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

Sunetra Sarkar

Department of Aerospace Engineering

Sayan Gupta

Department of Applied Mechanics

Santosh Krishna Kumar

Flutter (aerodynamics)

Time measurement

time series analysis

wakes

AEROELASTIC FLUTTER

AEROELASTIC RESPONSE

AEROELASTIC SYSTEM

Intermittency

INTERMITTENCY ROUTE

Numerical investigations

Time-scales

UNSTEADY WAKE

Aeroelasticity

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IIT Madras

Journal of Fluids and Structures

Fluid–elastic systems nearing dynamic instabilities are known to be sensitive to fluctuations in fluid flow. A cantilever plate in axial flow with random temporal fluctuations, is examined numerically for its dynamical behaviour. The numerical model comprises of a nonlinear structural model for the flexible plate, coupled with unsteady lumped vortex model for the fluid forces. As the mean flow velocity is increased, the system transitions to limit cycle oscillations from a state of rest, through a regime of intermittent oscillations. The conditions for onset and disappearance of intermittency are discussed and are interpreted using stochastic bifurcation theories. While the onset of intermittency is found to be unaffected by the time scales of the flow fluctuations, they are observed to affect the length of the intermittency regime. The effect of plate flexibility on intermittency is also discussed. © 2019 Elsevier Ltd

Intermittency in a cantilever plate in a randomly fluctuating fluid flow

The phenomenon of multiplicative noise induced intermittency (mNII) observed due to random temporal fluctuations in the system parameters of nonlinear dynamical systems is investigated using maps. Generic conditions for the onset and disappearance of mNII are derived. The role of correlation in the noise is investigated. The phenomenon of mNII is interpreted in terms of stochastic bifurcations as well. The developments are verified through numerical results for a set of simple1-d maps. Further, through a numerical example involving an aeroelastic system, it is shown that the proposed results are applicable for analysis of complicated engineering systems by considering Poincáre maps from the times histories of the response. © 2019 Elsevier Ltd

International Journal of Non-Linear Mechanics

Multiplicative noise induced intermittency in maps

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

International Journal of Mechanical Sciences

Effect of stochastic parametric noise on vortex induced vibrations

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

Aeroelastic systems with hardening nonlinearity exhibit supercritical Hopf bifurcation when the flow velocity exceeds a critical velocity leading to self-sustaining large amplitude limit cycle oscillations known as flutter. This study investigates the effects of irregular fluctuations in the flow on the dynamical stability characteristics of a two-degree-of-freedom pitch-plunge aeroelastic system with hardening nonlinearity. Dynamical or D-bifurcations are investigated through the computation of the largest Lyapunov exponent, while phenomenological or P-bifurcation analysis is carried out by examining the structure of the joint probability density function of the response quantities and their instantaneous time derivatives. The qualitative nature of P-bifurcation analysis makes it difficult to pinpoint the regimes of different response dynamics. In the light of this difficulty, a quantitative analysis using the Shannon entropy measure has been undertaken to quantify the P-bifurcation regime. This regime is shown to be coincident with the intermittency regime observed in the response time histories prior to flutter oscillations in fluctuating flows. © 2018, Springer Science+Business Media B.V., part of Springer Nature.

Nonlinear Dynamics

Intermittency in pitch-plunge aeroelastic systems explained through stochastic bifurcations

Wind tunnel tests on a NACA 0012 airfoil have been carried out to study the transition in aeroelastic response from an initial state characterised by low-amplitude aperiodic fluctuations to aeroelastic flutter when the system exhibits limit cycle oscillations. An analysis of the aeroelastic measurements reveals multi-fractal characteristics in the pre-flutter regime. This has not been studied in the literature. As the flow velocity approaches the flutter velocity from below, a gradual loss in multi-fractality is observed. Measures based on the generalised Hurst exponents are developed and are shown to have the potential to warn against impending aeroelastic flutter. The results of this study could be useful for health monitoring of aeroelastic structures. © 2016 Elsevier Ltd

Fulltext

Multi-fractality in aeroelastic response as a precursor to 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

Journal of Wind Engineering and Industrial Aerodynamics

Empirical modelling of the bifurcation behaviour of a bridge deck undergoing across-wind galloping

An analytical and experimental investigation into limit-cycle oscillations of an aeroelastic system

Subcritical, nontypical and period-doubling bifurcations of a delta wing in a low speed wind tunnel

Physical mechanism of intermittency route to aeroelastic flutter

Journal	Journal of Fluids and Structures
Publisher	Academic Press
ISSN	08899746
Open Access	No