Showing papers on "Vortex lattice method published in 1996"

A vortex lattice method for high-speed planning

Abstract: SUMMARY A three-dimensional numerical model using vortex lattice methods (VLMs) is developed to solve the steady planing problem. Planing hydrodynamics have similarities to the aerodynamic swept wing problem-the hdamental difference being the existence of a free surface. Details of the solution scheme are discussed, including the special features of the VLM used here in obtaining accurate flows at the leading and side edges. Computational results are presented and compared with existing theories and experiments.

Steady waves and forces about a yawing flat plate

Abstract: An analysis of the flow about a free-surface-piercing yawed plate in steady drift, or in steady turning motion, is presented. The problem is studied by means of an inviscid rotational flow model, based on an integral representation for the velocity field in terms of source and surface vorticity distributions. The related integral equations are numerically solved by coupling a source panel technique together with the vortex lattice method for treating the surface vorticity integrals. The free surface conditions are linearized with respect to the double model lifting flow, and the vortex shedding has been modeled in a simple but effective manner. A special effort has been put in the validation of the present approach by comparing the obtained results with several experimental data and previous numerical computations. The keel vortex shedding, as was expected by virtue of the low draft-to-length ratio typically encountered in naval architecture, has been found crucial for the prediction of the hydrodynamic forces on a maneuvering ship.

Measurements in Vortex Wakes Shed by Conventional and Modified Subsonic Aircraft

Abstract: A theoretical and experimental program is underway at NASA Ames Research Center to first obtain a better understanding of the hazard posed by the vortex wakes of subsonic transports, and then to develop methods on how to modify the wake-generating aircraft in order to make the vortices less hazardous. This paper summarizes results obtained in the 80- by 120-Foot Wind Tunnel at NASA Ames Research Center on the characteristics of the vortex wakes that trail from 0.03 scale models of a B-747 and of a DC-10. Measurements are first described that were taken in the wakes with a hot-film anemometer probe, and with wings that range in size from 0.2 to 1.0 times the span of the wake generating models at downstream distances of 81 ft and 162 ft. behind the wake-generating model; i.e., at scale distances of 0.5 and 1.0 mile. The data are then used to evaluate the accuracy of a vortex-lattice method for prediction of the loads induced on following wings by vortex wakes.

Compressible potential flow around a helicopter rotor in arbitrary motion

Abstract: To consider compressible transonic flow effects a vortex lattice method for the computation of helicopter rotor flow is coupled with a field panel method. For this purpose Cartesian grids are used which are not adapted to the contour and only discretise the domain of the nonlinear flow. The basis of this procedure is the separation of the full potential equation into the Laplacian operator and the nonlinear terms. Several methods, including the panel-clustering technique are presented to reduce the computational effort of the method. The developed program ROFPM is validated at several experimental test cases of the Caradonna rotor.

Wing in heaving oscillatory motion

Abstract: A wing in steady flight near a wavy surface, such as in the case of a large, transoceanic wingship, is simulated by a wing oscillating in heave near a flat surface. In accord with the wingship, small AR and slight camber are considered. In the present study, consideration is limited to the case when the wavelength of the surface is much greater than the chord of the wing and the slope of the surface is much less than unity. The numerical simulation predicts that the mean aerodynamic loads on a wing executing a simple-harmoni c heaving motion are higher than the corresponding loads on the same wing in steady flight at the mean height and the same angle of attack. These preliminary results suggest that it would be beneficial to fly near the waves and that doing so would improve the aerodynamic efficiency. Also included in the present results are numerical simulations of the wakes that show the strong influences of the ground and the oscillations on their behavior.

Calculation of influence of propeller slipstream on marine rudder performance

Abstract: An advanced computational method was applied to calculate forces and moments on a rudder located in a propeller slipstream. The vortex lattice method was applied for a screw propeller to determine velocity distribution in the propeller slipstream. Surface vorticity distribution was used to calculate pressure distribution on the rudder surface. A series of calculations were performed to validate the method and the computer program. The results were compared with available experimental data and the reliability of the presented model was confirmed.

Numerical calculation of interaction of propeller slipstream on flowfield over a complete aircraft

Abstract: A lifting surface analysis of vortex lattice method is completed. The coupling between the slipstream and the flow over the aircraft is obtained by using the Von Neumann condition at each control point of the aircraft and propeller panels. The slipstream contraction which definitely exists behind the propeller has been accounted. Practical computations for Y\|7 aircraft with double propellers show that the influences of the slipstream on the lift characteristics with deflected flaps and on the pitching moment coefficient of the complete aircraft are quite obvious. The computation results are in good agreement with experiment data. This method can also be applied to multi and single propeller aircraft.

Numerical Analysis of a Horizontal Axis Wind Turbine Rotor with Winglets.

Abstract: The objective of present study is to show the aerodynamic effectivity of a horizontal axis wind turbine rotor blades with winglets by means of numerical analysis. The winglet used in this study is considered to be an inclined extension of the blade. For the numerical analysis a vortex lattice method with a free wake model was used because the model can be fitted to an arbitrary blade shape and needs no empirical parameter about wake geometry. The calculations were made on the flow held in the rotor wake and the rotor performance, and the results were compared between the rotors with and without winglets. In order to examine the structural effects, the flap bending moment was also compared, The results shows that small installation angle of winglets is found to cause a larger increase in the power coefficient and a smaller increase in the flap bending moment than radially extended rotor blades.

Application of the Vortex Lattice CFD Method to Obtaining Unsteady Aerodynamic Predictions About Wing/Fuselage/Pylon/Store Configuration Including Store Separation

Abstract: : The Unsteady Vortex Lattice Method (UVLM) was used to predict the geometry of the wake and the aerodynamics loads on the Wing/Fuselage/Pylon/ Store (W/F/P/S) configuration in an incompressible flow. The main emphasis was placed on a practical and cost effective engineering solution of the complex problem with a reasonable computational efficiency allowing the computer code to run on personal computers. The mutual interaction between the configuration and the wake flow, which is not known a priori, was studied. Due to the interaction between the configuration and its wake, a new longitudinal vortex develops between the wing-tip and the wing-fuselage junction. It was shown that the wake development influences the unsteady aerodynamic forces acting both on the wing and the external store. Computed flow field simulations were presented for various angle of attack conditions. The effect of the pylon/store location on the wing aerodynamic coefficients were investigated. The external store separation under the influence of the wake rollup was modeled by considering the full mutual interaction between the store and the W/F/P system. The results show that the method is capable of simulating the unsteady aerodynamic interference between the moving store and the flow around the W/F/P configuration.

Generalized vortex lattice method for oscillating lifting surfaces in planar supersonic flow

Abstract: The velocity potential formulation is used to study the behavior of lifting surfaces oscillating harmonically in supersonic flow. Here the basic singularity is a doublet of constant density, corresponding to quadrilateral panels distributed over the wing and wake. The advantage of the present method over others available in the literature is the unification of the solution for the subsonic and supersonic regimes. Steady-state results are presented for both rectangular and delta wings, and they compare very well with analytical results when the wing aspect ratio is varied. Unsteady results along the symmetry line for a pitching rectangular wing are also presented, and again the agreement is excellent with bidimensional values. Finally, generalized aerodynamic coefficients are calculated and compared with several results available in the literature.

Convergence of Panel Methods Using Discrete Vortices around Two-Dimensional Bodies

Abstract: The panel method in which vorticity is distributed on the surface of a body, the surface vorticity distribution method such like the vortex lattice method, is useful for determining the Euler flow past bodies and recently has been applied to complicated body shapes together with the vortex method. In the present paper we demonstrate the mathematical analysis of two panel methods for inviscid incompressible flow past two-dimensional bluff bodies, the pointwise and piecewise-linear distributions of vortices. The governing singular integral equation is derived by distributing vortices on the surface of the body and the simultaneous linear algebraic equations are derived by approximating the integral equation by the discretization of vorticity distribution. The analytic solution of the linear algebraic equations is obtained and the accuracy of the approximate solution is discussed. We show that : (1) there exists an appropriate collocation point, (2) the accuracy of the vortices on the surface of the body is of the order of 1/n where n is the panel number and (3) the eigensolution of the governing integral equation cannot be obtained using these schemes unless they are modified.