101,975 research outputs found
Dynamic Identification of a Solid Rocket Motor From Firing Test Using Operational Modal Analysis
No full textThe main objective of this paper is the identification of the dynamic properties of a solid rocket motor using data recorded during a firing test, characterized by a mass variation due to the burning propeller. The dynamic identification of the motor is provided by applying Operational Modal Analysis (OMA) methodologies able to estimate the modal parameters of the structure undergoing its operative conditions. The sensitivity of the OMA approaches to deal with structures characterized by time-dependent parameters is evaluated through a numerical simulation. Moreover, a comparison between the estimates from different state-of-the-art approaches in OMA (operating in both time and frequency domain) are provided. The capabilities of the OMA methods to track the changes in the natural frequencies, damping ratios and mode shapes of the first stage of the Vega lunch vehicle will be investigated in order to asses the overall efficiency of such approaches. Copyright © 2011 by G. Coppotelli, C. Grappasonni, C. Di Trapani
On the effects of structural modifications in the large wind turbine dynamics
No full textIn this paper a numerical procedure for the investigation of the dynamic behavior of large wind turbines is developed. The aeroelastic modeling is capable to take into account the dynamic environment in which the wind turbines operate by considering the effects of the atmospheric boundary layer and the weight load of the rotating blades. The aerodynamic loads are simulated by the 2D quasi-steady aerodynamic formulation, derived from Greengberg's theory, whereas the structural dynamics of the flexible wind-turbine blade, undergoing significant elastic displacements, has been described by a nonlinear flap-lag-torsion slender-beam differential model. The loading condition and the kinematic effects are described for different configurations of the rotor using a tapered and twisted blade representative of commercial installations. Then, blade structural modifications, represented by structural weakening, are introduced to investigate the effects of local delamination (damages) of the composite compound on the dynamic response of the system giving then useful information concerning the fatigue life of the system. Sensitivity analyses have been performed varying the spanwise location and the magnitude of the reduction of stiffness. Moreover, the effects of a blade mounting error in the pitch angle have been investigated highlighting the critical loading arising from an aerodynamically unbalanced rotor. Copyright © 2011 by L. Balis Crema, G. Coppotelli, C. Grappasonni
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
OMA analysis for the identification of the damping properties of a sloshing fluid
No full textIn this paper, the modal characteristics of a sloshing fluid at different operating conditions are estimated using Operational Modal Analysis (OMA) methods. The chosen system is characterized by a plexiglass tank filled with water mimicking the fuel tank inside an aircraft wing. Specifically, the considered tests are aimed at evaluating the sensitivity of the eigen-properties of the fluid to the random excitation levels and the liquid filling levels. Such experimental investigations are carried out at the environmental test facility available at the Department of Mechanical and Aerospace Engineering of the University of Rome “La Sapienza” which provided the closed-loop control vibrations at the base of the tank. Because the chosen tank system can be considered fully uncoupled with the dynamics of the sloshing fluid, these tests can be considered as a preliminary laboratory representation of the fluid sloshing inside an actual aircraft wing, thus contributing to the formation of the experimental database for the European funded project SLOshing Wing Dynamics (SLOWD)
Identification of the modal masses of an UAV structure in operational environment
No full textIn the framework of Operational Modal Analysis, several methods have been developed to estimate the modal parameters, that is natural frequencies, damping ratios, and mode shapes of a structure in its operative conditions. However, it is not possible to directly estimate the modal masses associated to each mode shape due to the unknown excitation. The modal masses are usually evaluated from the analysis of the change of the modal parameters by testing the structure in correspondence of two mass configurations. In this paper the efficiency and the accuracy of two procedures for the estimate of the modal masses are assessed by performing laboratory vibration tests. Vibration response data recorded during flight tests of an unmanned aerial vehicle (UAV) are used for this purpose as well as a mass changing device was developed to produce the mass variation of the structure. Results from the traditional input/output experimental modal analysis and the operational modal analysis are compared in terms of modal parameters, modal masses, and synthesized frequency response functions
A data-driven approach for rapid detection of aeroelastic modes from flutter flight test based on limited sensor measurements
No full textFlutter flight test involves the evaluation of the airframe’s aeroelastic stability by applying artificial excitation on the aircraft lifting surfaces. The subsequent responses are captured and analyzed to extract the frequencies and damping characteristics of the system. However, noise contamination, turbulence, non-optimal excitation of modes, and sensor malfunction in one or more sensors make it time-consuming and corrupt the extraction process. In order to expedite the process of identifying and analyzing aeroelastic modes, this study implements a time-delay embedded Dynamic Mode Decomposition technique. This approach is complemented by Robust Principal Component Analysis methodology, and a sparsity promoting criterion which enables the automatic and optimal selection of sparse modes. The anonymized flutter flight test data, provided by the fifth author of this research paper, is utilized in this implementation. The methodology assumes no knowledge of the input excitation, only deals with the responses captured by accelerometer channels, and rapidly identifies the aeroelastic modes. By incorporating a compressed sensing algorithm, the methodology gains the ability to identify aeroelastic modes, even when the number of available sensors is limited. This augmentation greatly enhances the methodology’s robustness and effectiveness, making it an excellent choice for real-time implementation during flutter test campaigns
Some results of GARTEUR Action Group HC-AG 19 on methods for improvement of structural dynamic finite element models
No full textThe issue of vibration in helicopters is of major concern to operators. This requires close attention to the vehicle dynamics. The ability to faithfully simulate and optimise vehicle response, structural modifications, vehicle updates, the addition of stores and equipment is the key to producing a low vibration helicopter. GARTEUR Action Group, HC-AG 14, concluded that helicopter dynamic models are still deficient in their capability to predict airframe vibration. The AG looked at the methods for improving the model correlation with modal test data along with the suitability of existing shake test methods. The helicopter structure tested in AG14 was suspended in the laboratory. However, this is not the operational environment where there are very significant mass, inertia and gyroscopic effects from the rotor systems. Nowadays, modal analysis consists of two principal approaches: experimental modal analysis (EMA) and operational modal analysis (OMA). The EMA evaluates the modal parameters by considering that the excitation and the response of the system are both measurable. The OMA evaluates the modal parameters using only the measured response. The lack of knowledge of the input is replaced by the assumption that the input is a distributed stochastic load, constant in a broad frequency band, e.g. white noise, and uncorrected in space. This hypothesis, nevertheless, is restrictive in rotorcraft applications, because in these cases the load is characterized by harmonic components, i.e. deterministic signals, originating from the rotating parts. A new action group HC-AG19 was formed to study the benefit of using in-flight dynamic data for improving finite element models. Methodologies were assessed to evaluate vibration measurements from flight tests. The objective is to extract modal parameters and demonstrate that the dynamic model can be updated using this data. This paper presents one of the approaches developed by the University of Rome "La Sapienza"
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