Showing papers on "Vortex lattice method published in 2008"

Relaxed-Wake Vortex-Lattice Method Using Distributed Vorticity Elements

Abstract: A lifting-surface method is presented that uses elements having distributed vorticity to model lifting surfaces and their shed wakes. Using such distributed vorticity elements allows the representation of a force-free continuous wake-vortex sheet that is free of numerical singularities and is thus robust in its numerical rollup behavior. Unlike other potential-flow methods that use discrete vortex filaments having solid-core models at their centers to avoid problems with the singularities, the numerical robustness of the new method is achieved without the subsequent solution being dependent on the choice of a cutoff distance or core size. The computed loads compare well with results of classical theory and other potential-flow methods. Its numerical robustness, computational speed, and ability to predict loads accurately make the new method ideal for the investigation of applications in which the loadings on a lifting surface depend strongly on the influence of the wake and its shape, as is the case for the two application examples presented: formation flight and rotating-wing systems.

Effect of Stiffness Uncertainties on the Flutter of a Cantilever Wing

Abstract: This paper deals with the investigation of the influence of spanwise distribution of bending and torsion stiffness uncertainties on the flutter behavior of an aeroelastic wing using a stochastic finite element approach. The analysis adopted a numerical algorithm to simulate unsteady, nonlinear, incompressible flow (based on the unsteady vortex lattice method) interacting with linear aeroelastic structure in the absence of uncertainties. The airflow and wing structure are treated as elements of a single dynamic system. Parameter uncertainties are represented by a truncated Karhunen-Love expansion. Both perturbation technique and Monte Carlo simulation are used to establish the boundary of stiffness uncertainty level, at which the wing exhibits flutter in the form of limit-cycle oscillations and above which the wing experiences dynamic instability. The analysis also includes the limitation of perturbation solution for a relatively large level of stiffness uncertainty.

Aeroelastic Analysis of Membrane Wings

Abstract: The physics of flapping is very important in the design of agile MAVs. Such MAVs have to efficiently harness thrust and lift from flapping while maintaining a controllable configuration. The aerodynamics of a pitching and plunging flexible wing is simulated using a 3-D, free wake, vortex lattice method (VLM), and structural characteristics of the wing is simulated as a membrane supported by a rigid frame. The aerodynamics is validated by comparing the results from the VLM model for constant angle of attack flight and periodic plunging flight with analytical results, existing 2-D VLM and doublet lattice method. The aeroelasticity is studied by varying parameters affecting the flow as well as parameters affecting the structure. The results show that the aerodynamic loads increase for increased deformation, and vice-versa. For a step change in angle of attack, the membrane oscillates about the steady state deformation, influence the loads and eventually converges to steady state airloads and structural deformations. For prescribed oscillations in plunge, the membrane deformations and loads transition into a periodic steady state which lead to thrust. The thrust increases with deformation and thus the flexibility of the structure is beneficial in this case. Aeroelastic tailoring of the structure to optimize the efficiency of flapping is recommended for design of next generation MAVs.

Analytic and Experimental Investigation of Dihedral Configurations of Three-Winglet Planforms

Abstract: An analytic and experimental effort was undertaken to assess the effectiveness and efficiency of three winglets mounted chordwise to the tip of a rectangular wing. The winglets, with an aspect ratio of 4.6, were mounted on a half-span wing having an effective aspect ratio of 6.29. 13 configurations of varying dihedral arrangements were analyzed with a vortex lattice method and tested in a low-speed wind tunnel at a Reynolds number of 600,000. While the analytic method provided fair agreement with the experimental results, the predicted trends in lift, drag, and (to a lesser degree) pitching moment were in good agreement. The analytic distributions of wake velocity, circulation, and downwash angle verified that highly nonplanar configurations tended to reduce and diffuse the regions of highest circulation and to create more moderate downwash angles in the wake. This was manifest as an overall drag reduction. More specifically, the results showed that the winglets could be placed in various optimum orientations to increase the lift coefficient as much as 65% at the same angle of attack, decrease the drag coefficient as much as 54% at the same lift coefficient, or improve the maximum L/D by up to 57%. The most dramatic findings from this study show that positioning the winglet dihedral angles had the result of adjusting the magnitude and slope of the pitching moment coefficient. These observations suggest that multiple winglet dihedral variations may be feasible for use as actively controlled surfaces to improve the performance of aircraft at various flight conditions and to "tune" the longitudinal stability characteristics of the configuration.

Study on Optimization of Anti-erosion Rudder Section of Large Container Ship by Genetic Algorithm

Abstract: This paper describes the optimization of the rudder section by the genetic algorism based on VLM(Vortex Lattice Method) and panel method. The devel oped propeller-rudder analysis program has been validated by comparing with experimen tal data. The research extends to optimize the anti-erosion rudder section of the large container ship. The object function is the amount of pressure at leading edge of rudder wh ich is closely related with erosion phenomena. The optimized rudder has been compared with conventional rudder with NACA 0021 section by analyzing with the developed program. The finally optimized section has low and mild pressure distribution in comparison with the N ACA rudder. The experiments is expected to be carried out for the validation of the present optimization and more parametric study of section geometry is also expected to be conducted in the near future. ※Keywords: Rudder section optimization(방향타 단면최적화), Genetic algorithm(유전자 알고리즘), Rudder cavitation(방향타 캐비테이션), Combined analysis method of vortex lattice and panel(양력면과 양력판의 복합 해석 방법)접수일: 2008년 5월 6일, 승인일: 2008년 7월 29일†교신저자 : kmcprop@pusan.ac.kr, 051-510-2401

Sail Optimization for Upwind Sailing: Application in a Tornado, the Olympic Class Catamaran

Abstract: A study of a boat's motion is carried out in order to analyze the aerodynamic properties of the optimal sail for obtaining the maximum velocity when sailing to windward. The mechanics study shows the optimal C L and C D for a given sail and how the shape of the aerodynamic polar of the sail should be. A parametrical analysis of the aerodynamics of a sail is then carried out varying the maximum camber, position of the maximum camber in the chord direction and position of the maximum camber in the mast direction. The parametric analysis is done numerically with a vortex lattice method (VLM) and experimentally in a wind tunnel. The results show that the influence of the relevant parameters studied can be reduced to the variation of two parameters, A and B, defining the polar of the sail, C D = B + A 2 C L 2; and the influence of parameters A and B on the maximum VMG obtainable are calculated.

Calculation of Low Aspect Ratio Wing Aerodynamics by Using Nonlinear Vortex Lattice Method

Abstract: new computational procedure for the Non-Linear Vortex Lattice Method (NLVLM) is suggested in this work. Conventional procedures suggested so far usually involves inner iteration loop to update free vortex shape and an under-relaxation based iteration loop to determine the free vortex shape. In this present work, we suggest a new formula based on quasi-steady concept to fix free vortex shape which eliminates the need for inner iteration loop. Further, the ensemble averaging of the induced velocities for a given free vortex segment evaluated at each iteration significantly improves the convergence property of the algorithm without resorting to the under-relaxation technique. Numerical experiments over several low aspect ratio wings are carried out to obtain optimal empirical parameters such as the length of the free vortex segment, the vortex core radius, and the rolled-up wake length. ĀĀ

Static Aeroelastic Analysis of Hingeless Rotor System in Hover Using Free-Wake Method

Abstract: The static aeroelastic analysis of composite hingeless rotor blades in hover was performed using free-wake method. Large deflection beam theory was applied to analyze blade motions as a one-dimension beam. Anisotropic beam theory was applied to perform a cross-sectional analysis for composite rotor blades. Aerodynamic loads were calculated through a three-dimensional aerodynamic model which is based on the unsteady vortex lattice method. The wake geometry in hover was described using a time-marching free-wake method. Numerical results of the steady-state deflections for the composite hingeless rotor blades were presented and compared with those results based on two-dimensional quasi-steady strip theory and prescribed wake method. It was shown that wakes affect the steady-state deflections.

Wind tunnel study of a grid fin stabilized guided projectile

Abstract: There is a need for methods/techniques to analyze the aerodynamics of grid fins at high angle of attack. The existing methods to predict or compute the aerodynamic parameters for grid fins are inadequate at high angles of attack. The present study attempts to extend the understanding of the complex aerodynamics associated with grid fins. The analysis draws its conclusion from wind tunnel test results of five basic grid fin configurations. A vortex lattice solution was used to model the aerodynamics of the chosen grid fins. Next the analytical and empirical relations as discussed in open literature were applied to model grid fin aerodynamics at high angle of attack. The aerodynamic coefficients were computed. These coefficients were then compared with the experimental results obtained using the five grid fin configurations. To improve the matching at high angle of attack a proposed approach using Kirchoff’s theory along with vortex lattice method was applied. The results show that this approach can advantageously be used to model lattice/grid fin aerodynamics.

Unsteady Aerodynamic Coefficients Obtained by a Compressible Vortex Lattice Method

Abstract: Unsteady solutions for the aerodynamic coefficients of a thin airfoil in compressible subsonic or supersonic flows were studied. The lift, the pitch moment, and pressure coefficients were obtained numerically for the following motions: the indicial response (unit step function) of the profile, i.e., a sudden change in the angle of attack; a thin profile penetrating into a sharp edge gust (for several gust velocity ratios); a thin profile penetrating into a one-minus-cosine gust and sinusoidal gust, i.e., a typical gust used in commercial aircraft design; oscillating airfoil; and also the interaction of the profile with a convected (from convection phenomenon) vortex passing under the profile, a phenomenon known in literature as BVI (Blade Vortex Interaction) or for a profile case AVI (Airfoil Vortex Interaction). The present work uses a numerical approach based on vortex singularity. The numerical model was created through the profile discretization in uniform segments and the compressible flow vortex singularity was used. The results available in the literature are based on approximated exponential equations, or computed via Computational Fluid Dynamics (CFD). Thus, the purpose of this method is to obtain a more accurate computation compared to those of approximated equations, and quite faster than those done via CFD. The results yielded by the present methodology were also compared with solutions available in the literature. The results were obtained for subsonic and supersonic flow in compressible environment.