The role of parametric noise in altering the nonlinear dynamical behaviour of a circular cylinder undergoing vortex induced vibrations (VIV) is examined in this paper. Presence of parametric noise in the nonlinear VIV system is seen to bring in major qualitative and quantitative changes in response dynamics compared to its deterministic behaviour. Noise is seen to excite multiple frequencies in the response in the pre and post lock-in regimes and bring in new dynamical states. Noise intensity is seen to play a major role in dictating the qualitative nature of the new dynamical features that arise due to noise. Presence of low intensity noise gives rise to aperiodicity that resembles a quasi-periodic behaviour. Noise induced intermittency, dynamical state in which a system switches randomly between two near-by attractors is observed under the effect of high intensity noise. Nonlinear time series tools have been found effective in characterising the dynamical differences of different qualitative states for the present problem. Recurrence based time series tools have been especially insightful. Their adoption in a stochastic VIV system is also novel to the best of the authors’ knowledge and provides a fresh perspective to the problem. © 2019 Elsevier Ltd

Sunetra Sarkar

Department of Aerospace Engineering

M. S. Aswathy

Circular cylinders

Fluid structure interaction

Stochastic systems

time series analysis

Vortex flow

Dynamical behaviours

HIGH-INTENSITY NOISE

Intermittency

MULTIPLE FREQUENCY

NONLINEAR TIME SERIES

QUANTITATIVE CHANGES

STOCHASTIC NOISE

Vortex Induced Vibration

vibration analysis

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IIT Madras

International Journal of Mechanical Sciences

Interaction of fluid forces with flexible structures is often prone to dynamical instabilities, such as aeroelastic flutter. The onset of this instability is marked by sustained large amplitude oscillations and is detrimental to the structure's integrity. Therefore, investigating the possible physical mechanisms behind the onset of flutter instability has attracted considerable attention within the aeroelastic community. Recent studies have shown that in the presence of oncoming fluctuating flows, the onset of flutter instability is presaged by an intermediate regime of oscillations called intermittency. Further, based on the intensity of flow fluctuations and the relative time scales present in the flow, qualitatively different types of intermittency at different flow regimes have been reported hitherto. However, the coupled interaction between the pitch (torsion) and plunge (bending) modes during the transition to aeroelastic flutter has not been explored. With this, we demonstrate with a mathematical model that the onset of flutter instability under randomly fluctuating flows occurs via a mutual phase synchronization between the pitch and the plunge modes. We show that at very low values of mean flow speeds, the response is by and large noisy and, consequently, a phase asynchrony between the modes is present. Interestingly, during the regime of intermittency, we observe the coexistence of patches of synchronized periodic bursts interspersed amidst a state of desynchrony between the pitch and the plunge modes. On the other hand, at the onset of flutter, we observe a complete phase synchronization between the pitch and plunge modes. This study concludes by utilizing phase locking value as a quantitative measure to demarcate different states of synchronization in the aeroelastic response. © 2019 Author(s).

Fulltext

Chaos

Synchronization of pitch and plunge motions during intermittency route to aeroelastic flutter

Wind tunnel experiments carried out on a pitch-plunge aeroelastic system in the presence of fluctuating flows reveal that flutter instability is presaged by a regime of intermittency. It is observed that as the flow speed gradually increases towards the flutter speed, there appears intermittent bursts of periodic oscillations which become more frequent as the wind speed increases and eventually the dynamics transition into fully developed limit cycle oscillations, marking the onset of flutter. The signature from these intermittent oscillations are exploited to develop measures that forewarn a transition to flutter and can serve as precursors. This study investigates a suite of measures that are obtained directly from the time history of measurements and are hence model independent. The dependence of these precursors on the size of the measured data set and the time required for their computation is investigated. These measures can be useful in structural health monitoring of aeroelastic structures. © 2018 Elsevier Ltd

Journal of Sound and Vibration

Investigations on precursor measures for aeroelastic flutter

Intermittency has been observed in the response of aeroelastic systems in the presence of flow fluctuations. This study focuses on developing an understanding of the physical mechanisms that lead to intermittency in such systems. Specifically, the role of time scales of the input flow fluctuations is investigated. Numerical investigations reveal that flow fluctuations with predominantly long time scales in the pre-flutter regime lead to “on–off” type intermittency. On the other hand, rapid fluctuations constituting of small time scales lead to another qualitatively different intermittency, which is referred to in this paper as “burst” type intermittency. It is further shown that the unsteady wake effects play a crucial role in the burst type intermittency. Measures derived from time series analysis of the aeroelastic response are proposed to identify the different dynamical states quantitatively. © 2017 Elsevier Ltd

Journal of Fluids and Structures

Physical mechanism of intermittency route to aeroelastic flutter

The aeroelastic response of a NACA 0012 airfoil in the flow regimes prior to flutter is investigated in a wind tunnel. We observe intermittent bursts of periodic oscillations in the pitch and plunge response, that appear in an irregular manner from a background of relatively lower amplitude aperiodic fluctuations. As the flow speed is increased, the intermittent bursts last longer in time until eventually transitioning to a fully developed periodic response, indicating the onset of flutter. The repeating patterns in the measured response are visualized using recurrence plots. We show that statistics of the recurrence states extracted from these plots can be used to develop model-free precursors that forewarn an impending transition to flutter, well before its onset. © 2015 Elsevier Ltd.

Precursors to flutter instability by an intermittency route: A model free approach

A procedure to reconstruct two phase porous media, given the porosity and the two point correlation function of such media is described. The random media are modelled as a discrete valued random field Z(x→), which takes value 1 in regions of pores and 0 in regions of solid phase. The field Z(x→) is obtained by applying a non-linear filter - Nataf's transformation - to a correlated Gaussian random field Y(x→). The two point correlation function RYY of the Gaussian field Y is related to the two point correlation function RZZ of the field Z and can be calculated by expanding the bivariate Gaussian probability in terms of Hermite polynomials. The correlation function of the Gaussian field is decomposed into eigenfunctions and eigenvalues required by the Karhunen-Lóeve expansion. The eigenfunctions and eigenvalues are used to generate as many samples of the Gaussian field as required and the discrete field corresponding to each such sample can be obtained by applying the non-linear filter mentioned above. The method was tested by generating a large number of samples of one and two dimensional Debye random media using different porosities and different correlation lengths and the statistics of the ensemble was found to agree favourably with the input data. Also one and two dimensional 'chess board' patterns were reconstructed to see how well the geometry is reconstructed. The one dimensional case was reconstructed very accurately, whereas the two dimensional case, though not very satisfactory, indicates that the method captures some of the essentials of the geometry. The method also has the advantage that it gives an analytical framework for the porous media in terms of the random fields. These random fields could be used for further studies related to porous media. © 2013 Elsevier Ltd.

Probabilistic Engineering Mechanics

Reconstruction of 2-D porous media using Karhunen-Lóeve expansion

Volume 2: Prof. Carl Martin Larsen and Dr. Owen Oakley Honoring Symposia on CFD and VIV

Quantifying Uncertainties in Phenomenological Model of Two-Dimensional VIV Using Multivariate Monte Carlo Simulations

Effect of stochastic parametric noise on vortex induced vibrations

Journal	Data powered by TypesetInternational Journal of Mechanical Sciences
Publisher	Data powered by TypesetElsevier Ltd
ISSN	00207403
Open Access	No