Showing papers on "Vortex lift published in 2008"

Leading-edge vortex improves lift in slow-flying bats.

Abstract: Staying aloft when hovering and flying slowly is demanding. According to quasi-steady-state aerodynamic theory, slow-flying vertebrates should not be able to generate enough lift to remain aloft. Therefore, unsteady aerodynamic mechanisms to enhance lift production have been proposed. Using digital particle image velocimetry, we showed that a small nectar-feeding bat is able to increase lift by as much as 40% using attached leading-edge vortices (LEVs) during slow forward flight, resulting in a maximum lift coefficient of 4.8. The airflow passing over the LEV reattaches behind the LEV smoothly to the wing, despite the exceptionally large local angles of attack and wing camber. Our results show that the use of unsteady aerodynamic mechanisms in flapping flight is not limited to insects but is also used by larger and heavier animals.

Near- and far-field aerodynamics in insect hovering flight: an integrated computational study.

Abstract: We present the first integrative computational fluid dynamics (CFD) study of near- and far-field aerodynamics in insect hovering flight using a biology-inspired, dynamic flight simulator. This simulator, which has been built to encompass multiple mechanisms and principles related to insect flight, is capable of 'flying' an insect on the basis of realistic wing-body morphologies and kinematics. Our CFD study integrates near- and far-field wake dynamics and shows the detailed three-dimensional (3D) near- and far-field vortex flows: a horseshoe-shaped vortex is generated and wraps around the wing in the early down- and upstroke; subsequently, the horseshoe-shaped vortex grows into a doughnut-shaped vortex ring, with an intense jet-stream present in its core, forming the downwash; and eventually, the doughnut-shaped vortex rings of the wing pair break up into two circular vortex rings in the wake. The computed aerodynamic forces show reasonable agreement with experimental results in terms of both the mean force (vertical, horizontal and sideslip forces) and the time course over one stroke cycle (lift and drag forces). A large amount of lift force (approximately 62% of total lift force generated over a full wingbeat cycle) is generated during the upstroke, most likely due to the presence of intensive and stable, leading-edge vortices (LEVs) and wing tip vortices (TVs); and correspondingly, a much stronger downwash is observed compared to the downstroke. We also estimated hovering energetics based on the computed aerodynamic and inertial torques, and powers.

Aeromechanics of Membrane Wings with Implications for Animal Flight

Abstract: Bats and other flying mammals are distinguished by thin, compliant membrane wings. In an effort to understand the dependence of aerodynamic performance on membrane compliancy, wind-tunnel tests of low-aspect-ratio, compliant wings were conducted for Reynolds numbers in the range of 0.7-2.0 x 10 5 . The lift and drag coefficients were measured for wings of varying aspect ratio, compliancy, and prestrain values. In addition, the static and dynamic deformations of compliant membrane wings were measured using stereo photogrammetry. A theoretical model for membrane camber due to aerodynamic loading is presented, indicating that the appropriate nondimensional parameter describing the problem is a Weber number that compares the aerodynamic load to the membrane elasticity. Excellent agreement between the theory and experiments is found. Measurements of aerodynamic performance show that, in comparison with rigid wings, compliant wings have a higher lift slope, maximum lift coefficients, and a delayed stall to higher angles of attack. In addition, they exhibit a strong hysteresis both around a zero angle of attack as well as around the stall angle. Unsteady membrane motions were also measured, and it is observed that the membrane vibrates with a spatial structure that is closely related to the free eigenmodes of the membrane under tension and that the Strouhal number at which the membrane vibrates rises with the freestream velocity, coinciding with increasing multiples of the natural frequency of the membrane.

Wind turbine blades with vortex generators

Abstract: An advantageous new design of a wind turbine blade and rotor is obtained by providing one, two or more parallel rows of sub-boundary layer vortex generators, whereby a blade is obtained, which is resistant to stall and provides for a high maximum lift coefficient CL,max of the blades and a slender blade design, a low socalled radius specific solidity of the rotor. The very high lift coefficient CL can reduce the necessary blade area and loads or/and increase the length of the blade and maintain the original loads with higher production. The row or rows of sub-boundary layer vortex generators are in a preferred embodiment of the invention provided in combination with Gurney Flaps generating a very high lift coefficient CL with a relative gentle stall at very high angle of attack.

Dynamic stall control by leading edge vortex generators

Abstract: A new concept of passive dynamic-stall control was developed and tested on an OA209 rotorcraft airfoil during two wind-tunnel test campains in 2004 and 2005. Small vortex generators are mounted at the leading edge of the rotor blade. At low incidence they are located close to the stagnation point and do not impact the flow field. At high angles of attack the so-called Leading Edge Vortex Generators (LEVoGs) induce longitudinal vortices which impact the suction side flow. It is shown that the use of LEVoGs can significantly increase the overall time-averaged lift while an unwanted negative pitching-moment peak is reduced compared with the clean blade case. Furthermore, overall drag is reduced at dynamic-stall conditions. Detailed analysis of the flow field by Particle Image Velocimetry and Infrared Thermography show that this is achieved by a disturbance of the dynamic-stall vortex and therefore separation is partially prevented.

Lift from Spanwise Flow in Simple Flapping Wings

Abstract: Spanwise flow contributes to lift in thin flat-plate zero-pitch-angle flapping wings in quiescent air. It is reasonable to maintain only the kinematics and mechanical complexity absolutely necessary in developing flapping-wing micro air vehicles. This study continues the quantification of the lift generated from a flapping motion of absolute minimum complexity thought to be capable of generating lift. A flapping-wing micro air vehicle with rectangular planform wings fabricated in-house (semispan aspect ratios from 1.5 to 4.0) was used to quantify the contributions to lift from flow along the span of wings at numerous points throughout the flapping cycle under a variety of operating conditions (3-6 Hz and Reynolds numbers of 6000-15,000). These experiments were performed for several aspect ratios for flat-plate and spanwise-cambered wings. The lift force was quantified experimentally using a force transducer and a high-speed camera. Digital particle image velocimetry was used to determine the lift contributions of spanwise flow to the total measured lift. Additionally, the presence of spanwise camber was shown to affect the transient lift behavior.

Slow rotating wind turbine rotor with slender blades

Abstract: An advantageous new design of a wind turbine designed to a slow rotation of the rotor is obtained by providing one, two or more parallel rows of sub-boundary layer vortex generators, whereby a blade is obtained, which is resistant to stall and provides for a high maximum lift coefficient CL,maχ of the blades and a slender blade design, a low so-called radius specific solidity of the rotor. The very high lift coefficient CL allows for an advantageous new design of a rotor operated with a low tip speed of the blades, which reduces the generation of aerodynamic noise. The row or rows of sub-boundary layer vortex generators are in a preferred embodiment of the invention provided in combination with Gurney Flaps generating a very high lift coefficient CL with a relative gentle stall at very high angle of attack.

Experimental Investigations on Leading-Edge Vortex Structures for Flow over Non-Slender Delta Wings

Abstract: The dye injection and hydrogen bubble visualization techniques are used to investigate the dual-vortex structure including its development, breakdown and the spatial location of vortex core over nonslender delta wings It is concluded that the dual-vortex structure can be affected significantly by sweep angle and Reynolds number, and generated only at small angle of attack The angle between the projection of outer vortex core on delta wing surface and the root chord line has nothing to do with the Reynolds Number and angle of attack, but has simple linear relation with the sweep angle of the model tested

Stall Behaviour of the EUROLIFT High Lift Configurations

Abstract: *Based on a previous analysis of the high lift performance of a commercial aircraft-type high lift configuration in terms of lift curves and drag polars and their Reynolds-number dependency a detailed study of the corresponding stall behavior is carried out. It is part of extensive experimental research activities on the aerodynamics of high lift configurations within the European projects EUROLIFT (I) and II. The investigations are conducted using the KH3Y wind tunnel model (DLR F11), which is representative for a wide-body twin-jet commercial aircraft. The model is designed for a step by step complexity increase up to a complete high lift configuration including pylon, nacelle, and nacelle strake. The wind tunnel data have been gathered in the European Transonic Windtunnel ETW in two different test campaigns. The Reynolds-number range extends from Re ~ 2.3 x 10 6 up to Re ~ 25 x 10 6 . To analyze the stall behavior spanwise pressure distributions at maximum lift and at lift breakdown are compared for two limiting Reynolds-numbers for each of the four complexity stages of the KH3Y configuration. The investigation reveals that for the clean high lift wing without nacelle stall is triggered at the outboard sections of the fixed wing. When the nacelles are added the lift breakdown starts on the fixed wing inboards of the nacelle. Adding a nacelle strake alleviates the lift breakdown inboards of the nacelle, while lift breakdown still occurs around the spanwise position of the nacelle on the fixed wing. For none of the four configurations a significant change of the stall type is observed for the considered Reynolds-number conditions. Yet, the investigation of the most complex configuration with strake reveals, that the effectiveness of the strake and its interaction with the flow on the fixed wing is subject to Reynolds-number influences.

Initial Experiments and Analysis of Blunt-Edge Vortex Flows

Abstract: A review is presented of the initial experimental results and analysis that formed the basis the Vortex Flow Experiment 2 (VFE-2). The focus of this work was to distinguish the basic effects of Reynolds number, Mach number, angle of attack, and leading edge bluntness on separation-induced leading-edge vortex flows that are common to slender wings. Primary analysis is focused on detailed static surface pressure distributions, and the results demonstrate significant effects regarding the onset and progression of leading-edge vortex separation.

Flow pattern around the rigid cephalic shield of the devonian agnathan errivaspis waynensis (pteraspidiformes: heterostraci)

Abstract: Palaeozoic armoured agnathans (or ostraco- derms) are characterised by having an external, bone shield enclosing the anterior part of their bodies, which demon- strate great diversity of both forms and sizes. The functional significance of these cephalic shields remains unclear (they may have been a functional analogue of the vertebral col- umn, or merely afforded protection). Here we assess the importance of the cephalic shield in terms of locomotion. In order to do this, we have studied flow patterns of the Devo- nian heterostracan Errivaspis waynensis (White, 1935), using an anatomically correct model of E. waynensis positioned at different pitching angles. The fluid flow was visualised in a wind tunnel, using planar light sheet techniques, adding va- porised propylene glycol to the fluid. The flow pattern over the cephalic shield of Errivaspis is dominated by the forma- tion of leading-edge vortices (LEVs). When the model was positioned at angles of attack of -2 degrees or higher a pair of nearly symmetrical, counter-rotating primary vortices were produced, which flowed downstream over the upper surface of the cephalic shield. At moderate angles of attack, LEVs remained attached to the dorsal surface, but, as the angle of attack increased above 7 degrees, vortices began to separate from the surface at posterior locations. At a high angles of attack (around 12 degrees or 13 degrees), vortex breakdown (or vortex burst) occured. The body-induced vortical flow around the cephalic shield is very similar to the that described over delta wing aircraft. This strategy generates lift forces through vortex generation (vortex lift). Based on this analogue and knowing that Errivaspis lacked pectoral fins or any other obvious control surfaces, vortex lift forces added through this mechanism may have played a major role in the locomotion of these primitive fishes, not only to counteract the negative buoyancy of the fish, but also as a means of manoeuvring.

Closed-Loop Control of Vortex Shedding on a Two-Dimensional Flat-Plate Airfoil at a Low Reynolds Number ∗

Abstract: Open- and closed-loop control of vortex shedding in two-dimensional flow over a flat plate at high angles of attack is numerically investigated at a Reynolds number of 300. Unsteady actuation is modeled as a body force near the leading or trailing edge, and is directed either upstream or downstream. For moderate angles of attack, sinusoidal forcing at the natural shedding frequency results in phase locking, with a periodic variation of lift at the same frequency. However, at sufficiently high angles of attack, subharmonics of the forcing frequency are also excited and the average lift over the forcing period varies from cycle to cycle in a complex manner. It is observed that the periods with the highest averaged lift are associated with particular phase difference between the forcing and the lift. We design a feedback algorithm to lock the forcing with the phase shift associated with the highest period-averaged lift. It is shown that the compensator results in a stable phaselocked limit cycle for a larger range of forcing frequencies than the open-loop control, and that it is able to stabilize otherwise unstable high-lift limit cycles that cannot be obtained with open-loop control. For example at an angle of attack of 40 ◦ , the feedback controller can increase the averaged lift coefficient from 1.35 to 2.43, an increase of 80%.

Control of a Semi-Circular Planform Wing in a "Gusting" Unsteady Freestream Flow: I - Experimental Issues

Abstract: Active flow control is used to modify the lift, drag and pitching moments on a semicircular wing during “gusting” flow conditions. A longitudinal oscillating flow component has an amplitude of 10 percent of the freestream speed and a frequency giving k = 0.048 (f =0.2 Hz). The aspect ratio of the wing is AR = 2.54, and the chord Reynolds number of the wing is 70,600. Pulsed-blowing flow control actuation occurs along the leading edge of the airfoil via 16 spatially localized micro-valve actuators. Feed-forward control based on a quasi-steady lift model is used to stabilize lift fluctuations generated by an oscillating free stream, which simulates the longitudinal component of a gusting flow. The quasi-steady system model reduces the amplitude of the fundamental and first harmonics of lift oscillations, but does not account for time delays. The time delay between the lift and the freestream oscillation was measured to be τu + = 4.8. The time delay between the lift and the actuator input signal was found to be τa + = 11.3.

The immersed boundary projection method and its application to simulation and control of flows around low-aspect-ratio wings

Abstract: First, we present a new formulation of the immersed boundary method that is algebraically identical to the traditional fractional step algorithm. This method, called the immersed boundary projection method, allows for the simulations of incompressible flows over arbitrarily shaped bodies under motion and/or deformation in both two and three dimensions. The no-slip condition along the immersed boundary is enforced simultaneously with the incompressibility constraint through a single projection. The boundary force is determined implicitly without any constitutive relations for the rigid body formulation, which in turn allows the use of high CFL numbers in our simulations compared to past methods. Next, the above immersed boundary projection method is used to analyze three-dimensional separated flows around low-aspect-ratio flat-plate wings. A number of simulations highlighting the unsteady nature of the separated flows are performed for Re = 300 and 500 with various aspect ratios, angles of attack, and planform geometries. The aspect ratio and angle of attack are found to have a large influence on the stability of the wake profile and the force experienced by the low-aspect-ratio wing. At early times, following an impulsive start, topologies of the wake vortices are found to be the same across different aspect ratios and angles of attack. Behind low-aspect-ratio rectangular plates, leading-edge vortices form and eventually separate as hairpin vortices following the start-up. This phenomenon is found to be similar to dynamic stall observed behind pitching plates. The detached structure would then interact with the tip vortices, reducing the downward velocity induced by the tip vortices acting upon the leading-edge vortex. At large time, depending on the aspect ratio and angles of attack, the wakes reach one of the three states: (i) a steady state, (ii) a periodic unsteady state, or (iii) an aperiodic unsteady state. We have observed that the tip effects in three-dimensional flows can stabilize the flow and also exhibit nonlinear interaction with the shedding vortices. At last, we apply steady blowing to separated flows behind the low-aspect-ratio rectangular wings. The objective of the flow control is to enhance lift at post-stall angles of attack by changing the dynamics of the wake vortices. This controller strengthens the tip vortices by engulfing the trailing-edge vortex sheet to increase the downward thrust and the downward induced velocity onto the leading-edge vortices. The tip vortices that are traditionally considered as an aerodynamic nuisance, have been used favorably to increase lift in post-stall flows for the considered low-aspect-ratio wings.

Interaction of a Fin Trailing Vortex with a Downstream Control Surface

Abstract: A subscale experiment has been constructed using fins mounted on one wall of a transonic wind tunnel to investigate the influence of fin trailing vortices upon downstream control surfaces. Data were collected using a fin balance instrumenting the downstream fin to measure the aerodynamic forces of the interaction, combined with stereoscopic particle image velocimetry to determine vortex properties. The fin balance data show that the response of the downstream fin essentially is shifted from the baseline single-fin data dependent upon the angle of attack of the upstream fin. FreestreamMach number and the spacing between fins have secondary effects. The velocimetry shows the increase in vortex strength with upstream fin angle of attack, but no variation with Mach number can be discerned in the normalized velocity data. Correlations between the force data and the velocimetry indicate that the interaction is fundamentally a result of an angle of attack superposed upon the downstream fin by the vortex shed from the upstream fin tip. The Mach number influence arises from differing vortex lift on the leading edge of the downstream fin even when the impinging vortex is Mach invariant.

Thickness Effect on Low-Aspect-Ratio Wing Aerodynamic Characteristics at a Low Reynolds Number

Abstract: This paper presents the study of aerodynamic performance about low-aspect-ratio wings at a low Reynolds number in wind tunnel testing. The aerodynamic properties, including lift, total drag, lift-to-drag ratio and induced drag were measured and analyzed for detailed investigations. Two forms of nonlinear equations of lift curves were reported for comparison. The effect of airfoil thickness was found to be significant on aerodynamic characteristics for all wings tested. The lift due to tip vortices was prominent for wings of AR =1.0 and their stall angles were all larger than 20°, which was mainly augmented by tip vortices shed from the wing tips.

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

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

Reactive Flow Control of Delta-Wing Vortex

Abstract: In this paper, the reactive flow control of delta-wing leading-edge vortices using along-core pulse-widthmodulation flow injection is presented. Leading-edge vortices on the upper surface of a delta wing can augment lift. Manipulating breakdown points of leading-edge vortices can effectively change the delta wing’s lift and drag and generate attitude-control torque. In this paper, a black-box dynamic model for active flow control of vortex breakdown points is identified from wind-tunnel data using a model scheduling method. Based on the identified model, a closed-loop active flow controller is developed. Simulation and real-time wind-tunnel test show that the closed-loop controller can effectively manipulate the upper surface pressure of the delta wing, which indicates that the closed-loop controller can effectively control vortex breakdown points.

Low Re, High Α Wing Aerodynamics for Micro Air Vehicle Applications

Abstract: Driven by the need to develop suitable aerodynamic designs for micro air vehicles, results fr om a CFD investigation of low Reynolds number, high angle -of -attack wings are discussed. Results are presented for the effects of planform shape, camber, and angle -of attack at Reynolds number of 500. A drag polar is presented and shows a lift peak at 20 o . A tip vortex moves streamlines away from the tip and toward the root, thus leaving a dead fluid region. A spiral vortex and recirculating flow are also observed at high angles of attack.

Force and Moment Measurements of a Transonic Fin-Wake Interaction.

Abstract: Force and moment measurements have been made on an instrumented subscale fin model at transonic speeds in Sandia’s Trisonic Wind Tunnel to ascertain the effects of Mach number and angle of attack on the interaction of a trailing vortex with a downstream control surface. Components of normal force, bending moment, and hinge moment were measured on an instrumented fin downstream of an identical fin at Mach numbers between 0.85 and 1.24, and combinations of angles of attack between -5o and 10o for both fins. The primary influence of upstream fin deflection is to shift the downstream fin’s forces in a direction consistent with the vortex-induced angle of attack on the downstream fin. Secondary nonlinear effects of vortex lift were found to increase the slopes of normal force and bending moment coefficients when plotted versus fin deflection angle. This phenomenon was dependent upon Mach number and the angles of attack of both fins. The hinge moment coefficient was also influenced by the vortex lift as the center of pressure was pushed aft with increased Mach number and total angle of attack.

The Aerodynamics of Low Sweep Delta Wings

Abstract: The aerodynamics of wings with moderately swept wings continues to be a challenging and important problem due to the current and future use in military aircraft. And yet, there is very little work devoted to the understanding of the aerodynamics of such wings. The problem is that such wings may be able to sustain attached flow next to broken-down delta-wing vortices, or stall like two-dimensional wings, while shedding vortices with generators parallel to their leading edge. To address this situation we studied the flow field over diamond-shaped planforms and sharp-edged finite wings. Possible mechanisms for flow control were identified and tested. We explored the aerodynamics of swept leading edges with no control. We presented velocity and vorticity distributions along planes normal and parallel to the free stream for wings with diamond shaped planform and sharp leading edges. We also presented pressure distributions over the suction side of the wing. Results indicated that in the inboard part of the wing, an attached vortex can be sustained, reminiscent of delta-wing type of a tip vortex, but further in the outboard region 2-D stall dominated even at 13° AOA and total stall at 21° AOA. To explore the unsteady flow field and the effectiveness of leadingedge control of the flow over a diamond-planform wing at 13° AOA, we employed Particle Image Velocimetry (PIV) at a Reynolds number of 43,000 in a water tunnel. Our results indicated that two-D-like vortices were periodically generated and shed. At the same time, an underline feature of the flow, a leading edge vortex was periodically activated, penetrating the separated flow, eventually emerging downstream of the trailing edge of the wing. To study the motion and its control at higher Reynolds numbers, namely 1.3 x 10 we conducted experiments in a wind tunnel. Three control mechanisms were employed, an oscillating mini-flap, a pulsed jet and spanwise continuous blowing. A finite wing with parallel leading and trailing edges and a rectangular tip was swept by 0°, 20°, and 40° and the pulsed jet employed as is control mechanism. A wing with a diamond-shaped-planform, with a leading edge sweep of 42°, was tested with the mini-

Lift characteristic of the flapping wing micro air vehicle

Abstract: In order to exploringly investigate lift mechanism of flapping-wing MAV(Micro Air Vehicle),the lift test of flapping-wing MAV is performed in special micro air vehicle wind tunnel of NPU(Northwestern Polytechnical University).The influences of flapping frequency,wind velocity,angle of attack and camber of flapping-wing on lift characteristic are investigated.The relation between phase-angle of flapping and the magnitude of lift force is investigated.The result of wind tunnel test provides guideline in aircraft design and aerodynamic layout design of flapping-wing MAV.

Biomimetic Flight and Flow Control: Learning from the Birds

Abstract: In this paper we consider the various methods employed by birds to generate lift and control it. We focus on three particular aspects, namely the method that a bird employs to compensate the transport lags, the method of rapid lift generation employed and growth of lift to a steady value and finally the angle of attack at which a bird flies to generate maximum lift. Based on the study of these methods we establish mathematical control models for compensating the transport lags, and establish a constraint for the aeroelastic tailoring of a wing to maintain a steady angle of attack even when flexible modes of vibration are present. Finally the unsteady aerodynamic modeling of vortex flows for active control applications is discussed.

Analysis of bat wings for morphing

Abstract: The morphing of wings from three different bat species is studied using an extension of the Weissinger method. To understand how camber affects performance factors such as lift and lift to drag ratio, XFOIL is used to study thin (3% thickness to chord ratio) airfoils at a low Reynolds number of 100,000. The maximum camber of 9% yielded the largest lift coefficient, and a mid-range camber of 7% yielded the largest lift to drag ratio. Correlations between bat wing morphology and flight characteristics are covered, and the three bat wing planforms chosen represent various combinations of morphological components and different flight modes. The wings are studied using the extended Weissinger method in an "unmorphed" configuration using a thin, symmetric airfoil across the span of the wing through angles of attack of 0°-15°. The wings are then run in the Weissinger method at angles of attack of -2° to 12° in a "morphed" configuration modeled after bat wings seen in flight, where the camber of the airfoils comprising the wings is varied along the span and a twist distribution along the span is introduced. The morphed wing configurations increase the lift coefficient over 1000% from the unmorphed configuration and increase the lift to drag ratio over 175%. The results of the three different species correlate well with their flight in nature.

Analysis of the Influence of Airfoil's Camber on the Performance Based on FLUENT

Abstract: Airfoil's Camber has significant impact on its performance.Based on the standard k-e model which is widely used in engineering,the airfoils of different cambers are simulated.The numerical results show that both lift and drag force increase as the airfoil's camber increases.However,the increasing extent of lift coefficient decreases gradually.Best lift-to-drag ratio increases first,then the trend is decreasing.As the airfoil's camber increases,the minimum pressure of airfoil suction surface decreases,but the pressure is not below the fluid's vaporization-pressure.

Unsteady flow Weis-Fogh mechanism used for roll stabilization at zero speed

Abstract: Since the Weis-Fogh mechanism can produce lifting force at zero speed,so it can be used to reduce ship roll at zero speed.The Weis-Fogh lift model,which was established according to potential flow theory,neglects the effects of separation vortexes on the leading-edge,hence it doesn't simulate flow fields better than the vortex mathematical model.To rectify this,a new Weis-Fogh lift model based on a point vortex model was developed.Lift force owing to the unfolding of the Weis-Fogh mechanism was computed through numerical calculations.A comparison was made between the lift generated by Weis-Fogh mechanisms with separative vortex and the lift by Weis-Fogh mechanisms without separative vortex.It is found that the former lift is much bigger than the latter.Finally,after comparing the lift generated from these vortex models with data from experiments,it was obvious that the point vortex model better simulates the lift produced over wings.