VALIDATING GLOBAL STRUCTURAL DAMPING MODELS FOR DYNAMIC ANALYSES

Abstract: Finite Element (FE) models grow in terms of detail and complexity. They strive to provide a more precise mass and stiffness distribution in order to achieve better load prediction capabilities. However, they also need to include damping models to achieve better results for dynamic loads analyses. This is why experiments are usually carried out to quantify global damping ratios of the final structure and include them in the analytical model for further calculations. Yet, especially for large aerospace structures assembled from different substructures, the experimental determination of damping ratios for the assembled structure may be impossible or ineconomical. Therefore, a consistent approach to predict the damping properties of assembled structures is desirable. In this work, FE models of a laboratory test structure and its two substructures are built up. Modal tests are carried out on the substructures. On the basis of correlated substructure modal damping ratios, global proportional damping models are applied on substructure level in order to build proportional substructure damping matrices, construct a nonproportional, full structure damping matrix and thus predict the damping properties for the fully assembled structure. The approach is validated with the help of experimental results from a modal test on the fully assembled laboratory test structure. Because of the unsatisfactory reproduction of the substructure damping properties by the selected damping models and the outcome for the assembled structure in this work, an additional investigation on computational model updating of damping parameters on substructure level is carried out on a simulated plate

Determination of nutrient salts by automatic methods both in seawater and brackish water: the phosphate blank

9 páginas, 2 tablas, 2 figurasThe main inconvenience in determining nutrients in seawater by automatic methods is simply solved: the preparation of a suitable blank which corrects the effect of the refractive index change on the recorded signal. Two procedures are proposed, one physical (a simple equation to estimate the effect) and the other chemical (removal of the dissolved phosphorus with ferric hydroxide).Support for this work came from CICYT (MAR88-0245 project) and Conselleria de Pesca de la Xunta de GaliciaPeer reviewe

Solar Impulse – How to validate the numerical model of a superlight aircraft with A340 dimensions!

"The Solar Impulse project ... aims to have an airplane that can take off and fly autonomously, day and night, propelled uniquely by solar energy, right around the world without fuel or pollution. An unachievable goal without pushing the current technological limits in all fields..." This citation from the Solar Impulse homepage describes the engineering challenges for the aircraft development. The particular design and the use of novel materials made it possible to produce an aircraft with 63m wing span with only 1.7 t of weight including pilot. This superlight design demands very accurate testing procedures. In addition, the magnitude of the apparent mass of the surrounding air in relation to the structural mass requires a new approach for the model validation process to take these effects into account. This paper will give insight in a two step model validation strategy. The dynamic characteristics of a) the load carrying structure and b) the full aircraft equipped with solar panels were determined experimentally in two ground vibration test (GVT) campaigns. The modal parameters extracted from GVT were used to update the FE model of the load carrying structure at first and of the aircraft ready for flight afterwards. In detail, this paper presents the procedure for GVT on an aircraft with eigenfrequencies considerably below 1 Hz. For the model updating strategy, the approach for the analytical determination of eigenfrequencies and mode shapes with the apparent mass effects included is presented

Linking Analysis to Test - Parameter Identification and Validation

This article gives a brief overview of procedures and of some exemplary results of validating analytical models using experimental data. The presentation is restricted to procedures currently established by experience with large order industrial applications. The basic numerical techniques for updating the parameters of industrial type Finite Elemente (FE) models are described. Sources of modeling and test data uncertainties are addressed together with related inquirements with respect to the quality of the initial analytical model and the test data. Obstacles to ensure the prediction capability of the updated models to untested situations are discussed. The described applications include a helicopter, an aero-engine and a civil aircraft structure

Frequency domain decomposition for manual and automated tracking of modes using flight data

When flight tests are performed, no input information are available and therefore Operational Modal Analysis (OMA) approaches can be used to estimate the modal parameters. Nowadays, the main focus for these techniques lays in the possibility of real-time and automatic modal parameter estimates so as to reduce time and costs. In this paper, a new approach for the automated tracking of modes is proposed, based on the Frequency Domain Decomposition (FDD) technique and the Modal Assurance Criterion (MAC). Such methodology has been applied on a third generation aircraft, with the aim of following the evolution of modes shapes and natural frequencies, when the system is subjected to different operating conditions. For the study, firstly a GVT has been carried out, and then different stationary flight configurations have been investigated. The recorded flight data have been processed with both traditional FDD and the proposed automated Frequency Domain Decomposition Tracking technique (FDDT). From the comparison of the obtained results, a good correlation has been observed, suggesting further improvements of the FDDT method for real-time applications. A final study has been carried out, in order to assess the applicability of the FDD technique on pre-flight phases, like the taxi or the engine start segments. In this way the possibility of substituting traditional GVTs with such tests has been investigated. The presented work has been carried out in collaboration with the Experimental Flight Department of the Italian Air Force