Aeroelastic Simulation of Stall Flutter Undergoing High ‎and Low Amplitude Limit Cycle Oscillations

In the present study, we investigate the phenomenon of transition of a thermoacoustic system involving two-phase flow, from aperiodic oscillations to limit cycle oscillations. Experiments were performed in a laboratory scale model of a spray combustor. A needle spray injector is used to generate a droplet spray having one-dimensional velocity field. This simplified design of the injector helps in keeping away the geometric complexities involved in the real spray atomizers. We investigate the stability of the spray combustor in response to the variation of the flame location inside the combustor. Equivalence ratio is maintained constant throughout the experiment. The dynamics of the system is captured by measuring the unsteady pressure fluctuations present in the system. As the flame location is gradually varied, self-excited high-amplitude acoustic oscillations are observed in the combustor. We observe the transition of the system behavior from low-amplitude aperiodic oscillations to large amplitude limit cycle oscillations occurring through intermittency. This intermittent state mainly consists of a sequence of high-amplitude bursts of periodic oscillations separated by low-amplitude aperiodic regions. Moreover, the experimental results highlight that during intermittency, the maximum amplitude of bursts, near to the onset of intermittency, is as much as three times higher than the maximum amplitude of the limit cycle oscillations. These high-amplitude intermittent loads can have stronger adverse effects on the structural properties of the engine than the low-amplitude cyclic loading caused by the sustained limit cycle oscillations. Evolution of the three different dynamical states of the spray combustion system (viz., stable, intermittency, and limit cycle) is studied in three-dimensional phase space by using a phase space reconstruction tool from the dynamical system theory. We report the first experimental observation of type-II intermittency in a spray combustion system. The statistical distributions of the length of aperiodic (turbulent) phase with respect to the control parameter, first return map and recurrence plot (RP) techniques are employed to confirm the type of intermittency.

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Identifying route to stall flutter through stochastic bifurcation analysis

MATEC Web of Conferences ◽

10.1051/matecconf/201821102011 ◽

2018 ◽

Vol 211 ◽

pp. 02011

Author(s):

Rajagopal V Bethi ◽

Sai Vishal Reddy Gali ◽

J Venkatramani

Keyword(s):

Fluid Flow ◽

Limit Cycle ◽

Bifurcation Analysis ◽

Period Doubling ◽

Stochastic Bifurcation ◽

Nonlinear Phenomenon ◽

Limit Cycle Oscillations ◽

Viscous Effects ◽

Stall Flutter ◽

The Stability

The interaction of an elastic structure such as an airfoil and fluid flow can give rise to nonlinear phenomenon such as limit cycle oscillations, period doubling or chaos. These phenomena are indicated by a change in the stability behaviour of the dynamical known as bifurcations. Presence of viscous effects in the fluid flow can give rise to flow separation which causes a stability change in the system that is identified to happen via a Hopf bifurcation. In such cases, the airfoil exhibits limit cycle oscillations which are torsionally dominant, known as stall flutter. Despite identifying the route to stall flutter under uniform flow conditions, investigating a stall problem under stochastic wind has received minimal attention. The ability of fluctuating flows to change the stability boundaries and disrupt the route to flutter, compels the need to carry out a stochastic analysis of stalling airfoils. Study of stall flutter in such systems under the influence of a time varying sinusoidal gust is undertaken and the route to flutter is identified by carrying out a stochastic bifurcation analysis.

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Intermittency Route to Combustion Instability in a Laboratory Spray Combustor

Volume 4A: Combustion, Fuels and Emissions ◽

10.1115/gt2015-42919 ◽

2015 ◽

Cited By ~ 1

Author(s):

S. A. Pawar ◽

R. Vishnu ◽

M. Vadivukkarasan ◽

M. V. Panchagnula ◽

Sujith Raman

Keyword(s):

Phase Space ◽

Limit Cycle ◽

Maximum Amplitude ◽

High Amplitude ◽

Scale Model ◽

Spray Combustion ◽

Dynamical System Theory ◽

Limit Cycle Oscillations ◽

Combustion System ◽

Low Amplitude

In the present study, we investigate the phenomenon of transition of a thermoacoustic system involving two-phase flow, from aperiodic oscillations to limit cycle oscillations. Experiments were performed in a laboratory scale model of a spray combustor. A needle spray injector is used to generate a droplet spray having one dimensional velocity field. This simplified design of the injector helps in keeping away the geometric complexities involved in the real spray atomizers. We investigate the stability of the spray combustor in response to the variation of the flame location inside the combustor. Equivalence ratio is maintained constant throughout the experiment. The dynamics of the system is captured by measuring the unsteady pressure fluctuations present in the system. As the flame location is gradually varied, self-excited high amplitude acoustic oscillations are observed in the combustor. We observe the transition of the system behaviour from low amplitude aperiodic oscillations to large amplitude limit cycle oscillations occurring through intermittency. This intermittent state mainly consists of a sequence of high-amplitude periodic bursts separated by low amplitude aperiodic regions. Moreover, the experimental results highlight that during intermittency, the maximum amplitude of bursts oscillations, near to the onset of intermittency, is as much as three times higher than the maximum amplitude of the limit cycle oscillations. These high amplitude intermittent loads can have stronger adverse effects on the structural properties of the engine than the low amplitude cyclic loading caused by the sustained limit cycle oscillations. Evolution of the three different dynamical states of the spray combustion system (viz. stable, intermittency and limit cycle) are studied in three-dimensional phase space by using a phase space reconstruction tool from the dynamical system theory. We report the first experimental observation of type-II intermittency in a spray combustion system. The statistical distributions of the length of aperiodic (turbulent) phase with respect to the control parameter, first return map and recurrence plot techniques are employed to confirm the type of intermittency.

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