Showing papers on "Vortex lattice method published in 2013"

Modified Unsteady Vortex-Lattice Method to Study Flapping Wings in Hover Flight

Abstract: Fil: Roccia, Bruno Antonio. Universidad Nacional de Cordoba; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina

A refined structural model for static aeroelastic response and divergence of metallic and composite wings

Abstract: A refined beam model with hierarchical features is in this work extended to the static aeroelastic analysis of lifting surfaces made of metallic and composite materials. The refined structural one-dimensional (1D) theory is based on the Carrera Unified Formulation and it permits to take into account any cross-section deformation, including warping effects. The vortex lattice method is employed to provide aerodynamic loadings along the two in-plane wing directions (wing span and wing cross-section). Applications are obtained by developing a coupled aeroelastic computational model which is based on the finite element method. The accuracy of the proposed 1D model is shown by a number of applications related to various wings made of metallic and composite materials. The effect of the cross-section deformation is evaluated on the aeroelastic static response and divergence of the considered wings. The need of higher-order expansions is underlined as well as the limitations of beam results which are based on classical theories. Comparison with results obtained by existing plate/shell aeroelastic models shows that the present 1D model could result less expensive from the computational point of view with respect to shell cases. The beneficial effects of aeroelastic tailoring in the case of wings made of composite anisotropic materials are also confirmed by the present analysis.

Static Aeroelastic Response of Wing-Structures Accounting for In-Plane Cross-Section Deformation

Abstract: In this paper, the aeroelastic static response of flexible wings with arbitrary cross-section geometry via a coupled CUFXFLR5 approach is presented Refined structural one-dimensional (1D) models, with a variable order of expansion for the displacement field, are developed on the basis of the Carrera Unified Formulation (CUF), taking into account cross-sectional deformability A three-dimensional (3D) Panel Method is employed for the aerodynamic analysis, providing more accuracy with respect to the Vortex Lattice Method (VLM) A straight wing with an airfoil cross-section is modeled as a clamped beam, by means of the finite element method (FEM) Numerical results present the variation of wing aerodynamic parameters, and the equilibrium aeroelastic response is evaluated in terms of displacements and in-plane cross-section deformation Aeroelastic coupled analyses are based on an iterative procedure, as well as a linear coupling approach for different free stream velocities A convergent trend of displacements and aerodynamic coefficients is achieved as the structural model accuracy increases Comparisons with 3D finite element solutions prove that an accurate description of the in-plane crosssection deformation is provided by the proposed 1D CUF model, through a significant reduction in computational cost

Global-Local Optimization of Flapping Kinematics in Hovering Flight

Abstract: The kinematics of a hovering wing are optimized by combining the 2-d unsteady vortex lattice method with a hybrid of global and local optimization algorithms. The objective is to minimize the required aerodynamic power under a lift constraint. The hybrid optimization is used to efficiently navigate the complex design space due to wing-wake interference present in hovering aerodynamics. The flapping wing is chosen so that its chord length and flapping frequency match the morphological and flight properties of two insects with different masses. The results suggest that imposing a delay between the different oscillatory motions defining the flapping kinematics, and controlling the way through which the wing rotates at the end of each half stroke can improve aerodynamic power under a lift constraint. Furthermore, our optimization analysis identified optimal kinematics that agree fairly well with observed insect kinematics, as well as previously published numerical results.

Geometry Based Flight Dynamics Modelling of Unmanned Airplanes

Abstract: This paper presents estimation algorithms for flight dynamics of small unmanned aircraft. The estimates are mainly based on geometric information. Optional information taken into account are airfoil polars, a few mass characteristics and static thrust measurements. The aerodynamics dataset is estimated using the Vortex Lattice Method. Weight-and-balance estimates are driven by assuming constant mass per surface. Propulsion is estimated based on typical characteristics. The model is compared to higher-fidelity models, namely wind-tunnel measurements and inertia measurements. The differences to the reference are typically below 20% of the aerodynamic coefficients and below 10% of the reference inertia.

Hydrodynamic Optimization of Marine Propeller Using Gradient and Non-Gradient- based Algorithms

Abstract: Here a propeller design method based on a vortex lattice algorithm is developed, and two gradient-based and non-gradient-based optimization algorithms are implemented to optimize the shape and efficiency of two propellers. For the analysis of the hydrodynamic performance parameters, a vortex lattice method was used by implementing a computer code. In the first problem, one of the Sequential Unconstraint Minimization Techniques (SUMT) is employed to minimize the torque coefficient as an objective function, while keeping the thrust coefficient constant as a constraint. Also, chord distribution is considered as a design variable, namely 11 design variables. In the second problem, a modified Genetic algorithm is used. The objective function is to maximize efficiency by considering the design variables as non-dimensional blade's chord and thickness distribution along the blade, namely 22 design variables. The hydrodynamic performance analyzer code is modified by a higher order Quasi-Newton scheme. Also, a hybrid function is used to improve the accuracy of the convergence. The solution of the optimization problems showed that a nearly 13% improvement in efficiency and a nearly 15% decrease in torque coefficient for the first propeller, as well as nearly 10% improvement for efficiency of the later propeller, is possible.

Numerical analysis of cavitating propeller and pressure fluctuation on ship stern using a simple surface panel method “SQCM”

Abstract: This paper presents a calculation method for the pressure fluctuation induced by a cavitating propeller. This method consists of two steps: the first step is the calculation of propeller sheet cavitation, and the second step is the calculation of pressure fluctuation on the ship stern. It is for practicality that we divide the method into two steps but do not calculate these steps simultaneously. This method is based on a simple surface panel method “SQCM” which satisfies the Kutta condition easily. The SQCM consists of Hess and Smith type source panels on the propeller or cavity surface and discrete vortices on the camber surface according to Lan’s QCM (quasi-continuous vortex lattice method). In the first step, the cavity shape is solved by the boundary condition based on the free streamline theory. In order to get the accurate cavity shape near the tip of the propeller blade, the cross flow component is taken into consideration on the boundary condition. In the second step, we calculate the cavitating propeller and the hull surface flow simultaneously so as to calculate the pressure fluctuation including the interaction between the propeller and the hull. At that time, the cavity shape is changed at each time step using the calculated cavity shape gotten by the first step. Qualitative agreements are obtained between the calculated results and the experimental data regarding cavity shape, cavity volume and low order frequency components of the pressure fluctuation induced by the cavitating propeller.

A Vortex-Lattice Method for the Prediction of Unsteady Performance of Marine Propellers and Current Turbines

Abstract: In this paper, the unsteady hydrodynamic analysis of marine propellers and horizontal-axis tidal current turbines is performed by using a vortex lattice method (VLM). A fully unsteady wake alignment algorithm is implemented into the VLM to satisfy the force-free condition on the propeller and turbine wake surfaces. It was found that the position of the trailing wake is very important in predicting the performance of propellers or turbines in steady or unsteady flow. The effects of a non-linear interaction between the inflow and the propeller/turbine blades have been taken into account by using a hybrid viscous/potential flow method, which couples the potential flow solver for the unsteady analysis of the propeller/turbine and a viscous flow solver for the prediction of the viscous flow field around them. The present method is then applied to predict unsteady hydrodynamic performance of a propeller and a horizontal-axis tidal current turbine. The predicted unsteady forces of a propeller subject to an inclined inflow are compared with those from experiments. The hydrodynamic performance of tidal turbine in yawed flow for various yaw angles is investigated. The numerical results are compared with existing experimental data.

GPGPU implementation and benchmarking of the unsteady vortex lattice method

Abstract: The unsteady vortex lattice method is widely known for its robustness in modeling inviscid flow fields and aerodynamic loading. This method can be used to model a variety of configurations including multiple flapping wings in unsteady flows. One of the key computational algorithms used in this model can be expressed as nested loops or as a linear algebra based matrix vector product. Both forms are compared, and several different implementations including Matlab, C, and a GPGPU version are compared. The GPGPU based implementation is faster than the Matlab implementation by almost three orders of magnitude, and faster than the C implementation by almost two orders of magnitude. Details of the implementation are provided. The linear algebra implementation is determined to consume excess memory without a corresponding increase in computation performance. The fastest GPGPU implementation is applied to the study of ground effect, and separately, to the roll up of a vortex filament. Each of these cases would be intractable to compute without the acceleration offered by the GPGPU.

Model Reduction in Flexible-Aircraft Dynamics with Large Rigid-Body Motion

Abstract: This paper investigates the model reduction, using balanced realizations, of the unsteady aerodynamics of maneuvering flexible aircraft. The aeroelastic response of the vehicle, which may be subject to large wing deformations at trimmed flight, is captured by coupling a displacement-based, flexible-body dynamics formulation with an aerodynamic model based on the unsteady vortex lattice method. Consistent linearization of the aeroelastic problem allows the projection of the structural degrees of freedom on a few vibration modes of the unconstrained vehicle, but preserves all couplings between the rigid and elastic motions and permits the vehicle fiight dynamics to have arbitrarily-large angular velocities. The high-order aerodynamic system, which defines the mapping between the small number of generalized coordinates and unsteady aerodynamic loads, is then reduced using the balanced truncation method. Numerical studies on a representative high-altitude, long-endurance aircraft show a very substantial reduction in model size, by up to three orders of magnitude, that leads to model orders (and computational cost) similar to those in conventional frequency-based methods but with higher modeling fidelity to compute maneuver loads. Closed-loop results for the Goland wing finally demonstrate the application of this approach in the synthesis of a robust flutter suppression controller.

Comparative investigation of vlm codes for joined-wing analysis

Abstract: The Vortex lattice method (VLM) is a popular aerody namic analysis method used at the early stages of a ircraft design. In this paper a comparative investigation of several computer progr ams applying the VLM is carried out and their abili ty to simulate non conventional wing configurations is evaluated. It w as found that Athena Vortex Lattice (AVL) code calc ulated the most correct results, but other programs like Tornado provide useful supp lemental information.

Aeroservoelastic modelling and active control of very large wind turbine blades for gust load alleviation.

Abstract: The increased flexibility of wind turbine blades necessitates not only accurate predictions of the aeroelastic effects, but also requires active control techniques to overcome potentially damaging loadings and oscillations. An aeroservoelastic model, capturing the structural response and the unsteady aerodynamics of very large rotors, will be used to demonstrate the potential of closed-loop load alleviation using aerodynamic control surfaces. The structural model is a geometrically-nonlinear composite beam, which is linearised around equilibrium rotating conditions and coupled with a linearised 3D Unsteady Vortex Lattice Method (UVLM) with prescribed helicoidal wake. This provides a direct higher fidelity solution to BEM for the dynamics of deforming rotors in attached flow conditions. The resulting aeroelastic model is in a state-space formulation suitable for control synthesis. Flaps are modeled directly in the UVLM formulation and LQG controllers are finally designed to reduce fatigue by about 26% in the presence of continuous turbulence. Trade-offs between reducing root-bending moments (RBM) and suppressing the negative impacts on torsion due to flap deployment will also be investigated.

Aeroelastic Analysis of a Folding Wing: Comparison of Simple and Higher Fidelity Models for a Wide Range of Fold Angles

Abstract: The goal of folding wing research is to enable wing shape changes during flight in order to optimize aircraft performance over a multitude of mission segments. However, the additional mechanisms needed to implement the morphing capability tends to increase the weight, reduce the stiffness in comparison, and make it more susceptible to aeroelastic effects. In addition, the drastic geometric changes in itself affect the dynamics and aeroelastic behavior of the wing. This paper explores the effect of large geometric changes on the natural frequencies and modes, and subsequent effects on the flutter onset. The structural dynamics analysis compares beam theory results versus ANSYS finite element results, and the aeroelastic analysis compares results from using Theodorsen unsteady strip theory versus those obtained using unsteady vortex lattice method. This paper shows that the flutter onset of folding wings can be predicted using simplified beam dynamics and strip theory aerodynamics, as well as ANSYS structural analysis coupled with unsteady vortex lattice aerodynamics. However, when natural frequencies begin to migrate due to large changes in geometry, special care needs to be taken when studying the aeroelastic behavior.

Improved Computation of Induced Drag for Wakes of Arbitrary Shape

Abstract: In 1976, Blackwell provided a robust method for calculating the induced drag of an arbitrarily shaped wake at the Trefftz plane. However, to acheive an accuracy within one percent, the required discretization may exceed the number of panels that can be used in a typical vortex lattice method or panel method. Alternatively, a method given by Lundry fits a sine series to a set of loading points and obtains high accuracy by computing induced drag using the lifting-line theory formulation. Unfortunately, this is valid only for planar wakes. Therefore, the authors developed a hybrid method that uses Lundry’s method for sine series fitting to interpolate additional lifting elements to then use with Blackwell’s method for a general wake shape. The predictions from these three methods were compared for a prescribed loading and the hybrid method demonstrated significant improvement over Blackwell’s method. However, when the predictions were implemented following a loads calculation from an aerodynamics model, the improvement was reduced.

Aerodynamic analysis for control and simulation of unmanned aerial vehicles

Abstract: This research project is focused on the aerodynamic modeling of unmanned aerial vehicles (UAV) and its application to determine the aircraft aerodynamic characteristics. The aerodynamic modeling procedure of the UAV is based on the vortex lattice method (VLM). The goal is the computation of the aerodynamic stability and control derivatives for composing the flight vehicle equations of motion. The vortex lattice method code that will be used is the Tornado VLM Toolkit, a MATLAB based freeware program developed at KTH- Sweden. The aerodynamic (stability) derivatives shall be validated by comparing the calculated derivatives with the corresponding aerodynamic derivatives identified from experimental flight data. The unmanned aerial vehicles that is used for this research is called Vector-P. During the flight several maneuvers have been done to retrieve good data. The maneuvers that were done are the doublet and the 3-2-1-1 maneuver. By doing four identifications and one validation the longitudinal aerodynamic derivatives of the Vector-P are determined. Next the results of the flight testing are compared with the result of Tornado. Unfortunately only one derivative is more or less the same with both methods. The difference in the aerodynamic coefficients will have a great influence on the fly performance of the Vector-P. In Matlab a manoeuvre was simulated with both coefficients. The two manoeuvres were very different from each other and the velocity in both cases was not a match either. The main reason is that the Cmα is calculated too high by Tornado.

Aeroelastic Optimization of a High Aspect Ratio Wing

Abstract: Two fundamental aspects of aircraft wings are considered in this work: their aerodynamics and their structural properties. Although originating in different disciplines, these two aspects should be studied together because structural behaviour influences aerodynamics and vice-versa, leading to what is known as the aeroelastic behaviour of the wing. With respect to aerodynamics, lift and drag are the forces that allow the airplane to take off and sustain itself in the air. Although the absence of drag would make it impossible for the airfoil to fly, engineers generally seek to minimize it, as an increase in drag implies an overhead on aircraft maneuvers and greater fuel consumption. Drag is minimized by long elliptical wings, but such wings are difficult to manufacture in comparison to other shapes. Long wings are generally more aerodynamic, but a longer span naturally implies greater structural weight, thus reducing flight range. The range formulas of Breguet relate the lift and drag produced by the wing, the amount of fuel available and the weight of the aircraft to the maximum distance the aircraft can fly. Aeroelasticity, on the other hand, is concerned with the fact that when in flight, the wing structure is under the influence of several forces that deform its original shape. Understanding these forces and how they change the aerodynamic behaviour of the airfoil is very important. In particular, such forces and the corresponding deformation may create a positive feedback loop, and the wing may bend so much that it breaks. Accurately modelling the aeroelastic behaviour of a given wing may be computationally very demanding. Therefore, less accurate but simpler models are used for optimization purposes at the initial design stages. Such models must, nevertheless, remain valid to a certain extent, in order for optimized preliminary designs be useful at a later stage. In this work, the integration between wing aeroelastic models and optimizers is considered, with a view to allowing more accurate and more computationally demanding wing models to be used for optimization. This was accomplished through two different approaches. In the first approach, the precision at certain intermediate steps was reduced without affecting the output. More specifically, Gauss-Seidel iterations were used to achieve faster but less precise solutions for systems of linear equations arising in given model. In the

Aerodynamic Reduced-Order Modeling without Static Correction Requirement Based on Body Vortices

Abstract: The objective of this research is to propose a new reduced-order modeling method. This approach is based on fluid eigenmodes and body vortices without using static correction. The vortex lattice method (VLM) is used to analyze unsteady flows over two-dimensional airfoil and three-dimensional wing. Eigenanalysis and reduced-order modeling are performed using a conventional method with static correction and an unconventional one without the static correction. Numerical examples are proposed to demonstrate the performance of the present method. The results show that the new method can be considered an alternative way to perform the reduced-order models of unsteady flow.

Hydrodynamic Optimization of Marine Propeller and Numerical Investigation of Contra-rotating Propeller

Abstract: Here, two approaches have been accomplished. First, a propeller design method based on vortex lattice algorithm is developed and a gradient based optimization algorithm is implemented to optimize the shape and efficiency of a propeller. Second, a method for analysis of a Contra-Rotating propeller (CRP) has been developed. For analysis of the hydrodynamic performance parameters, a vortex lattice method was used by implementing an open-source code which is called OpenProp. One of the Sequential Unconstraint Minimization Techniques (SUMT) is employed to minimize the torque coefficient as an objective function, while keeping the thrust coefficient constant as a constraint. Also, chord distribution is considered as a design variable. A DTMB 4119 propeller has been optimized to achieve a lower torque coefficient than the original value. The scheme presented here is more efficient and less time consuming with respect to conventional methods. Solution of the optimization problem showed that nearly 13% improvement for propeller efficiency and nearly 15% decrease in torque coefficient is possible. The method presented for Contra-Rotating propeller (CRP) analysis is called coupled. Cavitation analysis has been done to show the robustness of the scheme.

Performance Analysis of Flapping Wings in Formation Flights

Abstract: In this paper, we investigate the aerodynamic aspects of formation flights. We do so by simulating the flow over flapping wings flying in grouping arrangement and in proximity of each others using the unsteady vortex lattice method. The results show that flying in V-shaped line formation at optimal spacing enables significant increase in the lift and thrust and savings in the power consumption. This is mainly due to the interaction between the trailing birds and the previously-shed wake vorticity from the leading bird.

Validation of a numerical simulation tool for aircraft formation flight.

Abstract: The use of formation flight for increased fuel efficiency has received a lot of attention in the last couple of years. This paper covers a numerical simulation of a NASA test flight utilizing a formation of two F18A Hornet aircraft. The numerical simulation was made using an adapted version of the vortex lattice method TORNADO, allowing for several aircraft to be simulated in a trimmed condition. The numerical results showed good agreement with the flight test data. Some discrepancies due to the numerical model not covering viscous diffusion was found as expected but not quantified or analyzed.