Reactive Rayleigh-Taylor Turbulence

The Rayleigh-Taylor (RT) instability develops and leads to turbulence when a heavy fluid falls under the action of gravity through a light one. We consider this phenomenon accompanied by a reactive transformation between the fluids, and study with Direct Numerical Simulations (DNS) how the reaction (flame) affects the turbulent mixing in the Boussinesq approximation. We discuss "slow" reactions where the characteristic reaction time exceeds the temporal scale of the RT instability. In the early turbulent stage, effects of the flame are distributed over a maturing mixing zone, whose development is weakly influenced by the reaction. At later times, the fully mixed zone transforms into a conglomerate of pure-fluid patches of sizes proportional to the mixing zone width. In this "stirred flame'' regime, temperature fluctuations are consumed by reactions in the regions separating the pure-fluid patches. This DNS-based qualitative description is followed by a phenomenology suggesting that thin turbulent flame is of a single-fractal character, and thus distribution of the temperature field is strongly intermittent.Comment: 14 pages, 4 figure

Ultimate photo-induced Kerr rotation achieved in semiconductor microcavities

Photoinduced Kerr rotation by more than $\pi /2$ radians is demonstrated in planar quantum well microcavity in the strong coupling regime. This result is close to the predicted theoretical maximum of $\pi$. It is achieved by engineering microcavity parameters such that the optical impedance matching condition is reached at the smallest negative detuning between exciton resonance and the cavity mode. This ensures the optimum combination of the exciton induced optical non-linearity and the enhancement of the Kerr angle by the cavity. Comprehensive analysis of the polarization state of the light in this regime shows that both renormalization of the exciton energy and the saturation of the excitonic resonance contribute to the observed optical nonlinearities.Comment: Shortened version prepared to submit in Phys. Rev. Letter

On the existence of traveling waves in the 3D Boussinesq system

We extend earlier work on traveling waves in premixed flames in a gravitationally stratified medium, subject to the Boussinesq approximation. For three-dimensional channels not aligned with the gravity direction and under the Dirichlet boundary conditions in the fluid velocity, it is shown that a non-planar traveling wave, corresponding to a non-zero reaction, exists, under an explicit condition relating the geometry of the crossection of the channel to the magnitude of the Prandtl and Rayleigh numbers, or when the advection term in the flow equations is neglected.Comment: 15 pages, to appear in Communications in Mathematical Physic

Flame Evolution During Type Ia Supernovae and the Deflagration Phase in the Gravitationally Confined Detonation Scenario

We develop an improved method for tracking the nuclear flame during the deflagration phase of a Type Ia supernova, and apply it to study the variation in outcomes expected from the gravitationally confined detonation (GCD) paradigm. A simplified 3-stage burning model and a non-static ash state are integrated with an artificially thickened advection-diffusion-reaction (ADR) flame front in order to provide an accurate but highly efficient representation of the energy release and electron capture in and after the unresolvable flame. We demonstrate that both our ADR and energy release methods do not generate significant acoustic noise, as has been a problem with previous ADR-based schemes. We proceed to model aspects of the deflagration, particularly the role of buoyancy of the hot ash, and find that our methods are reasonably well-behaved with respect to numerical resolution. We show that if a detonation occurs in material swept up by the material ejected by the first rising bubble but gravitationally confined to the white dwarf (WD) surface (the GCD paradigm), the density structure of the WD at detonation is systematically correlated with the distance of the deflagration ignition point from the center of the star. Coupled to a suitably stochastic ignition process, this correlation may provide a plausible explanation for the variety of nickel masses seen in Type Ia Supernovae.Comment: 14 pages, 10 figures, accepted to the Astrophysical Journa

Flame Enhancement and Quenching in Fluid Flows

We perform direct numerical simulations (DNS) of an advected scalar field which diffuses and reacts according to a nonlinear reaction law. The objective is to study how the bulk burning rate of the reaction is affected by an imposed flow. In particular, we are interested in comparing the numerical results with recently predicted analytical upper and lower bounds. We focus on reaction enhancement and quenching phenomena for two classes of imposed model flows with different geometries: periodic shear flow and cellular flow. We are primarily interested in the fast advection regime. We find that the bulk burning rate v in a shear flow satisfies v ~ a*U+b where U is the typical flow velocity and a is a constant depending on the relationship between the oscillation length scale of the flow and laminar front thickness. For cellular flow, we obtain v ~ U^{1/4}. We also study flame extinction (quenching) for an ignition-type reaction law and compactly supported initial data for the scalar field. We find that in a shear flow the flame of the size W can be typically quenched by a flow with amplitude U ~ alpha*W. The constant alpha depends on the geometry of the flow and tends to infinity if the flow profile has a plateau larger than a critical size. In a cellular flow, we find that the advection strength required for quenching is U ~ W^4 if the cell size is smaller than a critical value.Comment: 14 pages, 20 figures, revtex4, submitted to Combustion Theory and Modellin

Capturing the Fire: Flame Energetics and Neutronizaton for Type Ia Supernova Simulations

We develop and calibrate a realistic model flame for hydrodynamical simulations of deflagrations in white dwarf (Type Ia) supernovae. Our flame model builds on the advection-diffusion-reaction model of Khokhlov and includes electron screening and Coulomb corrections to the equation of state in a self-consistent way. We calibrate this model flame--its energetics and timescales for energy release and neutronization--with self-heating reaction network calculations that include both these Coulomb effects and up-to-date weak interactions. The burned material evolves post-flame due to both weak interactions and hydrodynamic changes in density and temperature. We develop a scheme to follow the evolution, including neutronization, of the NSE state subsequent to the passage of the flame front. As a result, our model flame is suitable for deflagration simulations over a wide range of initial central densities and can track the temperature and electron fraction of the burned material through the explosion and into the expansion of the ejecta.Comment: 21 pages, 24 figures, to appear in Ap

Model Flames in the Boussinesq Limit: The Effects of Feedback

We have studied the fully nonlinear behavior of pre-mixed flames in a gravitationally stratified medium, subject to the Boussinesq approximation. Key results include the establishment of criterion for when such flames propagate as simple planar flames; elucidation of scaling laws for the effective flame speed; and a study of the stability properties of these flames. The simplicity of some of our scalings results suggests that analytical work may further advance our understandings of buoyant flames.Comment: 11 pages, 14 figures, RevTex, gzipped tar fil

Nanoscale investigation of polymer cement concretes by small angle neutron scattering

An analysis of dense cements, such as polymer cement concrete, is made to produce original innovative components for different types of constructing materials. These materials present good functional properties (ageing resistance, crack formation resistance, hardness, and stability of mechanical modules) and can be used for various applications. In this paper, experimental tests on Portland cement with added γ-Al 2 O 3 and redispersible dry polymer performed using small angle neutron scattering are reported. The objective of the investigation was to assess the key parameters of the material (e.g., porosity, fractal dimensions, and size distribution) at the nanoscale level as well as to obtain useful structural information for expanding the possibility of applications. The results obtained can contribute to the optimisation of the consistency of the material, the design of operating conditions of elements of structures and facilities, and the design of the procedures that support ecological criteria and enhance quality and safety levels. © 2017 Walter de Gruyter GmbH, Berlin/Boston

Kolmogorov scaling and intermittency in Rayleigh-Taylor turbulence

The Rayleigh--Taylor (RT) turbulence is investigated by means of high resolution numerical simulations. The main question addressed here is on whether RT phenomenology can be considered as a manifestation of universality of Navier--Stokes equations with respect to forcing mechanisms. At a theoretical level the situation is far from being firmly established and, indeed, contrasting predictions have been formulated. Our first aim here is to clarify the above controversy through a deep analysis of scaling behavior of relevant statistical observables. The effects of intermittency on the mean field scaling predictions is also discussed.Comment: 4 pages, 5 figure