Showing papers on "Aerodynamic force published in 2008"

Influence of wing kinematics on aerodynamic performance in hovering insect flight

Abstract: The influence of different wing kinematic models on the aerodynamic performance of a hovering insect is investigated by means of two-dimensional time-dependent Navier–Stokes simulations. For this, simplified models are compared with averaged representations of the hovering fruit fly wing kinematics. With increasing complexity, a harmonic model, a Robofly model and two more-realistic fruit fly models are considered, all dynamically scaled at Re = 110. To facilitate the comparison, the parameters of the models were selected such that their mean quasi-steady lift coefficients were matched. Details of the vortex dynamics, as well as the resulting lift and drag forces, were studied. The simulation results reveal that the fruit fly wing kinematics result in forces that differ significantly from those resulting from the simplified wing kinematic models. In addition, light is shed on the effect of different characteristic features of the insect wing motion. The angle of attack variation used by fruit flies increases aerodynamic performance, whereas the deviation is probably used for levelling the forces over the cycle.

Skin design studies for variable camber morphing airfoils

Abstract: This paper identifies the desirable attributes of a flexible skin of a morphing wing. The study is conducted using airfoil camber morphing as an example. The ideal flex-skin would be highly anisotropic, having a low in-plane axial stiffness but a high out-of-plane flexural stiffness. Reduced skin axial stiffness allows morphing at low actuation cost. However, for some substructure and actuation designs, a lower limit on the skin's in-plane axial stiffness may be required to prevent unacceptable global camber deformation under aerodynamic loads. High flexural stiffness prevents local deformation of skin sections between supports due to aerodynamic pressure loads, and avoids buckling of skin sections under compression as the airfoil cambers under actuation force. For the camber morphing application the strain levels in the flex-skin are not expected to exceed around 2%. If the axial stiffness of the flex-skin is reduced significantly, it may be necessary to consider aerodynamic stiffness (negligible vis-a-vis structural stiffness for classical airfoils) to accurately calculate deformation under loading. The approach followed in the study can be used to identify specifications for the skin and then reverse engineer and design highly anisotropic composite skins that meet the specifications.

3D flow around a rectangular cylinder: a computational study.

Abstract: The aim of this paper is to provide a contribution to the analysis of the 3D, high Reynolds number, turbulent, separated and reattached flow around a fixed sharp-edged rectangular cylinder with a chord-to-depth ratio equal to 5. The work is developed in the perspective of the benchmark on the aerodynamics of a rectangular cylinder (BARC), in terms of an exploratory computational study. First, the adopted flow modelling and computational approach are shortly described. Second, the obtained main aerodynamic integral parameters are compared with other results proposed in the literature. Hence, the 3D flow features around the nominally 2D cylinder are investigated by means of both proper orthogonal decomposition and coherence function of the side-surface fluctuating pressure field. Once the main 2D nature of the flow has been pointed out, some of the 2D mechanisms that are responsible for the variation of the fluctuating aerodynamic forces are scrutinised: the computational approach post-processing facilities are employed to look for significant relationships between the flow structures, the pressure field and the aerodynamic force components.

Wing kinematics measurement and aerodynamics of hovering droneflies.

Abstract: The time courses of wing and body kinematics of three freely hovering droneflies ( Eristalis tenax ) were measured using 3D high-speed video, and the morphological parameters of the wings and body of the insects were also measured. The measured wing kinematics was used in a Navier–Stokes solver to compute the aerodynamic forces and moments acting on the insects. The time courses of the geometrical angle of attack and the deviation angle of the wing are considerably different from that of fruit flies recently measured using the same approach. The angle of attack is approximately constant in the mid portions of a half-stroke (a downstroke or upstroke) and varies rapidly during the stroke reversal. The deviation angle is relatively small and is higher at the beginning and the end of a half-stroke and lower at the middle of the half-stroke, giving a shallow U-shaped wing-tip trajectory. For all three insects considered, the computed vertical force is approximately equal to the insect weight (the difference is less than 6% of the weight) and the computed horizontal force and pitching moment about the center of mass of the insect are approximately zero. The computed results satisfying the equilibrium flight conditions, especially the moment balance condition, validate the computation model. The lift principle is mainly used to produce the weight-supporting vertical force, unlike the fruit flies who use both lift and drag principles to generate the vertical force; the vertical force is mainly due to the delayed stall mechanism. The magnitude of the inertia power is larger than that of the aerodynamic power, and the largest possible effect of elastic storage amounts to a reduction of flight power by around 40%, much larger than in the case of the fruit fly.

Integrative Model of Drosophila Flight

Abstract: This paper presents a framework for simulating the flight dynamics and control strategies of the fruit fly Drosophila melanogaster. The framework consists of five main components: an articulated rigid-body simulation, a model of the aerodynamic forces and moments, a sensory systems model, a control model, and an environment model. In the rigid-body simulation the fly is represented by a system of three rigid bodies connected by a pair of actuated ball joints. At each instant of the simulation, the aerodynamic forces and moments acting on the wings and body of the fly are calculated using an empirically derived quasi-steady model. The pattern of wing kinematics is based on data captured from high-speed video sequences. The forces and moments produced by the wings are modulated by deforming the base wing kinematics along certain characteristic actuation modes. Models of the fly’s visual and mechanosensory systems are used to generate inputs to a controller that sets the magnitude of each actuation mode, thus modulating the forces produced by the wings. This simulation framework provides a quantitative test bed for examining the possible control strategies employed by flying insects. Examples demonstrating pitch rate, velocity, altitude, and flight speed control, as well as visually guided centering in a corridor are presented.

Flexible-Membrane Airfoils at Low Reynolds Numbers

Abstract: An experimental study was conducted to assess the benefits of using flexible-membrane airfoils/wings at low Reynolds numbers for micro air vehicle applications compared with using a conventional rigid airfoil/wing. In addition to measuring aerodynamic forces acting on flexible-membrane airfoils/wings, a high-resolution particle image velocimetry system was used to conduct flowfield measurements to quantify the transient behavior of vortex and turbulent flow structures around the flexible-membrane airfoils/wings to elucidate the associated underlying fundamental physics. The aerodynamic force measurements revealed that flexible-membrane airfoils could provide better aerodynamic performance compared with their rigid counterpart at low Reynolds numbers. The flexibility (or rigidity) of the membrane skins of the airfoils was found to greatly affect their aerodynamic performance. Particle image velocimetry measurements elucidated that flexible-membrane airfoils could change their camber (i.e., crosssectional shape) automatically to adapt incoming flows to balance the pressure differences on the upper and lower surfaces of the airfoils, therefore suppressing flow separation on the airfoil upper surfaces. Meanwhile, deformation of the flexible-membrane skins was found to cause significant airfoil trailing-edge deflection (i.e., lift the airfoil trailing edge up from its original designed position), which resulted in a reduction of the effective angles of attack of the flexible-membrane airfoils, thereby delaying airfoil stall at high angles of attack. The nonuniform spanwise deformation of the flexible-membrane skins of the flexible-membrane airfoils was found to significantly affect the characteristics of vortex and turbulent flow structures around the flexible-membrane airfoils.

Review of zero-net-mass-flux jet and its application in separation flow control

Abstract: Since the zero-net-mass-flux (ZNMF) jet was first used as a laboratory flow control method in 1990’s, it has attracted much attention. The ZNMF jet has unique features such as compact actuator, no requirement for external air supply, complex piping, etc., and becomes a hot topic research subject in fluid mechanics. This review introduces the state of the art in the development of ZNMF jet in the quiescent fluid, the interaction of the ZNMF jet with the cross flow and its application in the separation flow control. The evolution of the vortex ring/pair and the spacial flow structure of the ZNMF in quicent fluid or cross flow are presented, as well as the key parameter effects. At last, the applications of ZNMF jet in the wake control of the circular cylinder, the separation control on the airfoil and the aerodynamic force or moment control on MAV/UAV are presented.

Simultaneous Excitation of Multiple-Input/Multiple-Output CFD-Based Unsteady Aerodynamic Systems

Abstract: A significant improvement to the development of computational-fluid-dynamics-based unsteady aerodynamic reduced-order models is presented. The improvement involves the simultaneous excitation of the structural modes of the computational-fluid-dynamics-based unsteady aerodynamic system. This improvement enables the computation of the unsteady aerodynamic state-space model using a single computational fluid dynamics execution, independent of the number of structural modes. Two new types of input functions are presented that can be used for the simultaneous excitation of the unsteady aerodynamic system via the excitation of the structural modes. Results are presented for a semispan configuration using the CFL3Dv6.4 code.

Multitarget Error Estimation and Adaptivity in Aerodynamic Flow Simulations

Abstract: Important quantities in aerodynamic flow simulations are the aerodynamic force coefficients including the pressure induced and the viscous stress induced drag, lift, and moment coefficients. In addition to the exact approximation of these quantities it is of increasing importance, in particular in the field of uncertainty quantification, to estimate the error in the computed quantities. In recent years a posteriori error estimation and goal-oriented refinement approaches have been developed for the accurate and efficient computation of single target quantities. The current approaches are based on computing an adjoint solution related to each of the specific target quantities under consideration. In this paper we extend this approach to the accurate and efficient computation of multiple target quantities. Instead of computing multiple adjoint solutions, one for each target functional, the new approach is based on the solution to one discrete adjoint problem and one discrete error problem. This way only two auxiliary problems are required irrespective of the number of target functionals. The practical performance of this approach is demonstrated for a laminar compressible flow. In particular, the proposed approach is compared to the standard approach of error estimation and goal-oriented refinement as well as to residual-based refinement. The performance of the algorithms is measured in terms of computing resources required for meeting industrial as well as academic accuracy requirements on the computed force coefficients.

Experimental Investigation on the Aerodynamic Characteristics of a Bio-mimetic Flapping Wing with Macro-fiber Composites:

Abstract: This study describes the development of a bio-mimetic flapping wing and the aerodynamic characteristics of a flexible flapping wing. First, the flapping wing is designed to produce flapping, twisting, and camber motions by using a bio-mimetic design approach. A structural model for a macro-fiber composite (MFC) actuator is established, and structural analysis of a smart flapping wing with the actuator is performed to determine the wing configuration for maximum camber motion. The analysis model is verified with the experimental data of the smart flapping wing. Second, aerodynamic tests are performed for the smart flapping wing in a subsonic wind tunnel, and the aerodynamic forces are measured for various test conditions. Additionally, the effects of camber and chordwise wing flexibility on unsteady and quasi-steady aerodynamic characteristics are discussed. The experimental results demonstrate that the effect of the camber generated by the MFC produces sufficient aerodynamic benefit. It is further found that chordwise wing flexibility is an important parameter in terms of affecting aerodynamic performance, and that lift produced in a quasi-steady flow condition is mostly affected by the forward speed and effective angle of attack.

Experimental investigation on spiked body in hypersonic flow

Abstract: A spike attached to a hemispherical body drastically changes its flowfield and influences aerodynamic drag in a hypersonic flow. It is, therefore, a potential candidate for drag reduction of a future high-speed vehicle. The effect of the spike length, shape, spike nose configuration and angle-of-attack on the reduction of the drag is experimentally studied with use of hypersonic wind-tunnel at Mach 6. The effects of geometrical parameters of the spike and angle-of-attack on the aerodynamic coefficient are analysed using schlieren picture and measuring aerodynamic forces. These experiments show that the aerodisk is superior to the aerospike. The aerodisk of appropriate length, diameter and nose configuration may have the capability for the drag reduction. The inclusion of an aero disk at the leading edge of the spike has an advantage for the drag reduction mechanism if it is at an angle-of-attack, however consideration to be given for increased moment resulting from the spike is required.

Bioinspired Corrugated Airfoil at Low Reynolds Numbers

Abstract: An experimental study was conducted to investigate the flow behavior around a bioinspired corrugated airfoil compared with a traditional streamlined airfoil and a flat plate at the chord Reynolds number of Re 34; 000 to explore the potential application of such bioinspired corrugated airfoils for micro air vehicle applications. The experiments were conducted in a low-speed wind tunnel. A high-resolution particle image velocimetry system was used to conduct detailed flowfield measurements to quantify the transient behavior of vortex and turbulent flow structures around the studied airfoils. The particle image velocimetry measurement results demonstrated clearly that the corrugated airfoil has better performance over the streamlined airfoil and the flat plate in preventing largescale flow separation and airfoil stall at low Reynolds numbers. It was found that the protruding corners of the corrugated airfoil would act as turbulators to generate unsteady vortex structures to promote the transition of the separated boundary-layer flow from laminar to turbulent. The unsteady vortex structures trapped in the valleys of the corrugated cross section would pump high-speed fluid from outside to near-wall regions to provide sufficient kinetic energy for the boundary layer to overcome adverse pressure gradients, thus discouraging large-scale flow separations and airfoil stall. Aerodynamic force measurements further confirmed the possibility of using such bioinspired corrugated airfoils in micro air vehicle designs to improve their flight agility and maneuverability.

Aerodynamic Modeling of the Wing-Propeller Interaction for a Tail-Sitter Unmanned Air Vehicle

Abstract: This paper considers the aerodynamic modeling of a twin-engine tail-sitter unmanned air vehicle that relies on wing- and fin-mounted control surfaces submerged in the propeller slipstreams for control during low-speed vertical flight The aerodynamic forces on this vehicle are predicted using a full azimuthal blade-element solution for the propellers combined with a fixed-wake panel-method model of the vehicle itself. When the flow components determined from the propeller solution are superimposed on the external flowfield and integrated with the panel-method model, the aerodynamic forces can be determined for the full vehicle, including slipstream effects, which are dominant in low-speed flight. The modeling described in this paper has been used extensively for multidisciplinary optimization work, as well as in the construction of a comprehensive aerodynamic database for the vehicle. This database encompasses all the aerodynamic force and moment coefficients and their derivatives for over 4300 separate flight conditions and has been extensively used for vehicle simulation and control design purposes.

Experimental investigation of wing-in-ground effect with a NACA6409 section

Abstract: The aerodynamic characteristics of NACA6409 in the vicinity of the ground were experimentally studied in a wind tunnel. Lift and drag forces, the pitching moment, and the center of pressure were measured with respect to various major aerodynamic parameters, such as the ground clearance, the angle of attack, the aspect ratio (AR), and the endplate type, which resulted in a total number of 420 conditions. In addition, a smoke trace test was conducted to visualize the flow pattern around NACA6409 in the vicinity of the ground. As a result of the ground effect and the influence of the endplate, the lift-to-drag ratio increased at low ground clearance and the center of pressure moved forward to the leading edge; that is, the endplate and ground effects were equivalent to the aerodynamic advantage that results from increasing the AR of the wing. Extended experimental results are useful for understanding the aerodynamic characteristics influenced by each aerodynamic parameter during ground effect as well as for verifying numerical simulation.

Effects of unsteady deformation of flapping wing on its aerodynamic forces

Abstract: Effects of unsteady deformation of a flapping model insect wing on its aerodynamic force production are studied by solving the Navier-Stokes equations on a dynamically deforming grid. Aerodynamic forces on the flapping wing are not much affected by considerable twist, but affected by camber deformation. The effect of combined camber and twist deformation is similar to that of camber deformation. With a deformation of 6% camber and 20° twist (typical values observed for wings of many insects), lift is increased by 10% ∼ 20% and lift-to-drag ratio by around 10% compared with the case of a rigid flat-plate wing. As a result, the deformation can increase the maximum lift coefficient of an insect, and reduce its power requirement for flight. For example, for a hovering bumblebee with dynamically deforming wings (6% camber and 20° twist), aerodynamic power required is reduced by about 16% compared with the case of rigid wings.

Wind turbine with pitch control arranged to reduce life shortening loads on components thereof

Abstract: A wind turbine is presented where the operation lifetime of the main bearing is extended by relieving the main bearing by reducing the mean bending moment on the bearing by means of individual pitch control of the blades of the rotor so as to create an aerodynamic mean tilt moment on the rotor by means of aerodynamic forces on the blades, the tilt moment at least partly counteracting the bending moment caused by the overhang load forces on the main bearing from the gravitational pull on the rotor mass.

Nonequilibrium Hypersonic Aerothermodynamics Using the Direct Simulation Monte Carlo And Navier-Stokes Models

Abstract: : Hypersonic flight vehicles are a current topic of interest in both civilian and military research. NASA is currently designing a Crew Transport Vehicle (CTV) [44, 69] and Crew Exploration Vehicle (CEV) [32] to replace the space shuttle; reentry vehicles are, by definition, hypersonic vehicles. Military requirements for reconnaissance and surveillance, as well as the mission of the United States Air Force to rapidly project power globally makes the design of a hypersonic plane that can quickly traverse the globe very attractive [102]. The design of hypersonic vehicles requires accurate prediction of the surface properties while in flight. These quantities are typically the heat flux, pressure and shear stress, from which the aerodynamic forces and moments can be calculated. These variables govern not only the aerodynamic performance of the vehicle, but also determine the selection and sizing of the thermal protection system (TPS), which protects the vehicle from the extreme temperatures encountered at hypersonic velocities. The geometry of a vehicle, and in particular, the nose and the leading edges of wings and other aerodynamic surfaces, is a critical consideration in a vehicle's design. Aerodynamic heating is inversely proportional to the square root of the 1 radius at the stagnation point; hence, historically most vehicles have had blunted noses and leading edges to reduce the thermal loads to acceptable levels. Recently, however, a class of materials, designated Ultra-High Temperature Ceramic (UHTC) composites, has been developed that can withstand temperatures as high as 3500 K [57, 78]. Materials such as these allow the use of much sharper leading edges.

Computational Study of Flexible Wing Ornithopter Flight

Abstract: This paper presents the development and evaluation of a computational fluid dynamics based methodology to predict the aerodynamic forces produced by a flexible flapping wing. The computational fluid dynamics analysis code solves the compressible Reynolds-averaged form of the Navier-Stokes equations on structured curvilinear grids. A grid deformation algorithm is devised that deforms the body-conforming volume grid at each time step consistent with the measured wing motions. This algorithm is based on geometric considerations and is both computationally efficient and capable of handling very large deformations. This methodology is validated using experimental data obtained from a test on an ornithopter with flexible wings. Test data include measurements of the wing surface deformations as well as the generated forces in the horizontal and vertical directions. Correlation with test data shows good agreement with measured vertical force and satisfactory agreement with measured horizontal force at low flapping frequencies. However, the prediction accuracy degrades with an increase in Happing frequency. Evidence of resonance in the vehicle system was detected from the analysis of the experimental data. Unmodeled inertial effects from the vehicle body and support mounts may be one of the contributors to disagreement between the data and analysis.

Hypersonic Flow Control Using Surface Plasma Actuator

Abstract: Plasma-fluid-dynamic interaction has been shown to be a viable mechanism for hypersonic flow control. An effective and verified flow control process using direct current surface discharge is summarized. The operating principle is based on a small electromagnetic perturbation to the growth rate of the displacement thickness of a shear layer that is strongly amplified by a subsequent pressure interaction. The aerodynamic control is delivered in less than a millisecond time frame and produces no parasitic effect when deactivated. The magnitude of the resultant aerodynamic force and moment can be significant and does not require a large amount of power for plasma generation to overcome the inefficient ionizing process, thus reducing the weight of a high-speed vehicle. The electromagnetic perturbation is derived from a surface gas discharge with or without an externally applied magnetic field. An embedded plasma actuator near the leading edge of a flat plate has produced high surface pressure equivalent to more than a 5 deg flow deflection at Mach 5, and the flow control effectiveness will increase with an increasing oncoming Mach number. The detailed flow structure of weakly ionized airstreams has been investigated by a combination of experimental effort and computational simulation solving the magneto-fluid-dynamic equations in the low magnetic Reynolds number limit with a drift-diffusion plasma model. The identical plasma actuator is investigated as a variable geometry cowl of a hypersonic inlet. All phenomena are replicated by computational results and are fully validated by experimental observations.

An aeroelastic analysis of a flexible flapping wing using modified strip theory

Abstract: The present study proposed a coupling method for the fluid-structural interaction analysis of a flexible flapping wing. An efficient numerical aerodynamic model was suggested, which was based on the modified strip theory and further improved to take into account a high relative angle of attack and dynamic stall effects induced by pitching and plunging motions. The aerodynamic model was verified with experimental data of rigid wings. A reduced structural model of a rectangular flapping wing was also established by using flexible multibody dynamics, so as to consider large flapping motions and local elastic deformations. Then, the aeroelastic analysis method was developed by coupling these aerodynamic and structural modules. To measure the aerodynamic forces of the rectangular flapping wing, static and dynamic tests were performed in a low speed wind-tunnel for various flapping pitch angles, flapping frequencies and the airspeeds. Finally, the aerodynamic forces predicted by the aeroelastic analysis method showed good agreement with the experimental data of the rectangular flapping wing.

Design Optimization of a Controllable Camber Rotor Airfoil

Abstract: A conformable airfoil is proposed as an alternative to trailing-edge flaps used for active helicopter vibration reduction through high-frequency changes in camber. The design consists of several compliant mechanisms of predetermined topology that are placed serially within the airfoil along the chord, aft of the leading-edge spar. A shape optimization approach is used to design the compliant mechanisms, in which the objective is to maximize trailing-edge deflection while minimizing airfoil deflections due to aerodynamic loads. Solutions were obtained using a sequential linear programming method coupled with a finite element analysis. An optimized shape is predicted to achieve a trailing-edge deflection of ±6.0 mm or a ±4.6- deg equivalent flap deflection angle using the tip deflection objective. Results indicate that the deflection is dependent on the amount of passive material allowed and the objective function used. The aerodynamic loads are found to cause only small deformations in comparison with those caused by the actuation. Prototype fabrication and bench-top tests demonstrated that rotor airfoil camber is controllable using the proposed concept.