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Showing papers on "Lift-induced drag published in 2013"


Journal ArticleDOI
TL;DR: In this paper, a leading-edge-protuberance technique was used to manipulate flow around the airfoil by replacing the straight leading edge of a baseline airframe with a sinusoidal wavy airframe, leading to a maximum 25.0 and 39.2% increase in lift coefficient and lift-to-drag ratio, respectively.
Abstract: This paper presents an experimental investigation of the control of airfoil aerodynamics at a low Reynolds number of 5×104 within a wide range of attack angle α using a leading-edge-protuberance technique. The essence of the technique is to manipulate flow around the airfoil by replacing the straight leading edge of a baseline airfoil with a sinusoidal wavy airfoil. Whereas the lift and drag forces and the lift-to-drag ratio were measured using a three-component force balance, the flow was mainly measured using a particle-image velocimetry. The sinusoidal protuberances effectively suppressed airfoil stall, and the corresponding aerodynamic performance was impaired to some extent. Meanwhile, control significantly improved the airfoil aerodynamics in the poststall α region, for example, 16<α<70 deg, leading to a maximum 25.0 and 39.2% increase in lift coefficient and lift-to-drag ratio, respectively, and a maximum 20.0% decrease in drag coefficient. The protuberances may influence control performance in a ...

102 citations


Proceedings ArticleDOI
07 Jan 2013
TL;DR: In this article, the authors presented the findings of a study conducted tn 2010 by the NASA Innovation Fund Award project entitled "Elastically Shaped Future Air Vehicle Concept". The study presents three themes in support of meeting national and global aviation challenges of reducing fuel burn for present and future aviation systems.
Abstract: This paper presents the findings of a study conducted tn 2010 by the NASA Innovation Fund Award project entitled "Elastically Shaped Future Air Vehicle Concept". The study presents three themes in support of meeting national and global aviation challenges of reducing fuel burn for present and future aviation systems. The first theme addresses the drag reduction goal through innovative vehicle configurations via non-planar wing optimization. Two wing candidate concepts have been identified from the wing optimization: a drooped wing shape and an inflected wing shape. The drooped wing shape is a truly biologically inspired wing concept that mimics a seagull wing and could achieve about 5% to 6% drag reduction, which is aerodynamically significant. From a practical perspective, this concept would require new radical changes to the current aircraft development capabilities for new vehicles with futuristic-looking wings such as this concept. The inflected wing concepts could achieve between 3% to 4% drag reduction. While the drag reduction benefit may be less, the inflected-wing concept could have a near-term impact since this concept could be developed within the current aircraft development capabilities. The second theme addresses the drag reduction goal through a new concept of elastic wing shaping control. By aeroelastically tailoring the wing shape with active control to maintain optimal aerodynamics, a significant drag reduction benefit could be realized. A significant reduction in fuel burn for long-range cruise from elastic wing shaping control could be realized. To realize the potential of the elastic wing shaping control concept, the third theme emerges that addresses the drag reduction goal through a new aerodynamic control effector called a variable camber continuous trailing edge flap. Conventional aerodynamic control surfaces are discrete independent surfaces that cause geometric discontinuities at the trailing edge region. These discontinuities promote vorticities which result in drag rises as well as noise sources. The variable camber trailing edge flap concept could provide a substantial drag reduction benefit over a conventional discrete flap system. Aerodynamic simulations show a drag reduction of over 50% could be achieved with the flap concept over a conventional discrete flap system.

80 citations


Journal ArticleDOI
TL;DR: Palacio et al. as mentioned in this paper proposed a non-dimensional distance between aerofoil mid-chord and center of rotation to measure the angle of attack of a single wing.
Abstract: α angle of attack, rad Γ circulation, m2s−1 λ wake wavelength, m ρ∞ free-stream air density, kgm −3 τ panel tangential vector ω angular velocity, rad s−1 a non-dimensional distance between aerofoil mid-chord and centre of rotation A panel area, m b semi-chord, m ∆b panel span, m B wingspan, m c aerofoil chord, m ∆c panel chord, m C Theodorsen’s function, C(k) = F (k) + iG(k) Cd sectional drag coefficient CD wing drag coefficient Cl sectional lift coefficient Cs sectional leading-edge suction coefficient ∗Graduate Student, Department of Aeronautics. AIAA Student Member. †Senior Lecturer, Department of Aeronautics. E-mail: rpalacio@imperial.ac.uk. AIAA Member. ‡Lecturer, Department of Mechanical Engineering Sciences. AIAA Member.

76 citations


Proceedings ArticleDOI
03 Nov 2013
TL;DR: In this paper, a nonlinear control scheme for multirotor helicopters that takes first-order drag effects into account explicitly is presented, based on a dynamic model including the blade flapping and induced drag forces.
Abstract: This paper presents the design and evaluation of a nonlinear control scheme for multirotor helicopters that takes first-order drag effects into account explicitly. A dynamic model including the blade flapping and induced drag forces is presented. Based on this model, a hierarchical nonlinear controller is designed to actively compensates for the nonlinear effects these drag forces. Reported simulation and experimental results indicate the significant performance improvement of the proposed drag-augmented control scheme with respect to a conventional nonlinear controller. For completeness, an offline procedure allowing for efficiently identifying the drag parameters is proposed.

69 citations


Journal ArticleDOI
TL;DR: In this article, closed-form relations for the induced-drag and lift coefficients acting on a wing in ground effect to those acting on the same wing outside the influence of ground effect are presented.
Abstract: Closed-form relations are presented for estimating ratios of the induced-drag and lift coefficients acting on a wing in ground effect to those acting on the same wing outside the influence of ground effect. The closed-form relations for these ground-effect influence ratios were developed by correlating results obtained from numerical solutions to Prandtl’s lifting-line theory. Results show that these influence ratios are not unique functions of the ratio of wing height to wingspan, as is sometimes suggested in the literature. These ground-effect influence ratios also depend on the wing planform, aspect ratio, and lift coefficient.

54 citations


Journal ArticleDOI
TL;DR: In this article, a volume integration of the Lamb vector (cross product of the vorticity and the velocity vector) is proposed to calculate the lift and lift-induced drag in steady incompressible high-Reynolds-number flows.
Abstract: Calculations of the lift and lift-induced drag in steady incompressible high-Reynolds-number flows are proposed in terms of volume integration of the Lamb vector (cross product of the vorticity and the velocity vector). Although conventional far-field methods can compute the profile drag by the volume integral, induced drag is usually obtained by Maskell’s method, which requires a two-dimensional plane cut from three-dimensional data. Using the Lamb vector integration, plane data reconstruction is not necessary, and a definition of the lift-induced drag in viscous flows is obtained. Furthermore, an insight of the drag production is provided by the analysis of the Lamb vector field. Numerical solutions of the flow around arbitrary-shaped bodies can be analyzed by the present method. In particular, the analysis of an elliptic wing is here proposed. The acquired aerodynamic forces are evaluated in comparison with the results of classical methods.

53 citations


Book
23 Jul 2013
TL;DR: The influence of lift offset on the performance of several rotorcraft configurations is explored, considering tandem and side-by-side rotorcraft as well as wing-rotor lift share.
Abstract: The influence of lift offset on the performance of several rotorcraft configurations is explored. A lift-offset rotor, or advancing blade concept, is a hingeless rotor that can attain good efficiency at high speed, by operating with more lift on the advancing side than on the retreating side of the rotor disk. The calculated performance capability of modern-technology coaxial rotors utilizing a lift offset is examined, including rotor performance optimized for hover and high-speed cruise. The ideal induced power loss of coaxial rotors in hover and twin rotors in forward flight is presented. The aerodynamic modeling requirements for performance calculations are evaluated, including wake and drag models for the high speed flight condition. The influence of configuration on the performance of rotorcraft with lift-offset rotors is explored, considering tandem and side-by-side rotorcraft as well as wing-rotor lift share.

50 citations


Journal ArticleDOI
TL;DR: In this paper, the aerodynamic impact of various fuel saving devices used in a commercial vehicle (i.e., semi-trailer truck) was measured using a 1/10th scale model.

47 citations


Journal ArticleDOI
TL;DR: In this article, a CFD study on drag reduction in supersonic flow with opposing jet has been conducted, where flow field parameters, reattachment point position and surface pressure distributions are obtained and validated with experiments.

46 citations


Journal ArticleDOI
TL;DR: In this article, a parametric study of the flow regarding the slant angle of the flaps is performed from pressure and force measurements as well as particle image velocimetry.
Abstract: The separated flow past the square-back model used in the experiments of Ahmed et al. (1984) is controlled using flaps at the end of the top and bottom faces. A parametric study of the flow regarding the slant angle of the flaps is performed from pressure and force measurements as well as particle image velocimetry. When the bottom flap orientation is fixed, variations in the top slant angle indicate a quadratic dependence of drag versus lift. This relationship presents self-similarities when modifying the bottom flap angle. It is furthermore observed that the lift is an affine function of both slant angles and the drag is a second-order polynomial containing a coupling term between the two angles. The evolution of the drag, depending on both angles, is discussed. The contribution of the wake size, lift-induced drag as well as the local drag induced by the inclination of the flaps is interpreted.

41 citations


Journal ArticleDOI
TL;DR: In this article, an approach to extend the classical methods by allowing the bending constraints to be imposed at different lift coefficients than that at which induced drag is minimized is presented. But this approach requires the bending constraint at a limiting structural load condition, such as a maneuver lift coefficient.
Abstract: The previous works of Prandtl, Jones, and Klein and Viswanathan addressed the problem of determining the lift distribution that minimizes induced drag for a given lift and specified bending moment. In these formulations, bending moment is considered to be a surrogate for wing weight. These classical methods require the bending constraints to be imposed at the same lift coefficient at which drag is minimized. In practice, however, it is commonly desired to minimize drag at a representative cruise lift coefficient while imposing the bending constraints at a limiting structural load condition, such as a maneuver lift coefficient. This paper presents an approach to extend the classical methods by allowing the bending constraints to be imposed at different lift coefficients than that at which induced drag is minimized. An example for a wing planform similar to that of a Boeing 737 shows that the penalty for optimizing induced drag at the maneuver lift coefficient as implied in the classical methods results in ...

Journal ArticleDOI
TL;DR: In this paper, the aerodynamics of a sailing yacht with different sail trims are presented, derived from simulations performed using computational fluid dynamics, and a verification and validation of the computed aerodynamic forces and pressure distributions are performed.

Journal ArticleDOI
TL;DR: In this paper, wind tunnel testing was carried out to evaluate the reduction of induced drag using wing-tip blowing, fixed and adaptive multi-winglets, with additional investigations performed on a half-body model of a trainer aircraft.
Abstract: Wind tunnel testing was carried out to evaluate the reduction of induced drag using wing-tip blowing, fixed and adaptive multi-winglets. Different arrangements of each device were tested on a semi-span wing, with additional investigations performed on a half-body model of a trainer aircraft. Results include: aerodynamic coefficients; surface pressure distributions; and wake surveys with a multi-hole Pitot probe. The influence of each wing-tip arrangement on aircraft performance was estimated, considering the energy requirements of the blowing system. All devices tested were able to minimize induced drag; however, for practical applications in commercial airplanes, multi-winglets are a more promising alternative due to the large amount of energy required by wing-tip blowing.

Book
02 Aug 2013
TL;DR: In this article, three configurations of the test van were evaluated using a box-shaped test van, a two-axle truck, and a tractor-semitrailer combination.
Abstract: Between 1973 and 1982, the NASA Dryden Flight Research Center conducted "coast-down" tests demonstrating means for reducing the drag of trucks, buses, and motor homes. Numerous configurations were evaluated using a box-shaped test van, a two-axle truck, and a tractor-semitrailer combination. Results from three configurations of the test van are of interest now in view of a trucking industry goal of a 0.25 drag coefficient for tractor-semitrailer combinations. Two test van configurations with blunt-base geometry, similar to present day trucks (one configuration has square front comers and the other has rounded front comers), quantify the base drag increase associated with reduced forebody drag. Hoemer's equations predict this trend; however, test van results, reinforced by large-scale air vehicle data, indicate that Hoemer's formula greatly underestimates this dependence of base drag on forebody efficiency. The demonstrated increase in base drag associated with forebody refinement indicates that the goal of a 0.25 drag coefficient will not be achieved without also reducing afterbody drag. A third configuration of the test van had a truncated boattail to reduce afterbody drag and achieved a drag coefficient of 0.242. These results are included here and references are identified for other means of reducing afterbody drag.

Journal ArticleDOI
TL;DR: In this paper, a generalization of the commonly used drag-prediction and decomposition method to unsteady flows is presented, designed for three-dimensional viscous, subsonic, and transonic flows, is defined for both inertial and noninertial coordinate systems.
Abstract: Far-field drag-prediction and decomposition methods are powerful tools that increase the accuracy of the drag coefficient computed from computational fluid dynamics results by removing the spurious drag caused by numerical procedures. Furthermore, these methods allow a physical decomposition of the drag in terms of viscous, wave, and induced drag. However, they are currently limited to steady flows. This paper presents a generalization of the commonly used drag-prediction and decomposition method to unsteady flows. This generalized method, designed for three-dimensional viscous, subsonic, and transonic flows, is defined for both inertial and noninertial coordinate systems and allows drag decomposition to be performed on either static or moving/rotating meshes. This generalization also allows the drag caused by the unsteady fluctuations of the flow to be identified.

Journal ArticleDOI
TL;DR: In this paper, a car geometry named Ahmed body with a rear slant angle of 35° was tested in a wind tunnel around a simplified car geometry with pulsed jet actuators located 5 × 10−3 m from the top of the rear window.
Abstract: Aerodynamic drag control by pulsed jets is tested in a wind tunnel around a simplified car geometry named Ahmed body with a rear slant angle of 35°. Pulsed jet actuators are located 5 × 10−3 m from the top of the rear window. These actuators are produced by a pressure difference ranging from 1.5 to 6.5 × 105 Pa. Their excitation frequency can vary between 10 and 550 Hz. The analysis of the control effects is based on wall visualizations, aerodynamic drag coefficient measurements, and the velocity fields obtained by 2D PIV measurements. The maximum drag reduction is 20 % and is obtained for the excitation frequency F j = 500 Hz and for the pressure difference ∆P = 1.5 × 105 Pa. This result is linked with a substantial reduction in the transverse development of the longitudinal vortex structures coming from the left and right lateral sides of the rear window, with a displacement of the vortex centers downstream and with a decrease in the transverse rotational absolute values of these structures.

Journal ArticleDOI
TL;DR: In this paper, the authors numerically investigate the influence of sinusoidal flow control on the von Karman vortex shedding behind a circular cylinder in two-dimensional flow and examine their effects on the wake and the corresponding change in drag on the cylinder.
Abstract: We numerically investigate the influence of sinusoidal flow control on the von Karman vortex shedding behind a circular cylinder in two-dimensional flow. Actuator location, direction, frequency, and amplitude are varied to examine their effects on the wake and the corresponding change in drag on the cylinder. We place focus on the conditions for which the cylinder wake locks onto the actuation frequency. The lock-on region is found to be consistent with stability horns observed in oscillator dynamics. Under certain conditions, the actuation reduces drag by elongating the wake structure to appear more streamlined than the wake without flow control. In other cases, the use of actuation led to less streamlined wakes, resulting in no significant drag reduction or for some instances in a drag increase. Purely steady and oscillatory actuation components are examined to highlight their individual influence on the lock-on and drag characteristics. We also note that low frequency oscillations are observed for cases in the neighborhood of the lock-on boundaries due to the competition between low and high-drag states.

Journal ArticleDOI
TL;DR: In this article, numerical simulations of the three-dimensional flow around a modelled insect wing were performed to investigate the performance in flapping flight and to provide insight into the vortex dynamics and associated force generation.

Journal ArticleDOI
TL;DR: In this article, fluidic actuators were used to prevent flow separation and increase the lift and decrease the drag of a NACA 0021 airfoil during rapid flap deflections, and a significant gain in lift and a substantial reduction in drag were achieved within a short time period.
Abstract: Unsteady aerodynamic loads were measured on a NACA 0021 airfoil during rapid flap deflections. Typical dynamic-stall effects like an overshoot in lift, drag, and pitching moment have been observed in the absence of active flow control. Final flap-deflection angles and the rate of their deflections were varied at a constant angle of attack. Stall prevention or delay was achieved by fluidic actuators that generated discrete sweeping jets over the upper surface of a deflected simple flap. Static experiments proved the effectiveness of these actuators in preventing flow separation, thus increasing the lift and decreasing the drag of the airfoil. Dynamic tests proved their efficacy in dynamic stall. A significant gain in lift and a substantial reduction in drag were achieved within a short time period, even when the flap deflection was high.

Journal ArticleDOI
TL;DR: A far-field drag prediction and decomposition method has been applied to the results of the AIAA Drag Prediction Workshop 5 held in Louisiana during the summer of 2012 as discussed by the authors.
Abstract: A far-field drag prediction and decomposition method has been applied to the results of the AIAA Drag Prediction Workshop 5 held in Louisiana during the summer of 2012. The method has two principal advantages: it allows the removal of spurious drag inherent to computational fluid dynamics solutions, and it allows the decomposition of drag into viscous, wave, and induced physical drag components. This research shows that accurate drag coefficients can be predicted on coarse grids when the spurious drag is extracted with the far-field method and that these results are closer to experimental values than drag coefficients computed on finer meshes when spurious drag is not extracted. The research also investigated the reasons behind the lift and drag losses found by some participants in the workshop. It is shown that the lift loss is caused by the boundary-layer separation at the wing root, inducing a reduction of 20% of the shock wave drag and a significant change in the wing loading. The initiation of buffet...

Journal ArticleDOI
TL;DR: In this article, the flight of a small bird under the influence of the ground effect is numerically investigated with a complete three-dimensional model including the bird's body and wings.
Abstract: The flight of a small bird under the influence of the ground effect is numerically investigated with a complete three-dimensional model including the bird's body and wings. The flight mode is not the conventional steady gliding flight but an unsteady flight consisting of flapping, twisting, and folding motions. As the bird approaches the ground, the average lift force gradually increases while the average drag force decreases. At a particular distance, the average lift force increases by approximately 47%, whereas the average drag force decreases by nearly 20%, relative to the absence of the ground effect. Because of the ground, the improved aerodynamic performance in flapping flight is much more significant than in steady flight, in which the modification of the lift-drag ratio is typically less than 10%. On the basis of the flow field, regardless of the presence or absence of the ground, there exists no evidence for an obstruction of a wing-tip vortex, which is a remarkable phenomenon and accounts for the improved performance in steady flight. The extent of the region of high pressure beneath the wing in the near-ground case seems to surpass that in the far-ground case, accounting for the greater lift and thrust forces in the near-ground case. This air cushion beneath the wing, known as the cram effect, is the dominant factor of the ground effect on a flapping bird.

Journal ArticleDOI
TL;DR: In this paper, the tumbling motion of freely falling plates with aspect ratio ranging from 2 to 10 was studied using both experimental measurement and simplified lifting line theory, where the trajectories of plates were recorded by high-speed video camera and analyzed to obtain instantaneous and average kinematic and aerodynamic parameters, including the descent angle, rotating speed, descent velocities, lift and drag coefficients, and their fluctuation spectrums.
Abstract: The tumbling motion of freely falling plates with aspect ratio ranging from 2 to 10 is studied using both experimental measurement and simplified lifting line theory. The trajectories of plates are recorded by high-speed video camera and analysed to obtain instantaneous and average kinematic and aerodynamic parameters, including the descent angle, rotating speed, descent velocities, lift and drag coefficients, and their fluctuation spectrums. A double period rotation with period doubling is observed here for some range of aspect ratio and the double frequency is determined from a Fourier analysis. By adding a correction from the inducing effect of trailing vortices and wing-tip vortices to the corresponding two-dimensional force and torque expressions, a simplified kinematic model is obtained which successfully predicts the qualitative influence of aspect ratio on the averaged kinematics, that the descent angle and the vertical descent velocity component are decreasing functions of the aspect ratio, while the rotation speed and horizontal velocity component are increasing functions of the aspect ratio. For decreasing aspect ratio, the induced drag due to the trailing and tip vortices reduces the lift to drag ratio and thus increases the descent angle, while the dissipative torque due to translational induced drag decreases the rotation speed.

Proceedings ArticleDOI
10 Jun 2013
TL;DR: In this paper, the effects of geometrical features on harbor seal whiskers on its force reduction ability were studied and the results showed that the undulations on minor and major axes are necessary in reducing the lift forces.
Abstract: This paper studies the effects of geometrical features on harbor seal whiskers on its force reduction ability. Each feature in the seal whisker is sequentially stripped from the original whisker shape. Four whisker-like geometries are created from such methodology. 3D simulations of flow around the structures are performed and the resulting forces on the structures are non-dimensionalized. The lift and drag coefficients of these structures are compared to the real whisker case. The undulations on minor and major axes are found to be necessary in reducing the lift forces. Existence of only one of the undulations fails to weaken the flow and break the vortex tubes and braids in the wake. The offset angle between leading edge and trailing edge are found to have slight effects on the lift and drag coefficients. The reduction in drag coefficient is found to be dependent on the existence of undulations in minor axis. The force responses of whisker-like geometries with undulations on one of their axes are found to be periodic. When the force response on whisker-like geometries with both undulations are found to be chaotic. Vortex shedding frequency of the structure is observed to decrease considerably when no offset angle is introduced into the geometry. This may result in longer lifespan of the structure.

Journal ArticleDOI
TL;DR: In this paper, a simple drag formula based on two empirical drag coefficients given by Mendez et al. (1999) and Mendez and Losada (2004) is used to estimate the drag force on a vegetation field in shoaling conditions under non-breaking and breaking random waves.

Journal ArticleDOI
TL;DR: In this paper, an experimental parametric study of synthetic jet array actuation to reduce the aerodynamic drag of a threedimensional simplified car was performed using two configurations of an Ahmed body with 25° and 35° slant angles.
Abstract: This paper describes an experimental parametric study of synthetic jet array actuation to reduce the aerodynamic drag of a threedimensional simplified car By using two configurations of an Ahmed body with 25° and 35° slant angles, we performed wind tunnel tests under different conditions of synthetic jet array Several parameters, namely, jet location, jet direction, jet momentum coefficient, jet driving frequency, and number and position of activated jets within the actuator array, were considered The total aerodynamic drag coefficients were compared, and the rear wake flows were studied by using the data obtained from rear surface pressure distribution and flow visualization tests Results of the parametric study show that the aerodynamic drag exhibits different behavior depending on the location of the jet for each slanted model Jet direction, jet momentum coefficient, and jet driving frequency affect only the amount of change in the aerodynamic drag The distribution of the activated jets also affects jet efficiency

Journal ArticleDOI
TL;DR: The polymer-induced breakdown of large-scale Taylor vortex structures leading to drag enhancement in viscoelastic turbulent Taylor-Couette flows is reported for the first time and a simple mechanism for this striking flow transition is proposed, paving the way for a mechanistic understanding of polymer- induced structure and drag modifications in high-Re turbulent curvilinear flows.
Abstract: We report for the first time the polymer-induced breakdown of large-scale Taylor vortex structures leading to drag enhancement in viscoelastic turbulent Taylor-Couette flows. Specifically, we demonstrate that upon the addition of trace amounts of soluble high molecular weight macromolecules the Newtonian large-scale Taylor vortices are replaced by small-scale vortices in the inner and outer cylinder wall regions. This flow transition and a commensurate drag increase of up to 62% are facilitated by the presence of large polymeric normal stresses in a narrow region immediately close to the outer wall. A simple mechanism for this striking flow transition is proposed with the aim of paving the way for a mechanistic understanding of polymer-induced structure and drag modifications in high-Re turbulent curvilinear flows.

Proceedings Article
14 Nov 2013
TL;DR: The aerodynamic characteristics of lift coefficient, drag coefficient and lift-to-drag ratio were compared and it was found that each winglet configuration at a particular AOA had different CL, CD and L/D values, indicating that fixed winglets do not provide optimum aircraft performance at different phases of flight.
Abstract: Aircraft performance is highly affected by induced drag caused by wingtip vortices. Winglets, referred to as vertical or angled extensions at aircraft wingtips, are used to minimise vortices formation to improve fuel efficiency. This paper describes a wind tunnel experiment and a Computational Fluid Dynamics (CFD) analysis, performed on a rectangular wing prototype (with and without winglet) of NACA 653218 aerofoil section. The objectives of the analysis were to compare the aerodynamic characteristics and to investigate the performance of winglet at cant angles 0°, 30°, 45° and 60° at various angles of attack (AOA). The experimental analysis was performed in a closed-loop wind tunnel at sea-level conditions and free-stream velocity of 35 m/s. The CFD simulations were performed at low subsonic flow speed in ANSYS CFX solver using Finite Volume Method. Spalart-Allmaras turbulence model and 3-dimensional unstructured tetrahedral mesh were used to compute the flow around the model. The aerodynamic characteristics of lift coefficient (CL), drag coefficient (CD) and lift-to-drag ratio (L/D) were compared and it was found that each winglet configuration at a particular AOA had different CL, CD and L/D values, indicating that fixed winglets do not provide optimum aircraft performance at different phases of flight.

Dissertation
01 Jan 2013
TL;DR: In this paper, an experimental study of the turbulent wake past different geometries is performed by increasing the complexity from axisymmetric bodies to road vehicles, and two kinds of coherent wake motions are likely to be observed.
Abstract: An experimental study of the turbulent wake past different geometries is performed by increasing the complexity from axisymmetric bodies to road vehicles. Whatever the geometry is, two kinds of coherent wake motions are likely to be observed. First, at timescales of the order of 5D/U, D and U being the characteristic size and velocity of the flow respectively, the wake may generate periodic oscillations. These coherent motions are usually associated with the interaction of two facing shear layers of opposite vorticity. As the corresponding frequencies rely at first order on the distance between the shear layers, two distinct frequencies are reported when the afterbody has a cross-flow aspect ratio different than 1. These unsteady global modes seem to weaken when the Reynolds number and the complexity of the geometry increase. The second type of coherent motions corresponds to the development of stationary cross-flow instabilities. They are linked to the symmetry breaking modes observed in laminar regimes and their domains of appearance are defined from geometry considerations in the cases of parallelepiped bodies in ground proximity. These instabilities are responsible for strong asymmetries in the instantaneous flow and may generate bistable dynamics with a characteristic time scale of the order of 1000D/U. The study of these phenomena, combined with sensitivity analyses to small perturbations, places the diminution of the cross-flow asymmetries of the instantaneous wake as a relevant strategy for drag reduction. In particular, it is found that both local and global pressure gradients on the sides of the body are source of streamwise vortices increasing the drag. Parabolic dependences between the drag and the cross-flow forces are reported suggesting similarities with the mechanisms of induced drag that are well-known in aeronautics. Consequently, as they often generate significant wake asymmetries, the development of the cross-flow instabilities is identified as a drag contributor. On the contrary, the part of the drag related to the periodic global modes seems to be negligible especially for complex geometries at high Reynolds number.

Book ChapterDOI
20 Mar 2013

Journal ArticleDOI
TL;DR: In this article, the authors focused on the function of an under-body diffuser applied to a sedan and a wagon car, and analyzed the diffuser behavior in its application to lift and drag forces on a vehicle in ground proximity.
Abstract: Reducing resistance forces all over the vehicle is the most sustainable way to reduce fuel consumption. Aerodynamic drag is the dominating resistance force at highway speeds, and the power required to overcome this force increases by the power three of speed. The exterior body and especially the under-body and rear-end geometry of a passenger car are significant contributors to the overall aerodynamic drag. To reduce the aerodynamic drag it is of great importance to have a good pressure recovery at the rear. Since pressure drag is the dominating aerodynamic drag force for a passenger vehicle, the drag force will be a measure of the difference between the pressure in front and at the rear. There is high stagnation pressure at the front which requires a base pressure as high as possible. The pressure will recover from the sides by a taper angle, from the top by the rear wind screen, and from the bottom, by a diffuser. It is not necessarily the case that an optimized lower part of the rear end for a wagon-type car has the same performance as for a sedan or hatch-back car. This study focused on the function of an under-body diffuser applied to a sedan and wagon car. The diffuser geometry was chosen from a feasibility stand-point of a production vehicle such as a passenger car. The fluid dynamic function and theory of the automotive under-body diffuser working as a drag reduction device is discussed. The flow physics of the under-body and the wake was analyzed to understand the diffuser behaviour in its application to lift and drag forces on a vehicle in ground proximity. This work is mainly a numerical analysis that uses the traditional CFD approach from the automotive industry. Results from this study show a potential to reduce aerodynamic drag of the sedan car approximately 10%, and the wagon car by 2-3 %. The possible gain was much bigger for the sedan vehicle and the optimum occurs at a higher diffuser angle. This was most likely due to the fact that the sedan car in its original shape produced more lift force than the wagon, a wagon usually produces very little lift or even down-force. Lift forces were also reduced with the use of under-body covers with diffuser. The down-force increased, or lift force decreased, linearly with increased diffuser angle, and the trend was the same for both sedan and wagon rear ends. Flow analysis of the wake showed the importance of how the wake is balanced.