An analytical-based method for studying the nonlinear evolution of localized vortices in planar homogenous shear flows

Recent experimental and numerical studies have shown that the interaction between a localized vortical disturbance and the shear of an external base flow can lead to the formation of counter-rotating vortex pairs and hairpin vortices that are frequently observed in wall bounded and free turbulent shear flows as well as in subcritical shear flows. In this paper an analytical-based solution method is developed. The method is capable of following (numerically) the evolution of finite-amplitude localized vortical disturbances embedded in shear flows. Due to their localization in space, the surrounding base flow is assumed to have homogeneous shear to leading order. The method can solve in a novel way the interaction between a general family of unbounded planar homogeneous shear flows and any localized disturbance. The solution is carried out using Lagrangian variables in Fourier space which is convenient and enables fast computations. The potential of the method is demonstrated by following the evolved structures of large amplitude disturbances in three canonical base flows, including simple shear, plane stagnation (extensional) and pure rotation flows, and a general case. The results obtained by the current method for plane stagnation and simple shear flows are compared with the published results. The proposed method could be extended to other flows (e.g. geophysical and rotating flows) and to include periodic disturbances as well.     

Single-scale wavelet representation of turbulence dynamics: Formulation and Navier-Stokes regularity

The ‘triad interactions’, arising from the spectral resolution of the nonlinear terms in the Navier-Stokes equations, have so far not been substantially modified in the wavelet representation. In this paper, the multiscale interactions are captured by exact expressions evaluated at a single scale of the Mexican hat wavelet coefficients: the larger-scale terms as a volume integral of nearby wavelet coefficients, and the smaller-scale contributions as iterated Laplacians of the coefficient at the point of interest. As a result, the Navier-Stokes equations are expressed exactly at a single scale. This facilitates the evaluation of the dominant Hölder exponent near singularities. From the scaling properties of wavelet coefficients, it is shown that Euler dynamics would generate stronger singularities for any h<1, but that viscous dynamics would not unless h<−1 (a discontinuous case). We discuss how this conclusion could be affected by boundary conditions.     

Chemically resolved photovoltage measurements in CdSe nanoparticle films

Light-induced chemically resolved electrical measurements (CREM) under controlled electrical conditions are used to study photovoltaic effects at selected regions in nanocrystalline CdSe-based films. The method, based on X-ray photoelectron spectroscopy (XPS), possesses unique capabilities for exploring charge trapping and charge transport mechanisms, combining spectrally filtered input signals with photocurrent detection and with a powerful, site-selective, photovoltage probe.     

Revisiting the boundary layer structure used in Craig and Gordon's model of isotope fractionation in evaporation

Craig and Gordon's (CG) model of isotope fractionation in evaporation was derived more than 40 years ago and was based on the turbulent boundary layer structure model acceptable at that time. That view suggested that turbulent flows consist of eddies with a wide range of length scales moving randomly in the flow domain. There is evidence that some parameters in CG model do not fully correspond to data in the literature. Owing to advances in fluid dynamics research techniques, it has been shown in recent decades that the apparent chaotic flow in turbulent boundary layers is in fact governed by few well-organised structures. This article reviews the major characteristics of these coherent structures based on available results from experimental, numerical and theoretical studies of turbulent and laminar boundary layers. The review points on some differences between past and present views of the boundary layer structure and on their relation and possible influence on power laws in CG model.     

Electron emission in superfluid and low temperature vapor phase helium

Tungsten filaments used as sources of electrons in a low-temperature liquid or gaseous helium environment have remarkable properties of operating at thousands of degrees kelvin in surroundings at temperatures of order 1 K. We provide an explanation of this performance in terms of important changes in the thermal transport mechanisms. The behavior can be cast as a first-order phase transition.     

On the geometry of isomonodromic deformations

This note examines the geometry behind the Hamiltonian structure of isomonodromy deformations of connections on vector bundles over Riemann surfaces. The main point is that one should think of an open set of the moduli of pairs (V,∇)$(V,\nabla )$ of vector bundles and connections as being obtained by “twists” supported over points of a fixed vector bundle _V0$V_0$ with a fixed connection _∇0${\nabla }_0$; this gives two deformations, one, isomonodromic, of (V,∇)$(V,\nabla )$, and another induced from the isomonodromic deformation of (_V0,_∇0)$(V_0,{\nabla }_0)$. The difference between the two will be Hamiltonian.     

An abinitio study of protonation and alkylation of aminopyridine

STO-3G abinitio calculations show that the first site of protonation in 2-, 3- and 4-aminopyridines is the cyclic nitrogen, whatever the position of the amino group. There is an excellent linear relationship between the calculated proton affinities and the experimental pK_a values (r = 0.995). The protonation site is also confirmed by π→π* transition energy variations as a function of position of the amino-group due to protonation; these effects are connected with the MO structures. Basicity and reactivity in alkylation reveal lone pair localization.