arXiv subjects

A. Berera

Publications and source records attributed to A. Berera.

12 recordsLinked to original sources

arXiv2025

Rogue Vertical Drafts in the Mesosphere and Lower Thermosphere: Evidence and Implications

Observational evidence of extreme vertical velocities (|w| ge 12.5 m/s and at times greater than 50 m/s) in the mesosphere and lower thermosphere (MLT), has emerged in recent years. We refer to these events as Rogue Vertical Drafts (RVDs). They exceed five standard deviations of observed vertical velocities and appear as paired updraft-downdraft structures in varicose mode. Four-dimensional observations reveal that RVDs are intermittent, recurrent, and unpredictable. On average, they are expected to occur every sim 12 days during summer over Northern Norway, assuming a 1000 s interval. Different instruments may capture only portions of these events, for example, only upward or downward drafts when restricted to a single altitude range. Despite their rarity, their magnitudes and frequency suggest potential impacts on dust-sized matter escaping from planets, natural and anthropogenic space material, and MLT climate and processes. We propose that RVDs are a fundamental yet under-recognized feature of the MLT, underscoring the need for global observations to assess their prevalence and significance.

physics.ao-ph↗

arXiv2017

Chaotic properties of a turbulent isotropic fluid

By tracking the divergence of two initially close trajectories in phase space in an Eulerian approach to forced turbulence, the relation between the maximal Lyapunov exponent $λ$, and the Reynolds number $Re$ is measured using direct numerical simulations, performed on up to $2048^3$ collocation points. The Lyapunov exponent is found to solely depend on the Reynolds number with $λ\propto Re^{0.53}$ and that after a transient period the divergence of trajectories grows at the same rate at all scales. Finally a linear divergence is seen that is dependent on the energy forcing rate. Links are made with other chaotic systems.

physics.flu-dyn↗

arXiv2015

Self-organization and transition to turbulence in isotropic fluid motion driven by negative damping at low wavenumbers

We observe a symmetry-breaking transition from a turbulent to a self-organized state in direct numerical simulation of the Navier-Stokes equation at very low Reynolds number. In this self-organized state the kinetic energy is contained only in modes at the lowest resolved wavenumber, the skewness vanishes, and visualization of the flows shows a lack of small-scale structure, with the vorticity and velocity vectors becoming aligned (a Beltrami flow).

physics.flu-dyn↗

arXiv2015

Energy transfer and dissipation in forced isotropic turbulence

A model for the Reynolds number dependence of the dimensionless dissipation rate $C_{\varepsilon}$ was derived from the dimensionless Kármán-Howarth equation, resulting in $C_{\varepsilon}=C_{\varepsilon, \infty} + C/R_L + O(1/R_L^2)$, where $R_L$ is the integral scale Reynolds number. The coefficients $C$ and $C_{\varepsilon,\infty}$ arise from asymptotic expansions of the dimensionless second- and third-order structure functions. This theoretical work was supplemented by direct numerical simulations (DNSs) of forced isotropic turbulence for integral scale Reynolds numbers up to $R_L=5875$ ($R_λ=435$), which were used to establish that the decay of dimensionless dissipation with increasing Reynolds number took the form of a power law $R_L^n$ with exponent value $n = -1.000\pm 0.009$, and that this decay of $C_{\varepsilon}$ was actually due to the increase in the Taylor surrogate $U^3/L$. The model equation was fitted to data from the DNS which resulted in the value $C=18.9\pm 1.3$ and in an asymptotic value for $C_\varepsilon$ in the infinite Reynolds number limit of $C_{\varepsilon,\infty} = 0.468 \pm 0.006$.

physics.flu-dyn↗

arXiv2014

Spectral analysis of structure functions and their scaling exponents in forced isotropic turbulence

The pseudospectral method, in conjunction with a new technique for obtaining scaling exponents $ζ_n$ from the structure functions $S_n(r)$, is presented as an alternative to the extended self-similarity (ESS) method and the use of generalized structure functions. We propose plotting the ratio $|S_n(r)/S_3(r)|$ against the separation $r$ in accordance with a standard technique for analysing experimental data. This method differs from the ESS technique, which plots $S_n(r)$ against $S_3(r)$, with the assumption $S_3(r) \sim r$. Using our method for the particular case of $S_2(r)$ we obtain the new result that the exponent $ζ_2$ decreases as the Taylor-Reynolds number increases, with $ζ_2 \to 0.679 \pm 0.013$ as $R_λ \to \infty$. This supports the idea of finite-viscosity corrections to the K41 prediction for $S_2$, and is the opposite of the result obtained by ESS. The pseudospectral method also permits the forcing to be taken into account exactly through the calculation of the energy input in real space from the work spectrum of the stirring forces.

physics.flu-dyn↗

arXiv2011

Shear viscous effects on the primordial power spectrum from warm inflation

We compute the primordial curvature spectrum generated during warm inflation, including shear viscous effects. The primordial spectrum is dominated by the thermal fluctuations of the radiation bath, sourced by the dissipative term of the inflaton field. The dissipative coefficient Υ, computed from first principles in the close-to-equilibrium approximation, depends in general on the temperature T, and this dependence renders the system of the linear fluctuations coupled. Whenever the dissipative coefficient is larger than the Hubble expansion rate H, there is a growing mode in the fluctuations before horizon crossing. However, dissipation intrinsically means departures from equilibrium, and therefore the presence of a shear viscous pressure in the radiation fluid. This in turn acts as an extra friction term for the radiation fluctuations that tends to damp the growth of the perturbations. Independently of the T functional dependence of the dissipation and the shear viscosity, we find that when the shear viscous coefficient ζ_s is larger than 3 ρ_r/H at horizon crossing, ρ_r being the radiation energy density, the shear damping effect wins and there is no growing mode in the spectrum.

astro-ph.CO↗

arXiv2007

Gauge symmetry and Slavnov-Taylor identities for randomly stirred fluids

The path integral for randomly forced incompressible fluids is shown to have an underlying Becchi-Rouet-Stora (BRS) symmetry as a consequence of Galilean invariance. This symmetry must be respected to have a consistent generating functional, free from both an overall infinite factor and spurious relations amongst correlation functions. We present a procedure for respecting this BRS symmetry, akin to gauge fixing in quantum field theory. Relations are derived between correlation functions of this gauge fixed, BRS symmetric theory, analogous to the Slavnov-Taylor identities of quantum field theory.

hep-th↗

arXiv2002

Non-Gaussian Signatures in the Cosmic Background Radiation from Warm Inflation

We calculate the bispectrum of the gravitational field fluctuations generated during warm inflation, where dissipation of the vacuum potential during inflation is the mechanism for structure formation. The bispectrum is non--zero because of the self--interaction of the scalar field. We compare the predictions with those of standard, or `supercooled', inflationary models, and consider the detectability of these levels of non--Gaussianity in the bispectrum of the cosmic microwave background. We find that the levels of non--Gaussianity for warm and supercooled inflation are comparable, and over--ridden by the contribution to the bispectrum due to other physical effects. We also conclude that the resulting bispectrum values will be undetectable in the cosmic microwave background for both the MAP and Planck Surveyor satellites.

astro-ph↗

arXiv2000

Perturbation Spectra in the Warm Inflationary Scenario

We investigate the phenomenology of warm inflation and present generic results about the evolution of the inflaton and radiation fields. The general conditions required for warm inflation to take place are derived and discussed. A comprehensive approach is presented for the generation of thermally induced adiabatic and isocurvature perturbations and the amplitude of their spectra calculated. In addition we derive the ratio of tensor-to-scalar perturbations and find the spectral indices for adiabatic, isocurvature and tensor perturbations formed in the warm inflationary era. These results are presented in a simplified and compact approach that is generally applicable. Our results are illustrated by inflation models with polynomial and exponential potentials. We compare our analytic results against numerical models and find excellent agreement. Finally, presently available data is used to put constraints on warm inflation and we discuss how near--future observations may distinguish the warm inflationary scenario from standard supercooled inflation. The main observable difference is the different scalar-to- tensor ratio, and that the consistency relation between this and the slope of tensor perturbations does not hold for warm inflation.

astro-ph↗

arXiv1997

The Limiting Curve of Leading Particles from Hadron-Nucleus Collisions at Infinite A

We argue that as the atomic number of the target nucleus $A \to \infty$, the multiplicity of leading particles in hadron-nucleus collisions tends to a finite limit. The limiting multiplicities for various particle production are computed for both proton and pion projectiles. Signatures at finite A are discussed. Data from 100 GeV/c central hadron-nucleus collisions are analyzed and found to be in qualitative agreement with this picture.

hep-ph↗

arXiv1994

Multi-color QCD at High Energies and Exactly Solvable Lattice Theories

We examine the generalized leading-logarithmic approximation (LLA) equations for compound states of n-reggeized gluons. It is shown that in multi-color QCD, when $N_c \rightarrow \infty$, these equations have a sufficient number of conservation laws to be exactly solvable. Holomorphic factorization of the wave functions is used to reduce the corresponding quantum mechanical problem to the solution of the one-dimensional Heisenberg model with the spins being the generators of the M$\ddot{o}$bius group of conformal transformations.

hep-ph↗

arXiv1994

Stochastic fluctuations and structure formation in the Universe

It is shown that the evolution of the density perturbations during certain eras of substantial entropy generation in the universe can be described in the scheme of the KPZ equation. Therefore, the influence on cosmological structure formation by stochastic forces arising from various dissipations can be studied through the universal characteristics of surface growth in $d=3+1$ dimensions. We identify eras of strong stochastic fluctuations and describe dynamically how these other dissipative sources of noise, besides initial (inflationary) quantum fluctuations, generate seeds of density perturbation with power law spectrum, including the Harrison-Zeldovich spectrum.

astro-ph↗