Showing papers on "Lift-induced drag published in 2014"

Drag Reduction Through Distributed Electric Propulsion

Abstract: One promising application of recent advances in electric aircraft propulsion technologies is a blown wing realized through the placement of a number of electric motors driving individual tractor propellers spaced along each wing. This configuration increases the maximum lift coefficient by providing substantially increased dynamic pressure across the wing at low speeds. This allows for a wing sized near the ideal area for maximum range at cruise conditions, imparting the cruise drag and ride quality benefits of this smaller wing size without decreasing takeoff and landing performance. A reference four-seat general aviation aircraft was chosen as an exemplary application case. Idealized momentum theory relations were derived to investigate tradeoffs in various design variables. Navier-Stokes aeropropulsive simulations were performed with various wing and propeller configurations at takeoff and landing conditions to provide insight into the effect of different wing and propeller designs on the realizable effective maximum lift coefficient. Similar analyses were performed at the cruise condition to ensure that drag targets are attainable. Results indicate that this configuration shows great promise to drastically improve the efficiency of small aircraft.

Skin-friction drag reduction in the turbulent regime using random-textured hydrophobic surfaces

Abstract: Technologies for reducing hydrodynamic skin-friction drag have a huge potential for energy-savings in applications ranging from propulsion of marine vessels to transporting liquids through pipes. The majority of previous experimental studies using hydrophobic surfaces have successfully shown skin-friction drag reduction in the laminar and transitional flow regimes (typically Reynolds numbers less than ≃106 for external flows). However, this hydrophobicity induced drag reduction is known to diminish with increasing Reynolds numbers in experiments involving wall bounded turbulent flows. Using random-textured hydrophobic surfaces (fabricated using large-length scalable thermal spray processes) on a flat plate geometry, we present water-tunnel test data with Reynolds numbers ranging from 106 to 9 × 106 that show sustained skin-friction drag reduction of 20%–30% in such turbulent flow regimes. Furthermore, we provide evidence that apart from the formation of a Cassie state and hydrophobicity, we also need a low surface roughness and an enhanced ability of the textured surface to retain trapped air, for sustained drag reduction in turbulent flow regimes. Specifically, for the hydrophobic test surfaces of the present and previous studies, we show that drag reduction seen at lower Reynolds numbers diminishes with increasing Reynolds number when the surface roughness of the underlying texture becomes comparable to the viscous sublayer thickness. Conversely, test data show that textures with surface roughness significantly smaller than the viscous sublayer thickness and textures with high porosity show sustained drag reduction in the turbulent flow regime. The present experiments represent a significant technological advancement and one of the very few demonstrations of skin-friction reduction in the turbulent regime using random-textured hydrophobic surfaces in an external flow configuration. The scalability of the fabrication method, the passive nature of this surface technology, and the obtained results in the turbulent regime make such hydrophobic surfaces a potentially attractive option for hydrodynamic skin-friction drag reduction.

Airfoil longitudinal gust response in separated vs. attached flows

Abstract: Airfoil aerodynamic loads are expected to have quasi-steady, linear dependence on the history of input disturbances, provided that small-amplitude bounds are observed. We explore this assertion for the problem of periodic sinusoidal streamwise gusts, by comparing experiments on nominally 2D airfoils in temporally sinusoidal modulation of freestream speed in a wind tunnel vs. sinusoidal displacement of the airfoil in constant freestream in a water tunnel. In the wind tunnel, there is a streamwise unsteady pressure gradient causing a buoyancy force, while in the water tunnel one must subtract the inertial load of the test article. Both experiments have an added-mass contribution to aerodynamic force. Within measurement resolution, lift and drag, fluctuating and mean, were in good agreement between the two facilities. For incidence angle below static stall, small-disturbance theory was found to be in good agreement with measured lift history, regardless of oscillation frequency. The circulatory component of fluctuating drag was found to be independent of oscillation frequency. For larger incidence angles, there is marked departure between the measured lift history and that predicted from Greenberg's formula. Flow visualization shows coupling between bluff-body shedding and motion-induced shedding, identifiable with lift cancellation or augmentation, depending on the reduced frequency. Isolating the buoyancy effect in the wind tunnel and dynamic tares in the water tunnel, and theoretical calculation of apparent-mass in both cases, we arrive at good agreement in measured circulatory contribution between the two experiments whether the flow is attached or separated substantiating the linear superposition of the various constituents to total lift and drag, and supporting the idea that aerodynamic gust response can legitimately be studied in a steady freestream by oscillating the test article.

Turbulent wake past a three-dimensional blunt body. Part 2. Experimental sensitivity analysis

Abstract: The sensitivity of the flow around three-dimensional blunt geometry is investigated experimentally at Reynolds number . Vertical and horizontal control cylinders are used to disturb the natural flow which is the superposition of two reflectional symmetry breaking states (see Part 1 of this study, Grandemange, Gohlke & Cadot, J. Fluid Mech., vol. 722, 2013b, pp. 51–84). When the perturbation breaks the symmetry of the set-up, it can select one of the two asymmetric topologies so that a mean side force is found. When the reflectional symmetry is preserved, some positions of horizontal and vertical control cylinders alter the natural bi-stability of the flow which may result in drag reduction. In addition, it is found that the horizontal perturbation affects the lift force especially when the top and bottom mixing layers are disturbed. The ability of the disturbances to suppress the bi-stable behaviour is discussed and, introducing a formalism of induced drag, a quantification of the impact on the drag of the cross-flow forces observed for the natural bi-stable wake is suggested. Finally, a general concept for a control strategy of separated flows past three-dimensional bluff bodies can be drawn up from these analyses.

Clap and fling mechanism with interacting porous wings in tiny insect flight

Abstract: The aerodynamics of flapping flight for the smallest insects such as thrips is often characterized by a 'clap and fling' of the wings at the end of the upstroke and the beginning of the downstroke. These insects fly at Reynolds numbers (Re) of the order of 10 or less where viscous effects are significant. Although this wing motion is known to augment the lift generated during flight, the drag required to fling the wings apart at this scale is an order of magnitude larger than the corresponding force acting on a single wing. As the opposing forces acting normal to each wing nearly cancel during the fling, these large forces do not have a clear aerodynamic benefit. If flight efficiency is defined as the ratio of lift to drag, the clap and fling motion dramatically reduces efficiency relative to the case of wings that do not aerodynamically interact. In this paper, the effect of a bristled wing characteristic of many of these insects was investigated using computational fluid dynamics. We performed 2D numerical simulations using a porous version of the immersed boundary method. Given the computational complexity involved in modeling flow through exact descriptions of bristled wings, the wing was modeled as a homogeneous porous layer as a first approximation. High-speed video recordings of free-flying thrips in take-off flight were captured in the laboratory, and an analysis of the wing kinematics was performed. This information was used for the estimation of input parameters for the simulations. Compared with a solid wing (without bristles), the results of the study show that the porous nature of the wings contributes largely to drag reduction across the Re range explored. The aerodynamic efficiency, calculated as the ratio of lift to drag coefficients, was larger for some porosities when compared with solid wings.

Flow topology in the wake of a cyclist and its effect on aerodynamic drag

Abstract: Three-dimensional flows around a full-scale cyclist mannequin were investigated experimentally to explain the large variations in aerodynamic drag that are measured as the legs are positioned around the crank cycle. It is found that the dominant mechanism affecting drag is not the small variation in frontal surface area over the pedal stroke but rather due to large changes in the flow structure over the crank cycle. This is clearly shown by a series of detailed velocity field wake surveys and skin friction flow visualizations. Two characteristic flow regimes are identified, corresponding to symmetrical low-drag and asymmetrical high-drag regimes, in which the primary feature of the wake is shown to be a large trailing streamwise vortex pair, orientated asymmetrically in the centre plane of the mannequin. These primary flow structures in the wake are the dominant mechanism driving the variation in drag throughout the pedal stroke. Topological critical points have been identified on the suction surfaces of the mannequin’s back and are discussed with velocity field measurements to elucidate the time-average flow topologies, showing the primary flow structures of the low- and high-drag flow regimes. The proposed flow topologies are then related to the measured surface pressures acting on the suction surface of the mannequin’s back. These measurements show that most of the variation in drag is due to changes in the pressure distribution acting on the lower back, where the large-scale flow structures having the greatest impact on drag develop.

Invariant Formulation for the Minimum Induced Drag Conditions of Nonplanar Wing Systems

Abstract: Under the hypotheses of linear potential flow and rigid wake aligned with the freestream, a configuration-invariant analytical formulation for the induced drag minimization of single-wing nonplanar systems is presented. Following a variational approach, the resulting Euler–Lagrange integral equation in the unknown circulation distribution is obtained. The kernel presents a singularity of the first order, and an efficient computational method, ideal for the early conceptual phases of the design, is proposed. Munk’s theorem on the normalwash and its relation with the geometry of the wing under optimal conditions is naturally obtained with the present method. Moreover, Munk’s constant of proportionality, not provided in his original work, is demonstrated to be the ratio between the freestream velocity and the optimal aerodynamic efficiency. The augmented Munk’s minimum induced drag theorem is then formulated. Additional induced drag theorems are demonstrated following the derivations of this invariant proced...

Computational Fluid Dynamics Study of the Effect of Leg Position on Cyclist Aerodynamic Drag

Abstract: An experimental and numerical analysis of cycling aerodynamics is presented The cyclist is modeled experimentally by a mannequin at static crank angle; numerically, the cyclist is modeled using a computer aided design (CAD) reproduction of the geometry Wind tunnel observation of the flow reveals a large variation of drag force and associated downstream flow structure with crank angle; at a crank angle of 15 deg, where the two thighs of the rider are aligned, a minimum in drag is observed At a crank angle of 75 deg, where one leg is at full extension and the other is raised close to the torso, a maximum in drag is observed Simulation of the flow using computational fluid dynamics (CFD) reproduces the observed variation of drag with crank angle, but underpredicts the experimental drag measurements by approximately 15%, probably at least partially due to simplification of the geometry of the cyclist and bicycle Inspection of the wake flow for the two sets of results reveals a good match in the downstream flow structure Numerical simulation also reveals the transient nature of the entire flow field in greater detail In particular, it shows how the flow separates from the body of the cyclist, which can be related to changes in the overall drag

Efficiency of Lift Production in Flapping and Gliding Flight of Swifts

Abstract: Many flying animals use both flapping and gliding flight as part of their routine behaviour. These two kinematic patterns impose conflicting requirements on wing design for aerodynamic efficiency and, in the absence of extreme morphing, wings cannot be optimised for both flight modes. In gliding flight, the wing experiences uniform incident flow and the optimal shape is a high aspect ratio wing with an elliptical planform. In flapping flight, on the other hand, the wing tip travels faster than the root, creating a spanwise velocity gradient. To compensate, the optimal wing shape should taper towards the tip (reducing the local chord) and/or twist from root to tip (reducing local angle of attack). We hypothesised that, if a bird is limited in its ability to morph its wings and adapt its wing shape to suit both flight modes, then a preference towards flapping flight optimization will be expected since this is the most energetically demanding flight mode. We tested this by studying a well-known flap-gliding species, the common swift, by measuring the wakes generated by two birds, one in gliding and one in flapping flight in a wind tunnel. We calculated span efficiency, the efficiency of lift production, and found that the flapping swift had consistently higher span efficiency than the gliding swift. This supports our hypothesis and suggests that even though swifts have been shown previously to increase their lift-to-drag ratio substantially when gliding, the wing morphology is tuned to be more aerodynamically efficient in generating lift during flapping. Since body drag can be assumed to be similar for both flapping and gliding, it follows that the higher total drag in flapping flight compared with gliding flight is primarily a consequence of an increase in wing profile drag due to the flapping motion, exceeding the reduction in induced drag.

Time-Averaged Aerodynamics of Sharp and Blunt Trailing-Edge Static Airfoils in Reverse Flow

Abstract: Two-dimensional wind-tunnel experiments have been conducted on three airfoils held at static angles of attack through 360 deg at a Reynolds number of Re=1.1×105 to evaluate the influence of trailing-edge shape on time-averaged force and flowfield measurements. The present study focuses on airfoil performance in reverse flow to advance the understanding of this flow regime for high-speed helicopter applications. It is shown that the drag of a NACA 0012 airfoil in reverse flow is more than twice as large compared to forward flow due to early flow separation, similar to a flat plate. Two blunt trailing-edge airfoils are considered in this work: an elliptical airfoil and the DBLN-526. Both airfoils exhibit a rapid increase in lift at low angles of attack in both forward and reverse flows. The drag of the elliptical airfoil in reverse flow is significantly lower than the NACA 0012 for 5<αrev<17 deg. Lift was calculated via a circulation box method applied to time-averaged flowfield measurements and compared t...

Lift-drag and flow structures associated with the "clap and fling" motion

Abstract: The present study focuses on the analysis of the fluid dynamics associated with the flapping motion of finite-thickness wings. A two-dimensional numerical model for one and two-winged “clap and fling” stroke has been developed to probe the aerodynamics of insect flight. The influence of kinematic parameters such as the percentage overlap between translational and rotational phase ξ, the separation between two wings δ and Reynolds numbers Re on the evolvement of lift and drag has been investigated. In addition, the roles of the leading and trailing edge vortices on lift and drag in clap and fling type kinematics are highlighted. Based on a surrogate analysis, the overlap ratio ξ is identified as the most influential parameter in enhancing lift. On the other hand, with increase in separation δ, the reduction in drag is far more dominant than the decrease in lift. With an increase in Re (which ranges between 8 and 128), the mean drag coefficient decreases monotonously, whereas the mean lift coefficient decreases to a minimum and increases thereafter. This behavior of lift generation at higher Re was characterized by the “wing-wake interaction” mechanism which was absent at low Re.

Vortex-induced drag and the role of aspect ratio in undulatory swimmers

Abstract: During cruising, the thrust produced by a self-propelled swimmer is balanced by a global drag force. For a given object shape, this drag can involve skin friction or form drag, both being well-documented mechanisms. However, for swimmers whose shape is changing in time, the question of drag is not yet clearly established. We address this problem by investigating experimentally the swimming dynamics of undulating thin flexible foils. Measurements of the propulsive performance together with full recording of the elastic wave kinematics are used to discuss the general problem of drag in undulatory swimming. We show that a major part of the total drag comes from the trailing longitudinal vortices that roll-up on the lateral edges of the foils. This result gives a comparative advantage to swimming foils of larger span thus bringing new insight to the role of aspect ratio for undulatory swimmers.

Vortex-induced drag and the role of aspect ratio in undulatory swimmers

Abstract: During cruising, the thrust produced by a self-propelled swimmer is balanced by a global drag force. For a given object shape, this drag can involve skin friction or form drag, both being well-documented mechanisms. However, for swimmers whose shape is changing in time, the question of drag is not yet clearly established. We address this problem by investigating experimentally the swimming dynamics of undulating thin flexible foils. Measurements of the propulsive performance together with full recording of the elastic wave kinematics are used to discuss the general problem of drag in undulatory swimming. We show that a major part of the total drag comes from the trailing longitudinal vortices that roll-up on the lateral edges of the foils. This result gives a comparative advantage to swimming foils of larger span thus bringing new insight to the role of aspect ratio for undulatory swimmers.

Drag increase at the very start of drag reducing flows in a rotating cylindrical double gap device

Abstract: In this note we present some results concerning the time required for turbulent structures to achieve their steady state, called here the developing time. Notably, there is a drag increase at the very start of the test. Such a drag increase is strongly dependent on the concentration, molecular weight, temperature, Reynolds number, and molecule conformation before the test start-up. The analysis conducted here improves the understanding of the way drag reduction evolves over time, which was considered in Pereira et al. (2013).

Aerodynamic efficiency study of 2D airfoils and 3D rectangular wing in heavy rain via two-phase flow approach:

Abstract: Heavy rainfall greatly affects the aerodynamic performance of the aircraft. Aerodynamic efficiency degradation due to the heavy rain has been the cause of many aircraft accidents. We have studied t...

Co-Flow Jet Airfoil Trade Study Part II : Moment and Drag

Abstract: This paper is Part II of a parametric study on CFJ airfoils. In the first part of the paper, the CFJ airfoil suction surface shape is modified to reduce or overcome the nose-down moment. In the second part of the paper, the injection and suction sizes and Cµ are varied to increase the CFJ airfoil thrust generation. For both parts, the resulting effects on the lift, drag, moment and energy consumption is analyzed. The two dimensional flow is simulated using steady and unsteady Reynolds Average Navier-Stokes (RANS). A 5th order WENO scheme for the inviscid flux, a 4th order central differencing model for the viscous terms and the one equation SpalartAllmaras model for the turbulence are used to resolve the flow. The Mach number is 0.15 and Reynolds number is 6.4 × 10 6 . The nose-down moment of the CFJ airfoils was successfully reduced with the use of reflex camber while negative drag was achieved with a thinner airfoil, and a reduced injection size. Increasing Cµ further reduces the drag, but at the cost of a much higher energy consumption and reduced corrected aerodynamic efficiency. The minimum drag achieved isCD = 0.033 and the highest moment achieved is CM = 0.031.

Unsteady lift and drag characteristics of cavitating Clark Y-11.7% hydrofoil

Abstract: Unsteady cavitating flow and lift/drag characteristics of a two-dimensional Clark Y- 11.7% hydrofoil are experimentally investigated in order to clarify the relation between the lift drop mechanism and the unsteady cavity behavior. Unsteady lift and drag forces are measured by strain gauges attached on the cantilever supporting the hydrofoil, assuming the negligible bending moment. In combination with the above force measurements, the cavitating flow is filmed from both top and side simultaneously using two high speed video cameras. It is clearly observed that, in larger attack angle conditions (4-10 degrees), the time-averaged lift coefficient slightly increases from that in the non-cavitating condition. After the slight increase, the lift gradually decreases then its steep decrease starts to occur. On the other hand, in a small attack angle case (2 degrees), little increase of the lift is observed, and just after that the sudden lift drop occurs. From the instantaneous frequency spectra of the lift, the followings are found; during the slight increase of the lift, the cavity is being a partial cavity in almost steady state, but during the subsequent gradual lift decrease, the partial cavity oscillates with cloud cavity shedding, in other word, the partial cavity oscillation occurs, whose frequency decreases with the growth of the cavity. During the sudden lift drop, the low frequency transitional cavity oscillation occurs, in which the cavity dramatically changes between partial and super cavities. The typical events of cavity behavior during the cavitation instabilities are found to be able to be related with the behavior of instantaneous lift force and pressure.

Analytical Decomposition of Wing Roll and Flapping Using Lifting-Line Theory

Abstract: A decomposed Fourier-series solution to Prandtl’s classical lifting-line theory is used to examine the effects of rigid-body roll and small-angle wing flapping on the lift, induced-drag, and power coefficients developed by a finite wing. This solution shows that, if the flapping rate for any wing is large enough, the mean induced drag averaged over a complete flapping cycle will be negative, that is, the wing flapping produces net induced thrust. For quasi-steady flapping in pure plunging, the solution predicts that wing flapping has no net effect on the mean lift. A significant advantage of this analytical solution over commonly used numerical methods is the utility provided for optimizing wing-flapping cycles. The analytical solution involves five time-dependent functions that could all be optimized to maximize thrust, propulsive efficiency, and/or other performance measures. Results show that, by optimizing only one of these five functions, propulsive efficiencies exceeding 90% can be attained. For the...

Measuring the Drag Force on a Falling Ball

Abstract: The effect of the aerodynamic drag force on an object in flight is well known and has been described in this and other journals many times. At speeds less than about 1 m/s, the drag force on a sphere is proportional to the speed and is given by Stokes' law. At higher speeds, the drag force is proportional to the velocity squared and is usually small compared with the gravitational force if the object mass is large and its speed is low. In order to observe a significant effect, or to measure the terminal velocity, experiments are often conducted with very light objects such as a balloon 1,2 or coffee filter 3 or muffin cup, 4 or are conducted in a liquid rather than in air. The effect of the drag force can also be increased by increasing the surface area of the object.

Effect of the vortex dynamics on the drag coefficient of a square back Ahmed body: Application to the flow control

Abstract: A vortex generated behind a simplified vehicle induces a pressure force at the back wall that contributes to a significant part of the drag coefficient. This pressure force depends on two parameters: the distance of the vortex to the wall and its amplitude or its circulation. Therefore there are two ways to reduce the drag coefficient: pushing the vortices away from the wall and changing their amplitude or their dynamics. Both analytical studies and numerical simulations show that these two actions decrease the pressure force and consequently reduce the drag coefficient. The first action is achieved by an active control procedure using pulsed jets and the second action is achieved by a passive control procedure using porous layers that change the vortex shedding. The best drag coefficient reduction is obtained by coupling the two procedures.

Effects of rear spoilers on ground vehicle aerodynamic drag

Abstract: Purpose – The purpose of this paper is to examine the aerodynamic effects of rear spoiler geometry on a sports car. Today, due to economical, safety and even environmental concerns, vehicle aerodynamics play a much more significant role in design considerations and rear spoilers play a major role in this area. Design/methodology/approach – A 2-D vehicle geometry of a race car is created and solved using the computational fluid dynamics (CFD) solver FLUENT version 6.3. The aerodynamic effects are analyzed under various vehicle speeds with and without a rear spoiler. The main results are compared to a wind tunnel experiment conducted with 1/18 replica of a Nascar. Findings – By the CFD analysis, the drag coefficient without the spoiler is calculated to be 0.31. When the spoiler is added to the geometry, the drag coefficient increases to 0.36. The computational results with the spoiler are compared with the experimental data, and a good agreement is obtained within a 5.8 percent error band. The uncertainty a...

Far-Field Induced Drag Prediction Using Vorticity Confinement Technique

Abstract: The goal of this study is to show the efficiency of the wake-integral technique coupled with vorticity confinement method for the prediction of induced drag by inviscid numerical modeling. While the vorticity confinement method has previously been used to capture trailing vortices, its quantitative effect on improving wake-integral drag predictions has not been investigated. In the present study, parameters of vorticity confinement are tuned and the advantages of using a variable confinement parameter are demonstrated. A combination of vorticity confinement with the wake-integral technique indicates that the computational fluid dynamics-predicted induced drag for wing with and without winglet becomes insensitive to wake-integration plane location, the computed drag approaches its theoretical lifting-line value, and the spurious entropy drag is suppressed.

Fast Low-Fidelity Wing Aerodynamics Model for Surrogate-Based Shape Optimization

Abstract: Variable-fidelity optimization (VFO) can be efficient in terms of the computational cost when compared with traditional approaches, such as gradient-based methods with adjoint sensitivity information. In variable-fidelity methods, the direct optimization of the expensive high-fidelity model is replaced by iterative re-optimization of a physics-based surrogate model, which is constructed from a corrected low-fidelity model. The success of VFO is dependent on the reliability and accuracy of the low-fidelity model. In this paper, we present a way to develop a fast and reliable low-fidelity model suitable for aerodynamic shape of transonic wings. The low-fidelity model is component based and accounts for the zero-lift drag, induced drag, and wave drag. The induced drag can be calculated by a proper method, such lifting line theory or a panel method. The zero-lift drag and the wave drag can be calculated by two-dimensional flow model and strip theory. Sweep effects are accounted for by simple sweep theory. The approach is illustrated by a numerical example where the induced drag is calculated by a vortex lattice method, and the zero-lift drag and wave drag are calculated by MSES (a viscous-inviscid method). The low-fidelity model is roughly 320 times faster than a high-fidelity computational fluid dynamics models which solves the Reynolds-averaged Navier-Stokes equations and the Spalart-Allmaras turbulence model. The responses of the high-and low-fidelity models compare favorably and, most importantly, show the same trends with respect to changes in the operational conditions (Mach number, angle of attack) and the geometry (the airfoil shapes).