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# Department of Aerospace Engineering

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- PublicationDetecting deterministic nature of pressure measurements from a turbulent combustor(02-12-2015)
;Tony, J. ;Gopalakrishnan, E. A. ;Sreelekha, E.Show more Identifying nonlinear structures in a time series, acquired from real-world systems, is essential to characterize the dynamics of the system under study. A single time series alone might be available in most experimental situations. In addition to this, conventional techniques such as power spectral analysis might not be sufficient to characterize a time series if it is acquired from a complex system such as a thermoacoustic system. In this study, we analyze the unsteady pressure signal acquired from a turbulent combustor with bluff-body and swirler as flame holding devices. The fractal features in the unsteady pressure signal are identified using the singularity spectrum. Further, we employ surrogate methods, with translational error and permutation entropy as discriminating statistics, to test for determinism visible in the observed time series. In addition to this, permutation spectrum test could prove to be a robust technique to characterize the dynamical nature of the pressure time series acquired from experiments. Further, measures such as correlation dimension and correlation entropy are adopted to qualitatively detect noise contamination in the pressure measurements acquired during the state of combustion noise. These ensemble of measures is necessary to identify the features of a time series acquired from a system as complex as a turbulent combustor. Using these measures, we show that the pressure fluctuations during combustion noise has the features of a high-dimensional chaotic data contaminated with white and colored noise.Show more - PublicationReissnerâ€™s new mixed variational principle applied to laminated cylindrical shells(01-01-1992)
; Varadan, T. K.Show more Reissner’s new mixed variational theorem, which allows independentinterpolation, through the thickness, of the three transverse stresses besides that of the three displacements, is applied here to derive a higher-order theory of laminated orthotropic cylindrical shells. The accuracy of the theory is verified by comparison with three-dimensional elasticity solutions. It is shown that Reissner’s principle does not directly lead to accuratetransverse shear stress predictions, but requires the use of the equilibrium equations of three-dimensional elasticity as is common in the conventional displacement approach. © 1992 ASME.Show more - PublicationBifurcation analysis and observation of intermittency in combustion-driven thermoacoustic oscillations(01-12-2012)
;Kabiraj, L.Show more Thermoacoustic oscillations occur due to the interaction between the acoustic field in the combustor and the unsteady heat release rate. This interaction gives rise to a wide variety of dynamical states. In our experiments on a ducted laminar premixed flame, a prototypical thermoacoustic system, we observed presence of type-II intermittency in addition to limit cycle oscillation and quasi-periodic oscillation. Each dynamical state of the system are investigated based on the analysis of the reconstructed phase space and recurrence plots. The intermittent state resulted from the bifurcation of a quasi-periodic state and further, was followed by flame blowout. The dynamical behavior of the flame during intermittency is also discussed through flame imaging. Copyright © 2012 by L. Kabiraj and R. I. Sujith.Show more - PublicationStability analysis of simply-supported rectangular plates under non-uniform uniaxial compression using rigorous and approximate plane stress solutions(01-05-2006)
;Jana, P.Show more The non-classical problem of buckling of a simply-supported rectangular plate due to various types of non-uniform compressive edge loads is analysed here. For each case, the elasticity solution for the internal in-plane stress field is obtained rigorously using a superposition of Airy's stress functions and also approximately using extended Kantorovich method. Subsequently, the convergent buckling loads are obtained using Galerkin's method. Results are presented to highlight the dependence of the total buckling load and the corresponding buckled shape on the edge load distribution, as well as to illustrate the applicability of the approximate plane stress solutions. © 2006 Elsevier Ltd. All rights reserved.Show more - PublicationMechanism of pulsations of a triple flame in mixing layer arising due to thermo-diffusive instability with Lewis number greater than unity(01-01-2016)
;Bhatt, David S.Show more Pulsating behaviour is observed in laminar flames of different kinds ranging from edge flames in a mixing layer to premixed flames even in partially premixed triple flame. This study focuses to study the physical mechanisms behind the pulsating behaviour of triple flames. A numerical simulation is performed for a typical case of high Lewis number and low Damköhler number, where pulsating instability is observed. The flame undergoes a limit cycle oscillations. Last cycle of oscillation is analyzed to study the physical mechanisms. Temperature, and concentration filed data is plotted for different phases of the cycle. Vector plots representing the heat and mab diffusive flux gives further insight into the flow of reactants into the flame zone and also the flow of heat away from the reaction zone. Also a closer look at the field variables at the triple point is made. With the help of these plots, some insight is obtained in understanding the underlying mechanism driving the thermo-diffusive oscillations.Show more - PublicationExperimental investigation of combustion dynamics in a turbulent syngas combustor(01-01-2017)
;Baraiya, Nikhil Ashokbhai ;Nagarajan, BaladandayuthapaniShow more In the present work, the proportion of carbon monoxide to hydrogen is widely varied to simulate different compositions of synthesis gas and the potential of the fuel mixture to excite combustion oscillations in a laboratory-scale turbulent bluff body combustor is investigated. The effect of parameters such as the bluff body location and equivalence ratio on the selfexcited acoustic oscillations of the combustor is studied. The flame oscillations are mapped by means of simultaneous highspeed CH and OH chemiluminescence imaging along with dynamic pressure measurement. Mode shifts are observed as the bluff body location or the air flow Reynolds number/overall equivalence ratio are varied for different fuel compositions. It is observed that the fuel mixtures that are hydrogen-rich excite high amplitude pressure oscillations as compared to other fuel composition cases. Higher H2 content in the mixture is also capable of exciting significantly higher natural acoustic modes of the combustor so long as CO is present, but not without the latter. The interchangeability factor Wobbe Index is not entirely sufficient to understand the unsteady flame response to the chemical composition.Show more - PublicationAxisymmetric vortex breakdown: A barrier to mixing(21-03-2019)
;Sharma, ManjulShow more In this paper, we investigate mixing in flows dominated by bubble-type vortex breakdown. Three-dimensional Navier-Stokes equations in cylindrical polar coordinates are solved to simulate flow in a cylindrical container. We have found that in steady regime of the flow, the vortex breakdown bubble is axisymmetric and apparent non-axisymmetric features observed in experiments are artifacts of imperfections in experimental set-ups. We also find that the heteroclinic manifold joining hyperbolic points of the vortex breakdown bubble is stable in the absence of any perturbation and no chaotic advection was found within vortex breakdown bubble. This makes the vortex breakdown bubble impermeable to outer fluid and hence, the vortex breakdown bubble inhibits mixing. We conclude that symmetry is a barrier to mixing.Show more - PublicationUniversality in the emergence of oscillatory instabilities in turbulent flows(01-01-2020)
;Pavithran, Induja ;Unni, Vishnu R. ;Varghese, Alan J.; ;Saha, Abhishek ;Marwan, NorbertKurths, JÃ¼rgenShow more Self-organization driven by feedback between subsystems is ubiquitous in turbulent fluid mechanical systems. This self-organization manifests as emergence of oscillatory instabilities and is often studied in different system-specific frameworks. We uncover the existence of a universal scaling behaviour during self-organization in turbulent flows leading to oscillatory instability. Our experiments show that the spectral amplitude of the dominant mode of oscillations scales with the Hurst exponent of a fluctuating state variable following an inverse power law relation. Interestingly, we observe the same power law behaviour with a constant exponent near -2 across various turbulent systems such as aeroacoustic, thermoacoustic and aeroelastic systems.Show more - PublicationOnset of thermoacoustic instability in turbulent combustors: An emergence of synchronized periodicity through formation of chimera-like states(25-01-2017)
;Mondal, Sirshendu ;Unni, Vishnu R.Show more Thermoacoustic systems with a turbulent reactive flow, prevalent in the fields of power and propulsion, are highly susceptible to oscillatory instabilities. Recent studies showed that such systems transition from combustion noise to thermoacoustic instability through a dynamical state known as intermittency, where bursts of large-amplitude periodic oscillations appear in a near-random fashion in between regions of low-amplitude aperiodic fluctuations. However, as these analyses were in the temporal domain, this transition remains still unexplored spatiotemporally. Here, we present the spatiotemporal dynamics during the transition from combustion noise to limit cycle oscillations in a turbulent bluff-body stabilized combustor. To that end, we acquire the pressure oscillations and the field of heat release rate oscillations through high-speed chemiluminescence images of the reaction zone. With a view to get an insight into this complex dynamics, we compute the instantaneous phases between acoustic pressure and local heat release rate oscillations. We observe that the aperiodic oscillations during combustion noise are phase asynchronous, while the large-amplitude periodic oscillations seen during thermoacoustic instability are phase synchronous. We find something interesting during intermittency: patches of synchronized periodic oscillations and desynchronized aperiodic oscillations coexist in the reaction zone. In other words, the emergence of order from disorder happens through a dynamical state wherein regions of order and disorder coexist, resembling a chimera state. Generally, mutually coupled chaotic oscillators synchronize but retain their dynamical nature; the same is true for coupled periodic oscillators. In contrast, during intermittency, we find that patches of desynchronized aperiodic oscillations turn into patches of synchronized periodic oscillations and vice versa. Therefore, the dynamics of local heat release rate oscillations change from aperiodic to periodic as they synchronize intermittently. The temporal variations in global synchrony, estimated through the Kuramoto order parameter, echoes the breathing nature of a chimera state.Show more - PublicationOptimum air turbulence intensity for polydisperse droplet size growth(31-07-2019)
;Kumar, M. Shyam; Show more The growth of the average size of liquid droplets suspended in a turbulent air flow is of paramount importance in several natural and engineering systems. Here we present an experimental study of the effects of air flow turbulent intensity on the size growth of water droplets in a polydisperse droplet field. For a given initial distribution of droplets in the size range of 0-120 μm diameter, we identify an optimum air turbulent intensity that maximizes the rate at which the average droplet diameter increases with time. The observed trend is understood in terms of droplet collision rate statistics, droplet clustering, and the existence of a crossover diameter, below and above which the number of droplets decreases and increases in time, respectively. We show that the onset of clustering suppresses the intuitive effect of an increase in droplet collision rate with air turbulent intensity, resulting in the existence of an optimum air turbulent intensity that maximizes the average droplet size growth rate due to droplet coalescence. Our results bear consequences for the understanding of warm rain initiation from clouds and the design of engines with improved combustion characteristics.Show more