1,721,042 research outputs found
Conceptual Approach to Unconventional Airship Design and Synthesis
No full textThis paper presents a novel design methodology to be used in the evaluation of the main features of advanced unconventional airship configuration. Similarly to the process used in aircraft design, the concept of Volumes Fractions (VF) is introduced to estimate airship weights, dimensions, and performances, in an early design phase. The paper presents the complete methodology, with tables and constants to help unconventional airship designer in preliminary design considerations. Volumes and weight of candidate solutions are obtained through an iterative method within a user-friendly tool requiring graphical and straightforward mathematical operations. The solutions are ranked based on procedures aiming at satisfying the customer needs and expectations provided as inputs. A case study highlighting a step by step methodology process is presented, and the approach followed to select the final solution is documented. The method is easy to use and to implement rapidly providing significant amount of data. A parametric approach is used such that the evolution in materials and technology, new configurations, and modern power and energetic solutions can be considered by simply performing a parameter sweep to perform sensitivity analysis
Dynamic identification of wind turbine system under operational conditions using FBG transducers
No full textThe development of an identification procedure of the dynamic properties of a composite horizontal axis wind turbine blade using data recorded during its operative working conditions is reported in this article. The operational modal analysis method based on the properties of the Hilbert Transform applied to response signals, represented in the frequency domain, has been extended to deal with systems characterized by harmonic component excitations blended with white noise spectra loading. The unknown harmonic contributions are identified and their effects on the time responses removed for a clearer estimate of the modal parameters. The considered data are gained from a Fiber Bragg Grating (FBG) integrated with the wind turbine blade. The FBG transducers have many advantages over other conventional sensors. Providing real-time information about structural integrity and operational load can be used in conjunction with appropriate methods to decrease the overall energy cost of wind turbines by optimizing the maintenance, yielding maximum service life of the wind turbine at minimum maintenance cost. This paper discusses the capabilities of the proposed operational modal analysis procedure, included in the Natural Input Modal Analysis, NIMA, jointly used with data from FBG transducers to track the changes in the natural frequencies, damping ratios and mode shapes of the rotating wind turbine blade for a possible use in structural health monitoring. Experimental data are provided by the wind tunnel test campaign carried out at the Mechanical and Aeronautical Engineering Department of Clarkson University. © 2012 by P. Marzocca, G. Coppotelli. Published by the American Institute of Aeronautics and Astronautics, Inc
Added masses computation for unconventional airships and aerostats through geometric shape evaluation and meshing
No full textThe modern development in design of airships and aerostats has led to unconventional configurations quite different from the classical ellipsoidal and spherical ones. This new class of air-vehicles presents a mass-to-volume ratio that can be considered very similar to the density of the fluid displaced by the vehicle itself, and as a consequence, modeling and simulation should consider the added masses in the equations of motion. The concept of added masses deals with the inertia added to a system, since an accelerating or decelerating body moving into a fluid displaces a volume of the neighboring fluid. The aim of this paper is to provide designers with the added masses matrix for more than twenty Lighter Than Air vehicles with unconventional shapes. Starting from a CAD model of a given shape, by applying a panel-like method, its external surface is properly meshed, using triangular elements. The methodology has been validated by comparing results obtained with data available in literature for a known benchmark shape, and the inaccuracies of predictions agree with the typical precision required in conceptual design. For each configuration, a CAD model and a related added masses matrix are provided, with the purpose of assisting the practitioner in the design and flight simulation of modern airships and scientific balloons
Detection of nonlinearities in plates via higher-order spectra: numerical and experimental studies
No full textHigher-order spectral (HOS) analysis tools are employed to extract the nonlinear dynamic response features of elastic and laminated plates by using both physics-based mechanical plate models and experimental data. Bispectral and trispectral densities are computed to highlight the presence and relative importance of quadratic and cubic nonlinearities. The former are associated with the presence of asymmetry either in the excitation or in the mechanical response of predeflected plates while the latter are due to midplane stretching effects. Besides the detection of these structural nonlinearities in perfect (baseline) fully clamped plates, the changes of such nonlinearities induced by the presence of small inertial imperfections (i.e., lumped masses) are identified and exploited to localize the imperfections. The numerical and experimental investigations are carried out both on isotropic and laminated composite plates subject to Gaussian white noise excitation. The effectiveness of the HOS-based procedure for detection of the nonlinearities is fully demonstrated for both types of plates
Geometrically exact plate models for system identification via Higher-Order Spectra extracted from nonlinear dynamic responses
No full textPlates are widely used in a broad range of engineering applications and particularly as building elements for aeronautical, mechanical, marine, and civil structures. In the great majority of applications, thin linearly elastic plate-like structures with isotropic properties are used. In the last decades, structures with orthotropic or anisotropic properties, such as composite laminated multilayered plates, are becoming widely accepted. Due to their out-of-plane flexibility, these structures might be prone to large rotations, displacements and strains, as it usually occurs in the post-buckling regime or when they are excited near resonance. In these cases, consistent mechanical models capable of capturing their static and dynamic nonlinear behaviors are needed [1-3]. However, the use of truncated nonlinear models – such as the widely used Föppl-von Kármán model which considers the stretching of the mid-plane as the dominant nonlinearity but neglects the nonlinear curvatures, shear deformations, and rotary inertia – may turn out to be inaccurate. The exemplary cases are when one aims at a careful nonlinear system identification for damage detection or at a vibration control problem. In this work, we propose a geometrically exact model capable of describing thin isotropic, orthotropic, or laminated multilayered plates. First, we show, by means of experiments (see, e.g., Fig. 1), that the equilibrium response under a quasi-static monotonically increasing point load is predicted with greater accuracy by the proposed theory compared to the Föppl-von Kármán theory. Then, with the purpose of characterizing the nonlinear behavior of the plate, we study the nonlinear dynamic properties of the responses under resonant as well as single- and multi-tone excitations, employing a Higher-Order Spectral analysis [4]. We show that the prediction of the nonlinearities in the dynamic response agrees well with our experimental results. These results pave the way to the implementation of a nonlinear system identification technique based on Higher-Order Spectra of the response obtained through the geometrically exact equations of motion
Flutter of suspension bridges under nonuniform wind profiles
No full textA parametric one-dimensional model of suspension bridges is employed to investigate the effects of arbitrary wind
distribution in the onset of flutter. A total Lagrangian formulation is adopted to obtain the equations of motion
expressed in terms of incremental kinematic variables and linearized about the prestressed aeroelastic equilibrium.
The kinematics for the deck feature the vertical and the horizontal displacements of its elastic axis and the torsional
rotations of the cross sections, the latter, are assumed rigid in their own plane. The elasto-geometric stiffness of the
cables is obtained by condensing the bridge equilibrium under the self-weight in the longitudinal direction. The
equations of motion are coupled with an incompressible unsteady aerodynamic defined through a set of indicial
functions developed, by means of computational fluid dynamics simulations, for the cross section of the Runyang
suspension bridge. The numerical treatment of the aeroelastic system of equations is performed using the finite
element approach
Heuristic Algorithms Applied to Multidisciplinary Design Optimization of Unconventional Airship Configuration
No full textThis paper describes the multidisciplinary optimization of an airship with unconventional configuration. The shape of the airship is based upon two semi-ellipsoids, whose axis ratios can be altered for optimization purpose. The parameters to optimize are volume, ratio between longitudinal and lateral semi-axis, ratio between vertical and lateral semi-axis, percentage of the top surface covered by photovoltaic films, and dimension of the tail. The objective of the optimization is to reduce the mass of the airship by keeping the equilibrium between buoyancy and weight as a constraint, reaching the design speed while maintaining the static longitudinal stability of the vehicle. The mathematical model developed to evaluate airship features includes the computation of the ballonet volume, a weight breakdown, considerations about the energy storage for night operations, the power system, and the stability. Six heuristic optimization strategies have been applied to achieve the best solution; some case studies have been developed, and the final optimal configurations found by algorithms have been analyzed to validate the optimization framework. The approach demonstrates that the heuristic optimization strategies used are good tools for the conceptual design of unconventional airship since this problem requires a multidisciplinary approach and several parameters including aerodynamics, propulsion, mass breakdown, aerostatics, and stability. These parameters are strongly dependent on each other and they must be considered together to obtain an optimum and balanced design
Integrated sensor system for structural integrity and load monitoring of wind turbines
No full textThe development and laboratory scale testing of a Fiber Bragg Grating (FBG) based distributed strain sensor system for structural health monitoring (SHM) and real time load monitoring of a wind turbine is reported. The key objective of the study is to characterize the performance of the sensor system under various loading conditions in order to determine its effectiveness for such applications. Loading conditions considered in this study include quasi-static loading, vibrations and wind flow from wind tunnel. For quasi-static loading condition a single instrumented blade was tested under various loading levels, and the strains measured using FBG system were compared with conventional strain gages. Both free and forced vibration studies were conducted on an instrumented single blade. The obtained dynamic response in terms of natural frequencies and mode shapes were compared with those obtained from numerical models. Finally, a 1.828 meters wind turbine rotor with three instrumented blades was tested under the wind flow generated from a wind tunnel. The sensor data transmitted wirelessly were analyzed for dominant frequencies and correlated with other information collected using other methods, such as accelerometers on the tower. The results from the present study demonstrate that the FBG based strain sensor system has high potential to be used as SHM and real time load monitoring systems
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