Течія рідини в циліндричному каналі з діафрагмами прямокутного профілю

The flow of a viscous incompressible liquid in a cylindrical duct with two serial diaphragms of a rectangular profile is studied by the numerical solution of the unsteady Navier-Stokes equations. The discretization procedure is based on the finite volume method using the TVD scheme for the discretization of the convective terms and second order accurate in both space and time difference schemes. The resulting system of non-linear algebraic equations is solved by the PISO algorithm. It is shown that the fluid flow in the region between the diaphragms is non-stationary and is characterized by the presence of an unstable shear layer under the certain parameters. A series of ring vortices is formed in the shear layer that causes quasi-periodic self-sustained oscillations of the velocity field in the vicinity of the orifice of the second diaphragm. In comparison with the case of rounded diaphragms, an increase in the maximum jet velocity is observed, which in turn leads to an increase in the frequency of self-sustained oscillations and a decrease in the Reynolds numbers at which quasi-periodic oscillations are excited. Pages of the article in the issue: 76 - 81 Language of the article: UkrainianДосліджено течію в’язкої нестисливої рідини у циліндричному каналі з двома послідовно розташованими діафрагмами прямокутного профілю на основі чисельного розв’язання нестаціонарних рівнянь Нав’є-Стокса. Показано, що при певних параметрах течія рідини в області між діафрагмами нестаціонарна і характеризується наявністю нестійкого зсувного шару, у якому утворюється послідовний ряд кільцевих вихорів, що викликають квазіперіодичні автоколивання поля швидкості в околі отвору другої діафрагми. У порівнянні з випадком округлого профілю діафрагм спостерігається збільшення максимальної швидкості струменя, що своєю чергою призводить до збільшення частоти автоколивань та зменшення значень чисел Рейнольдса, при яких збуджуються квазіперіодичні коливання

CHARACTERISTICS OF SPECIALIZED SINGLE-PHASE HIGH VOLTAGE DOUBLER RECTIFIER

Introduction. To obtain a high voltage direct current, voltage multipliers with a number of cascades of three or more are widely used. At the same time, for voltage levels of 100…200 kV there are several advantages of using a specialized single-phase high voltage doubler rectifier. Problem. The main difficulty is that at the moment mathematical modeling has not been worked out for describing modes that use the built-in R, C-filter, as well as a nonlinear load in the form of Zener diodes. Goal. Generalization of the results of the authors' previous publications on the development of an analytical method for calculating the modes of a typical high-voltage direct current installation based on a specialized single-phase voltage doubler rectifier. Methodology. Compilation of a system of algebraic linear and nonlinear equations that describe the current and voltage modes in the elements of a typical high-voltage direct current installation with a nonlinear load. Results. It is shown that with the use of linearization of the current-voltage characteristics of Zener diodes used in the load circuits of a typical high-voltage direct current installation, an analytical solution for the voltages and currents in its elements can be obtained. Originality. The theoretical basis of the complex solution of the system of equations for the currents, voltages and power of the elements of a typical high-voltage direct current installation with the account of nonlinear pulsations is formulated for the first time. Practical value. The obtained theoretical results can be used for calculations, design, optimization of the modes for a wide range of high-voltage direct current installations of technical, technological, and measuring purposes in the range up to 100...200 kV

Soil methane sink capacity response to a long-term wildfire chronosequence in Northern Sweden

Boreal forests occupy nearly one fifth of the terrestrial land surface and are recognised as globally important regulators of carbon (C) cycling and greenhouse gas emissions. Carbon sequestration processes in these forests include assimilation of CO2 into biomass and subsequently into soil organic matter, and soil microbial oxidation of methane (CH4). In this study we explored how ecosystem retrogression, which drives vegetation change, regulates the important process of soil CH4 oxidation in boreal forests. We measured soil CH4 oxidation processes on a group of 30 forested islands in northern Sweden differing greatly in fire history, and collectively representing a retrogressive chronosequence, spanning 5000 years. Across these islands the build-up of soil organic matter was observed to increase with time since fire disturbance, with a significant correlation between greater humus depth and increased net soil CH4 oxidation rates. We suggest that this increase in net CH4 oxidation rates, in the absence of disturbance, results as deeper humus stores accumulate and provide niches for methanotrophs to thrive. By using this gradient we have discovered important regulatory controls on the stability of soil CH4 oxidation processes that could not have not been explored through shorter-term experiments. Our findings indicate that in the absence of human interventions such as fire suppression, and with increased wildfire frequency, the globally important boreal CH4 sink could be diminished

The Abundance of Pink-Pigmented Facultative Methylotrophs in the Root Zone of Plant Species in Invaded Coastal Sage Scrub Habitat

Pink-pigmented facultative methylotrophic bacteria (PPFMs) are associated with the roots, leaves and seeds of most terrestrial plants and utilize volatile C1 compounds such as methanol generated by growing plants during cell division. PPFMs have been well studied in agricultural systems due to their importance in crop seed germination, yield, pathogen resistance and drought stress tolerance. In contrast, little is known about the PPFM abundance and diversity in natural ecosystems, let alone their interactions with non-crop species. Here we surveyed PPFM abundance in the root zone soil of 5 native and 5 invasive plant species along ten invasion gradients in Southern California coastal sage scrub habitat. PPFMs were present in every soil sample and ranged in abundance from 102 to 105 CFU/g dry soil. This abundance varied significantly among plant species. PPFM abundance was 50% higher in the root zones of annual or biennial species (many invasives) than perennial species (all natives). Further, PPFM abundance appears to be influenced by the plant community beyond the root zone; pure stands of either native or invasive species had 50% more PPFMs than mixed species stands. In sum, PPFM abundance in the root zone of coastal sage scrub plants is influenced by both the immediate and surrounding plant communities. The results also suggest that PPFMs are a good target for future work on plant-microorganism feedbacks in natural ecosystems

Degradation of a benzene–toluene mixture by hydrocarbon-adapted bacterial communities

We examined the rate of degradation of a benzene–toluene mixture in aerobic microcosms prepared with samples of an aquifer that lies below a petrochemical plant (SIReN, UK). Five samples exposed to different concentrations of benzene (from 0.6 to 317 mg l−1) were used. Fast degradation (approx. 1–6 mg l−1 day−1) of both contaminants was observed in all groundwater samples and complete degradation was recorded by the seventh day except for one sample. We also identified the microbial community in each of the samples by culture-independent techniques. Two of the less impacted samples harbour the aerobic benzene degrader Pseudomonas fluorescens, while Acidovorax and Arthrobacter spp. were found in the most polluted sample and are consistent with the population observed in situ. Hydrogenophaga was found in the deepest sample while Rhodoferax spp. were recovered in an alkaline sample (pH 8.4) and may also be implicated in benzene degradation. Time series analysis shows that each of the samples has a different community but they remain stable over the degradation period. This study provides new information on a well not previously studied (no. 309s) and confirms that adapted communities have the ability to degrade hydrocarbon mixtures and could be used in further bioaugmentation approaches in contaminated sites

Microbial diversity and biogeochemical cycling in soda lakes

Soda lakes contain high concentrations of sodium carbonates resulting in a stable elevated pH, which provide a unique habitat to a rich diversity of haloalkaliphilic bacteria and archaea. Both cultivation-dependent and -independent methods have aided the identification of key processes and genes in the microbially mediated carbon, nitrogen, and sulfur biogeochemical cycles in soda lakes. In order to survive in this extreme environment, haloalkaliphiles have developed various bioenergetic and structural adaptations to maintain pH homeostasis and intracellular osmotic pressure. The cultivation of a handful of strains has led to the isolation of a number of extremozymes, which allow the cell to perform enzymatic reactions at these extreme conditions. These enzymes potentially contribute to biotechnological applications. In addition, microbial species active in the sulfur cycle can be used for sulfur remediation purposes. Future research should combine both innovative culture methods and state-of-the-art ‘meta-omic’ techniques to gain a comprehensive understanding of the microbes that flourish in these extreme environments and the processes they mediate. Coupling the biogeochemical C, N, and S cycles and identifying where each process takes place on a spatial and temporal scale could unravel the interspecies relationships and thereby reveal more about the ecosystem dynamics of these enigmatic extreme environments

Numerical simulation of the flow of viscous incompressible fluid through cylindrical cavities

The flow of viscous incompressible fluid in a cylindrical duct with two serial diaphragms is studied by the numerical solution of the unsteady Navier–Stokes equations. The discretization procedure is based on the finite volume method using the TVD scheme for the discretization of the convective terms and second order accurate in both space and time difference schemes. The resulting system of non-linear algebraic equations is solved by the PISO algorithm. It is shown that the fluid flow in the region between the diaphragms is nonstationary and is characterized by the presence of an unstable shear layer under certain parameters. A series of ring vortices is formed in the shear layer that causes quasi-periodic self-sustained oscillations of the velocity and pressure fields in the orifice of the second diaphragm. There can be four self-sustained oscillation modes depending on the length of the cavity formed by the diaphragms. With the increase in the distance between the diaphragms, the frequency of oscillations decreases within the same self-oscillation mode and rises sharply with the switch to the next mode.Key words: direct numerical simulation, cylindrical cavity, duct with diaphragms, vortical structures, self-sustained oscillations.Pages of the article in the issue: 218-221Language of the article: Ukrainia