148 research outputs found

    Modal Identification of Bladed Disks by Time-Frequency Analysis of the Nonsynchronous Response

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    In some circumstances, it is impossible to exploit resonance crossings to identify the modal properties of rotor disks. In these cases, the identification process must rely on nonsynchronous vibrations and becomes challenging for two reasons. First, the signals are weak (compared to the levels measured during resonance crossings) and random, thus an averaging procedure is necessary. Second, the dynamical system is time-variant due to the variation of the rotor speed. This paper presents a modal identification procedure formulated in the framework of the time-frequency analysis. A region of the time-frequency plane is stretched to map the system into a fictitious linear time-invariant (LTI) system. Then, the power spectral density function (PSD) of the response is computed by an averaging procedure. Finally, the modal properties are estimated through an output-only modal identification algorithm. The procedure is applied to simulated and experimental data regarding a bladed disk of a steam turbine

    Nonlinear model standardization for the analysis and design of nonlinear systems with multiple equilibria

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    In engineering practice, a nonlinear system stable about several equilibria is often studied by linearizing the system over a small range of operation around each of these equilibria, and allowing the study of the system using linear system methods. Theoretically, for operations beyond a small range but still within the stable regime of an equilibrium, the system behaves nonlinearly, and can be described and investigated using the Volterra series approach. However, there is still no available approach that can systematically transform the model of a nonlinear system into a form that can be studied over the whole stable regime about an equilibrium so as to facilitate the system study using the Volterra series approach. This transformation is, in the present study, referred to as nonlinear model standardization, which is the extension of the well-known concept of linearization to the nonlinear case. In this paper, a novel approach to nonlinear model standardization is proposed for nonlinear systems that can be described by a Nonlinear AutoRegressive model with eXogeneous input (NARX) or a nonlinear differential equation (NDE) model. The proposed approach is then used in three case studies covering the applications in nonlinear system analysis, nonlinear system design, and nonlinearity compensation, respectively, demonstrating the significance of the proposed nonlinear model standardization in a wide range of engineering practices

    Logical Signal Processing: a Fourier Analysis of Temporal Logic

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    What is the frequency content of temporal logic formulas? That is, when we monitor a signal against a formula, which frequency bands of the signal are relevant to the logic and should be preserved, and which can be safely discarded? This question is relevant whenever signals are filtered or compressed before being monitored, which is almost always the case for analog signals. To answer this question, we focus on monitors that measure the robustness of a signal relative to a specification in Signal Temporal Logic. We prove that robustness monitors can be modeled using Volterra series. We then study the Fourier transforms of these Volterra representations, and provide a method to derive the Fourier transforms of entire formulas. We also make explicit the measurement process in temporal logic and re-define it on the basis of distributions to make it compatible with measurements in signal processing. Experiments illustrate these results. Beyond compression, this work enables the integration of temporal logic monitoring into common signal processing toolchains as just another signal processing operation, and enables a common formalism to study both logical and non-logical operations in the frequency domain, which we refer to as Logical Signal Processing.Comment: 17 page

    Statistical and modal analysis of surface pressure fluctuations in tornado-like vortices

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    Surface pressure measurement is a general tool for evaluating wind flow qualitatively and quantitatively. Due to its complex temporal and spatial features, modal analysis is an interesting tool to be used for interpretation and discussion. The most common technique for modal representation is proper orthogonal decomposition (POD), also referred to as principal component analysis. However, it is believed that POD sometimes fails to extract meaningful features of the pressure field. To remove the non-physical POD modes and provide a closer physical description of the pressure field, an advanced method independent component analysis (ICA) is applied. Furthermore, these two methods are generalized in the frequency domain, called dynamic POD and dynamic ICA, to provide the temporal evolutions of coherent structures over the spatial domain. Modal analysis is used to isolate the different coherent structures in tornado-like vortices, e.g., wandering, vortex breakdown, and two-cell structure, and find the spectral characteristic of each phenomenon. Moreover, a comparison of modal analysis between the current paper and the previous paper on the velocity field {see Karami et al., ["Coherent structures in tornado-like vortices,"Phys. Fluids 31, 085118 (2019)]} presents new insight into the pressure-velocity correlation of the POD modes

    Leakage flow flutter in a low-speed axial-flow fan with shrouded blades

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    A large-amplitude axial vibration of a rotor fan with shrouded blades has been experimentally observed. The analysis of the Campbell diagram shows that the vibration is related to a backward-whirl vibrational mode of the rotor which is always present, with different amplitudes depending on the operating conditions and configuration. Modifications of the shroud roughness and insertion of small obstacles in the gap region have independently shown that leakage flow fluctuations constitute the excitation of the large-amplitude vibration. This indicates that the phenomenon is likely a flutter, as it is also suggested by the observed intermittency and aerodynamic stiffening. The feedback of the vibration on the flow is likely due to the variations of the gap size

    Coherent structures in tornado-like vortices

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    The dynamics of tornadolike vortices is investigated through a set of novel physical experiments and modal analyses for a wide range of swirl ratios (0.22 64 S 64 0.96). Various physical phenomena such as wandering, vortex breakdown, or transition from one-cell to two-cell structures are observed. To investigate the coherent structure of the tornado vortices, two different decomposition methods are applied: (i) proper orthogonal decomposition (POD), also referred to as principle component analysis, and (ii) a novel dynamic proper orthogonal decomposition to provide time evolutions of the POD modes. To foster the physical interpretation of these POD modes, we also applied modal decomposition on a simulated synthetic vortex. The results show that at low swirl ratios before vortex breakdown, the flow is characterized by a single vortex which is tilted at lower heights. For intermediate swirls, before vortex touchdown, the flow is characterized by a recirculation bubble with a single spiral rotating around it. By further increasing the swirl ratio, transition from a single spiral to a double spiral (one-cell to two-cell structures) occurs. Based on these observations, a simple vortex structure of tornadolike vortex is put forward which can be used to generate a low order, large scale turbulence model for these types of flows

    Probabilistic response of a bladed disk with uncertain geometry

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    Geometric uncertainties in the blade manufacturing process have important consequences in terms of dynamical properties of bladed disks. In this paper, we address the problem of modeling a full bladed disk composed by blades having uncertain geometry. The geometric imperfection of the blades is represented and analyzed according to a procedure previously presented by the authors, based on the principal component analysis (PCA) and the mesh morphing. The dynamical model of the full disk is constructed following the component mode synthesis (CMS) approach. The blade geometry is represented using a probabilistic model constructed from an experimental dataset. The effect of the geometric uncertainties is assessed using a linear uncertainty propagation approach, leading to a procedure that is fast enough to be embedded into a Monte Carlo simulation (MCS) loop

    Choice and Outcomes of Rate Control versus Rhythm Control in Elderly Patients with Atrial Fibrillation: A Report from the REPOSI Study

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    Background: Among rate-control or rhythm-control strategies, there is conflicting evidence as to which is the best management approach for non-valvular atrial fibrillation (AF) in elderly patients. Design: We performed an ancillary analysis from the \u2018Registro Politerapie SIMI\u2019 study, enrolling elderly inpatients from internal medicine and geriatric wards. Methods: We considered patients enrolled from 2008 to 2014 with an AF diagnosis at admission, treated with a rate-control-only or rhythm-control-only strategy. Results: Among 1114 patients, 241 (21.6%) were managed with observation only and 122 (11%) were managed with both the rate- and rhythm-control approaches. Of the remaining 751 patients, 626 (83.4%) were managed with a rate-control-only strategy and 125 (16.6%) were managed with a rhythm-control-only strategy. Rate-control-managed patients were older (p\ua0=\ua00.002), had a higher Short Blessed Test (SBT; p\ua0=\ua00.022) and a lower Barthel Index (p\ua0=\ua00.047). Polypharmacy (p\ua0=\ua00.001), heart failure (p\ua0=\ua00.005) and diabetes (p\ua0=\ua00.016) were more prevalent among these patients. Median CHA2DS2-VASc score was higher among rate-control-managed patients (p\ua0=\ua00.001). SBT [odds ratio (OR) 0.97, 95% confidence interval (CI) 0.94\u20131.00, p\ua0=\ua00.037], diabetes (OR 0.48, 95% CI 0.26\u20130.87, p\ua0=\ua00.016) and polypharmacy (OR 0.58, 95% CI 0.34\u20130.99, p\ua0=\ua00.045) were negatively associated with a rhythm-control strategy. At follow-up, no difference was found between rate- and rhythm-control strategies for cardiovascular (CV) and all-cause deaths (6.1 vs. 5.6%, p\ua0=\ua00.89; and 15.9 vs. 14.1%, p\ua0=\ua00.70, respectively). Conclusion: A rate-control strategy is the most widely used among elderly AF patients with multiple comorbidities and polypharmacy. No differences were evident in CV death and all-cause death at follow-up
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