Showing papers on "Vortex lattice method published in 1985"

Subsonic wing rock of slender delta wings

Abstract: Two recent experimental studies investigated the s elf-excited motion Of a flat deita wing that was free to roll about an axis parallel to its mid-span chord. In this paper these experiments are s imulated numerically. An unsteady vortex-lattice method is used to provide the aerodynamic loads, and the equation of motion is integrated by a prediction-corre ction scheme. The solution provides complete histories of the motion of the wing and,the fl owfi eld simultaneously, fully accounting for dynamic-aerodyna mic interaction. The present simulation predicts that the symmetri c configuration of the 1 eadi ng-edge vortex system becomes unstable as the anyle of attack increases. Sequential views of the computed, time-dependent shape of the leading-edge system Show how this causes a loss of roll damping at small a nyles of roll. Consequently, at sufficiently high incidence, s mall disturbances i ntroduced into the flowfield grow, causing wing rock to develop. For anyles of attack at onset and amplitudes and periods of the ensuing limit cycles, the numerical predictions and the experimental o bservations are in agreement. The simulation shows the influence of the experimental parameters and provides an explanation for the differences in the observations

Calculation of the Aerodynamic Forces on Automotive Lifting Surfaces

Abstract: A numerical technique was developed to investigate the performance of automotive lifting surfaces in close proximity to ground. The model is based on the Vortex Lattice Method and includes freely-deforming wake elements. The ground effect was simulated by reflection and both steady and unsteady pressures and loads on various wing planforms were considered. Calculated results are presented for wings having both positive and negative incidences, with and without ground effect. Also the transient lift of a wing in a plunging motion was analyzed in ground proximity and at a negative angle of attack. Finally the periodic lift fluctuations on the front winglet of a racing car, due to its suspension oscillations, were calculated and found to exceed approximately twice the steady-state value.

A vortex-lattice method for general, unsteady aerodynamics

Abstract: A general method of calculating unsteady, incompressible, inviscid, three-dimensional flows around arbitrary planforms has been developed. The method is an extension of the vortex-lattice technique. It is not limited by aspect ratio, camber, or angle of attack, as long as vortex breakdown does not occur above the surface of the wing and separation occurs only along sharp edges. As the wing performs arbitrary maneuvers, the position of the wake and the distribution of circulation on the wing and in the wake are obtained as functions of time. One desirable feature of the present method is its ability to treat steady lifting flows very efficiently. Several examples of steady and unsteady flows are presented. These include rectangular wings, with and without flaps, delta, and cropped delta wings.

Dynamic ground effects on a two-dimensional flat plate

Abstract: The effect of time-variant vortex shedding is simulated by a sequence of discrete vortices convecting downstream in the wake of a two-dimensional flat plate whose lifting condition is modeled by means of the quasi-vortex lattice method. The boundary condition of this problem is specified in such a way that the tangency condition on the surface of the flat plate is satisfied; the boundary condition also takes into account the effect of airfoil motion relative to the ground. Significant lift changes are shown to occur, due to the dynamic ground effect, that are crucial in aircraft takeoff and landing transitions.

Vortex trajectories and breakdown on wing-canard configurations

Abstract: Flow visualization and force measurement experiments are carried out on close-coupled wing-canard and wing-alone configurations. The effects of the canard sweep angle and longitudinal position on the leadingedge vortex trajectories, their breakdown characteristics, and the configuration aerodynamic coefficients, are studied. The effects of highly (75 deg) and moderately (56 deg) swept canards of equal area are compared. Results show that the canard displaces the leading-edge vortex of the wing upward and outboard in the vicinity of the trailing edge. Increasing the angle of attack results in an upward displacement of these vortices. The angle of attack for which the wing-vortex breakdown points cross the trailing edge on the wing-canard configurations is 8-10 deg higher than in the wing-alone configuration. Increasing the longitudinal separation between the canard and the wing reduces the wing-canard interference. Force measurements show that the normal-force coefficient of the wing—highly-swept-canard configuration at high incidence can be smaller than for the appropriate wing-alone coefficients. This may indicate that the strong leading-edge vortices of the highly-swept canard, which at high incidence are very close to the wing surface near the trailing edge, alter the leeside flow structure there and reduce the lift.

High angle-of-attack calculations of the subsonic vortex flow on slender bodies

Abstract: A nonlinear vortex-lattice method is utilized for the calculation of separated vortex flows over slender bodies at high incidence. Symmetric and asymmetric vortex flow cases are calculated showing good agreement with recently obtained experimental data for an ogive-cyclinder body at high Reynolds numbers. The only input needed for these calculations is the positions of the separation lines on the body. Studies of some of the numerical aspects of this method are described and conclusions are implemented to improve the calculations.

Aerodynamic tradeoff study of conventional, canard, and trisurface aircraft systems

Abstract: Conventional, canard, and three-surface aircraft configurations are investigated analytically to determine each configuration's induced and viscous drag under trimmed conditions. A three-surface vortex lattice method is used to trim the aircraft, as well as to predict the induced drag of each configuration. A vortex panel method in conjunction with the momentum integral boundary-layer method is used to predict inviscid and viscous characteristics. Parameters varied including wing to stabilator surface area ratio, static margin, canard to tail loading ratio, and CL^ . For all of the parameters considered, the conventional configuration had the highest ^tnm^' ^ e " stabilator aspect ratios, the CL . /CD of the conventional aircraft was the highest, whereas for the highest stabilator aspect ratio considered the canard configuration had the highest CL /Cp. The trisurface was superior to the canard at the lower aspect ratio with the canard becoming superior at the higher values.

Propeller aerodynamic performance by vortex-lattice method

Abstract: It is inappropriate to apply classical propeller theories to design an advanced turboprop (ATP) In this paper, the vortex-lattice method is applied to rotating blades It is assumed that the flow is inviscid and incompressible However, the compressibility effect is included in the calculations through the Prandtl-Glauert similarity rule The other properties characteristics of an ATP, ie, the effect of displacement velocities, the interference effect between blades, and the effect of flow deflection by a spinner and nacelle, are introduced into the calculations Powers, thrusts, and efficiencies of two kinds of ATP, SR-1 and SR-3, are obtained and compared with experimental values The numerical values show that they agree well with the experimental results of SR-1 However, the former is larger than those of SR-3 This is consistent with the calculations by Hamilton Standard The principal reason for this difference is that the portion of the blade near the tip is distorted by centrifugal force Using this method, the ATP performance can be calculated accurately below Mach 06 However, the transonic wing theory should be applied for performance estimates at Mach 07-08

Convergence characteristics of a vortex-lattice method for nonlinear configuration aerodynamics

Abstract: The convergence characteristics of a vortex-lattice method for the high-angle-of-attack, nonlinear aerodynamics of aircraft and missile configurations are studied parametrically. The solution for the vortex intensities is determined by the tangency boundary condition on all of the configuration surfaces, including the three-dimensional, rolled-up wakes that characterize such flowfields. The a priori unknown position of the wake that renders this problem nonlinear is determined by an iterative process. Since there is no proof for the existence and uniqueness of this process, this paper investigates the effects of several geometrical and numerical parameters on the converged solution. It was found that the convergence of the iterative solution procedure is usually rapid and is not affected by initial conditions of the first iteration and by the integration method of the wake streamlines. Grid refinement leads to a converged solution, but its final values vary with wing surface paneling and wake discretization schemes within some range in the vicinity of the experimental data.

VORSTAB: A computer program for calculating lateral-directional stability derivatives with vortex flow effect

Abstract: A computer program based on the Quasi-Vortex-Lattice Method of Lan is presented for calculating longitudinal and lateral-directional aerodynamic characteristics of nonplanar wing-body combination. The method is based on the assumption of inviscid subsonic flow. Both attached and vortex-separated flows are treated. For the vortex-separated flow, the calculation is based on the method of suction analogy. The effect of vortex breakdown is accounted for by an empirical method. A summary of the theoretical method, program capabilities, input format, output variables and program job control set-up are described. Three test cases are presented as guides for potential users of the code.

Aerodynamic canard/wing parametric analysis for general-aviation applications

Abstract: Vortex panel and vortex lattice methods have been utilized in an analytic study to determine the two- and three-dimensional aerodynamic behavior of canard and wing configurations. The purpose was to generate data useful for the design of general aviation canard aircraft. Essentially no two-dimensional coupling was encountered and the vertical distance between the lifting surfaces was found to be the main contributor to interference effects of the three-dimensional analysis. All canard configurations were less efficient than a forward wing with an aft horizontal tail, but were less sensitive to off-optimum division of total lift between the two surfaces, such that trim drag could be less for canard configurations. For designing a general aviation canard aircraft, results point toward large horizontal and vertical distance between the canard and wing, a large wing-to-canard area ratio, and with the canard at a low incidence angle relative to the wing.

Application of the vortex-lattice concept to flows with smooth-surface separation

Abstract: : Numerical schemes based on the vortex-lattice concept have had considerable success in treating the fully three-dimensional separated potential flow over low aspect-ratio thin wings at high incidence and the attached potential flow over bodies of more general shape. In principle, the vortex-lattice method is not limited in application ao long as convection dominates vorticity diffusion. A non-linear 3-D vortex-lattice method was developed which treats the steady separated flow over prolate bodies with open separation moving through an inviscid incompressible fluid. The strength and position of the body wake were found as part of the solution. Flows with smooth-surface separation were considered as opposed to flows with sharp-edge separation treated with the vortex-lattice concept in the past. Results for flow over an inclined ogive-cylinder demonstrated the technique. For attached flow, comparisons are presented of the results from the vortex-lattice method using optimal and average control point locations with the results of the source-distribution method and with experimental data. The same panel arrangement was used calculating both methods. The results of the present method are somewhat more sensitive to panel arrangement than are those for the source-distribution method. Also, the effect of control point location varies dramatically as the incidence of the body is changed. For separated flow, results of the vortex-lattice method were compared with experimental data and with the results of a typical 2-D analogy. The present method agrees favorably with the experimental data windward of a separation line.

Lift due to thickness for low aspect ratio wings in incompressible flow

Abstract: The problem under consideration is a numerical study of the effects of thickness on lift for low aspect ratio wings in steady incompressible inviscid flow at moderate angles of attack. At these angles of attack the flow separates along the leading edge giving rise to a lift substantially higher than that computed by classical attached flow potential theory. The problem is treated as a perturbation expansion in a small thickness parameter. The lifting elements of the flow are modeled using a nonlinear vortex lattice method which replaces the leading and trailing edge vortex sheets by segmented straight vortex filaments. The thickness elements of the flow are modeled with a mean plane source distribution and a modification to the wing boundary conditions. Results are obtained for wings with biconvex and NACA 0012 sections which compare well with available experimental data. The important observation that the effect of thickness is to decrease the lift is made.