Showing papers on "Aerodynamic force published in 1996"
TL;DR: In this article, a tethered moth's flapping wings are simulated using an unsteady aerodynamic panel method and accounts for wing flexibility using a finite element model, and a switching pattern is discovered where the magnitude and direction of the aerodynamic force are decoupled, thereby pointing to a means whereby control is achieved.
Abstract: The mechanization of flapping-wing flight is addressed. A tethered moth's flapping wings are simulated using an unsteady aerodynamic panel method and accounts for wing flexibility using a finite element model. The resultant simulation code delineates both the aerodynamic and inertial forces acting on flapping, flexible wings undergoing arbitrary motion in the presence of large-scale vortices and establishes the importance of including the wake in the unsteady analysis of flapping flexible wings. A switching pattern is discovered where the magnitude and direction of the aerodynamic force are decoupled, thereby pointing to a means whereby control is achieved. Overall, important groundwork necessary for the establishment of the principles of flapping-wing flight is laid, leading to the development of a highly agile, alternative flight technology.
108 citations
TL;DR: This paper responds to research into the aerodynamics of flapping wings and to the problem of the lack of an adequate method which accommodates large-scale trailing vortices with the main advantages of an unsteady aerodynamic panel method.
Abstract: This paper responds to research into the aerodynamics of flapping wings and to the problem of the lack of an adequate method which accommodates large-scale trailing vortices. A comparative review is provided of prevailing aerodynamic methods, highlighting their respective limitations as well as strengths. The main advantages of an unsteady aerodynamic panel method are then introduced and illustrated by modelling the flapping wings of a tethered sphingid moth and comparing the results with those generated using a quasi-steady method. The improved correlations of the aerodynamic forces and the resultant graphics clearly demonstrate the advantages of the unsteady panel method (namely, its ability to detail the trailing wake and to include dynamic effects in a distributed manner).
106 citations
TL;DR: In this article, the effect of Reynolds number on the aerodynamic characteristics of an airfoil with ground effect in viscous flow is investigated by numerical method, based on the standard k-e turbulence model, generalized body-fixed coordinates and the finite volume method.
Abstract: The effect of Reynolds number on the aerodynamic characteristics of an airfoil with ground effect in viscous flow is investigated by numerical method. A numerical scheme, based on the standard k-e turbulence model, generalized body-fixed coordinates and the finite volume method, is developed to solve the two-dimensional wing-in-ground problem hi viscous flow. The steady, incompressible Navier-Stokes equations are solved using a grid generation program developed by the authors, and the PHOENICS code. Some numerical results are presented to show the effects of Reynolds number, ground clearance, and angles of attack on the aerodynamic characteristics of a NACA 4412 airfoil.
96 citations
84 citations
TL;DR: In this paper, the authors investigate the feasibility of a new type of multiple component force balance for measurements on models in hypervelocity hows of millisecond duration, which extends the concept of the single component stress-wave force balance to measurement of axial force, normal force, and pitching moment.
Abstract: The authors investigate the feasibility of a new type of multiple-component force balance for measurements on models in hypervelocity hows of millisecond duration. The balance extends the concept of the single component stress-wave force balance to measurement of axial force, normal force, and pitching moment. Numerical modeling of the performance of the balance shows that coupled deconvolution techniques can be used to decouple the signals from axial strain measurements in the balance to determine the applied loads. Experiments performed in the T4 free-piston shock tunnel on a sharp cone at incidence indicate that the prototype balance performs satisfactorily, forces being measured to within 11% of their theoretical values, and the location of the line of force being measured to within 2.1% of the theoretical location as a fraction of model chord.
76 citations
72 citations
TL;DR: In this article, a collation of the results of several investigators for the aerodynamic forces and moments on lorries and railway containers in high cross winds, which were obtained in a variety of different wind tunnel simulations are mainly considered.
71 citations
01 Jul 1996
TL;DR: In this paper, a force analysis of a cylindrical liquid element subjected to an aerodynamic drag force was performed and the results indicated that for larger injection velocity conditions liquid jets penetrate relatively far into the crosse fields and exhibit surface breakup processes before the column breaks.
Abstract: The breakup processes of liquid jets injected into subsonic air crosse ows were experimentally studied. Test liquids, injector diameters, and air Mach numbers were varied to provide a wide range of jet operation conditions. Results indicate that for larger injection velocity conditions liquid jets penetrate relatively far into the crosse ows and exhibit surface breakup processes before the column breaks. Liquid column trajectories were correlated by liquid/air momentum e ux ratios based on a force analysis of a cylindrical liquid element subjected to an aerodynamic drag force. Drag coefe cients were inferred from the column trajectories and were found to exhibit a weak dependence on liquid viscosity. The heights of the column fracture points were correlated using the time required for an analogous droplet to complete an aerodynamic secondary breakup process. The success of the resulting correlation justie es the assumption that the aerodynamic forces acting on a droplet and those acting on a liquid column have similar effects. This result, combined with the trajectory correlation, leads to the conclusion that the liquid column always breaks at the same streamwise location, in agreement with the present experimental observation.
68 citations
TL;DR: In this article, wind tunnel tests were made to evaluate the lift and drag of rectangular planform wings at a Reynolds number lower than that attained by Schmitz (Re≥4.2 x 10 4 )
Abstract: Wind tunnel tests were made to evaluate the lift and drag of rectangular planform wings at a Reynolds number lower than that attained by Schmitz (Re≥4.2 x 10 4 )
60 citations
51 citations
TL;DR: In this article, the authors investigated the active control of panel flutter with piezoelectric transducers and including linearized potential flow aerodynamics, which was accomplished by approximating the aerodynamic generalized forces with infinite impulse response filters.
Abstract: This article investigates the active control of panel flutter with piezoelectric transducers and including linearized potential flow aerodynamics. The aerodynamic modeling is accomplished by approximating the aerodynamic generalized forces with infinite impulse response filters. These filters are coupled to the in vacuo panel dynamic system in feedback, thus, creating a coupled, aeroelastic system. The panel model is developed from a Rayleigh-Ritz approach and includes the mass and stiffness effects of a piezoelectric transducer. Acting as a self-sensing actuator, the piezoelectric transducer is used to implement direct rate feedback control. Results of an analytical implementation of this control system demonstrate a significant increase in the flutter boundaries.
TL;DR: In this paper, the nonlinear indicial response (NIR) methodology and the concept of critical states accompanied by changes in the flow structure and topology were used to provide a rational framework for the analyses and modeling of smoke flow.
Abstract: Swept and delta wings maneuvering at moderate and high angles of attack produce highly nonlinear and often discontinuous aerodynamic forces and moments that are difficult to model. The nonlinear indicial response (NIR) methodology and the concept of critical states accompanied by changes in the flow structure and topology could provide a rational framework for the analyses and modeling of these flows. The analysis of surface oil-flow photographs and laser light sheet high-speed video images of smoke flow has been performed. The correlation of the structural and topological changes in the flow with force and moment data follows. Critical states are often accompanied by changes in the flow topology and not all topological changes produce measurable changes hi the forces and moments, however, a useful relationship may exist.
TL;DR: In this paper, the effects of aerodynamic forces upon the orbits of satellites can be isolated, using precise orbital analysis (POA), when examining the orbit of a single satellite, the solution is restricted by the limited observational data, atmospheric model errors and the inevitable correlations between the atmospheric and aerodynamic models.
01 Apr 1996
TL;DR: In this paper, a test was conducted on a model of the NACA 0012 airfoil section with a solid upper surface or a porous upper surface with a cavity beneath for passive venting.
Abstract: A test was conducted on a model of the NACA 0012 airfoil section with a solid upper surface or a porous upper surface with a cavity beneath for passive venting. The purposes of the test were to investigate the aerodynamic characteristics of an airfoil with full-chord porosity and to assess the ability of porosity to provide a multipoint or self-adaptive design. The tests were conducted in the Langley 8-Foot Transonic Pressure Tunnel over a Mach number range from 0.50 to 0.82 at chord Reynolds numbers of 2 x E+06, 4 x E+06, and 6 E+06. The angle of attack was varied from -1 degree to 6 degrees. At the lower Mach numbers, porosity leads to a dependence of the drag on the normal force. At subcritical conditions, porosity tends to flatten the pressure distribution, which reduces the suction peak near the leading edge and increases the suction over the middle of the chord. At supercritical conditions, the compression region on the porous upper surface is spread over a longer portion of the chord. In all cases, the pressure coefficient in the cavity beneath the porous surface is fairly constant with a very small increase over the rear portion. For the porous upper surface, the trailing edge pressure coefficients exhibit a creep at the lower section normal force coefficients, which suggests that the boundary layer on the rear portion of the airfoil is significantly thickening with increasing normal force coefficient.
TL;DR: This paper is focused on the analysis of the intersection between the air-flow field around a piece of cloth and the motion of the cloth in order to animate realistically the cloth motion in the air flow.
Abstract: This paper is focused on the analysis of the intersection between the air-flow field around a piece of cloth and the motion of the cloth in order to animate realistically the cloth motion in the air flow. Two force models, based on aerodynamic theory, have been established to calculate the force distribution of the air flow on a cloth surface. The quasi-steady force model deals with the cases in which the free stream velocity of the relative air flow changes slightly or slowly, whereas the unsteady force model deals with unsteady air flows. The resulting force distributions are coupled with a cloth-deformation model based on structural dynamics to compute the displacement and deformation of cloth under these forces. This solution cycle is repeated and the time histories of the cloth motion can be obtained which form the basis of the computer animation of cloth motion in air flow. Animation results of the movement of a piece of curtain in free air under various air flow conditions are presented to compare the performances of the two force models.
TL;DR: In this paper, the authors deal with supersonic flutter analysis of laminated composite panels exposed to a temperature field and evaluate the aerodynamic force by considering the first-order high Mach number approximation to linear potential flow theory.
01 Aug 1996
TL;DR: In this article, high quality airloads data have been obtained on an instrumented UH-60A in flight and these data provide insight into the aerodynamic limiting behavior of the rotor.
Abstract: High quality airloads data have been obtained on an instrumented UH-60A in flight and these data provide insight into the aerodynamic limiting behavior of the rotor. At moderate weight coefficients and high advance ratio limiting performance is largely caused by high drag near the blade tip on the advancing side of the rotor as supercritical flow develops on the rotor with moderate to strong, shocks on both surfaces of the blade. Drag divergence data from two-dimensional airfoil tests show good agreement with the development of the supercritical flow regions. Large aerodynamic pitching moments are observed at high advance ratio, as well, and these pitching moments are the source of high torsional moments on the blade and control system loads. These loads occur on the advancing side of the disk and are not related to blade stall which does not occur for these weight coefficients. At high weight coefficients aerodynamic and structural limits are related to dynamic stall cycles that begin on the retreating side of the blade and, for the most severe conditions, carry around to the advancing side of the blade at the presumed first frequency of the blade/control system.
TL;DR: In this paper, the singular value decomposition of a block Hankel matrix constructed from the aerodynamic matrix impulse response was used to construct a system that has common global poles. But the singular values decomposition was not applied to the single-dimensional case.
Abstract: This technique employs the singular value decomposition of a block Hankel matrix constructed from the aerodynamic matrix impulse response resulting in a system that has common global poles
TL;DR: In this article, the effects of the aerodynamic actions on the body of automobiles running at high speed on randomly profiled roads are studied, and the results show that suspension parameters should be tuned to account for aerodynamic effects which become more and more effective as the vehicle speed increases.
Abstract: The effects of the aerodynamic actions on the body of automobiles running at high speed on randomly profiled roads are studied. The forces acting on the body are introduced as functions of the air speed and some of the state variables describing the vibrations in the vertical plane of the vehicle. The linearised analytical expressions of these forces assume a very interesting form: Sky-hook springs and sky-hook dampers can be used to model the aerodynamic forces acting in the vertical plane of the vehicle body.Numerical simulations and experimental tests have been carried out to investigate the effects of aerodynamic forces on the ride comfort and road holding of an automobile running at high speed on a randomly profiled road. Both theoretical and measured data state that these effects appear to be sensible starting from vehicle speed of 40-50 m/s. Suspension parameters should be tuned to account for aerodynamic effects which become more and more effective as the vehicle speed increases. Considerable improvement of ride comfort could be gained from a proper combined design of the suspension system and body shape. (A) For the covering abstract of the symposium see IRRD 882950.
TL;DR: In this paper, a high-speed moving-track test system was designed and built for aerodynamic testing of magnetically levitated vehicle (Maglev) models, and extensive flowfield testing around the track system confirmed its capability to properly simulate the flow around the high speed tracked vehicle.
Abstract: A high-speed moving-track test system was designed and built for the aerodynamic testing of magnetically levitated vehicle (Maglev) models. The moving-track test system was configured to match wind speeds up to 150 miles per hour and to extend a sufficient distance upstream and downstream of the test-vehicle model to provide a uniform flowfield at the model nose and to simulate proper closure of the flowfield at the rear of the model. Extensive flowfield testing around the track system confirmed its capability to properly simulate the flow around the high-speed tracked vehicle. The selected Maglev model was tested with the moving-track system, and aerodynamic forces and moments were measured. Hot-wire anemometer surveys were used to examine the flowfield behind the model and the turbulence levels and transition location on the vehicle nose. Flow visualization tests were also conducted to verify the nature of the flowfield found in the velocity surveys. Testing included direct measurement of skin friction at selected locations on the model. Separate tests were conducted with the track system removed from the tunnel. The resulting data provide a database for use in the design of Maglev systems and for comparison with computer analyses.
TL;DR: In this article, a full Navier-Stokes flow simulation is performed in a streamwise periodic module of bristles corresponding to the staggered square configuration to investigate the aerodynamic bristle force distributions in brush seals used in aircraft gas turbine engines.
Abstract: This paper analytically investigates the aerodynamic bristle force distributions in brush seals used in aircraft gas turbine engines. These forces are responsible for the onset of bristle tip lift-off from the rotor surface which significantly affects brush seal performance. In order to provide an enhanced understanding of the mechanisms governing the bristle force distributions, a full Navier-Stokes flow simulation is performed in a streamwise periodic module of bristles corresponding to the staggered square configuration. As is the case with a companion paper (Sharatchandra and Rhode, 1996), this study has the novel feature of considering the combined effects of axial (leakage) and tangential (swirl) flows. Specifically, the effects of intra-bristle spacing and bristle inclination angle are explored. The results indicate that the lifting bristle force increases with reduced intra-bristle spacing and increased inclination angle. It was also observed that increases in the axial or tangential flow rates increased the force component in the normal as well as the flow direction.
Abstract: Under a simplifying assumption of uncharged particles, the adhesion theory was used in conjunction with the boundary layer theory to derive an expression for the lower limit of velocity for the resuspension of fibrous particles from flat surfaces. The theory, albeit approximate, suggests that mineral fibers with diameters less than 9 μm diameter will not be resuspended by bulk air velocities less than 10 m/s. Since electrostatic forces, fiber orientation or high humidity will require a higher aerodynamic force to lift the fibers off a surface, this theoretical limit is conservative for environmental exposure considerations. The theoretical calculations were experimentally confirmed by observing the behavior of settled single fibers in turbulent air flow. The results of the theoretical analysis and the subsequent experimental confirmation suggests that the concerns about the aerodynamic resuspension of mineral fibers within a biologically significant size range are overstated.
TL;DR: In this article, new criteria for the ovalling onset of thin circular cylindrical shells exposed to a cross-flow were proposed, based on a balance between the negative aerodynamic damping and the shell structural damping coefficients.
01 Jan 1996
TL;DR: In this paper, a transient aeroelastic rotor response analysis for shipboard engage/disengage sequences is used to calculate quasi-steady or unsteady aerodynamic loads in linear and nonlinear regimes.
Abstract: : A previously developed transient aeroelastic rotor response analysis for shipboard engage/disengage sequences is utilized in the present research. The blade has elastic flap and torsion degrees of freedom and the equations of motion are discretized using the finite element method. The discretized equations of motion are integrated for a specified rotor speed run-up or run-down profile. Blade element theory is used to calculate quasi-steady or unsteady aerodynamic loads in linear and nonlinear regimes. Three deterministic wind gust distributions can be used to model the ship air wake environment. This analysis is modified to include a flap stop which restrains upper flap motion and a flap damper which damps flap hinge motion. In addition, an arbitrary gust model is incorporated into the analysis to enable more realistic airwake models. Validation studies are conducted using experimental data collected from a ship/helicopter model placed in a wind tunnel. Theoretical prediction show good agreement with experimental data for windward hub locations on the deck. A study of the effectiveness and feasibility of a flap damper placed at the flap hinge is conducted. It indicates that a flap damper is an effective and feasible method to reduce downward tip deflections for an H-46 if the flap stop angle is raised. One study of the effects of pilot controllable parameters shows that the H-46 throttle advancement rate reduces the maximum downward tip deflections for spatially varying gusts.
TL;DR: In this paper, a quasisteady method is presented where the results of steady computational fluid dynamics (CFD) calculations are used to obtain generalized aerodynamic forces for flutter analysis.
Abstract: A quasisteady method is presented where the results of steady computational fluid dynamics (CFD) calculations are used to obtain generalized aerodynamic forces for flutter analysis. For high-speed flows, the method provides a bridge between the computational efficiency, but relative, inaccuracies of piston theory and the greater accuracy, but high, computational cost of CFD flutter calculations. The method uses the structure's vibratory modes to modify the boundary conditions in the steady CFD calculations. Two steady CFD solutions are required per vibratory mode: one for the static part and one for the harmonic part of the pressure distribution. The pressure distributions of these solutions can be used to compute generalized aerodynamic forces necessary for flutter analysis. Sample two- and three-dimensional aerodynamic force calculations are provided demonstrating the method, and a flutter analysis of a National Aerospace Plane type wing is also discussed.
TL;DR: In this article, an implicit two-dimensional CFD solver is used to predict the rotor blade forced response due to the static pressure distortion from the downstream stator vanes and struts.
Abstract: A blade forced response prediction system has been developed using an implicit two-dimensional CFD solver to model the rotor blade forced response due to the static pressure distortion (potential disturbance) from the downstream stator vanes and struts. The CFD solver predicts the static pressure distortion upstream of the stator vanes and struts, which is used to calculate the induced velocity perturbation at the rotor inlet. Using the velocity perturbation and the blade's natural frequencies and mode shapes from a finite element model, the unsteady aerodynamic modal forces and the aerodynamic damping are calculated. A modal response solution is then performed. The results show that the stator vanes cause a significant amplification of the potential disturbances due to the struts. Effects of strut and vane modifications are examined using the analysis. A vane modification with an optimized flow angle distribution shows that the disturbance can be greatly reduced. Recent testing of the strut modification shows exceptional correlation with the prediction.
TL;DR: In this paper, the aerodynamic forces surrounding two bodies in relative motion in a fluid at rest in three dimensions were investigated using the unsteady boundary element method, which is very convenient for obtaining an approximate expression of the velocity potential, especially for practical use.
Abstract: In this paper a numerical analysis was made to investigate the aerodynamic forces surrounding two bodies in relative motion in a fluid at rest in three dimensions. The unsteady boundary element method was employed in the numerical calculations. This method is very convenient for obtaining an approximate expression of the velocity potential, especially for practical use. The passing-by of two spheres in an incompressible perfect fluid which extends to infinity is treated by the present method. The resultant pressure coefficients on two spheres passing each other in opposite directions are calculated and discussed numerically. Numerical examples are presented to show the validity of the present method. The method is also applied to the calculation of the passing-by of two trains in an open area in order to investigate its applicability.
TL;DR: In this article, a method for flutter analysis and design of missiles with pneumatic fin actuators is presented, which combines state-space aeroelastic models with nonlinear PNE models for time-domain simulations and for frequency-domain approximate solutions.
Abstract: A method for flutter analysis and design of missiles with pneumatic fin actuators is presented. The method combines state-space aeroelastic models with nonlinear pneumatic models for time-domain simulations and for frequency-domain approximate solutions. The missile and its fins are represented by normal modes with the generalized unsteady aerodynamic force coefficients approximated by rational functions. Time simulations are presented for various velocities, maneuver commands, and pneumatic parameters, and the system behavior at the flutter boundary is discussed. It is shown that the fundamental fin flutter speed is strongly dependent on the missile maneuver commands. The flutter mechanism is investigated with equivalent frequency-domain modes. A way to increase the flutter speeds by introducing chordwise-bending flexibility near the fin trailing edge is introduced and demonstrated.