Showing papers on "Vortex lattice method published in 1994"

Eigenanalysis of unsteady flow about airfoils, cascades, and wings

Abstract: A general technique for constructing reduced order models of unsteady aerodynamic flows about twodimensional isolated airfoils, cascades of airfoils, and three-dimensional wings is developed. The starting point is a time domain computational model of the unsteady small disturbance flow. For illustration purposes, we apply the technique to an unsteady incompressible vortex lattice model. The eigenmodes of the system, which may be thought of as aerodynamic states, are computed and subsequently used to construct computationally efficient, reduced order models of the unsteady flowfield. Only a handful of the most dominant eigenmodes are retained in the reduced order model. The effect of the remaining eigenmodes is included approximately using a static correction technique. An important advantage of the present method is that once the eigenmode information has been computed, reduced order models can be constructed for any number of arbitrary modes of airfoil motion very inexpensively. Numerical examples are presented that demonstrate the accuracy and computational efficiency of the present method. Finally, we show how the reduced order model may be incorporated into an aeroelastic flutter model.

Validation of Vortex-Lattice Method for Loads on Wings in Lift-Generated Wakes

Abstract: A study is described that evaluates the accuracy of vortex-lattice methods when they are used to compute the loads induced on aircraft as they encounter lift-generated wakes. The evaluation is accomplished by the use of measurements made in the 80 by 120 ft Wind Tunnel of the lift, rolling moment, and downwash in the wake of three configurations of a model of a subsonic transport aircraft. The downwash measurements are used as input for a vortex-lattice code in order to compute the lift and rolling moment induced on wings that have a span of 0.186, 0.510, or 1.022 times the span of the wake-generating model. Comparison of the computed results with the measured lift and rolling-moment distributions the vortex-lattice method is very reliable as long as the span of the encountering or following wing is less than about 0.2 of the generator span. As the span of the following wing increases above 0.2, the vortex-lattice method continues to correctly predict the trends and nature of the induced loads, but it overpredicts the magnitude of the loads by increasing amounts.

Helicopter rotor lift distributions for minimum induced power loss

Abstract: A method is described for solving the minimum-induced loss (MIL) rotor design problem. First, the generalized Betz condition for MIL rotors is developed. Because the resulting lift distributions would generally exceed the maximum blade lift coefficient on the retreating side of the rotor, the necessary conditions are extended to include constraints on the lift. A method for solving for the optimum lift distribution using finite elements is described. Numerical results are presented for a typical rotor in forward flight. The MIL rotor may have on the order of 10% less induced power loss than a typical unoptimized rotor.

Numerical simulation of steady and unsteady, vorticity-dominated aerodynamic interference

Abstract: The problem of modeling steady and unsteady aerodynamic interference is discussed. A configuration resembling the X-29 is used as an example. The general unsteady vortex-lattice method is used to model the flowfield. By considering the components operating alone and as members of the complete configuration, we demonstrate the importance of accurately simulating the wakes of the upstream components. The wakes of the canards as well as the canards themselves have a strong negative influence on the lift generated by the main wing; the main wing has a positive influence on the lift generated by the canards. The forward sweep of the main wing tends to focus the generated upwash in the vicinity of the canards. It is shown that the maximum influence of the vorticity shed from the canards does not develop until the shed vorticity convects downstream directly over the main wing. The general unsteady vortex-lattice method appears to be a reasonably accurate model of closely coupled, vorticity-dominated flowfields as long as the lines of the separation are known and vortex bursting does not occur near the wings.

Pitch-up characteristics for hsct class planforms: survey and estimation

Abstract: Low aspect ratio, highly-swept cranked delta and arrow wing planforms are often proposed for high-speed civil transports. These wing planforms offer low supersonic drag without suffering greatly from low liftldrag ratios in low-speed flight. They can, however, suffer from pitch-up at modest angles of attack (as low as 5' angle of attack) during low-speed flight due to leading edge vortex influence, flow separation and vortex breakdown. This paper describes an investigation conducted to study past research on the longitudinal aerodynamic characteristics of highlyswept cranked wing planforms and a new method to estimate pitch-up. The survey of past research placed emphasis on 1) understanding the problem of pitch-up, and 2) ascertaining the effects of leading and trailing edge flaps. The estimation method uses a vortex lattice method to calculate the inviscid flow solution. Then, the results are adjusted to account for flow separation on the outboard wing section by imposing a limit on the equivalent 2-D sectional lift coefficient. The method offers a means of making low cost estimates of the non-linear pitching moment characteristics of slender, cranked arrow wing configurations. Numerous comparisons with data are included.

Vortex-Wing Interaction of a Close-Coupled Canard Configuration

Abstract: The thin-layer Navier-Stokes equations are solved numerically to investigate the effects of canard vertical position on a close-coupled, canard-wing-body configuration at a transonic Mach number of 0.90 and angles of attack ranging from — 2 to 12 deg. Canard-wing interactions are investigated for the canard positioned above, coplanar with, and below the wing (high-, mid- and low-canard positions, respectively). The computational results show favorable canard-wing interactions for the high- and midcanard configurations. The unfavorable lift and drag characteristics for the low-canard configuration are examined by analyses of the low-canard flowfield structure and found to be directly attributed to the interaction between the canard vortex and the wing surface. At relatively low angles of attack, the low-canard vortex passes under the wing surface and induces low pressures on the wing lower surface. As angle of attack is increased, the low-canard vortex impacts the wing surface and is split into two distinct vortices.

Investigation on Propeller-Rudder Interaction by Numerical Methods

Abstract: The axial forces in a propeller-rudder interactive system working in uniform flow are theoretically studied: In a steady linear method the propeller is represented by bound and free vortex systems, the rudder by vortex and source filaments and the rudder wake by free vortex filaments A lifting-line analysis method predicts the propeller characteristics A vortex lattice method determines the rudder vortex strengths Comparisons of the axial forces with systematic experiments show a qualitative agreement A thick rudder located in the accelerating flow just behind a propeller experiences a pressure drag It is shown that this drag is an internal force which is counterbalanced by part of the increased propeller thrust Due to the tangential velocities in the propeller slipstream a rudder thrust is created The rudder in the test cases recovers 39% of the rotational energy, transforms 14% into radial energy and leaves 26% behind The remaining 21% should in principle have transformed into axial energy but is missing according to the linear method The drawback of the linear method is that the boundary conditions at the free vortex system of the propeller are not satisfied A semi-nonlinear method is developed where the boundary condition that the static pressure is the same on the two sides of the free vortex sheet of a simplified propeller is fulfilled As a result the strength-density variations of the propeller free vortex sheet due to the rudder disturbance are taken into account The rudder thrust computed by the method is 4% lower than that given by a corresponding linear method for a propeller at C T =11 A nonlinear method is developed where also the boundary conditions of zero normal velocity at the propeller free vortex sheet is satisfied, resulting in a deformed slipstream A smaller, improved rudder thrust is obtained For a propeller at C T =11, the rudder thrust is 8% lower than that obtained by a corresponding linear method; For another propeller at C T =30, the rudder thrust is 21% lower than that obtained by a linear method

Phase transitions and connectivity in three-dimensional vortex equilibria

Abstract: The statistical mechanics of collections of closed self avoiding vortex loops on a lattice are studied. The system is related to the vortex form of the three dimensional XY model and to lattice vortex equilibrium models of turbulence. The system exhibits vortex connectivity and screening effects, and models in vorticity variables the superfluid transition. The equilibrium states of the system are simulated by a grand canonical Monte Carlo method. A set of geometric transformations for self-avoiding loops is developed. The numerical method employs histogram sampling techniques and utilizes a modification to the Metropolis flow which enhances efficiency. Results are given for a region in the temperature-chemical potential plane, where the chemical potential is related to the vortex fugacity. A line of second order transitions is identified at low temperature. The transition is shown to be a percolation threshold at which connected vortex loops of infinite size appear in the system. The nature of the transition supports the assumption that the lambda transition in bulk superfluid helium is driven by vortices. An asymptotic analysis is performed for the energy and entropy scaling of the system as functions of the system size and the lattice spacing. These estimates indicate that the infinite more » temperature line is a phase boundary between small scale fractal vortices and large scale smooth vortices. A suggestion is made that quantum vortices have uniform structure on the scale of the lattice spacing and lie in the positive temperature regime, while classical vortices have uniform structure on the scale of the domain and lie in the negative temperature regime. « less

A numerical study of the hydrodynamic forces developed by a marine rudder

Abstract: The flow around a ship's rudder has been modeled suing the vortex lattice method. The technique has been used to study the forces, particularly the side force, developed by the rudder in uniform and nonuniform flow. The model was extended to include the effect of a plate above the rudder in order to simulate the hull. A final part of the work was a study of the effect of the deadwood ahead of the rudder. The results obtained from the calculations have shown that a small side force can be created if the rudder is placed symmetrically along the propeller axis when the gap between the top of the rudder and the hull is not large. In addition, it was found that a side force could be generated by the proximity of the rudder to the deadwood.

Numerical calculation of stability derivatives of an aircraft

Abstract: Work presents application of methods based on the Prandtl-Glauert potential model of fluid to stability derivatives calculation of an aircraft in subsonic (Vortex Lattice Method) and supersonic (Characteristic Box Method) flow. Modeling of velocity field, next used in calculating the set of stability derivatives was shown. Results of calculation were compared with experimental data.

On propulsive performance of propeller and rudder system

Abstract: In this paper a numerical lifting surface method, ie, the vortex lattice method is applied for predicting the characteristics of the propeller. The propulsive performance of the propeller-rudder system is studied and a panel method is employed to describe the performance of the rudder behind the propeller. The drag of the rudder is calculated by boundary layer theory to enhance the accuracy of the prediction. Several examples are given and the calculated results and experimental data show good agreement.

Analysis of wings with multi-segmented flaps using a non-planar vortex lattice method

Abstract: An important problem of analysis of complex wing planforms with multisegmented is addressed. Nonplanar vortices are used to simulate the wing incidence and camber effects, whereas thickness effects are incorporated using constant source flat panels.Important aspects of the current work are modeling of flow near wing-flap junctures,use of symmetrical singularity for calculating the strengths of the thickness sources,and efficient programming,which is a prerequisite while using nonplanar vortices and a versatile geometric package which generates automatically optimum lattice layout for rapid numerical convergence. The method was validated against a number of test cases for which results are available from experiments and other theories.

Design of Supercavitating Propellers Using a Vortex Lattice Method

Abstract: This paper describes a rigorous design method for supercavitating propellers (SCPs). The lifting surface correction was directly performed using a vortex lattice method. A circulation distribution was used, which was calculated using Lerb's lifting line theory. Three SCPs were designed, manufactured and tested in a large cavitation tunnel. They all generated the required thrust at the design point except the SCP designed for a higher load condition. The efficiencies obtained were reasonably high for all propellers.

A simple calculation method for thick wing

Abstract: A simple method of calculation to obtain the characteristics of a thick wing is proposed. The method uses source distribution on the wing surface and vortex distribution arranged on the camber surface, according to Lan's quasi-continuous vortex lattice method. The usefulness of the method is shown in examples for 2-D and 3-D wings, and a rudder with the angle of attack behind a working propeller.