15,289 research outputs found
Efficiency of Higher Order Finite Elements for the Analysis of Seismic Wave Propagation
The analysis of wave propagation problems in linear damped media must take
into account both propagation features and attenuation process. To perform
accurate numerical investigations by the finite differences or finite element
method, one must consider a specific problem known as the numerical dispersion
of waves. Numerical dispersion may increase the numerical error during the
propagation process as the wave velocity (phase and group) depends on the
features of the numerical model. In this paper, the numerical modelling of wave
propagation by the finite element method is thus analyzed and dis-cussed for
linear constitutive laws. Numerical dispersion is analyzed herein through 1D
computations investigating the accuracy of higher order 15-node finite elements
towards numerical dispersion. Concerning the numerical analy-sis of wave
attenuation, a rheological interpretation of the classical Rayleigh assumption
has for instance been previously proposed in this journal
Lagrangian Time Series Models for Ocean Surface Drifter Trajectories
This paper proposes stochastic models for the analysis of ocean surface
trajectories obtained from freely-drifting satellite-tracked instruments. The
proposed time series models are used to summarise large multivariate datasets
and infer important physical parameters of inertial oscillations and other
ocean processes. Nonstationary time series methods are employed to account for
the spatiotemporal variability of each trajectory. Because the datasets are
large, we construct computationally efficient methods through the use of
frequency-domain modelling and estimation, with the data expressed as
complex-valued time series. We detail how practical issues related to sampling
and model misspecification may be addressed using semi-parametric techniques
for time series, and we demonstrate the effectiveness of our stochastic models
through application to both real-world data and to numerical model output.Comment: 21 pages, 10 figure
On the robustness of the average power ratios in damping estimation: application in the structural health monitoring of composites beams
In composites structures, cracking, delamination will cause changes in the measured dynamic response of structure and so on experimentally modal parameters. Estimation of damping in structural control often poses a difficult problem especially using broadband experiments. If these estimations are faulty, it is difficult to propose a robust Structural Health Monitoring (SHM) algorithm. Recently H.P. Yin introduced the optimal power ratios damping estimator. A new theoretical basis of the bandwidth method for the damping estimation from frequency response functions (in case of a single degree of freedom system) has been proposed. The main goal of this paper is to study the robustness of this enhanced damping estimator on simulated signal (sampling frequency, Signal to Noise Ratio and damping level/density), and also compare its performance with industrial improved estimator like “Polymax” on experimental Frequency Response Functions (FRFs). The pole shifts would be studied as a change in the frequency-damping plane function of level and density of damage
Motion magnification in coronal seismology
We introduce a new method for the investigation of low-amplitude transverse
oscillations of solar plasma non-uniformities, such as coronal loops,
individual strands in coronal arcades, jets, prominence fibrils, polar plumes,
and other contrast features, observed with imaging instruments. The method is
based on the two-dimensional dual tree complex wavelet transform
(DTWT). It allows us to magnify transverse, in the
plane-of-the-sky, quasi-periodic motions of contrast features in image
sequences. The tests performed on the artificial data cubes imitating
exponentially decaying, multi-periodic and frequency-modulated kink
oscillations of coronal loops showed the effectiveness, reliability and
robustness of this technique. The algorithm was found to give linear scaling of
the magnified amplitudes with the original amplitudes provided they are
sufficiently small. Also, the magnification is independent of the oscillation
period in a broad range of the periods. The application of this technique to
SDO/AIA EUV data cubes of a non-flaring active region allowed for the improved
detection of low-amplitude decay-less oscillations in the majority of loops.Comment: Accepted for publication in Solar Physic
Operational modal analysis of a spar-type floating platform using frequency domain decomposition method
System identification of offshore floating platforms is usually performed by testing small-scale models in wave tanks, where controlled conditions, such as still water for free decay tests, regular and irregular wave loading can be represented. However, this approach may result in constraints on model dimensions, testing time, and costs of the experimental activity. For such reasons, intermediate-scale field modelling of offshore floating structures may become an interesting as well as cost-effective alternative in a near future. Clearly, since the open sea is not a controlled environment, traditional system identification may become challenging and less precise. In this paper, a new approach based on Frequency Domain Decomposition (FDD) method for Operational Modal Analysis is proposed and validated against numerical simulations in ANSYS AQWA v.16.0 on a simple spar-type structure. The results obtained match well with numerical predictions, showing that this new approach, opportunely coupled with more traditional wave tanks techniques, proves to be very promising to perform field-site identification of the model structures
Identification of Bare-Airframe Dynamics from Closed-Loop Data Using Multisine Inputs and Frequency Responses
Amethod is presented for computing multiple-input multiple-output frequency responses of bare-airframe dynamics for systems excited using orthogonal phase-optimized multisines and including correlated data arising from control mixing or feedback control. The estimation was posed as the solution to an underdetermined system of linear equations, for which additional information was supplied using interpolation of the frequency responses. A simulation model of the NASA T-2 aircraft having two inputs and two outputs was used to investigate the method in the open-loop configuration and under closed-loop control. The method was also applied to flight test data from the X-56A aeroelastic demonstrator having five inputs and ten outputs and flying under closed-loop control with additional control allocation mixing. Results demonstrated that the proposed method accurately estimates the bare airframe frequency responses in the presence of correlated data from control mixing and feedback control. Results also agreed with estimates obtained using different methods that are less sensitive to correlated inputs
Seeing Tree Structure from Vibration
Humans recognize object structure from both their appearance and motion;
often, motion helps to resolve ambiguities in object structure that arise when
we observe object appearance only. There are particular scenarios, however,
where neither appearance nor spatial-temporal motion signals are informative:
occluding twigs may look connected and have almost identical movements, though
they belong to different, possibly disconnected branches. We propose to tackle
this problem through spectrum analysis of motion signals, because vibrations of
disconnected branches, though visually similar, often have distinctive natural
frequencies. We propose a novel formulation of tree structure based on a
physics-based link model, and validate its effectiveness by theoretical
analysis, numerical simulation, and empirical experiments. With this
formulation, we use nonparametric Bayesian inference to reconstruct tree
structure from both spectral vibration signals and appearance cues. Our model
performs well in recognizing hierarchical tree structure from real-world videos
of trees and vessels.Comment: ECCV 2018. The first two authors contributed equally to this work.
Project page: http://tree.csail.mit.edu
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