2,848 research outputs found
Multilayer network analysis to study complex inter-subsystem interactions in a turbulent thermoacoustic system
Thermoacoustic systems are complex systems where the interactions between the
hydrodynamic, acoustic and heat release rate fluctuations lead to diverse
dynamics such as chaos, intermittency, and ordered dynamics. Such complex
interactions cause catastrophically high-amplitude acoustic pressure
oscillations and the emergence of order in the spatio-temporal dynamics,
referred to as thermoacoustic instability. In this work, we use multilayer
networks to study the topology of inter-subsystem interactions between the
hydrodynamic, acoustic and combustion subsystems in a bluff-body stabilized
turbulent dump combustor during diverse dynamical states. We construct a
two-layered network where the layers represent the thermoacoustic power and
vorticity fields. The inter-layer links are determined using cross-variable
short-window correlations between vorticity and thermoacoustic power
fluctuations. We distinguish the topology of inter-layer networks during
different dynamical states using network properties such as assortativity and
link-rank distributions. During chaotic dynamics, interactions between
subsystems are non-localized and spread throughout the flow field of the
combustor. During the state of thermoacoustic instability (order), the
inter-layer network topology reveals dense inter-layer connections between
regions of coherent thermoacoustic power generation and regions where large
vortices are shed periodically. Furthermore, we show that such dense
inter-layer connections emerge in localized pockets in the recirculation zone
in the dump plane of the combustor much prior to the onset of order, that is,
during the state of intermittency. We identify these regions of intense
inter-subsystem interactions as hubs of the multilayer network, and conjecture
that targeted attack at such hubs disrupts inter-subsystem interactions and
thus helps mitigate thermoacoustic instability.Comment: Under review in JF
Novel Experimentally Observed Phenomena in Soft Matter
Soft materials such as colloidal suspensions, polymer solutions and liquid
crystals are constituted by mesoscopic entities held together by weak forces.
Their mechanical moduli are several orders of magnitude lower than those of
atomic solids. The application of small to moderate stresses to these materials
results in the disruption of their microstructures. The resulting flow is
non-Newtonian and is characterised by features such as shear rate-dependent
viscosities and non-zero normal stresses. This article begins with an
introduction to some unusual flow properties displayed by soft matter.
Experiments that report a spectrum of novel phenomena exhibited by these
materials, such as turbulent drag reduction, elastic turbulence, the formation
of shear bands and the existence of rheological chaos, flow-induced
birefringence and the unusual rheology of soft glassy materials, are reviewed.
The focus then shifts to observations of the liquid-like response of granular
media that have been subjected to external forces. The article concludes with
examples of the patterns that emerge when certain soft materials are vibrated,
or when they are displaced with Newtonian fluids of lower viscosities.Comment: 30 pages, 11 figures, invited review article, supplementary videos
may be obtained from the journal websit
A global study of enhanced stretching and diffusion in chaotic tangles
A global, finite-time study is made of stretching and diffusion in a class of chaotic tangles associated with fluids described by periodically forced two-dimensional dynamical systems. Invariant lobe structures formed by intersecting global stable and unstable manifolds of persisting invariant hyperbolic sets provide the geometrical framework for studying stretching of interfaces and diffusion of passive scalars across these interfaces. In particular, the present study focuses on the material curve that initially lies on the unstable manifold segment of the boundary of the entraining turnstile lobe.A knowledge of the stretch profile of a corresponding curve that evolves according to the unperturbed flow, coupled with an appreciation of a symbolic dynamics that applies to the entire original material curve in the perturbed flow, provides the framework for understanding the mechanism for, and topology of, enhanced stretching in chaotic tangles. Secondary intersection points (SIP's) of the stable and unstable manifolds are particularly relevant to the topology, and the perturbed stretch profile is understood in terms of the unperturbed stretch profile approximately repeating itself on smaller and smaller scales. For sufficiently thin diffusion zones, diffusion of passive scalars across interfaces can be treated as a one-dimensional process, and diffusion rates across interfaces are directly related to the stretch history of the interface.An understanding of interface stretching thus directly translates to an understanding of diffusion across interfaces. However, a notable exception to the thin diffusion zone approximation occurs when an interface folds on top of itself so that neighboring diffusion zones overlap. An analysis which takes into account the overlap of nearest neighbor diffusion zones is presented, which is sufficient to capture new phenomena relevant to efficiency of mixing. The analysis adds to the concentration profile a saturation term that depends on the distance between neighboring segments of the interface. Efficiency of diffusion thus depends not only on efficiency of stretching along the interface, but on how this stretching is distributed relative to the distance between neighboring segments of the interface
Chimera states: Coexistence of coherence and incoherence in networks of coupled oscillators
A chimera state is a spatio-temporal pattern in a network of identical
coupled oscillators in which synchronous and asynchronous oscillation coexist.
This state of broken symmetry, which usually coexists with a stable spatially
symmetric state, has intrigued the nonlinear dynamics community since its
discovery in the early 2000s. Recent experiments have led to increasing
interest in the origin and dynamics of these states. Here we review the history
of research on chimera states and highlight major advances in understanding
their behaviour.Comment: 26 pages, 3 figure
Microscopic phenomena and a modern approach to turbulence
The use of an arc driven shock tube as a technique in the study of turbulence and evidence to support a kinetic theory of turbulence are described. Topics covered include: (1) reaction rate distortion in turbulent flow; (2) turbulent bursts in a shock tube; (3) driver gas flow with fluctuations; (4) improving the Mach number capabilities of arc driver shock tubes; and (5) resonant absorption in an argon plasma at thermal equilibrium
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