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At least 19 recordsLinked to original sources
arXiv2007

Endogenous and exogenous dynamics of pressure fluctuations in an impinging entrained-flow gasifier

This paper has been withdrawn by the authors due to a fatal error in the analysis. The manuscript was submitted to Chemical Engineering Science. To clarify the situation, we copy the main comment from an anonymous referee here: "To my understanding, the authors analyze i = 1 ... 63 time series and calculate their mean and standard deviation. These time series correspond to individual, single ignition processes. Is this correct? If yes, these processes, as Fig. 3 shows very clearly, are not stationary, and the pressure difference (i.e., the signal) quickly decays to zero. In this case both the mean and the standard deviation are poorly defined, for example because both depend in a trivial fashion on the observation period T. I am not aware of any study (including those cited by the authors) which allows for any conclusion from such non-stationary signals. The results of Menezes and Barabasi are strictly only valid for stationary time series, and they cannot be applied at all in this case." We agree with this insightful comment that our data are not stationary and the method adopted in our manuscript does not apply. We do not see any possibility to correct this error and decide to withdraw it. We would like to thank gratefully the referee and apologize for any inconvenience caused by our oversight.

physics.flu-dyn↗
arXiv2008

Chaotic traveling rolls in Rayleigh-Bénard convection

In this paper we investigate two-dimensional (2D) Rayleigh-Bénard convection using direct numerical simulation in Boussinesq fluids of Prandtl number $P = 6.8$ confined between thermally conducting plates. We show through the simulation that in a small range of reduced Rayleigh number $r$ ($770 < r < 890$) the 2D rolls move chaotically in a direction normal to the roll axis. The lateral shift of the rolls may lead to global flow reversal of the convective motion. The chaotic traveling rolls are observed in simulation with {\it free-slip} as well as {\it no-slip} boundary conditions on the velocity field. We show that the traveling rolls and the flow reversal are due to an interplay between the real and imaginary parts of the critical modes.

nlin.CD↗
arXiv2007

Influence of the extent of the eigenstates of a system on the resonances formed through its coupling to a field

We examine resonances for two systems consisting of a particle coupled to a massless boson's field. The field is the free field in the whole space. In the first system, the particle is confined inside a ball. We show that besides the usual energy levels of the particle, which have become complex through the coupling to the field, other resonances are to be taken into account if the ball's radius is comparable to the particle's Compton wavelength. In the second system, the particle is in a finite-depth square-well potential. We study the way the resonances' width depends on the extent of the uncoupled particle's wave functions. In both cases, we limit ourselves to considering two levels of the particle only.

physics.flu-dyn↗
arXiv2008

Descriptor approach for eliminating spurious eigenvalues in hydrodynamic equations

We describe a general framework for avoiding spurious eigenvalues -- unphysical unstable eigenvalues that often occur in hydrodynamic stability problems. In two example problems, we show that when system stability is analyzed numerically using {\em descriptor} notation, spurious eigenvalues are eliminated. Descriptor notation is a generalized eigenvalue formulation for differential-algebraic equations that explicitly retains algebraic constraints. We propose that spurious eigenvalues are likely to occur when algebraic constraints are used to analytically reduce the number of independent variables in a differential-algebraic system of equations before the system is approximated numerically. In contrast, the simple and easily generalizable descriptor framework simultaneously solves the differential equations and algebraic constraints and is well-suited to stability analysis in these systems.

physics.comp-ph↗
arXiv2007

On dynamics of velocity vector potential in incompressible fluids

An elegant quaternionic formulation is given for the Lagrangian advection equation for velocity vector potential in fluid dynamics. At first we study the topological significance of a restricted conserved quantity viz., stream-helicity and later more realistic configuration of open streamlines is figured out. Also, using Clebsch parameterisation of the velocity vector potential yet another physical significance for the stream-helicity is provided. Finally we give a Nambu-Poisson formalism of the Lagrangian advection equation for velocity vector potential.

physics.flu-dyn↗
arXiv2007

Wind instability of a foam layer sandwiched between the atmosphere and the ocean

Wind shortwave instability of a foam layer between the atmosphere and the ocean is examined in order to reach greater understanding of the recent findings of the decrease in momentum transfer from hurricane winds to sea waves. The three-fluid configuration with the high contrasts in densities of the air, foam and water provides for an effective mechanism to stabilize the water surface.

physics.ao-ph↗
arXiv2007

Asymmetric and Moving-Frame Approaches to Navier-Stokes Equations

In this paper, we introduce a method of imposing asymmetric conditions on the velocity vector with respect to independent variables and a method of moving frame for solving the three dimensional Navier-Stokes equations. Seven families of non-steady rotating asymmetric solutions with various parameters are obtained. In particular, one family of solutions blow up at any point on a moving plane with a line deleted, which may be used to study turbulence. Using Fourier expansion and two families of our solutions, one can obtain discontinuous solutions that may be useful in study of shock waves. Another family of solutions are partially cylindrical invariant, contain two parameter functions of $t$ and structurally depend on two arbitrary polynomials, which may be used to describe incompressible fluid in a nozzle. Most of our solutions are globally analytic with respect to spacial variables.

physics.flu-dyn↗
arXiv2007

New Algebraic Approaches to Classical Boundary Layer Problems

In this paper, we use various ansatzes with undetermined functions and the technique of moving frame to find solutions with parameter functions modulo the Lie point symmetries for the classical non-steady boundary layer problems. These parameter functions enable one to find the solutions of some related practical models and boundary value problems.

physics.flu-dyn↗
arXiv2007

Signatures of fractal clustering of aerosols advected under gravity

Aerosols under chaotic advection often approach a strange attractor. They move chaotically on this fractal set but, in the presence of gravity, they have a net vertical motion downwards. In practical situations, observational data may be available only at a given level, for example at the ground level. We uncover two fractal signatures of chaotic advection of aerosols under the action of gravity. Each one enables the computation of the fractal dimension $D_{0}$ of the strange attractor governing the advection dynamics from data obtained solely at a given level. We illustrate our theoretical findings with a numerical experiment and discuss their possible relevance to meteorology.

nlin.CD↗
arXiv2008

Experimental studies of liquid-liquid dispersion in a turbulent shear flow

We study liquid-liquid dispersions in a turbulent Taylor - Couette flow, produced between two counterrotating coaxial cylinders. In pure Water and in counterrotation, Reynolds numbers up to 1.4 10^5 are reached. We first characterize the single-phase flow, in terms of threshold for transition to turbulence, scaling of the torque and measurements of the mean flow and of the Reynolds stress by stereoscopic PIV. We then study the increase of the dissipation in the two-phase flows and find that the torque per unit mass can be twice the torque for a single-phase flow. Long-time behaviours are also reported.

physics.flu-dyn↗
arXiv2008

Experimental studies of turbulent Taylor-Couette flows: single phase and liquid-liquid dispersions

We study liquid-liquid dispersions in a turbulent Taylor-Couette flow, produced between two counterrotating coaxial cylinders. In pure Water and in counterrotation, Reynolds numbers up to 1.4 10^5 are reached. The liquids we use are a low-viscous Oil and pure Water or a Sodium Iodide solution with a refractive index matched to that of Oil, in order to get transparent dispersions. We first characterize the single-phase flow, in terms of threshold for transition to turbulence, scaling of the torque and measurements of the mean flow and of the Reynolds stress by stereoscopic PIV. We then study the increase of the dissipation in the two-phase flows and find that the torque per unit mass can be twice the torque for a single-phase flow. Long-time behaviours are also reported.

physics.flu-dyn↗
arXiv2007

On the Rayleigh theorem for inflectional velocity instability of inviscid flows

It is exactly proved that the classical Rayleigh Theorem on inflectional velocity instability is wrong which states that the necessary condition for instability of inviscid flow is the existence of an inflection point on the velocity profile. It is shown that the disturbance amplified in 2D inviscid flows is necessarily 3D. After the break down of T-S wave in 2D parallel flows, the disturbance becomes a type of spiral waves which proceed along the streamwise direction. This is just the origin of formation of streamwise vortices.

physics.flu-dyn↗
arXiv2007

Modification of the turbulent energy cascade by polymer additives

By tracking small particles in the bulk of an intensely turbulent flow, we show that even a very small concentration of long-chain polymers disrupts the usual turbulent energy cascade. The polymers affect scales much larger than their physical size, from the dissipation range to the inertial range. The effect depends strongly on the polymer concentration. While the dissipative-scale statistics change continuously as the polymer concentration is increased, the inertial-range energy transfer rate is only altered by the polymer additives when the concentration is above a threshold (approximately 5 parts per million by weight for the polymer we used).

physics.flu-dyn↗
arXiv2009

Local and Nonlocal Dispersive Turbulence

We consider the evolution of a family of 2D dispersive turbulence models. The members of this family involve the nonlinear advection of a dynamically active scalar field, the locality of the streamfunction-scalar relation is denoted by $α$, with smaller $α$ implying increased locality. The dispersive nature arises via a linear term whose strength is characterized by a parameter $ε$. Setting $0 < ε\le 1$, we investigate the interplay of advection and dispersion for differing degrees of locality. Specifically, we study the forward (inverse) transfer of enstrophy (energy) under large-scale (small-scale) random forcing. Straightforward arguments suggest that for small $α$ the scalar field should consist of progressively larger eddies, while for large $α$ the scalar field is expected to have a filamentary structure resulting from a stretch and fold mechanism. Confirming this, we proceed to forced/dissipative dispersive numerical experiments under weakly non-local to local conditions. For $ε\sim 1$, there is quantitative agreement between non-dispersive estimates and observed slopes in the inverse energy transfer regime. On the other hand, forward enstrophy transfer regime always yields slopes that are significantly steeper than the corresponding non-dispersive estimate. Additional simulations show the scaling in the inverse regime to be sensitive to the strength of the dispersive term : specifically, as $ε$ decreases, the inertial-range shortens and we also observe that the slope of the power-law decreases. On the other hand, for the same range of $ε$ values, the forward regime scaling is fairly universal.

physics.flu-dyn↗
arXiv2007

The Lorentz force effect on the On-Off dynamo intermittency

An investigation of the dynamo instability close to the threshold produced by an ABC forced flow is presented. We focus on the on-off intermittency behavior of the dynamo and the counter-effect of the Lorentz force in the non-linear stage of the dynamo. The Lorentz force drastically alters the statistics of the turbulent fluctuations of the flow and reduces their amplitude. As a result much longer burst (on-phases) are observed than what is expected based on the amplitude of the fluctuations in the kinematic regime of the dynamo. For large Reynolds numbers, the duration time of the ``On'' phase follows a power law distribution, while for smaller Reynolds numbers the Lorentz force completely kills the noise and the system transits from a chaotic state into a ``laminar'' time periodic flow. The behavior of the On-Off intermittency as the Reynolds number is increased is also examined. The connections with dynamo experiments and theoretical modeling are discussed.

physics.flu-dyn↗
arXiv2007

A General Strategy for Physics-Based Model Validation Illustrated with Earthquake Phenomenology, Atmospheric Radiative Transfer, and Computational Fluid Dynamics

Validation is often defined as the process of determining the degree to which a model is an accurate representation of the real world from the perspective of its intended uses. Validation is crucial as industries and governments depend increasingly on predictions by computer models to justify their decisions. In this article, we survey the model validation literature and propose to formulate validation as an iterative construction process that mimics the process occurring implicitly in the minds of scientists. We thus offer a formal representation of the progressive build-up of trust in the model, and thereby replace incapacitating claims on the impossibility of validating a given model by an adaptive process of constructive approximation. This approach is better adapted to the fuzzy, coarse-grained nature of validation. Our procedure factors in the degree of redundancy versus novelty of the experiments used for validation as well as the degree to which the model predicts the observations. We illustrate the new methodology first with the maturation of Quantum Mechanics as the arguably best established physics theory and then with several concrete examples drawn from some of our primary scientific interests: a cellular automaton model for earthquakes, an anomalous diffusion model for solar radiation transport in the cloudy atmosphere, and a computational fluid dynamics code for the Richtmyer-Meshkov instability. This article is an augmented version of Sornette et al. [2007] that appeared in Proceedings of the National Academy of Sciences in 2007 (doi: 10.1073/pnas.0611677104), with an electronic supplement at URL http://www.pnas.org/cgi/content/full/0611677104/DC1. Sornette et al. [2007] is also available in preprint form at physics/0511219.

physics.comp-ph↗