Recent wind tunnel studies have shown that an aeroelastic system, prior to losing stability through flutter, goes through a regime where the response is characterized by intermittent bursts of periodic oscillations. The focus of this study is to investigate the reasons for this intermittent behavior through a numerical model. The studies indicate that the intermittency is observed only when the flow is accompanied by small random fluctuations. A stochastic bifurcation analysis is carried out to gain an understanding for this phenomenological interesting behavior. © 2016 The Authors.

Sayan Gupta

Department of Applied Mechanics

Sunetra Sarkar

Department of Aerospace Engineering

Bifurcation (mathematics)

Flutter (aerodynamics)

Hopf bifurcation

Stochastic systems

Wind tunnels

AEROELASTIC FLUTTER

FLUCTUATING FLOWS

Intermittency

INTERMITTENCY ROUTE

INTERMITTENT BEHAVIORS

Numerical investigations

PERIODIC OSCILLATION

STOCHASTIC BIFURCATION

Aeroelasticity

Fulltext

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IIT Madras

Procedia Engineering

This study carried out investigations on the bifurcation characteristics of a Duffing-Van der Pol (DVDP) oscillator subjected to white noise excitations. Dynamical (or D-) bifurcations are characterised by dramatic changes in the dynamical behaviour leading to topological changes in the phase portrait. An additional mode of bifurcation - phenomenological (or P-) bifurcation, is observed in stochastically excited systems when the stationary joint probability density function of the state variables undergo topological changes in the probability space. While D-bifurcation analysis is quantified in terms of the sign changes in the largest Lyapunov exponent, P-bifurcation analysis is usually qualitative and through visual inspection. In this study, a new quantitative measure for P-bifurcations based on the Shannon entropy is proposed. A comparison of the parameter regimes of the noisy DVDP oscillator identified by the three bifurcation methodologies have been presented. © 2016 Elsevier Ltd. All rights reserved.

Probabilistic Engineering Mechanics

Investigations on the bifurcation of a noisy Duffing-Van der Pol oscillator

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.

Journal of Fluids and Structures

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

The dynamic transition from combustion noise to combustion instability was investigated experimentally in two laboratory-scale turbulent combustors (namely, swirl-stabilized and bluff-body-stabilized backward-facing-step combustors) by systematically varying the flow Reynolds number. We observe that the onset of combustion-driven oscillations is always presaged by intermittent bursts of high-amplitude periodic oscillations that appear in a near-random fashion amidst regions of aperiodic low-amplitude fluctuations. These excursions to periodic oscillations last longer in time as operating conditions approach instability and finally the system transitions completely into periodic oscillations. A continuous measure to quantify this bifurcation in dynamics can be obtained by defining an order parameter as the probability of the signal amplitude exceeding a predefined threshold. A hysteresis zone was observed in the bluff-body-stabilized configuration that was absent in the swirl-stabilized configuration. The recurrence properties of the dynamics of intermittent burst oscillations were quantified using recurrence plots and the distribution of the aperiodic phases was examined. From the statistics of these aperiodic phases, robust early-warning signals of an impending combustion instability may be obtained. © 2014 Cambridge University Press.

Journal of Fluid Mechanics

Intermittency route to thermoacoustic instability in turbulent combustors

Nonlinear self-excited thermoacoustic oscillations appear in systems involving confined combustion in the form of coupled acoustic pressure oscillations and unsteady heat release rate. In this paper, we investigate the nonlinear transition undergone by thermoacoustic oscillations to flame blowout via intermittency, in response to variation in the location of the combustion source with respect to the acoustic field of the confinement. A ducted laminar premixed conical flame, stabilized on a circular jet exit with a fully developed exit velocity profile, was investigated. Transition to limit cycle oscillations from a non-oscillatory state was observed to occur via a subcritical Hopf bifurcation. Limit cycle oscillations underwent a further bifurcation to quasi-periodic oscillations characterized by the repeated formation of elongated necks in the flame that pinch off as pockets of unburned fuel-air mixture. The quasi-periodic state loses stability, resulting in an intermittent state identified as type II through recurrence analysis of phase space trajectories reconstructed from the acoustic pressure time trace. In this state, the flame undergoes repeated lift-off and reattachment. Instantaneous flame images suggest that the intermittent flame behaviour is influenced by jet flow dynamics. © 2012 Cambridge University Press.

Nonlinear self-excited thermoacoustic oscillations: Intermittency and flame blowout

A Modern Course in Aeroelasticity Solid Mechanics and Its Applications

Static Aeroelasticity

Journal of Wind Engineering and Industrial Aerodynamics

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

Numerical Investigations on Intermittency Route to Aeroelastic Flutter

Journal	Data powered by TypesetProcedia Engineering
Publisher	Data powered by TypesetElsevier Ltd
ISSN	18777058
Open Access	Yes