Lattice-Boltzmann simulations of the drag force on a sphere approaching a superhydrophobic striped plane

By means of lattice-Boltzmann simulations the drag force on a sphere of radius R approaching a superhydrophobic striped wall has been investigated as a function of arbitrary separation h. Superhydrophobic (perfect-slip vs. no-slip) stripes are characterized by a texture period L and a fraction of the gas area $\phi$. For very large values of h/R we recover the macroscopic formulae for a sphere moving towards a hydrophilic no-slip plane. For h/R=O(1) and smaller the drag force is smaller than predicted by classical theories for hydrophilic no-slip surfaces, but larger than expected for a sphere interacting with a uniform perfectly slipping wall. At a thinner gap, $h\ll R$ the force reduction compared to a classical result becomes more pronounced, and is maximized by increasing $\phi$. In the limit of very small separations our simulation data are in quantitative agreement with an asymptotic equation, which relates a correction to a force for superhydrophobic slip to texture parameters. In addition, we examine the flow and pressure field and observe their oscillatory character in the transverse direction in the vicinity of the wall, which reflects the influence of the heterogeneity and anisotropy of the striped texture. Finally, we investigate the lateral force on the sphere, which is detectable in case of very small separations and is maximized by stripes with $\phi=0.5$.Comment: 9 pages, 7 figure

Effective slip over superhydrophobic surfaces in thin channels

Superhydrophobic surfaces reduce drag by combining hydrophobicity and roughness to trap gas bubbles in a micro- and nanoscopic texture. Recent work has focused on specific cases, such as striped grooves or arrays of pillars, with limited theoretical guidance. Here, we consider the experimentally relevant limit of thin channels and obtain rigorous bounds on the effective slip length for any two-component (e.g. low-slip and high-slip) texture with given area fractions. Among all anisotropic textures, parallel stripes attain the largest (or smallest) possible slip in a straight, thin channel for parallel (or perpendicular) orientation with respect to the mean flow. For isotropic (e.g. chessboard or random) textures, the Hashin-Strikman conditions further constrain the effective slip. These results provide a framework for the rational design of superhydrophobic surfaces.Comment: 4+ page

Распознавание радиолокационных изображений, формируемых радиолокационными системами с синтезированной апертурой

In the field of radar remote sensing of the Earth, the problem of detecting and / or identifying spatially distributed targets against the background of a homogeneous surface is becoming increasingly important, for example, the tasks of the coast guard, monitoring of unauthorized forest logging, assessing the consequences of natural disasters, and others. This study is devoted to solving the problem of developing the optimal algorithm for making a decision on the class of a spatially distributed target based on data from side-scan radar systems with a synthetic aperture. A detailed description of the signal formation process in the fixed range channel is given, taking into account possible interference factors. Based on the statistical criterion by the method of maximum likelihood, the recognition algorithm is proposed, expressions for the formation of a feature vector are obtained, and the nonparametric decision rule is proposed. The algorithm has been tested on the example of recognizing three classes of spatially distributed targets that differ in size. В области дистанционного зондирования Земли в радиодиапазоне в последнее время приобретает все большую актуальность проблема обнаружения и/или идентификации пространственно-распределенных целей на фоне однородной поверхности. В качестве примера можно привести задачи береговой охраны, мониторинг несанкционированных лесных вырубок, оценка последствий стихийных бедствий и другие. Настоящая работа посвящена решению задачи синтеза оптимального алгоритма принятия решения о классе пространственно-распределенной цели по данным радиолокационных систем бокового обзора с синтезированной апертурой. Приведено подробное описание процесса формирования сигнала в фиксированном канале дальности с учетом возможных возмущающих факторов. На основе статистического критерия по методу максимального правдоподобия предложен алгоритм распознавания, получены выражения для формирования вектора признаков, а также предложено непараметрическое решающее правило. Предложенный алгоритм опробован на примере распознавания трех классов пространственно-распределенных целей, отличающихся размерами.Виноградова Н. С., Доросинский Л. Г. Распознавание радиолокационных изображений, формируемых радиолокационными системами с синтезированной апертурой. Ural Radio Engineering Journal. 2021;5(3):258–271. DOI: 10.15826/urej.2021.5.3.004.

Effectiveness of signal classification using chernoff and kailath boundary conditions

The probabilities calculation of correct and incorrect surface changes recognition observed by radars placed on satellites is an actual task. In the paper, analytical expressions for calculating the efficiency of classification based on the Chernoff and Kailath boundary ratios, are obtained. The expressions are illustrated by a number of practically useful examples of detecting and recognizing the changes in the radar image. © 2020 American Institute of Physics Inc.. All rights reserved

Contents of heavy metals in fructicose epiphytic lichens of Karelia as indicator of atmospheric transport of pollutants

Results of studies of heavy metals contents in fruticose epiphytic lichens in Karelia are presented and the influence of different sources on the elemental composition of lichens has been estimated. It has been shown, that long-range atmospheric transport influences strongly the accumulation of Pb, Zn, Cd, Sb. For Al, Fe and Co lithogenic source is the main one. In the Northern Karelia atmospheric transport of Cu, Co and Ni from metallurgic enterprises of the Murmansk Region is important source of these elements

Surface Roughness and Effective Stick-Slip Motion

The effect of random surface roughness on hydrodynamics of viscous incompressible liquid is discussed. Roughness-driven contributions to hydrodynamic flows, energy dissipation, and friction force are calculated in a wide range of parameters. When the hydrodynamic decay length (the viscous wave penetration depth) is larger than the size of random surface inhomogeneities, it is possible to replace a random rough surface by effective stick-slip boundary conditions on a flat surface with two constants: the stick-slip length and the renormalization of viscosity near the boundary. The stick-slip length and the renormalization coefficient are expressed explicitly via the correlation function of random surface inhomogeneities. The effective stick-slip length is always negative signifying the effective slow-down of the hydrodynamic flows by the rough surface (stick rather than slip motion). A simple hydrodynamic model is presented as an illustration of these general hydrodynamic results. The effective boundary parameters are analyzed numerically for Gaussian, power-law and exponentially decaying correlators with various indices. The maximum on the frequency dependence of the dissipation allows one to extract the correlation radius (characteristic size) of the surface inhomogeneities directly from, for example, experiments with torsional quartz oscillators.Comment: RevTeX4, 14 pages, 3 figure

Lattice Boltzmann simulations in microfluidics: probing the no-slip boundary condition in hydrophobic, rough, and surface nanobubble laden microchannels

In this contribution we review recent efforts on investigations of the effect of (apparent) boundary slip by utilizing lattice Boltzmann simulations. We demonstrate the applicability of the method to treat fundamental questions in microfluidics by investigating fluid flow in hydrophobic and rough microchannels as well as over surfaces covered by nano- or microscale gas bubbles.Comment: 11 pages, 6 figure

How the initiating ribosome copes with ppGpp to translate mRNAs

During host colonization, bacteria use the alarmones (p)ppGpp to reshape their proteome by acting pleiotropically on DNA, RNA, and protein synthesis. Here, we elucidate how the initiating ribosome senses the cellular pool of guanosine nucleotides and regulates the progression towards protein synthesis. Our results show that the affinity of guanosine triphosphate (GTP) and the inhibitory concentration of ppGpp for the 30S-bound initiation factor IF2 vary depending on the programmed mRNA. The TufA mRNA enhanced GTP affinity for 30S complexes, resulting in improved ppGpp tolerance and allowing efficient protein synthesis. Conversely, the InfA mRNA allowed ppGpp to compete with GTP for IF2, thus stalling 30S complexes. Structural modeling and biochemical analysis of the TufA mRNA unveiled a structured enhancer of translation initiation (SETI) composed of two consecutive hairpins proximal to the translation initiation region (TIR) that largely account for ppGpp tolerance under physiological concentrations of guanosine nucleotides. Furthermore, our results show that the mechanism enhancing ppGpp tolerance is not restricted to the TufA mRNA, as similar ppGpp tolerance was found for the SETI-containing Rnr mRNA. Finally, we show that IF2 can use pppGpp to promote the formation of 30S initiation complexes (ICs), albeit requiring higher factor concentration and resulting in slower transitions to translation elongation. Altogether, our data unveil a novel regulatory mechanism at the onset of protein synthesis that tolerates physiological concentrations of ppGpp and that bacteria can exploit to modulate their proteome as a function of the nutritional shift happening during stringent response and infection.Russian Foundation for Basic ResearchRevisión por pare

On discretization in time in simulations of particulate flows

We propose a time discretization scheme for a class of ordinary differential equations arising in simulations of fluid/particle flows. The scheme is intended to work robustly in the lubrication regime when the distance between two particles immersed in the fluid or between a particle and the wall tends to zero. The idea consists in introducing a small threshold for the particle-wall distance below which the real trajectory of the particle is replaced by an approximated one where the distance is kept equal to the threshold value. The error of this approximation is estimated both theoretically and by numerical experiments. Our time marching scheme can be easily incorporated into a full simulation method where the velocity of the fluid is obtained by a numerical solution to Stokes or Navier-Stokes equations. We also provide a derivation of the asymptotic expansion for the lubrication force (used in our numerical experiments) acting on a disk immersed in a Newtonian fluid and approaching the wall. The method of this derivation is new and can be easily adapted to other cases