Showing papers on "Lift-induced drag published in 2011"

Lift enhancement by means of small-amplitude airfoil oscillations at low Reynolds numbers

Abstract: Force and particle image velocimetry measurements were conducted on a NACA 0012 airfoil undergoing small-amplitude sinusoidal plunge oscillations at a poststall angle of attack and Reynolds number of 10,000. With increasing frequency of oscillation, lift increases and drag decreases due to the leading-edge vortices shed and convected over the suction surface of the airfoil. Within this regime, the lift coefficient increases approximately linearly with the normalized plunge velocity. Local maxima occur in the lift coefficient due to the resonance with the most unstable wake frequency, its subharmonic and first harmonic, producing the most efficient conditions for high-lift generation. At higher frequencies, a second mode of flowfield occurs. The leading-edge vortex remains nearer the leading edge of the airfoil and loses its coherency through impingement with the upward-moving airfoil. To capture this impingement process, high-fidelity computational simulations were performed that showed the highly transitional nature of the flow and a strong interaction between the upper and lower-surface vortices. A sudden loss of lift may also occur at high frequencies for larger amplitudes in this mode.

Aerodynamic Characteristics at Low Reynolds Numbers for Wings of Various Planforms

Abstract: The aerodynamic characteristics of various wing planforms at low Reynolds numbers (about 1 x 10 4 ) were studied by conducting wind-tunnel tests. These low Reynolds numbers correspond to the flights of small creatures, such as insects. Elliptical, rectangular, and triangular planforms with various aspect ratios were used in this study, as well as a swept rectangular (parallelogram) wing with an aspect ratio of four. The wing sections of all models were thin rectangular airfoils. The aerodynamic forces (lift and drag) and the pitching moment acting on the wing were measured for a wide range of angles of attack (including the maximum of 90 deg). Nonlinear characteristics of the lift coefficient were obtained, even at low angles of attack for high-aspect-ratio wings, whereas a small lift slope and a large maximum lift coefficient were obtained for low-aspect-ratio wings. The drag and the pitching moment coefficients also exhibited nonlinear characteristics. The effect of the Reynolds number based on the wing chord was comparatively small, but a distinctive phenomenon in low-Reynolds-number flow was observed in flow visualization using oil-film and smoke-wire methods.

Plastron induced drag reduction and increased slip on a superhydrophobic sphere

Abstract: On low contact angle hysteresis superhydrophobic surfaces, droplets of water roll easily. It is intuitively appealing, but less obvious, that when such material is immersed in water, the liquid will flow more easily across its surface. In recent experiments it has been demonstrated that superhydrophobic surfaces with the same high contact angle and low contact angle hysteresis may not, in fact, have the same drag reducing properties. A key performance parameter is whether the surface is able to retain a layer of air (i.e. a plastron) when fully immersed. In this report, we consider an analytical model of Stokes flow (i.e. low Reynolds number, Re, creeping flow) across a surface retaining a continuous layer of air. The system is based on a compound droplet model consisting of a solid sphere encased in a sheathing layer of air and is the extreme limit of a solid sphere with a superhydrophobic surface. We demonstrate that an optimum thickness of air exists at which the drag on this compound object is minimized and that the level of drag reduction can approach 20 to 30%. Physically, drag reduction is caused by the ability of the external flow to transfer momentum across the water–air interface generating an internal circulation of air within the plastron. We also show that the drag experienced by the plastron-retaining sphere can be viewed as equivalent to the drag on a non-plastron retaining sphere, but with the no-slip boundary condition replaced by a slip boundary condition. If the plastron layer becomes too thin, or the liquid-gas interface is rigidified, circulation is no longer possible and drag increases to the value expected for a solid object in direct contact with water. We discuss the implications of this physical understanding in terms of its general applicability to the intelligent design of drag reducing superhydrophobic surfaces at low Re. We emphasize that the length scales and connectivity of surface topography generating superhydrophobicity are also likely to determine whether a plastron is of a suitable size to reduce drag.

Aerodynamics of gliding flight in common swifts.

Abstract: Gliding flight performance and wake topology of a common swift (Apus apus L.) were examined in a wind tunnel at speeds between 7 and 11 m s(-1). The tunnel was tilted to simulate descending flight at different sink speeds. The swift varied its wingspan, wing area and tail span over the speed range. Wingspan decreased linearly with speed, whereas tail span decreased in a nonlinear manner. For each airspeed, the minimum glide angle was found. The corresponding sink speeds showed a curvilinear relationship with airspeed, with a minimum sink speed at 8.1 m s(-1) and a speed of best glide at 9.4 m s(-1). Lift-to-drag ratio was calculated for each airspeed and tilt angle combinations and the maximum for each speed showed a curvilinear relationship with airspeed, with a maximum of 12.5 at an airspeed of 9.5 m s(-1). Wake was sampled in the transverse plane using stereo digital particle image velocimetry (DPIV). The main structures of the wake were a pair of trailing wingtip vortices and a pair of trailing tail vortices. Circulation of these was measured and a model was constructed that showed good weight support. Parasite drag was estimated from the wake defect measured in the wake behind the body. Parasite drag coefficient ranged from 0.30 to 0.22 over the range of airspeeds. Induced drag was calculated and used to estimate profile drag coefficient, which was found to be in the same range as that previously measured on a Harris' hawk.

Aerodynamic Considerations in the Design of Truss-Braced-Wing Aircraft

Abstract: The paper describes a study of the effects of several key aerodynamic considerations on the conceptual design of minimum fuel / emissions, long range transport, truss-braced wing aircraft configurations. This unconventional configuration has a large benefit over conventional cantilever wing configurations. The truss system enables an increased aspect ratio with lower sweep, thickness ratio, and chords, thus exploiting natural laminar flow. The design problem is solved by an MDO process, which takes into account both aerodynamic and structural considerations. The paper contains several studies, each of which investigates the dependency of the design space on a specific aerodynamic parameter such as the extent of laminar flow on the wing, cruise Mach number, maximum cruise twodimensional lift coefficient, the supercritical characteristics of the airfoil, winglet influence, and intersection fairing design. In addition, various fuselage drag reduction technologies are investigated: fuselage relaminarization, surface riblets, tailless arrangements and Goldschmied apparatus. All of these studies illustrate large potential of the truss-braced wing along with additional drag reduction technologies, which may substantially decrease the fuel weight and vehicle emissions. The paper emphasizes the importance of appropriate wing section airfoils, which can satisfy various contradicting criteria of natural laminar flow, supercritical characteristics, high lift coefficient, and low drag coefficient. Finally, these studies illustrate the importance of fuselage drag reduction for a low induced drag, NLF aircraft like a truss-braced wing configuration.

Numerical investigation of air jets for dynamic stall control on the OA209 airfoil

Abstract: The design and numerical investigation of constant blowing air jets as fluidic control devices for helicopter dynamic stall control is described. Prospective control devices were first investigated using 3D RANS computations to identify effective configurations and reject ineffective configurations. Following this, URANS investigations on the dynamically pitching OA209 airfoil verified that configurations had been selected which reduced the peaks in pitching moment and drag while preserving at least the mean lift and drag from the clean wing. Two configurations using jets at 10% chord on the airfoil top were identified, and one configuration using a tangential slot at 10% chord on the airfoil top, with each configuration evaluated for two jet total pressures. For the best configuration, a reduction in the pitching moment peak of 85% and in the drag peak of 78% were observed, together with a 42% reduction in the mean drag over the unsteady pitching cycle.

Drag reduction of a 3D bluff body using plasma actuators

Abstract: The aim of this study is to reduce the drag of a simplified car geometry using surface dielectric barrier discharge actuators. Experiments were conducted in a wind tunnel for a low Reynolds number (6.7.105) with the Ahmed body reference (rear slant angle of 25°, zero yaw angle). The effect of steady and unsteady actuation on the flow topology was investigated carrying out 2C-PIV and 1D hot wire measurements. The efficiency of the actuators was characterised by stationary balance measurements. Drag reductions up to 8% were obtained by suppressing the separation bubble above the rear window. The results suggest that plasma actuators are simple to implement on a model and can provide useful information for automotive aerodynamics through parametric studies with parameters relevant for flow control (position, surface, frequency and duty cycle of the pulsed actuation).

CFD Analysis of Winglets at Low Subsonic Flow

Abstract:  Abstract— A winglet is a device attached at the wingtip, used to improve aircraft efficiency by lowering the induced drag caused by wingtip vortices. It is a vertical or angled extension at the tips of each wing. Winglets work by increasing the effective aspect ratio of a wing without adding greatly to the structural stress and hence necessary weight of the wing structure. This paper describes a CFD 3-dimensional winglets analysis that was performed on a rectangular wing of NACA653218 cross sectional airfoil. The wing is of 660 mm span and 121 mm chord and was analyzed for two shape configurations, semicircle and elliptical. The objectives of the analysis were to compare the aerodynamic characteristics of the two winglet configurations and to investigate the performance of the two winglets shape simulated at selected cant angle of 0, 45 and 60 degrees. The computational simulation was carried out by FLUENT 6.2 solver using Finite Volume Approach. The simulation was done at low subsonic flow and at various angles of attack using Spalart-Allmaras couple implicit solver. A comparison of aerodynamics characteristics of lift coefficient CL, drag coefficient CD and lift to drag ratio, L/D was made and it was found that the addition of the elliptical and semi circular winglet gave a larger lift curve slope and higher Lift-to-Drag Ratio in comparison to the baseline wing alone. Elliptical winglet with 45 degree cant angle was the best overall design giving about 8 percent increase in lift curve slope and the best Lift-to-Drag Ratio.

Aircraft Conceptual Design Optimization Considering Fidelity Uncertainties

Abstract: ARv = vertical tail aspect ratio ARw = wing aspect ratio CD;0 = parasite drag coefficient CL;max = maximum lift coefficient hcr = cruise altitude Ix = moment of inertia about x Iy = moment of inertia about y Iz = moment of inertia about z k = induced drag constant Mcr = cruise Mach number Me = aircraft empty mass Mf = fuel mass Mg = aircraft gross mass Mpl = payload mass Npax = number of passengers nfuse = fuselage configuration number R = range Sh = horizontal tail area STO = takeoff distance Sv = vertical tail aspect ratio Sw = wing area Tsl = sea-level thrust Va = approach speed Vs = stall speed xcg = longitudinal center of gravity w = wing dihedral angle "Me = empty mass error ratio "SFC = specific fuel consumption error ratio "Tavail = available thrust error ratio "Treq = required thrust error ratio h = horizontal tail aspect ratio w = wing sweep angle h = horizontal tail taper ratio v = vertical tail taper ratio w = wing taper ratio

Spurious Far-Field-Boundary Induced Drag in Two-Dimensional Flow Simulations

Abstract: A hypothesis is made and evidence is given of a spurious far-field-boundary-induced drag component in twodimensional flow numerical solutions, which is of a different nature from irreversible spurious drag. In the case of solutions to the Euler equations, analysis is carried out of its behavior with far-field boundary condition, lift coefficient, computational domain size, uniform grid refinement, far-field grid refinement, and flow regime (subcritical/transonic). Phenomenological breakdown of this drag component is derived through Taylor series expansion. Finally, the concept of far-field-boundary-induced drag is applied to the interpretation of some of the data contained inVassberg and Jameson’s results [Vassberg, J. C., and Jameson, A., “In Pursuit of GridConvergence for Two-Dimensional Euler Solutions,” Journal of Aircraft, Vol. 474, July–Aug. 2010, pp. 1152–1166]. Estimates of this component based on the aforementioned analysis are found to be coherentwith, and to provide insight into, Vassberg and Jameson’s results.

Drag reduction of a pickup truck by a rear downward flap

Abstract: The drag reduction of a pickup truck by a rear flap add-on was examined through CFD simulations and wind tunnel experiments. When installed at the rear edge of the roof, the flap increased the cabin back surface pressure coefficient, causing the downwash of the bed flow to be inclined on the tailgate. Thus, the attachment of the bed flow to the tailgate was eliminated; consequently, the drag coefficient was reduced with increasing flap length and downward angle despite the enlarged reverse flow in the wake. However, the drag coefficient did not decrease any further after a specific downward angle was reached because the bed flow increased the drag force at the tailgate and the flap lowered the pressure field above the flap. To maximize the drag reduction effect, the rear downward flap should be designed to have an optimum downward angle.

Application of piezoelectric fiber composite actuator to aircraft wing for aerodynamic performance improvement

Abstract: The application of actuator made of piezoelectric material, particularly the advanced piezoelectric fiber composite due to the rapid development of smart materials and structures and active control technology in aviation and aerospace industry, to aircraft for performance enhancements such as flight control, aerodynamic force optimization, structure weight reduction, and overall aircraft design represents a new challenge to researches. It is considered as one of the key technologies for developing future flight vehicle. An approach with virtual control surface instead of conventional control surface to control aerodynamic force distribution and flight performance by use of piezoelectric fiber composite actuators distributed on wing surface is presented here. Particularly, the design and implementation of increasing lift force, providing roll maneuver, decreasing induced drag and wing root moment in different flight environments by the same structure control platform are studied. The control effect and sensitivity are examined quantitatively. Generally speaking, better control effect can be obtained by making better use of aeroelastic character to enlarge the actuation strain produced by piezoelectric material.

Optimal wing twist distribution for roll control of MAVs

Abstract: The aerodynamic design optimisation of a Micro Air Vehicle (MAV) wing is performed to obtain the optimal anti-symmetric wing twist distribution for the roll control of the MAV’s wing instead of using conventional ailerons. This twist distribution should produce minimum induced drag and achieve a better roll response. The implementation of several anti-symmetric load distributions such as the half lemniscates and the Horten distributions is studied leading to an initial solution for the optimal distribution that could achieve better roll requirements. Multhopp’s method based on Prandtl’s classical lifting line theory is used for the determination of the spanwise load distribution required during the optimisation process. The optimisation process is based on the modified feasible directions gradient based optimisation algorithm implemented in the optimisation system, VisualDOC, given by Dr. Garret Vanderplaats. The proposed optimisation process is applied to the ‘BARQ’developed MAV which has successful flight in July 2009.

Effects of a trapped vortex cell on a thick wing airfoil

Abstract: The effects of a trapped vortex cell (TVC) on the aerodynamic performance of a NACA0024 wing model were investigated experimentally at Re = 106 and $$6.67\times 10^{5}$$ . The static pressure distributions around the model and the wake velocity profiles were measured to obtain lift and drag coefficients, for both the clean airfoil and the controlled configurations. Suction was applied in the cavity region to stabilize the trapped vortex. For comparison, a classical boundary layer suction configuration was also tested. The drag coefficient curve of the TVC-controlled airfoil showed sharp discontinuities and bifurcative behavior, generating two drag modes. A strong influence of the angle of attack, the suction rate and the Reynolds number on the drag coefficient was observed. With respect to the clean airfoil, the control led to a drag reduction only if the suction was high enough. Compared to the classical boundary layer suction configuration, the drag reduction was higher for the same amount of suction only in a specific range of incidence, i.e., α = −2° to α = 6° and only for the higher Reynolds number. For all the other conditions, the classical boundary layer suction configuration gave better drag performances. Moderate increments of lift were observed for the TVC-controlled airfoil at low incidence, while a 20% lift enhancement was observed in the stall region with respect to the baseline. However, the same lift increments were also observed for the classical boundary layer suction configuration. Pressure fluctuation measurements in the cavity region suggested a very complex interaction of several flow features. The two drag modes were characterized by typical unsteady phenomena observed in rectangular cavity flows, namely the shear layer mode and the wake mode.

Aircraft Drag Reduction: An Overview

Abstract: Drag reduction for aerial vehicles has a range of positive ramifications: reduced fuel consumption, larger operational range, greater endurance and higher achievable speeds. The aerodynamic drag breakdown of a transport aircraft at cruise shows that the skin friction drag and the lift-induced drag constitute the two main sources of drag, approximately one half and one third of the total drag. The paper summarizes the state of the art in aeronautical drag reduction for the `conventional’ drag components of viscous drag, drag due to lift and wave drag, and also will give an overview of the results obtained for the different mentioned topics and will try to evaluate the potential gains offered by the different technologies.

The Conflict of Aerodynamic Efficiency and Static Longitudinal Stability of Box Wing Aircraft

Abstract: The induced drag of box wing aircraft is assessed with the help of literature data. The theoretical foundations of static longitudinal stability and controllability are presented and applied to the box wing aircraft. The results are interpreted and put into practice with the help of a medium range box wing aircraft based on the Airbus A320. Stability in cruise is attained by increasing the ratio CL,1/CL,2 to a value of 1,74, which is the ratio of lift coefficients of the forward and the aft wing. According to biplane theory this results in a 3,4 % increase of induced drag. Applying aerodynamic theory for closed wing systems no increase would be expected. With the stated ratio of lift coefficients results a relatively small envelope for the center of gravity (CG). Consequently the aircraft is designed to be well balanced with regard to its CG. The individual CGs of the airframe, engines, fuel and payload are all located approximately at the same position. Hence the CG shift is minimized for different payload and fuel quantities.

Experimental Investigation of Bio-Inspired High Lift Effectors on a 2-D Airfoil

Abstract: Flaps mounted on the upper surface of an airfoil, called Lift Enhancing Effectors, have been shown to increase maximum lift and stall angle in wind tunnel tests. These effectors are fabricated from 0.35mm thick Mylar and are allowed to rotate freely about their leading edge. The tests were done in the NCSU Subsonic Wind Tunnel at a chord Reynolds number of 4 10 5 . The maximum lift coefficient was increased by up to 30% and stall was increased from 12o to at least 16o. Fabricating the effector out of stiff wood and fixing the deployment angle with respect to the airfoil surface caused the zero lift angle of attack to increase in proportion to the deployment angle. Drag tests on both the free-moving and fixed deployment effectors showed marked improvement in drag at high alpha. The fixed deployment angle effectors showed drag improvement at increasingly higher alpha as deployment angle was increased. Oil flow visualization was completed on the clean airfoil and the fixed deployment angle effectors. The surface flow pictured by these oil flow tests proved that the effector causes the separation point to move aft on the airfoil, as compared to the clean airfoil. This is thought to be the main mechanism by which the effectors improve both lift and drag. Finally CFD simulations were run and compared to the oil flow visualization. Results for separation point agree between oil flow and CFD, for most alphas. Lift tests indicate that increasing the deployment angle past 60o amounts to very little improvement in Cl. Drag tests show that the free-moving effector naturally produces a drag curve in between the curves for the 30o and 45o fixed effectors.

Experimental and numerical studies on a low Reynolds number airfoil for wind turbine blades

Abstract: Testing of a low Reynolds number airfoil designed for low speed horizontal axis wind turbine (HAWT) blades was performed to study its aerodynamic characteristics. Experiments were conducted at Reynolds numbers (Re) of 38,000 to 200,000 at angles of attack from -2 o to 20 o . The airfoil geometry was chosen after testing a number of profiles with XFOIL software. The pressure distribution, lift and drag coefficients and the flow characteristics were also studied with ANSYS-CFX software. The freestream turbulence level was increased from 1% to 5% and 10% which shifted the transition point on the upper surface upstream, resulting in increased skin friction drag for lower angles of attack due to a larger turbulent boundary layer region. The slope of the lift curve did not change much at higher turbulence levels; however, for higher angles of attack, the separation from the upper surface was delayed resulting in an increase in lift and a reduction in drag. The lift to drag ratio increased by 8% to 15% as a result of increasing the turbulence level in the angles of attack-range of interest.

Design analysis of a smaller-capacity straight-bladed VAWT with an asymmetric airfoil

Abstract: One of the main challenges of widespread applications of the smaller-capacity straight-bladed vertical axis wind turbine (SB-VAWT) is to design it in a cost-effective manner. The overall cost of the SB-VAWT primarily depends on the proper selection of an appropriate blade shape or airfoil to achieve desired aerodynamic performance. In this article, an attempt has been made to perform detailed design analysis with MI-VAWT1, which is a special purpose airfoil designed for smaller-capacity SB-VAWT. The present investigation has attempted to explore the viability of several innovative design concepts like alternative blade materials and incorporation of wing tip devices to reduce the induced drag of the blades of a SB-VAWT. It has been found that blades manufactured from pultruded fibre reinforced plastics (FRP) are superior to conventional aluminium. Also, the aerodynamic performance of MI-VAWT1 is better than that of conventionally used NACA 0015.

Three‐dimensional geometry of the narwhal (Monodon monoceros) flukes in relation to hydrodynamics

Abstract: : Flukes are distally located extensions of the tail, and from a biomechanical standpoint, function as a pair of wings (Vogel 1994). Flukes function to produce thrust generated as an anteriorly directed lift force as flukes oscillate vertically (Fish 1998a, b). Their cross-sections resemble hydrofoils. For a hydrofoil to be effective, a large lift must be produced while drag is minimized; this, in turn, increases the thrust generated (Weihs 1989, Vogel 1994). The hydrodynamic implications of fluke design can be studied by examining the cross-sections (i.e., parasagittal) of the flukes. Cross-section profiles taken along the horizontal axis exhibit what is a typical streamlined hydrofoil profile with a rounded leading edge and a long, tapered trailing edge. This shape is critical for the generation of lift for thrust, while minimizing induced drag (i.e., drag due to lift production; Lighthill 1970, Vogel 1994). The flukes are symmetrical about the chord (Lang 1966, Bose et al. 1990). The cross-sectional profile of the flukes is similar to symmetrical engineered foils (Fish 1998b). The similarity to engineered foils would imply that cetacean flukes would be capable of effectively generating large lift with low drag at higher angles of attack.

An accelerometer balance for the measurement of roll, lift and drag on a lifting model in a shock tunnel

Abstract: A force balance to measure roll, lift and drag on a lifting aerodynamic body in an ultrashort-duration hypersonic test facility, such as a shock tunnel, has been developed and tested on a flapped, blunt-nosed, triangular lifting body at a freestream Mach number of 8. The flow total enthalpy and the freestream unit Reynolds number were 0.83 MJ kg−1 and 0.98 million, respectively. The balance structure has a soft suspension that allows the model to have a free flight during the short-duration aerodynamic test. The balance was mounted inside the hollow model and was equipped with accelerometers to sense the aerodynamic moment and forces on the model. The measurements were carried out at different angles of incidence of the model and the acquired signals of the accelerometers were reduced to the aerodynamic moment and the force coefficients based on the theories of applied mechanics and aerodynamics. Also, the moment and force coefficients were theoretically calculated based on the Newtonian theory, which is an accepted analytical approach for hypersonic bodies. Good agreement has been observed between the experimental and the analytical results. The method of measurement of roll and lift, and the data on the rolling moment of a lifting body presented in this note are novel.

Numerical Investigation of Rotorcraft Fuselage Drag Reduction Using Active Flow Control

Abstract: The effectiveness of unsteady zero-net-mass-flux jets for fuselage drag reduction was evaluated numerically on a generic rotorcraft fuselage in forward flight with a rotor. Previous efforts have shown significant fuselage drag reduction using flow control for an isolated fuselage by experiment and numerical simulation. This work will evaluate a flow control strategy, that was originally developed on an isolated fuselage, in a more relevant environment that includes the effects of a rotor. Evaluation of different slot heights and jet velocity ratios were performed. Direct comparisons between an isolated fuselage and rotor/fuselage simulations were made showing similar flow control performance at a -3deg fuselage angle-of-attack condition. However, this was not the case for a -5deg angle-of-attack condition where the performance between the isolated fuselage and rotor/fuselage were different. The fuselage flow control resulted in a 17% drag reduction for a peak C(sub mu) of 0.0069 in a forward flight simulation where mu = 0:35 and CT/sigma = 0:08. The CFD flow control results also predicted a favorable 22% reduction of the fuselage download at this same condition, which can have beneficial compounding effects on the overall performance of the vehicle. This numerical investigation was performed in order to provide guidance for a future 1/3 scale wind tunnel experiment to be performed at the NASA 14-by 22-Foot Subsonic Tunnel.

Reliable Force Predictions for a Flapping-wing Micro Air Vehicle: A "Vortex-lift" Approach

Abstract: Vertical and horizontal force of a flapping-wing micro air vehicle (MAV) has been measured in slow-speed forward flight using a force balance. Detailed information on kinematics was used to estimate forces using a blade-element analysis. Input variables for this analysis are lift and drag coefficients. These coefficients are usually derived from steady-state measurements of a wing in translational flow. Previous studies on insect flight have shown that this method underestimates forces in flapping flight, mainly because it cannot account for additional lift created by unsteady phenomena. We therefore derived lift and drag coefficients using a concept for delta-wings with stably attached leading-edge vortices. Resulting lift coefficients appeared to be a factor of 2.5 higher than steady-flow coefficients, and match the results from previous (numerical) studies on instantaneous lift coefficients in flapping flight. The present study confirms that a blade-element analysis using force coefficients derived fro...

Compressibility Effects of Extended Formation Flight

Abstract: Aircraft own in formations may realize signi cant reductions in induced drag by ying in regions of wake upwash. However, most transports y at transonic speeds and compressibility e ects in formation ight are not well understood. This study uses an Euler solver to analyze the inviscid aerodynamic forces and moments of transonic wing/body con gurations ying in a 2-aircraft formation. We consider formations with large streamwise separation distances (10-50 wingspans) in an arrangement we term extended formation ight. Compressibility-related drag penalties in formation ight may be eliminated by slowing 2-3% below the nominal out-of-formation drag divergence Mach number, at xed lift coe cient or xed altitude. The latter option has the additional bene t that the aerodynamic performance of the formation improves slightly at higher lift coe cients. Optimal in-formation lift coe cients are not nearly as high as those estimated by incompressible analyses, but if not limited by engine performance, modest increases in altitude can yield further improvements in aerodynamic e ciency. Increasing the lateral separation of the aircraft can allow for slightly higher cruise speeds in exchange for higher induced drag. For the con gurations examined here, a 1-2% reduction in Mach number combined with a lateral spacing increase of 5% span achieves a total formation drag savings of about 10%.

Exploring the aerodynamic characteristics of a blown annular wing for vertical/short take-off and landing applications:

Abstract: This article explores, theoretically and experimentally, a new wingform, based on an annular wing wrapped around a radial flow generator, potentially creating a vehicle with no external moving parts, reduced vehicle aerodynamic losses compared to previous vertical/short take-off and landing technologies, and substantially eliminating induced drag. Concentrating on hovering and slow flight, it is shown that such a wing works best with a thick aerofoil section and appears to offer greatest potential at a micro-aerial vehicle scale. Experimental methods are described along with results, and this work shows that wing efficiency can be substantially improved by the use of upper surface blowing technology and the Coanda effect. The main causes of efficiency loss are annular flow expansion and problems with achieving acceptable slot heights. Experimental efficiency remains below theoretical efficiency, partly due to flow asymmetry but possibly also other factors. This work is also very early in the development o...

Aerodynamic analysis of drag reduction devices on the underbody for SAAB 9-3 by using CFD

Abstract: Environmental issues and increased fuel prices are driving forces for the automotive manufactures to develop more fuel efficient vehicles with lower emissions. Large investments are aimed at minimizing power needed for propulsion i.e. new downsized engines with new aerodynamic devices for drag reduction. For passenger cars the aerodynamic drag force is the dominating resistance force at higher velocity. The car body is often optimized for reducing the drag resistance but one region where the aerodynamic development has not reached its full potential is the design of the underbody. To explain the aerodynamic force in a simplified manner the resisting drag originates from the pressure difference between the stagnation pressure in the front and the base pressure at the rear. By reducing the difference in pressure the drag force will be reduced hence the fuel consumption will be reduced. A device to improve the aerodynamics that is used on sports- and racing-cars is a diffuser, with lower ground clearance the diffuser generates downforce and aid the braking, cornering and acceleration. Using a diffuser on a passenger car, with higher ground clearance, will improve the pressure recovery on the underbody and the base pressure will be increased. To get an effective diffuser a flat underbody is preferred which also contributes to reducing the resisting drag force. In this study the drag reduction effect of a diffuser with panels on the underbody have been studied on the SAAB 9-3 Sport sedan and the SAAB 9-3 Sports wagon. To measure the effect of altering the underbody Computational Fluid Dynamics (CFD) simulations has been performed for the analysis, i.e. no wind tunnel tests have been performed. The simulations showed that a great improvement of the aerodynamic drag force can be achieved with a flat underbody and a diffuser. It was also found that the rear-end of the vehicles has an effect of the diffusers effect, a steeper diffuser is to prefer on a rear-end with steep rear-windscreen e.g. the sedan. Different additional aerodynamic devices and diffuser designs was simulated to find the most effective drag reduction setup, it was found that the most effective configuration consisted of a diffuser with covered rear rims. A reason for the drag reduction was found that the turbulent crossflow through the rims was prevented which was advantageous for the pressure recovery and overall streamlining of the pressure wake behind the vehicle. This study has shown that there are still possibilities to improve the aerodynamics of vehicles, especially at the underbody. By implementing panels at the underbody and a diffuser the drag resistance can significantly be reduced and hence a lower fuel consumption.

Numerical simulation of aerodynamic drag for high-speed train bogie

Abstract: To study the aerodynamic drag properties of high-speed train bogies,an aerodynamics model of train was built.Based on 3D steady compressible N-S equation and turbulent model of k-e two equations,the aerodynamics properties of high-speed train running at 400 km·h-1 were numerically simulated by using finite volume method.The influence of train bottom shapes on bogie's aerodynamic drag was analyzed.Analysis result shows that the flow field structure of bogie region is very complex.Vortices emerge on both the forward and backward bogies.The aerodynamic drags of bogies are different.Under crosswind-free condition,the aerodynamic drag of the first bogie is more than 4 times of the fourth bogie.The aerodynamic drag of bogie takes up more than 20% of whole train aerodynamic drag,and it is more than 40% under crosswind.The maximum differences of bogie aerodynamic drags are more than 30% due to the different shapes of train bottom.The aerodynamic drags of bogies and whole train reduce due to changing train bottom shape appropriately.2 tabs,10 figs,16 refs.

Non Planar Span Loads for Minimum Induced Drag

Abstract: Future transport category aircraft for both military and commercial customers will need to offer significant reductions in fuel burn in comparison to current technology aircraft. Non-planar lifting system configurations, such as wings with large winglets, upswept wing tips, bi-planes and strut braced configurations have been proposed as promising from an induced drag reduction perspective. This paper surveys recent and historical literature as to the theoretical limits of drag reduction enabled by such configurations. This paper also suggests specific transverse span loads and their associated efficiencies that may be realized for use in the conceptual and preliminary design process. The authors find some configurations where practicable non-planar wings can attain the theoretical limits of efficiency. For other geometries, the performance approaching the theoretical limit of efficiency does not appear to be easily realized with a practical wing.