariance of the drag decomposition with respect to the size of the integration domain. Finally, a reference-point-invariant formulation is defined and tested for a 2-D transonic flow around an OAT15A airfoil and a 3-D flow around the NASA Common Research Model under cruise flight conditions.

Variable cant angle winglets for improvement of aircraft flight performance

Traditional winglets are designed as fixed devices attached at the tips of the wings. The primary purpose of the winglets is to reduce the lift-induced drag, therefore improving aircraft performance and fuel efficiency. However, because winglets are fixed surfaces, they cannot be used to control lift-induced drag reductions or to obtain the largest lift-induced drag reductions at different flight conditions (take-off, climb, cruise, loitering, descent, approach, landing, and so on). In this work, we propose the use of variable cant angle winglets which could potentially allow aircraft to get the best all-around performance (in terms of lift-induced drag reduction), at different flight phases. By using computational fluid dynamics, we study the influence of the winglet cant angle and sweep angle on the performance of a benchmark wing at Mach numbers of 0.3 and 0.8395. The results obtained demonstrate that by adjusting the cant angle, the aerodynamic performance can be improved at different flight conditions.

Box Wing and Induced Drag: Compressibility Effects in Subsonic and Transonic Regimes

Prandtl introduced the best wing system or Box Wing a century ago and showed its exceptional lift-induced drag performance with respect to wing systems having the same wingspan and lift (Prandtl, L., “Induced Drag of Multiplanes,” NACA TN-182, 1924.). In this work, and for the first time, their superior induced drag properties are verified in compressible and transonic regimes. The aerodynamic forces are obtained by analyzing computational fluid dynamics data, and the drag is subdivided in induced, viscous, and wave contributions. This decomposition is provided by a recently introduced aerodynamic force breakdown technique based on the vortex-force theory and allows an unambiguous definition of the lift-induced drag. The methodology is applicable to incompressible, compressible, and transonic flows. This paper will address whether the Best Wing system provides interesting induced drag performance also in transonic viscous flow.

Passive Wingtip Vortex Control by Using Tip-Mounted Half Delta Wings in Ground Effect

Abstract The ground effect on the wingtip vortex generated by a rectangular semiwing equipped with tip-mounted regular and reverse half delta wings was investigated experimentally. The passive tip vortex control always led to a reduced lift-induced drag as the ground was approached. In close ground proximity, the presence of the corotating ground vortex (GV) added vorticity to the tip vortex while the counter-rotating secondary vortex (SV) negated its vorticity level. The interaction of the GV and SV with the tip vortex and their impact on the lift-induced drag were discussed. Physical mechanisms responsible for the change in the vortex flow properties in ground effect were also provided.

Sweep and anisotropy effects on the viscous hydroelastic response of composite hydrofoils

Composite materials have become prevalent in hydrofoil design due to their favorable characteristics. Both material anisotropy and planform sweep are factors that affect hydrofoil performance. However, the interplay between these two factors has not been studied systematically. This paper investigates this interplay in the viscous hydroelastic response of linearly tapered composite hydrofoils with a modified NACA 0009 cross-section using high-fidelity hydrostructural simulations. The simulations are based on a Reynolds-averaged Navier-Stokes solver coupled to a structural finite-element model. Both sweep and off-axial fiber layup reduce the bending-mode natural frequencies due to the decrease in bending rigidity. Sweep reduces the form drag by suppressing trailing edge separation, but it can increase the lift-induced drag by causing the spanwise load distribution to deviate away from the ideal elliptical distribution. The spanwise load distribution can be tailored to reduce the induced drag and delay cavitation inception using sweep-induced geometric, material-induced bend-twist coupling, or both. Nevertheless, a poor combination of sweep and material configurations can accelerate static divergence, early separation, cavitation, noise, and material failures. Besides changing the structural response and resultant fluid-structure interaction, the material failure inception load and failure location depend directly on the fiber orientation due to the anisotropic characteristic of composites.