Layer Vortices Research Articles

On Turbulent Helicity in the Surface Layer of the Atmosphere

Synchronous measurements of vorticity and velocity in the boundary layer of the atmosphere were carried out using the original 3-component acoustic circulator developed at the A.M. Obukhov Institute of Physical Physics in 2019–2020. The measurements were carried out in summer at the Tsimlyansk scientific station (in 2021, 2022) at altitudes of 1.75 and 30 m. For different realizations, turbulent helicity has negative values on average, which is possibly due to the presence of local (breeze) winds. The observed spectra of turbulent helicity exhibit a slope close to –5/3, which corresponds to the transfer of helicity along the spectrum towards small scales (direct cascade). Spectrum slopes of –4/3 are also observed, and in the low-frequency region – –1, associated with the convective component, wind shear, and submesoscale structures. The components of the turbulent vortex flow are calculated. The helicity values agree with the previously measured and theoretical estimates obtained for neutral conditions.

Thermal effects of pretreatment of dark fermentation feedstocks in a vortex layer apparatus

Anaerobic digestion is an efficient way to process organic waste to produce biogas. At the same time, one of the stages of anaerobic digestion so called dark fermentation (DF), which makes it possible to obtain biohydrogen. To increase the bioavailability and efficiency of mass transfer between feedstock particles and hydrolytics and, as a result, increase the DF efficiency, it is advisable to pretreat organic waste by various methods. One of the most promising and energy efficient methods of preparing the feedstock for anaerobic fermentation, including DF, is its processing in the vortex layer apparatus (VLA). However, not all aspects of VLA application in anaerobic digestion feedstock pretreatment systems have been fully considered: in order to introduce the VLA into the organic waste DF system, it is necessary to determine, among other things, the energy characteristics of VLA under various modes of its operation, and the data obtained must be used in the development of the energy balance of the anaerobic processing system. Thus, the purpose of this work is to experimentally determine the thermal effects of pretreatment of DF feedstocks in VLA under various modes of its operation using the response surface methodology. To achieve this goal, an experimental setup was developed and created, which made it possible to conduct an experiment based on the central composite design (CCD) using Design-Expert software. The resulting models and their response surfaces have a determination coefficient of more than 0.99, which indicates their adequacy. The coefficient of conversion of electricity into heat on the VLA coil is on mean at the level of 0.54 ± 0.11, and the coefficient of useful use of heat VLA for heating the feedstock is 0.42 ± 0.06, which indicates a high potential for using waste heat. Thus, the obtained models of thermal effects of pretreatment of DF feedstocks in the VLA can be used to determine both the heat directly introduced into the feedstock during pretreatment and the heat removed by the heat exchanger to maintain the VLA operating mode. In this case, the heat removed by the heat exchanger can be used as a source of low-grade heat.

Investigation of the combined effect of control rods and forced rotation on a cylinder

A novel structure combining the application of control rods and forced rotation on a cylinder is proposed based on the cylindrical vibration suppression, and the combined structure is numerically simulated at a low Reynolds number of 200, an attack angle of 0°–105°, and a rotation rate of 0−1. The vortex-induced vibration responses, fluid forces, and cylindrical wake evolution are analyzed, and the VIV suppression is compared and discussed. The results show that the merging of the vortex layers on the cylinder and control rods promotes cylindrical vortex shedding, causing a high amplitude cylinder response. The cylinder vibrates at a low amplitude for no vortex layer merging. Rotation causes increased directional sensitivity of the control rod to cylindrical amplitude suppression. A 98%-cylinder amplitude suppression can be achieved by combining the control rod and rotation, while only 60% can be achieved by the control rods or rotation alone, indicating that the combined structure is highly effective for amplitude suppression.

Simulating the Process of Operation of Vortex Layer Electromagnetic Apparatus with Ferromagnetic Working Elements

Simulating the Process of Operation of Vortex Layer Electromagnetic Apparatus with Ferromagnetic Working Elements

Mixing enhancement of a compressible jet over a convex wall

A highly compressive effect would suppress the mixing of the shear layer in a convex wall jet. The spanwise distributed protrusions at the nozzle lip are employed to achieve mixing enhancement in this study. The mixing characteristics and enhancement mechanisms are numerically investigated by the delayed detached-eddy simulation method based on the two-equation shear-stress transport model. A widely applicable flow spatiotemporal analysis method, called proper orthogonal decomposition (POD), is used to gain further insight into the dynamical behaviours of the flow instability mode. The results reveal that the centrifugal effect maintains and amplifies the initial perturbations induced by the spanwise distributed heterogeneities, resulting in forced streamwise vortices. The instabilities induced by the streamwise vortices significantly increase the growth rate of the jet half-width and the shear layer vorticity thickness. The spanwise wavelength of the streamwise vortices is consistent with the spanwise distributed forced excitation. In addition, the spanwise meandering motion of the streamwise vortices is observed, which is usually associated with the streamwise travelling wave. This is further confirmed by the POD analysis of the spanwise velocity fluctuation in both stream-radial and stream-span sections. Also, the spatial distributions of the POD modes with the highest energy provide information on the secondary instability modes. Both sinuous and varicose types of disturbances are observed in the unforced jet, whereas the forced jet seems to be dominated by the sinuous type instability, which is more easily excited than the varicose type instability. Moreover, the turbulence intensity in the forced jet is also significantly enhanced as expected due to the earlier and stronger streamwise vortices and associated instabilities. The enhanced turbulent characteristics of the highly compressible condition tend to be isotropic, whereas in the unforced jet, it is anisotropic due to the strong compressibility suppressing the spanwise turbulent fluctuations.

Numerical simulation of the horizontal rotating cylinder and the air entrainment near the free surface

The flow around a cylinder near a free surface contains many physical phenomena and mechanisms, especially when the cylinder is moving. In this paper, the two-dimensional flow of a rotating cylinder near the free surface is simulated. The dynamic behavior of the rotating cylinder and the flow characteristics such as force coefficient, vortex structure, and air entrainment under different depth ratios, Fr numbers, and rotation ratios are discussed. In such problems, the rotational motion is rarely mentioned. The two-phase model is the sharp volume of the fluid method based on the two-dimensional incompressible Navier–Stokes equation. Combined with the spatially adaptive four-octree grid, the gas–liquid interface is reconstructed with the high-density ratio (ρwater/ρair = 816). The results show that, at α ≤ 2.0, the combined effect of the free surface and the rotation causes the wake vortex to produce a positive upper and negative lower distribution, which is contrary to the single condition. For the rotating cylinder, the existence of a free surface leads to the stable vortex layer at a low rotation ratio (α = 1.0), which only occurs at the high rotation ratio (2 < α < 4.2) without free surface. For the force coefficients, the simultaneous existence of the free surface and the rotation obviously changes the value and periodicity of the coefficients, which is different from the single condition. As for the air entrainment, it can be divided into two categories within the selected parameters: vortex entrainment caused by the parallel free surface and wake jet entrainment. In the latter model, the entailed bubbles have a wider distribution in space due to the influence of shedding vortices.

Use of a Vortex Layer Apparatus for the Preparation of Oiled Scale and Zinc-Containing Dust and Sludge for Briquetting

Use of a Vortex Layer Apparatus for the Preparation of Oiled Scale and Zinc-Containing Dust and Sludge for Briquetting

High-speed volumetric particle tracking measurements of unstable helical vortex pairs

We present results from an experimental study investigating pairs of helical vortices generated by a one-bladed rotor in hovering conditions. Time-resolved volumetric Lagrangian particle tracking measurements are conducted in a water tunnel to analyze the three-dimensional development of the vortex system. The vortex pairs are generated by a specific tip design, which allows splitting the single tip vortex into two vortices, whose characteristics depend on the geometric fin parameters. The objective of this procedure is the modification of the tip vortex structure, in order to minimize negative effects caused by fluid–structure interactions in applications involving rotors. Certain vortex configurations are affected by centrifugal instabilities, which result in an immediate pronounced growth of the vortex cores. As a consequence of the instability, secondary vortex structures are formed between the unstable cores. The presence of these structures results in an accelerated break-up of the cores, causing them to merge. In order to investigate the influence of the trailing vorticity layer shed from the inner part of the blade, two blade designs with different radial circulation distributions are considered. The measurements are able to track the evolution of the vorticity layer and the secondary structures, providing new insights into the instability of closely spaced vortex pairs with varying circulation ratios.

Modulational instability of two obliquely interacting interfacial waves in the presence of a basic current shear

Evolution equations are derived for two obliquely interacting interfacial waves in the presence of a basic current shear. Using these equations modulational instability analysis is carried out for a pair of obliquely interacting uniform wave trains considering both unidirectional and bidirectional perturbations. It is found that the region of instability reduces in the presence of a basic current shear. Variations in the perturbed wavenumber at marginal stability due to changes in wave steepness and vorticities in the two mediums are displayed graphically. The maximum growth rate of instability for unidirectional perturbations increases with the increase in vorticity in the lower layer, but the reverse effect is observed when the vorticity in upper layer increases. The maximum growth rate of instability decreases with the increase in the angle of interaction of the two wave packets when the angle is acute, but the effect is opposite when the angle is obtuse.

Wake control of a square cylinder: Impinging the vortex cores with dual flexible splitter plates

Numerous works have been conducted to study the wake control of a bluff body and more efficient strategies are still what we seek for. This paper studies the complex fluid–structure interaction (FSI) problem of a square cylinder mounted with dual flexible splitter plates from a new perspective: impinging the vortex core with flapping plate tip. The Reynolds number (Re) based on the side length of the square cylinder is fixed at 332.6. The splitter plates are attached at both sides of the square cylinder. Structural parameters, such as plate length and bending stiffness, are considered to achieve an optimal arrangement of this configuration. Compared to the traditional single splitter plate arrangement, only a short splitter length is needed for this dual splitter plates configuration. Flow visualization indicates that the drag reduction is associated with the increase of base pressure and the lift suppression is related to the inhabitation of the interaction of the shear layer and split effect of the tip vortices. The dimensionless bending stiffness plays a vital role in determining the behavior of the splitter plate. Two vibration modes, i.e., off-center vibration and asymmetry vibration modes are captured and the up and down splitter plates tend to vibration in-phase. The Lissajous trajectories of the free tip mainly exhibits arc-shape and sickle-shape. Given the performance of the square cylinder-dual splitter plates configuration, splitter length of l/L=1.5 is an optimal configuration, which is less affected by flexural rigidity. The maximum drag reduction could reach 23.1% and the lift force exerting on this configuration does not increase significantly, which is beneficial for the structural stability.

Vortex force decomposition—forces associated with individual elements of a vorticity field

The impulse theory used to calculate the force from a vorticity distribution in two-dimensional, incompressible flow, is re-cast with the aim of approximating, to a first order, the forces generated by a specific flow feature, such as a free vortex passing by an object. To achieve this, the force acting on the body is split up into several core contributions. The first component arises from the time variation of the body’s boundary layer. The second is generated by the advection of any free vorticity located in the flowfield by the object’s boundary layer vorticity. The final force contribution is due to new vorticity being shed. To test the theory, it is applied to two multi-body flowfields consisting of a circular cylinder and a flat plate wing at incidence in close proximity. Force balance measurements and planar particle image velocimetry data are simultaneously obtained at Reynolds numbers of 10,000 and 20,000. The forces acting on the cylinder are successfully recovered from the vorticity data using the derived formulation, verifying its accuracy. Subsequently, the proposed force formulation is used to create force heat maps that demonstrate how the location of a leading edge vortex affects its force contribution around a pitching NACA-0021 wing translating at a Reynolds number of 10, 000.

How tidal waves interact with convective vortices in rapidly rotating planets and stars

Context. The dissipation of tidal inertial waves in planetary and stellar convective regions is one of the key mechanisms that drive the evolution of star–planet and planet–moon systems. This dissipation is particularly efficient for young low-mass stars and gaseous giant planets, which are rapid rotators. In this context, the interaction between tidal inertial waves and turbulent convective flows must be modelled in a realistic and robust way. In the state-of-the-art simulations, the friction applied by convection on tidal waves is commonly modeled as an effective eddy viscosity. This approach may be valid when the characteristic length scales of convective eddies are smaller than those of the tidal waves. However, it becomes highly questionable in the case where tidal waves interact with potentially stable large-scale vortices such as those observed at the poles of Jupiter and Saturn. The large-scale vortices are potentially triggered by convection in rapidly-rotating bodies in which the Coriolis acceleration forms the flow in columnar vortical structures along the direction of the rotation axis. Aims. We investigate the complex interactions between a tidal inertial wave and a columnar convective vortex. Methods. We used a quasi-geostrophic semi-analytical model of a convective columnar vortex, which is validated by numerical simulations. First, we carried out linear stability analysis using both numerical and asymptotic Wentzel–Kramers–Brillouin–Jeffreys (WKBJ) methods. We then conducted linear numerical simulations of the interactions between a convective columnar vortex and an incoming tidal inertial wave. Results. The vortex we consider is found to be centrifugally stable in the range –Ωp ≤ Ω0 ≤ 3.62Ωp and unstable outside this range, where Ω0 is the local rotation rate of the vortex at its center and Ωp is the global planetary (stellar) rotation rate. From the linear stability analysis, we find that this vortex is prone to centrifugal instability with perturbations with azimuthal wavenumbers m = {0,1, 2}, which potentially correspond to eccentricity, obliquity, and asynchronous tides, respectively. The modes with m > 2 are found to be neutral or stable. The WKBJ analysis provides analytic expressions of the dispersion relations for neutral and unstable modes when the axial (vertical) wavenumber is sufficiently large. We verify that in the unstable regime, an incoming tidal inertial wave triggers the growth of the most unstable mode of the vortex. This would lead to turbulent dissipation. For stable convective columns, the wave-vortex interaction leads to the mixing of momentum for tidal inertial waves while it creates a low-velocity region around the vortex core and a new wave-like perturbation in the form of a progressive wave radiating in the far field. The emission of this secondary wave is the strongest when the wavelength of the incoming wave is close to the characteristic size (radius) of the vortex. Incoming tidal waves can also experience complex angular momentum exchanges locally at critical layers of stable vortices. Conclusions. The interaction between tidal inertial waves and large-scale coherent convective vortices in rapidly-rotating planets (stars) leads to turbulent dissipation in the unstable regime and complex behaviors such as mixing of momentum and radiation of new waves in the far field or wave-vortex angular momentum exchanges in the stable regime. These phenomena cannot be modeled using a simple effective eddy viscosity.

Investigating the dynamics of point helical vortices on a rotating sphere to model tropical cyclones

A general exact weak solution to the non-linear equation of the conservation of the absolute vorticity in a thin layer of an incompressible medium on a rotating sphere is proposed. It takes into account the helicity of the point vortices and the non-uniformity of the depth of the layer. This is used to develop a model of the observed interactions of spiral atmospheric vortices. The fusion of two-point helical vortices (HVs) on the rotating sphere is considered. We also analyze the prognostic applicability of the dynamics of the HVs for modeling the abrupt changes observed in the trajectories of tropical cyclones and their landfall in comparison with the traditional approach. The analytical condition for chiral symmetry violation related to the direction of the movement of the center of a helical cyclone is obtained.

Near-field evolution and scaling of shear layer instabilities in a reacting jet in crossflow

This study analyses the stability characteristics of the shear layer vortices (SLV) in a reacting jet in crossflow, analysing effects of flame position, momentum flux ratio ( $J$ ) and density ratio ( $S$ ). It utilizes 40 kHz particle image velocimetry to characterize the dominant SLV frequencies, streamwise evolution and convective/global stability characteristics for three different canonical configurations, one non-reacting and two reacting (‘R1’ and ‘R2’). In the non-reacting case, both convective and global instability is observed, depending upon $S$ and $J$ . Qualitatively similar $S$ dependencies occur for the R1 reacting case where the radial flame position lies outside the jet shear layer, albeit with slower SLV growth rates. When the flame lies inside the jet shear layer, the R2 reacting case, a qualitatively different behaviour is observed, as vorticity concentration in the shear layers is suppressed almost completely. Finally, we show that frequency and stability characteristics of the non-reacting and R1 cases can be scaled in a unified manner using a counter-current shear layer model. This model relates these SLV behaviours to a vorticity layer thickness, a velocity scale and an effective density ratio (noting that there are three distinct densities associated with the jet, the crossflow and the burned gases). These parameters were extracted from the data and used to collapse the frequency scaling, and to explain the transition to self-excited oscillatory behaviour.

Development of catalysts for anaerobic processing of liquid organic waste

In the work, the development of a catalyst carrier of biomaterial for anaerobic processing of liquid organic waste was carried out. A highly porous cellular material (НPСM) made of high-alumina ceramics obtained by thermal destruction of a polymer matrix impregnated with a high-alumina ceramic slip was used as the carrier base. The catalyst coating was applied by blowing a conductive powder through a carrier pre-coated with an adhesive composition or by impregnating the carrier with a suspension of conductive glaze followed by firing. The conductive powder was the product of milling grinding ferromagnetic bodies in a vortex layer apparatus. An experimental setup has been developed and manufactured, including a vortex layer apparatus, a blower, and a chamber with a catalyst carrier. The plant operation was optimized using the response surface methodology. It has been established that it is rational to vary the mass of working bodies to obtain the required concentration while reducing the power consumption of the plant by minimizing the duration of its operation. The next steps are to study the efficiency of an anaerobic bioreactor with an integrated HPCM with a catalyst coating formed by sputtering and glazing to produce biohydrogen and biomethane during anaerobic bioconversion of organic matter of various wastes.

Simulation of SFS2 using Embedded LES: Part-II Understanding the asymmetry in bistable airwake at zero yaw

The turbulent flow field of simplified frigate model SFS2 is numerically studied using an embedded Large Eddy Simulation (ELES) turbulence model in Ansys Fluent® 19.0 environment for its characterization. The focus of this research work is to understand the underlying reasoning of the generation of bistable airwake of the superstructure and to ascertain the influence of its upstream flow field such as bow region turbulent vortical structures, if any, on the nature and characteristics of airwake. The asymmetry in the mean airwake behind superstructure was attributed to “locking” and/or differential momentum flux on sides of superstructure in literature, however no concrete evidence of causation was reported. In this research, the in-depth analysis of turbulent boundary layer buildup, vorticity distribution, relative size and orientation of vortices on and along starboard and port sides of superstructure provide clear understanding and evidence of the influence of bow region flow field on airwake. It has been observed that dynamics of bow leading edge vortices and horseshoe vortex generated upstream of superstructure forward facing step, and their interaction are the main contributor towards the generation of asymmetrical airwake. The asymmetrical bias towards starboard side, which has also been observed in the previous experimental and numerical studies, can also be explained from this finding. That is the relatively larger turbulent eddies on starboard side generates more entrainment of the freestream upon reaching the flight deck compared to the portside. Later, the correlation and spectral analyses of the velocity measurements recorded on bow region and in the superstructure airwake supplements and validates the influence of bow region turbulent vortical structures on the airwake as dominant frequencies associated with bow vortical structures are also observed in airwake spectrum.

Suppression mechanism of Richtmyer–Meshkov instability by transverse magnetic field with different strengths

The Richtmyer–Meshkov instability (RMI) is caused by an incident planar shock wave impinging on the heavy-gas-density interface. We have numerically investigated the RMI controlled by different transverse magnetic-field strengths based on the ideal compressible magnetohydrodynamics (MHD) equations. The MHD equations are solved by the corner transport upwind + constrained transport algorithm, which guarantees a divergence-free constraint on the magnetic field. We discuss the flow characteristics and shock patterns in both classical hydrodynamic and MHD situations and verify our conclusions by comparing the experimental results with the numerical results. The results show that the magnetic field modifies the pressure-gradient distribution, and the baroclinic vorticity splits and attaches to the MHD shock waves. In addition, the results indicate that the interaction of shock wave and density interface changes the distribution of magnetic-field energy and distorts the magnetic induction line in the region of magnetic-field energy accumulation. The distortion of the magnetic induction lines alters the magnetic field gradient and creates a magnetic tension that produces a torque opposing that generated by the shear force on the vorticity layer, so the Kelvin–Helmholtz instability is effectively suppressed and no Kelvin–Helmholtz vortex appears on the vorticity layer. The result is that the interface instability is suppressed.

Analysis of the effects of climate change on the gyre in Lake Biwa, Japan

AbstractThe currents of gyre (circulation) flow play an important role in the transportation and mixing of dissolved or suspended nutrients and chemical substances. In recent years, climate change has affected the flow field in Lake Biwa. However, how much climate change can influence the flow field has not been clarified enough yet. In the present study, using a three-dimensional hydrodynamic model, we investigated the effect of changes in air temperature, wind speed, and precipitation on the change of the flow field in Lake Biwa. For sensitivity analysis, numerical simulations were performed for a baseline case using realistic meteorological data from 2007 to 2012, as well as hypothetical cases using meteorological data. The analysis revealed that the changes in air temperature and precipitation changed stratification and vorticity in the surface layer in each season. The strength of the stratification affected the gyre strength in the surface layer. The change in wind speed, on the other hand, changed the wind stress acting on the lake's surface, influencing the strength of gyre significantly. Increased wind speed lengthened the radius of the gyre in Lake Biwa, while decreased wind speed made it weaker above and below the thermocline.

РЕМОНТНЫЕ СТРОИТЕЛЬНЫЕ РАСТВОРЫ, ПОЛУЧЕННЫЕ ПЛАЗМЕННОЙ МОДИФИКАЦИЕЙ И ВИХРЕВОЙ АКТИВАЦИЕЙ

Introduction: The service life of reinforced-concrete structures can be increased with the use of effective repair compositions obtained by activating the original components. Purpose of the study: We aimed to develop effective repair compounds obtained by activating the original components. Methods: To process the original components, low-temperature non-equilibrium plasma (LTNP) and electromagnetic activation in a vortex layer device were used. In the course of the study, we used polypropylene, steel, glass, and basalt fiber and fiber made of structured ferromagnetic microwire. Electron microscopy and X-ray diffraction analysis were applied. Results: It was established that the combined use of the above methods for the modification of raw components makes it possible to improve the strength of these materials by more than 50%, which is due to the characteristics of structure formation in the developed compositions. For instance, LTNP increases the amount of portlandite and reduces the main phases of cement stone— C3S and β-C2S, and vortex activation contributes to an increase in the total number of crystalline phases. Quartz powder particles processed in an electromagnetic mill are characterized by layered structure, high surface roughness, large developed cracking, as well as inclusions as a result of impact action. All that improves the physical and mechanical properties of the resulting repair compositions by an average of 20%. Repair compositions additionally treated with plasma modification feature new hydrated formations on quartz grains.

WAYS OF IMPROVING THE VERTICAL LAYER APPARATUS FOR PROCESSING LIQUID PIG MANURE

Technologies for processing (manure processing) manure depend on the system and method of keeping animals. This is due to the physical properties of the obtained manure, primarily its moisture content. By using litter, solid (litter) manure is obtained, which has a moisture content of 60...80%. If litter is not used when keeping pigs, then manure without litter is obtained, which is divided into two groups by moisture content: solid (60-80%) and liquid (82-98%). Vortex layer devices are equipped with different types of working bodies and layout of the working area. When carrying out liquid-phase processes, using ferromagnetic elements as a working body, trap screens or labyrinths designed to keep ferromagnetic particles in the working area can be installed on the ends of the non-magnetic insert (or only at the outlet). Grinding and mixing can occur not only with the help of ferromagnetic particles, but also with the help of knives, tubes or a rotor. In these cases, the sieves act as filters (separators). Ferromagnetic elements are added to the working area using an electromagnetic dispenser. Analyzing the results of previous scientific studies, it can be stated that increasing the efficiency of the vortex layer devices is proposed due to the improvement of the design or the substantiation of the structural and technological parameters of individual systems, without taking into account their interconnection. It should be noted that most of the obtained results from the substantiation of structural and technological parameters are experimental and do not have a theoretical justification. In order to solve the problems, the structural and technological scheme of the vortex layer apparatus for liquid manure processing, which allows to obtain liquid organic fertilizers of high quality with increased productivity and level of automation, is substantiated.