Mechanism Of Vorticity Generation Research Articles

Solar Vortex Tubes. III. Vorticity and Energy Transport

This study investigated the mechanisms of vorticity generation and the role of vortex tubes in plasma heating and energy transport. Vortex tubes were identified using the instantaneous vorticity deviation technique in the MURaM data set of a simulated solar plage region of the solar photosphere. Within 3D kinetic vortex tubes, the misalignment of the magnetic pressure and the inverse of the density gradient, rather than baroclinic effects, primarily drive vorticity within the tubes. During their lifetime, vortices become less dense as the Lorentz force pushes plasma outwards against pressure gradients. In the simulated upper photosphere, the Lorentz force contributes to adiabatic cooling and heating by expanding or compressing the plasma around the vortex tubes. In turn, vortex motion affects the magnetic field, enhancing current generation and intensifying the Lorentz force, which may further increase adiabatic cooling and heating. Moreover, our results confirm that vortices can significantly boost viscous and ohmic heating on intergranular scales in the photosphere. They generate more magnetic than kinetic energy, with energy transport by Poynting flux notably nonuniform and dominant at the vortex boundaries. This creates energy circulation in which the net upwards Poynting flux can enhance chromospheric plasma heating and support chromospheric temperatures.

Baroclinic factor in the stirring of waters of a sinking thermal with surrounding fresh water

The results of studying the process of stirring of a sinking thermal (small volume of denser water) with surrounding water are presented. The analysis of laboratory flows in a hydroflume has been allowed to identify the main peculiarities of the stirring of the thermal and the surrounding water. The essence of one of these features concluded in the presence of many high-gradient layers in the density field. The phase of an advective-rotational nature of the stirring of thermal waters with different density has been identified. Detailing of the evolution of the flow density structure was performed using the calculated flows on a 2d-nonlinear model of the dynamics of a fluid that is inhomogeneous in density. Features of the density structure are revealed using the values of local density gradients for the internal nodes of the computational grid. The density gradients along and across the streamlines were calculated. Quantitative estimates of the repeatability of high-gradient layers have shown significant variability in the flow density field. An analysis of the potential stability of the selected local regions of the flow has been carried out. The calculation of the Reynolds (Re) and Richardson (Ri) numbers has been allowed to assume a laminar nature of the flow, with the exception of two small zones in the back part of the sinking thermal. The role of the baroclinic mechanism of vorticity generation as a structure-forming factor in the formation of a distinguished stage of advective-vortex movement within interpenetration of thermal with surrounding water has shown.

Vorticity dynamics in transcritical liquid jet breakup

A transcritical domain with a sharp two-phase interface may exist during the early times of liquid hydrocarbon fuel injection at supercritical pressure. Thus, two-phase dynamics are sustained before substantial heating of the liquid and drive the early three-dimensional deformation and atomisation. A recent study of a transcritical liquid jet showed distinct deformation features caused by interface thermodynamics, low surface tension and intraphase diffusive mixing. In the present work, the compressible vortex identification method $\lambda _\rho$ is used to study the vortex dynamics in a cool liquid n-decane transcritical jet surrounded by a hotter oxygen gaseous stream at supercritical pressures. The relationship between vortical structures and the liquid surface evolution is detailed, along with the vorticity generation mechanisms, including variable-density effects. The roles of hairpin and roller vortices in the early deformation of lobes, the layering and tearing of liquid sheets and the formation of fuel-rich gaseous blobs are analysed. At these high pressures, enhanced intraphase mixing and ambient gas dissolution affect the local liquid structures (i.e. lobes). Thus, liquid breakup differs from classical sub-critical atomisation. Near the interface, liquid density and viscosity drop by up to 10 % and 70 %, respectively, and the liquid is more easily affected by the vortical motion (e.g. liquid sheets wrap around vortices). Despite the variable density, compressible vorticity generation terms are smaller than the vortex stretching and tilting. Layering traps and aligns the vortices along the streamwise direction while mitigating the generation of new rollers.

On the streamwise vorticity generation and distribution in an angular particle wake

We investigate the streamwise vorticity generation mechanism and distribution pattern in an unbounded steady inertial flow past a fixed Platonic polyhedron. Three angular positions are selected: an edge facing the flow (E), a face facing the flow (F) and a vertex facing the flow (V). We provide compelling evidence that the generation of the streamwise vorticity is primarily caused by the tilting of the transverse vorticity that originates from the particle front surface. Each inclined face on the front surface generates a pair of opposite-signed streamwise vortices. They are advected to the particle wake and form a chiral vorticity pattern which preserves the symmetry of the particle front surface. Two particles at dual angular positions exhibit highly similar vorticity patterns. Our study reveals a striking similarity between the vorticity patterns and the far-field optics diffraction pattern of a light beam past a polygonal aperture. We discover the deterministic vorticity generation mechanism to predict the streamwise vorticity patterns based on the distribution of edges and inclined faces on the particle front surface. Conversely, the vorticity patterns themselves can serve as a diagnostic tool to infer the geometry of the opaque particle front surface. Additionally, the vorticity patterns can be used to predict the stable angular position of a freely settling angular particle, which tends to be such that the number of streamwise vorticity pairs in the wake is maximized.

Investigation of aspect ratio effects on flow characteristics and vorticity generation in shock-induced rectangular bubble

In shock-induced Richtmyer–Meshkov instability, the polygonal bubbles, including the rectangular interface can offer favorable circumstances for shock refraction research. Also, the aspect ratio is crucial in characterizing the physics behind this instability. Therefore, the aspect ratio effects on the flow characteristics and vorticity generation in the shock-induced rectangular light gas bubbles are investigated numerically in this study. This investigation is performed for a wide range of aspect ratios varying from 0.25 to 3.0. These effects are highlighted in the interface morphology, wave paradigms, vorticity generation, enstrophy evolution, and a qualitative examination of interface deformation. For simulating two-component gas flows, a high-order explicit modal discontinuous Galerkin approach is employed to solve a two-dimensional system of compressible inviscid Euler equations. Existing experimental results for shock-induced square and cylindrical light bubbles are used to validate the numerical model and solver. The results indicate that the effects of aspect ratio are critical in characterizing the interface morphology during the interaction between the shock wave and rectangular bubble. The interface morphology evolves faster as the aspect ratio increases, and the process of transverse jet formation is observed to be significantly different. The aspect ratio effects result in a substantial change in the flow field with bubble deformation, vortex development, vorticity generation, and transverse jet formation. The effects of aspect ratio are further investigated in detail through the mechanism of vorticity generation and enstrophy evolution. Finally, these effects on shock trajectories and interface characteristics over time are thoroughly explored.

Flow‐Topography Interaction Along the Spanish Slope in the Alboran Sea: Vorticity Generation and Connection to Interior Fronts

AbstractFlow‐topography interaction is examined using a realistic simulation at submesoscale resolution. The study focuses on a strong, eastward fresh current flowing along the Spanish margin in the Alboran Sea. The current develops large cyclonic vertical vorticity over the irregular slope and separates downstream at a headland; vertical vorticity is advected offshore and an interior density front is sustained by the presence of denser waters to the east. Using a depth‐integrated vorticity balance formulation, we evince the major role of bottom stress as a vorticity generation mechanism, in the form of a bottom stress divergence torque. Our proposed formulation provides an alternative to the classic transport curl balance for quantifying vorticity budgets around the oceans, using high resolution models with well resolved bottom boundary layers. Also as a result of topographic interaction, we show that the fronts observed in the basin interior could be originated along the margins prior to current separation. This is manifested by large values of the Lagrangian frontogenetic tendency for buoyancy gradients, spanning the vertical extent of the boundary current and extending toward the interior.

A multi-phase SPH simulation of hydraulic jump oscillations and local scouring processes downstream of bed sills

In the present study, we numerically focus on the vorticity generation mechanism in a scour hole developed downstream of a grade control structure in sand-bed channels. The liquid-granular flow has been simulated through a symplectic Weakly Compressible SPH scheme with a two-equation turbulence model, where the two phases are treated as continua with different rheological properties. The equilibrium configuration of flow field and bed topography is reached after a transient period in which the system oscillates between two flow patterns: (i) the scour hole shows a relatively steep hump downstream of it, increasing the free surface elevation and generating a B-jump type with a counterclockwise rotating macro-vortex structure which pushes the main flow towards the bed, where strong erosive action is observed; (ii) the scour shows a relatively modest inclination of the scour cavity, leading to the formation of a wave jump dominated by a clockwise rotating macro-vortex structure.

Diagnosing surfzone impacts on inner-shelf flow spatial variability using realistic model experiments with and without surface gravity waves

AbstractRip currents are generated by surfzone wave breaking and are ejected offshore inducing inner-shelf flow spatial variability (eddies). However, surfzone effects on the inner-shelf flow spatial variability have not been studied in realistic models that include both shelf and surfzone processes. Here, these effects are diagnosed with two nearly identical twin realistic simulations of the San Diego Bight over summer to fall where one simulation includes surface gravity waves (WW) and the other that does not (NW). The simulations include tides, weak to moderate winds, internal waves, submesoscale processes, and have surfzone width Lsz of 96(±41) m (≈ 1 m significant wave height). Flow spatial variability metrics, alongshore root mean square vorticity, divergence, and eddy cross-shore velocity, are analyzed in a Lsz normalized cross-shore coordinate. At the surface, the metrics are consistently (> 70%) elevated in the WW run relative to NW out to 5Lsz offshore. At 4Lsz offshore, WW metrics are enhanced over the entire water column. In a fixed coordinate appropriate for eddy transport, the eddy cross-shore velocity squared correlation betweenWWand NW runs is < 0.5 out to 1.2 km offshore or 12 time-averaged Lsz. The results indicate that the eddy tracer (e.g., larvae) transport and dispersion across the inner-shelf will be significantly different in the WW and NW runs. The WW model neglects specific surfzone vorticity generation mechanisms. Thus, these inner-shelf impacts are likely underestimated. In other regions with larger waves, impacts will extend farther offshore.

Vortex generation in the early Universe

Context. Accretion disks formed near primordial black holes can be sources of seed magnetic fields in the early Universe. In particular, the Biermann battery mechanism has been shown to generate primordial magnetic fields in an unmagnetized and turbulence-free accretion disk, but this depends on a delicate misalignment of density and pressure gradients in plasmas. Aims. We aim to reformulate the question of magnetogenesis in the context of plasma generalized vorticity and to search for a more robust mechanism of vorticity generation in the early Universe. Methods. We utilize the electro-vortical formalism in curved spacetime, which treats the plasma flow and electromagnetic field on an equal footing, and apply it to a thin accretion disk model near a rotating black hole. Results. We present a spacetime curvature-driven mechanism that persists even in the absence of the Biermann battery. We explore the vorticity and enstrophy generation rate dependencies on black hole masses and spin rates. Conclusions. Analysis indicates that the accretion disks around lower-mass, faster rotating black holes contribute the greatest amount to the enstrophy and vorticity generation rates from the spacetime curvature drive. The shorter turning radii at which the sign of the vorticity changes – corresponding with this region of phase space – may favor these length scales in vortical structure formation and subsequent evolution.

Measurement of Eulerian vorticity beneath rotating surface waves

In this study, we report experimental verification of the existence of an Eulerian contribution to vortices beneath rotating gravity surface waves. We are motivated by recent theoretical predictions regarding the generation of vortices by surface waves. The first prediction indicates that surface waves which produce vortex lattices at the liquid-gas interface can also produce vorticity beneath them and in the liquid bulk. The second prediction is the existence of an Eulerian contribution to the vorticity, owing to the liquid viscosity. First, we establish that we can generate a vortex lattice at the liquid-air interface using rotating waves, resulting from the superposition of two orthogonal phase-shifted standing waves. Using glycerol-water solution as the liquid, we then conduct Particle Image Velocimetry of tracer particles to measure the velocity and vorticity profiles of the flow beneath the interface. We found that vortices are indeed generated beneath these rotating surface waves. To verify the existence of the Eulerian vorticity, we first measure the vorticity at different values of the liquid layer z < 0. After this, we set a reference vorticity value at a specific z closest to the interface that does not intersect with the waves, and get its ratio with respect to the vorticities at other z. We introduce the computation of the normalized density curves of the vorticity ratios, and compare the peak values to the vorticity prefactor ratios obtained using the theoretical predictions. Our results are in close agreement with the theoretical analysis which accounts for this additional contribution. Our experiments shed light on the mechanism of vorticity generation beneath rotating surface waves.

Hydrodynamic regimes induced by nanosecond pulsed discharges in air: mechanism of vorticity generation

The mechanisms controlling the hydrodynamic effects induced by nanosecond pulsed discharges applied between two pin electrodes in air at atmospheric conditions are investigated experimentally and numerically. A cylindrical plasma kernel is formed between the electrodes during the discharge. After the discharge, schlieren images show that the cylindrical kernel evolves in different ways depending on the gap distance: (1) for short gap distances (), the cylindrical kernel collapses to form a torus-like structure. In this case, the flow field presents a significant source of vorticity; (2) for larger gaps (), the kernel retains its initial cylindrical shape and cools down primarily through heat diffusion. The objective of this work is to understand the mechanisms leading to the formation of these two hydrodynamic regimes. To this end, simulations were performed and compared with the experimental schlieren images. It is shown that the main source of vorticity is the baroclinic torque caused by the cylindrical blast wave that follows the fast energy addition during the discharge. Finally, a criterion is given to predict the occurrence of the two hydrodynamic regimes. This criterion is then validated against experimental results from the literature.

Pressure-gradient tailoring effects on the turbulent flame-vortex dynamics of bluff-body premixed flames

This paper explores the effects of pressure-gradient tailoring on the turbulent flame and vorticity generation mechanisms of premixed flames. A turbulent premixed flame stabilized by a bluff-body in a high-speed combustor is used for the investigation. The combustor pressure gradient is altered using a variable-geometry test section. The turbulent flame-flow field is measured and characterized using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. A Lagrangian tracking technique is applied to analyze the details of the flame-vortex interactions from the experimental data. Lagrangian fluid elements are tracked as they evolve across the flame. The vorticity mechanisms are decomposed along the Lagrangian trajectories to determine their relative balance under various pressure gradient conditions. It is demonstrated that the induced pressure-gradient affects the relative magnitudes of combustion-generated dilatation and baroclinic torque, as well as the vortex stretching. An increase in the magnitudes of the vorticity mechanisms is shown with the largest gain in baroclinicity for the augmented pressure gradient relative to the attenuated.

Experimental and Numerical Investigation of Pre-Breaking and Breaking Vorticity within a Plunging Breaker

The characteristics of vorticity induced just prior and at the splash-down of a plunging breaker on a 1:10 planar slope have been studied using wave flume experiments and numerical simulations. Laboratory experiments involved detailed measurements in the outer surf zone of both fluid velocities below trough level, achieved by a fibre-optic laser-Doppler anemometer, and water surface elevations, obtained by an ultrasound probe. A Weakly-Compressible Smoothed Particle (WCSPH) model, coupled with a two-equation model for turbulent stresses, has been employed for the numerical simulations. A thorough calibration of the SPH’s numerical parameters has been first performed, through comparison between numerical and experimental wave elevation and velocity data. Then, considering that time-averaged laboratory data shows a significant vorticity beneath the free surface in the wave pre-breaking region, the vorticity generation mechanism has been thoroughly analyzed by means of the numerical model. In the attempt of explaining the generation of vorticity as induced by flow separation, we also inspected the role of the streamwise flow deceleration and surface-parallel vorticity flux. In analogy with the case of spilling breakers a cause-effect relation has been observed between streamwise flow deceleration and vorticity generation. Numerical findings are positively confirmed by the experimental results.

Cross-shore variability and vorticity dynamics during wave breaking on a fixed bar

In the present experiments a volumetric particle-tracking system (V3V from TSI Inc.) allowed the measurement of the velocity fields generated by regular breaking waves past a fixed bar on a 1:10 rigid plane slope. The measurement volume extended from the wave crest to a portion of the domain below the wave trough, with two sets of monochromatic wave trains with different periods and heights. The aim of the present work is the quantification of the terms in the vorticity balance equation by including all the terms in a fully 3D approach. A possible new vorticity generation mechanism is revealed, that is amplified by the geometry of the laboratory flume and that awaits experimental validation in 3D wave tanks and in the field. The results are new and original and represent a data set for the comprehension of the effects of a submerged berm, for developing conceptual models of vorticity and for the calibration of numerical codes.

Large Eddy Simulations of CH4 Fire Plumes

Large eddy simulations of large-scale CH4 fire plumes (1.59-2.61 MW) with two different CFD packages, FireFOAM and FDS, are presented. It is investigated how the vorticity generation mechanism and puffing behavior of large-scale fire plumes differs from previously studied iso-thermal buoyant plumes of the same scale. In addition, the predictive capabilities of the turbulence and combustion models, currently used by the two CFD codes, to accurately capture the fire dynamics and the buoyancy-generated turbulence associated with large-scale fire plumes are evaluated. Results obtained with the two CFD codes, typically used for numerical simulations of fire safety applications, are also compared with respect to the average and rms velocities and temperatures, puffing frequencies, average flame heights and entrainment rates using experimental data and well-known correlations in literature. Furthermore, the importance of the applied reaction time scale model in combination with the Eddy Dissipation Model is examined. In particular, the influence of the considered mixing time scales in the predicted centerline temperatures is illustrated and used to explain the discrepancies between the two codes.

Study of coherent structures of turbulence with large wall-normal gradients in thermophysical properties using direct numerical simulation

Forced-convection heat transfer in a heated working fluid at a thermodynamic state near its pseudocritical point is poorly predicted by correlations calibrated with data at subcritical temperatures and pressures. This is suggested to be primarily due to the influence of large wall-normal thermophysical property gradients that develop in proximity of the pseudocritical point on the concentration of coherent turbulence structures near the wall. The physical mechanisms dominating this influence remain poorly understood. In the present study, direct numerical simulation is used to study the development of coherent vortical structures within a turbulent spot under the influence of large wall-normal property gradients. A turbulent spot rather than a fully turbulent boundary layer is used for the study, for the coherent structures of turbulence in a spot tend to be in a more organized state which may allow for more effective identification of cause-and-effect relationships. Large wall-normal gradients in thermophysical properties are created by heating the working fluid which is near the pseudocritical thermodynamic state. It is found that during improved heat transfer, wall-normal gradients in density accelerate the growth of the Kelvin-Helmholtz instability mechanism in the shear layer enveloping low-speed streaks, causing it to roll up into hairpin vortices at a faster rate. It is suggested that this occurs by the baroclinic vorticity generation mechanism which accelerates the streamwise grouping of vorticity during shear layer roll-up. The increased roll-up frequency leads to reduced streamwise spacing between hairpin vortices in wave packets. The density gradients also promote the sinuous instability mode in low-speed streaks. The resulting oscillations in the streaks in the streamwise-spanwise plane lead to locally reduced spanwise spacing between hairpin vortices forming over adjacent low-speed streaks. The reduction in streamwise and spanwise spacing between hairpin vortices causes them to interact more frequently by merging together and by breaking apart into smaller turbulence structures.

Numerical Study of a Vortex Ring Interacting with a Three-dimensional Convex Surface

A vortex ring impinging on a three-dimensional bump is studied using Large Eddy Simulation (LES) for a Reynolds number Re = 4×10 4 based on the initial diameter and translational speed of the vortex ring. The evolution of vortical structures are investigated and an array of flow phenomena are discovered, such as the generation and deformation of secondary vortex ring, formation of loop-like vortices, interaction of vortex rings and the instability and breakdown of vortical structures. The total enstrophy of the flow reasonably elucidates some typical phases of flow evolution. Based on the Fourier analysis of the vertical vorticity, the azimuthal instabilities of the primary vortex ring are studied. Furthermore, the mechanism of vorticity generation on the bump surface has been revealed based on analysis of the boundary vorticity flux.

Direct numerical simulation of lock-exchange density currents over the rough wall in slumping phase

Direct numerical simulation (DNS) is used to simulate lock-exchange flow to understand density currents over rough surfaces. This work is one of the first DNS to simulate density currents over rough walls. The simulations are performed at a Grashof number of 1.6×107. The non-dimensional height of the roughness elements with respect to the half-height of the channel is 0.12. Roughness reduces the speed of the front. Furthermore, the instabilities are significantly enhanced resulting in secondary instabilities that arise much earlier in time. Roughness introduces an additional vorticity generation mechanism which is comparable to the vorticity generated from Kelvin–Helmholtz and lobe–cleft type of instabilities. Flow exhibits significant differences near the leading edge or the nose of the front of the density currents due to roughness.

Sill effects on physical dynamics in eastern Long Island Sound

This study investigates how Mattituck Sill influences circulation patterns and physical dynamics in eastern Long Island Sound, a major estuary on the U.S. east coast. Observations show there is pronounced across-estuary transport in the area and suggest there may be subtidal anticyclonic flow around the sill. Model runs, with and without sill bathymetry, exhibit this across-estuary transport and anticyclonic circulation. Comparison between these runs indicates that the sill intensifies the anticyclonic circulation. This study finds the sill does not exert internal hydraulic control during neap, mean, or spring tidal conditions. Nevertheless, along-estuary exchange is reduced over the sill and across-estuary fluxes are increased. The Connecticut River plume enters close to the estuary mouth. The sill deflects more of the plume waters towards the mouth, causing less freshwater to take the long looping route through the estuary. The subtidal circulation balance around the sill indicates a barotropic balance between the tidal advection of tidal vorticity and friction. The subtidal vorticity balance indicates the net effect of tidal advection of relative vorticity is balanced with frictional curl associated with lateral speed gradients and vorticity dissipation. Previously developed scalings based on the circulation balance (Nature 290:549–555, 1981), frictional vorticity generation mechanisms (Deep-Sea Res 28:195–212, 1981), and tidal diffusion of potential vorticity (J Phys Oceanogr 29:821–827, 1999) are applicable to Mattituck Sill and predict circulation with a similar magnitudes to model results.

Tidal residual eddies and their effect on water exchange in Puget Sound

Tidal residual eddies are one of the important hydrodynamic features in tidally dominant estuaries and coastal bays, and they could have significant effects on water exchange in a tidal system. This paper presents a modeling study of tides and tidal residual eddies in Puget Sound, a tidally dominant fjord-like estuary in the Pacific Northwest coast, using a three-dimensional finite-volume coastal ocean model. Mechanisms of vorticity generation and asymmetric distribution patterns around an island/headland were analyzed using the dynamic vorticity transfer approach and numerical experiments. Model results of Puget Sound show that a number of large twin tidal residual eddies exist in the Admiralty Inlet because of the presence of major headlands in the inlet. Simulated residual vorticities near the major headlands indicate that the clockwise tidal residual eddy (negative vorticity) is generally stronger than the anticlockwise eddy (positive vorticity) because of the effect of Coriolis force. The effect of tidal residual eddies on water exchange in Puget Sound and its subbasins was evaluated by simulations of dye transport. It was found that the strong transverse variability of residual currents in the Admiralty Inlet results in a dominant seaward transport along the eastern shore and a dominant landward transport along the western shore of the inlet. A similar transport pattern in Hood Canal is caused by the presence of tidal residual eddies near the entrance of the canal. Model results show that tidal residual currents in Whidbey Basin are small in comparison to other subbasins. A large clockwise residual circulation is formed around Vashon Island near entrance of South Sound, which can potentially constrain the water exchange between the Central Basin and South Sound.