Layer Vortices Research Articles

The synergistic effect of the thickened digestate treatment in the vortex layer apparatus prior to its recirculation into the reactor on the characteristics of anaerobic bioconversion of organic waste

For the first time, the effect of thickened digestate treatment in a vortex layer apparatus (VLA) prior to its recirculation into anaerobic reactor on the efficiency of anaerobic bioconversion of an organic substrate was investigated. At the hydraulic retention time of the recirculated digestate in VLA for 30 seconds, of the substrate in the anaerobic bioreactor for 8 days and the recirculation coefficient of 1.5, the average specific yield of biogas increased by 75.2%, and the average volumetric methane production – by 16.8%. There was no increase in methane concentration in biogas, while the average concentration of hydrogen sulfide in biogas decreased by 8 times.

The effects of turbulence on the lean blowout mechanisms of bluff-body flames

The lean blowout mechanisms of premixed bluff-body flames are experimentally investigated at various turbulence intensities. Turbulence levels are varied using a novel turbulence generator, which combines static grid and fluidic jet impingement techniques. Three different turbulence levels are probed to study their effects on lean blowout. The three conditions span across the combustion regime diagram, from flamelets to broken reactions. For all three turbulence levels, the lean blowout process is induced through controlled fuel flow rate reduction. The transient blowout process is captured using three simultaneous high-speed diagnostic systems: particle image velocimetry (PIV), stereoscopic PIV (SPIV), and C2*/CH* species measurements. The two PIV systems are used to resolve the instantaneous velocity and vorticity fields, and the C2*/CH* species diagnostics allow for global equivalence ratios to be evaluated throughout the duration of blowout. The results show that the dynamics of lean blowout vary with turbulence intensity. At low turbulence levels, the flame experiences a global effect where the flame boundary interacts with the shear layer vorticity. This imparts high strain rates along the length of the flame, leading to blowout. As turbulence levels increase, the blowout mechanism becomes less dependent on flame and shear layer interactions and more driven by flame–turbulence interactions. At high turbulence conditions, flame-eddy interactions within the freestream augment flame stretching via increased flame straining and small-scale flame corrugations. Increased flame stretching disrupts the flame stabilization process, and ultimately results in blowout.

Inviscid Damping and Enhanced Dissipation of the Boundary Layer for 2D Navier–Stokes Linearized Around Couette Flow in a Channel

We study the 2D Navier-Stokes equations linearized around the Couette flow $(y,0)^t$ in the periodic channel $\mathbb T \times [-1,1]$ with no-slip boundary conditions in the vanishing viscosity $\nu \to 0$ limit. We split the vorticity evolution into the free evolution (without a boundary) and a boundary corrector that is exponentially localized to at most an $O(\nu^{1/3})$ boundary layer. If the initial vorticity perturbation is supported away from the boundary, we show inviscid damping of both the velocity and the vorticity associated to the boundary layer. For example, our $L^2_t L^1_y$ estimate of the boundary layer vorticity is independent of $\nu$, provided the initial data is $H^1$. For $L^2$ data, the loss is only logarithmic in $\nu$. Note both such estimates are false for the vorticity in the interior. To the authors' knowledge, this inviscid decay of the boundary layer vorticity seems to be a new observation not previously isolated in the literature. Both velocity and vorticity satisfy the expected $O(\exp(-\delta\nu^{1/3}\alpha^{2/3}t))$ enhanced dissipation in addition to the inviscid damping. Similar, but slightly weaker, results are obtained also for $H^1$ data that is against the boundary initially. For $L^2$ data against the boundary, we at least obtain the boundary layer localization and enhanced dissipation.

Intraseasonal variability of the Kuroshio east of Taiwan, China, observed by subsurface mooring during 2016–2017

The mean velocity structure and variability of the Kuroshio east of Taiwan, China, were investigated using mooring observations acquired at 23°N/122°E between January 2016 and May 2017. Power spectrum analysis reveals marked intraseasonal variability in 70-95 days in the meridional Kuroshio velocity in the upper 800 m. Hybrid Coordinate Ocean Model assimilation data adequately reproduce variations of the Kuroshio. Westward propagating mesoscale eddies that originated from the Subtropical Countercurrent play an important role in intraseasonal variability in the Kuroshio velocity. Cyclonic (anticyclonic) eddies are often associated with negative (positive) sea surface height anomaly, resulting in positive (negative) potential vorticity in the subsurface layer of the eddy center, which decreases (increases) isopycnal slope and then weakens (strengthens) the Kuroshio. When anticyclonic (cyclonic) eddies are active, the Kuroshio axis moves offshore (inshore). Corresponding to the Kuroshio intraseasonal variability (ISV), the variation of the intermediate water salinity also had a significant ISV induced by the mesoscale eddies.

Stochastic Lagrangian dynamics of vorticity. Part 2. Application to near-wall channel-flow turbulence

Abstract

Insights into the Aeroacoustic Noise Generation for Vertical Axis Turbines in Close Proximity

We present Large Eddy Simulations and aeroacoustic spectra for three configurations of increasing flow complexity: an isolated NACA0012 airfoil, an isolated rotating vertical axis wind turbine composed of three rotating airfoils and a farm of four vertical axis turbines (with identical characteristics as the isolated turbine), which are located in close proximity. The aeroacoustic signatures of the simulated airfoil and the isolated turbine are validated using published numerical and experimental data. We provide theoretical estimates to predict tonal frequencies, which are used to identify the main physical mechanisms responsible for the tonal signature and for each configuration and enable the categorisation of the main tonal aeroacoustic sources of vertical axis turbines operating in close proximity. Namely, we identify wake, vortex, blade passing and boundary layer phenomena and provide estimates for the associated tonal frequencies, which are validated with simulations. In the farm, we observe non-linear interactions and enhanced mixing that decreases tonal frequencies in favour of larger broadband amplitudes at low frequencies. Comparing the spectrum with that of the isolated turbine, only the blade passing frequency and the boundary layer tones can be clearly identified. Variations in acoustic amplitudes, tonal frequencies and sound directivities suggest that a linear combination of sources from isolated turbines is not enough to characterise the aeroacoustic footprint of vertical axiswind turbines located in close proximity, and that farms need to be considered and studied as different entities.

Stability and transition on a Coandă cylinder

An experimental stability and transition investigation of a centrifugally unstable wall jet blown from a slot over a circular Coandă cylinder was conducted using flow visualization and particle image velocimetry. Clear and unambiguous observations of spontaneously generated stationary streamwise structures, never observed previously, were analyzed using standard image processing techniques. These structures ultimately exhibited a secondary time-dependent wavy instability that was followed by transition to turbulence. A modified Görtler number, based on the slot height, was used as a basis for comparison to linear stability theory and was directly related to the Reynolds number via geometric scaling. Direct observation of the vortices allowed the identification of an upper limit to the critical Görtler number of 6.3 that bracketed the value of 3.5 from the linear stability theory. The vortical shear layer grew exponentially along the azimuth and was characterized by a Reynolds number dependent growth rate parameter. Extrapolation of the growth rate parameter to zero furnished a critical Görtler number of 3.1 ± 1.25 (95% CI) that compared remarkably well with the linear stability theory. The shear layer separation angle did not vary monotonically with the Reynolds number: as the Reynolds number increased, the thicker boundary layer was more susceptible to separation, and thus, the separation angle, relative to the slot, decreased. However, following transition to turbulence at higher Reynolds numbers (≈400), the high-momentum fluid near the wall, resulting from turbulent mixing, produced a subsequent increase in the separation angle.

Stationary water waves on rotational flows of two vortical layers

Stationary waves of constant shape and constant propagation speed on rotational flows of two layers are computed numerically. Two layers are assumed to be of distinct constant vorticity distributions. Three different kinds of waves of finite depth are considered: pure capillary, capillary-gravity, and gravity waves. The problem is formulated as a bifurcation problem, which involves many parameters and produces a complicated structure of solutions. We adopted a numerical method by which waves with stagnation points can be computed, and obtained variety of new solutions. It is also reported that the locations of the stagnation points vary curiously with the prescribed parameters and that they offer an interesting problem.

Characteristics of shearing motions in incompressible isotropic turbulence

Shearing motions in isotropic turbulence are studied with a triple decomposition of velocity gradient tensor. A mean flow around the shearing motions exhibits a thin shear-layer pattern sustained by a biaxial strain. The thickness of each shear layer is well predicted by Burgers' vortex layer. Interplay between the shear and biaxial strain causes enstrophy production and strain self-amplification.

System of preliminary treatment of substrates of methantenks in the apparatus of the vortex layer with heat recovery

The studying of energy balance for the analysis of thermal processes carried out at the stage of preliminary treatment of substrates is an urgent task. The energy balance is necessary for the subsequent studying the material balance of the developed system with heat recovery and for its energy analysis. The proposed method of substrate pre-treatment in the digester apparatus of the vortex layer allows to increase the biogas yield by 7-10 percent. At the same time, a new source of low-potential energy appears in the form of a heat carrier used for cooling the vortex layer apparatus, which, with the additional use of a heat pump, will reduce energy consumption to maintain the temperature mode of the methane tank. (Research purpose) The research purpose is in creation of technological shape of the system of preliminary treatment of substrate to anaerobic digestion, developing the energy balance of the proposed scheme and determine criteria to evaluate the proposed scheme for post-processing of experimental data. (Materials and methods) Authors used information research methods, including standard analytical methods of modern system approach, processing and analysis. The article presents analytical studies based on the well-known laws of heat engineering and thermodynamics. (Results and discussion) Based on the energy balance of the proposed system, the criteria for evaluating the system of pre-treatment of the substrate for anaerobic digestion with heat recovery of the vortex layer apparatus were determined: ensuring preheating of the initial substrate to the temperature of the anaerobic fermentation process; ensuring the temperature mode of operation of the coil of the vortex layer apparatus; ensuring the energy efficiency of the proposed system. (Conclusions) It was found that the functional dependencies based on the energy balance of the proposed system allow evaluating the effectiveness of this system. It was found that further selection of technological equipment and calculation of the material balance of the proposed system is carried out based on the energy balance.

Transition mechanisms in cross-flow-dominated hypersonic flows with free-stream acoustic noise

Transition to turbulence in high-speed flows is determined by multiple parameters, many of which are not fully understood, leading to problems in developing physics-based prediction methods. In this contribution, we compare transition mechanisms in configurations with unswept and swept leading edges that are exposed to free-stream acoustic disturbances. Direct numerical simulations are run at a Mach number of six with the same free-stream noise, consisting of either fast or slow acoustic disturbances, with two different amplitudes to explore the linear and nonlinear aspects of receptivity and transition. For the unswept configuration, receptivity follows an established mechanism involving synchronisation of fast acoustic disturbances with boundary-layer modes. At high forcing amplitudes, transition proceeds via the formation of streaks and their eventual breakdown. In the swept case, the process of streak-induced transition is modified by the presence of a cross-flow instability in the leading-edge region. Linear stability analysis confirms the presence of a cross-flow mode as well as weaker first and second mode waves. Both fast and slow types of forcing independently stimulate an unusual transition mechanism involving significantly narrower streaks than those arising from the cross-flow instability behind the swept leading edge or those induced nonlinearly in the unswept case. In the observed transition process, the cross-flow mode leads to a thin layer of streamwise vorticity that breaks up under the influence of high spanwise wavenumber disturbances. These disturbances first appear in the leading-edge region.

Reconciling different formulations of viscous water waves and their mass conservation

The viscosity of water induces a vorticity near the free surface boundary. The resulting rotational component of the fluid velocity vector greatly complicates the water wave system. Several approaches to close this system have been proposed. Our analysis compares three common sets of model equations. The first set has a rotational kinematic boundary condition at the surface. In the second set, a gauge choice for the velocity vector is made that cancels the rotational contribution in the kinematic boundary condition, at the cost of rotational velocity in the bulk and a rotational pressure. The third set circumvents the problem by introducing two domains: the irrotational bulk and the vortical boundary layer. This comparison puts forward the link between rotational pressure on the surface and vorticity in the boundary layer, addresses the existence of nonlinear vorticity terms, and shows where approximations have been used in the models. Furthermore, we examine the conservation of mass for the three systems, and how this can be compared to the irrotational case.

Vortex Layers of Small Thickness

AbstractWe consider a 2D vorticity configuration where vorticity is highly concentrated around a curve and exponentially decaying away from it: the intensity of the vorticity is O(1/ε) on the curve while it decays on an O(ε) distance from the curve itself. We prove that, if the initial datum is of vortex‐layer type, Euler solutions preserve this structure for a time that does not depend on ε. Moreover, the motion of the center of the layer is well approximated by the Birkhoff‐Rott equation. © 2020 Wiley Periodicals, Inc.

Dynamics of seeded blobs under the influence of inelastic neutral interactions

Field-aligned filaments, the so-called blobs, born at the edge of the magnetically confined region of tokamaks propagate radially outward into the scrape-off layer (SOL) region that allows for a substantial population of neutral particles compared to the region of confinement. The electrons and ions constituting the blob undergo both elastic and inelastic collisions with the neutral particles, and the latter leads to sources and sinks of the blob density, momentum, and heat. The influence of the inelastic collisions with neutrals on the evolution of seeded blobs is investigated numerically by the nHESEL drift-fluid model through a series of discrete scans in interactions, active source terms, and blob plasma parameters. In light of the results, the potential influence of local inelastic collisions on the SOL density shoulder formation is discussed. It is found that density sources increase the blob compactness, which delays the blob dispersion and decreases the dispersion rate. Density sources or momentum sinks also influence the blob dynamics by increasing the vorticity layer around the perturbation, whereas the pressure sources/sinks only affect the blob dynamics marginally. The change to the vorticity structure leads, in most cases, to a decrease in the radial velocity of the blob center of mass, although, at high source rates, a radial acceleration of the blob center of mass is also observed. Density sources may, thus, contribute to shoulder formation not only by increasing the density locally but also by changing the filament dynamics.

Formation of Recirculating Cores in Convectively Breaking Internal Solitary Waves of Depression Shoaling over Gentle Slopes in the South China Sea

AbstractThe formation of a recirculating subsurface core in an internal solitary wave (ISW) of depression, shoaling over realistic bathymetry, is explored through fully nonlinear and nonhydrostatic two-dimensional simulations. The computational approach is based on a high-resolution/accuracy deformed spectral multidomain penalty-method flow solver, which employs the recorded bathymetry, background current, and stratification profile in the South China Sea. The flow solver is initialized using a solution of the fully nonlinear Dubreil–Jacotin–Long equation. During shoaling, convective breaking precedes core formation as the rear steepens and the trough decelerates, allowing heavier fluid to plunge forward, forming a trapped core. This core-formation mechanism is attributed to a stretching of a near-surface background vorticity layer. Since the sign of the vorticity is opposite to that generated by the propagating wave, only subsurface recirculating cores can form. The onset of convective breaking is visualized, and the sensitivity of the core properties to changes in the initial wave, near-surface background shear, and bottom slope is quantified. The magnitude of the near-surface vorticity determines the size of the convective-breaking region, and the rapid increase of local bathymetric slope accelerates core formation. If the amplitude of the initial wave is increased, the subsequent convective-breaking region increases in size. The simulations are guided by field data and capture the development of the recirculating subsurface core. The analyzed parameter space constitutes a baseline for future three-dimensional simulations focused on characterizing the turbulent flow engulfed within the convectively unstable ISW.

Energy Efficiency of Pretreatment of Digester Synthetic Substrate in a Vortex Layer Apparatus

Introduction. Processing organic waste to reduce the anthropogenic impact on the environment remains an urgent task, one of the ways to solve which is the use of methods for bioconversion of organic matter of organic waste to produce gaseous energy carrier and high-quality organic fertilizers. One of the most important stages of anaerobic processing of organic waste in bioreactors is the stage of preliminary preparation of waste for fermentation, which can be carried out by a number of methods. However, the technical literature does not pay enough attention to the use of devices with a vortex layer for the preliminary processing of substrates. The aim of the work is to determine the energy efficiency of the organic waste pretreatment process in the vortex layer apparatus before anaerobic digestion. Materials and Methods. An experimental installation was developed to study the process of organic waste pretreatment. A mixture of organic fraction of municipal solid waste and tap water in the ratio of 300 g/l served as a substrate for treatment in the vortex layer apparatus. Results. The calculations confirmed that the condition for energy efficiency of processing substrate pretreatment of methane is met, despite the additional cost of electrical energy during the integration of this stage in the system of anaerobic processing. Discussion and Conclusion. Integration of the process of organic waste pretreatment in the vortex layer apparatus before fermentation in anaerobic bioreactors into the system of anaerobic treatment of organic waste can improve both the energy efficiency of the system and the level of anaerobic decomposition of organic matter of waste. The specific amount of biogas energy produced in the digestion process in an anaerobic bioreactor with pretreatment of the substrate in the apparatus of the vortex layer fully compensates the energy cost of pre-treatment of the substrate in the vortex layer apparatus. The practical significance of the work is confirmed by an increase in the specific yield of commercial energy by 70% compared to anaerobic treatment in traditional methane tanks.

Potentiodynamic and Galvanodynamic Regimes of Mass Transfer in Flow-Through Electrodialysis Membrane Systems: Numerical Simulation of Electroconvection and Current-Voltage Curve.

Electromembrane devices are usually operated in two electrical regimes: potentiodynamic (PD), when a potential drop in the system is set, and galvanodynamic (GD), when the current density is set. This article theoretically investigates the current-voltage curves (CVCs) of flow-through electrodialysis membrane systems calculated in the PD and GD regimes and compares the parameters of the electroconvective vortex layer for these regimes. The study is based on numerical modelling using a basic model of overlimiting transfer enhanced by electroconvection with a modification of the boundary conditions. The Dankwerts’ boundary condition is used for the ion concentration at the inlet boundary of the membrane channel. The Dankwerts’ condition allows one to increase the accuracy of the numerical implementation of the boundary condition at the channel inlet. On the CVCs calculated for PD and DG regimes, four main current modes can be distinguished: underlimiting, limiting, overlimiting, and chaotic overlimiting. The effect of the electric field regime is manifested in overlimiting current modes, when a significant electroconvection vortex layer develops in the channel.

РАБОТА РОТОРА В АППАРАТЕ ВИХРЕВОГО СЛОЯ

Vortex layer devices can be means of intensification of most processes. It is particularly effective to use these devices to produce rubber products, to accelerate chemical processes, to prepare raw materials in metallurgy, to use them in ore processing processes, to solve environmental problems in wastewater treatment and in many other areas. Such widespread use is possible due to the properties of the vortex layer, which, in addition to mechanical impact, also provides electrochemical effects. The efficiency of traditional devices of this class can be improved by using a new design. This design is characterized by the presence of a rotor in the working area, which actively affects electromagnetic processes. This article is devoted to the study of the rotor operation in the vortex layer device. The research is of a practical nature and consists in studying the operation of an experimental installation. The design of this installation makes it possible to identify patterns in the designated area of research. This is achieved by using easily removable rotors of different diameters, which allow getting a gap between the rotor and the inductor of different sizes. Also, the design of the experimental installation allows to enter a different number of ferromagnetic elements into the active zone. The main evaluation criterion for the study is the coefficient of energy use by the rotor. Estimating the absolute value of this coefficient, a conclusion is drawn about the futility of classical devices for mechanical impact on the product. The results of the study are to determine the dependence of the operating parameters on the gap between the rotor and the inductor, as well as the number of ferromagnetic elements in the device under study.

Effects of Fluctuating Magnetic Field on the Growth of the Kelvin‐Helmholtz Instability at the Earth's Magnetopause

AbstractAt the Earth's magnetopause, the Kelvin‐Helmholtz (KH) instability, driven by the persistent velocity shear between the magnetosheath and the magnetosphere, has been frequently observed during northward interplanetary magnetic field periods and considered as one of the most important candidates for transporting and mixing plasmas across the magnetopause. However, how this process interacts with magnetic field fluctuations, which persistently exist near the magnetopause, has been less discussed. Here we perform a series of 2‐D fully kinetic simulations of the KH instability at the magnetopause considering a power law spectrum of initial fluctuations in the magnetic field. The simulations demonstrate that when the amplitude level of the initial fluctuations is sufficiently large, the KH instability evolves faster, leading to a more efficient plasma mixing within the vortex layer. In addition, when the spectral index of the initial fluctuations is sufficiently small, the modes whose wavelength is longer than the theoretical fastest growing mode grow dominantly. The fluctuating magnetic field also results in the formation of the well‐matured turbulent spectrum with a −5/3 index within the vortex layer even in the early nonlinear growth phase of the KH instability. The obtained spectral features in the simulations are in reasonable agreement with the features in KH waves events at the magnetopause observed by the Magnetospheric Multiscale mission and conjunctively by the Geotail and Cluster spacecraft. These results indicate that the magnetic field fluctuations may really contribute to enhancing the wave activities especially for longer wavelength modes and the associated mixing at the magnetopause.

Mechanical activation effect on the biostability of modified cement compositions

The creation of materials with high strength, durability, and biostability is a relevant development area for modern building materials science. This paper studies a number of modified cement compositions obtained by the mechanical activation of the vortex layer device binder. A superplasticizer and carbon nanotubes of various structure and functionality were used as modifiers. The cement composition was determined in a laboratory kinetic unit designed by the authors. It is found that the biostability coefficient values of the cement composition samples obtained by the mechanical activation of the binder and with the introduction of the superplasticizer are 13% higher than the same control composition value. It is indicated that the additional introduction of single and multilayer carbon nanotubes increases the cement composition biostability by 50%. It is found that the highest biostability values are observed in cement compositions obtained by the mechanical activation of the binder with the combined introduction of the superplasticizer and multilayer (MWCNT) «Graphistrength» carboxyl-functionalized (–COOH) nanotubes. The x-ray phase analysis method found that during the exposure of the cement composition samples under study in a model citric acid medium, the portlandite is carbonized with calcite formed which causes a decrease in the compression and bending strength of the cement composition samples.