Columnar Vortices Research Articles

Evolution of downburst-like flows produced by an active-controlled multi-blade facility

The intricate dynamics of vortex structures within the downburst outflow region present significant challenges in studying flow evolutionary features, which are crucial for understanding the effect of such flow on various structures. This study aims to reveal the evolutionary features of downburst-like winds produced by an active-controlled multi-blade (AMBS) facility, using the particle image velocimetry tests and the large-eddy simulation (LES) studies. The numerical simulation indicates that the wind velocity profiles, nonstationary wind velocity time history, and the transient flow patterns of the downward flow impinging on the ground can be well simulated by the LES method. For stationary winds, a series of columnar vortices are produced, and the vortices tend to be more organized as the maximum velocity appears at a lower height. The proper orthogonal decomposition analysis manifests that the primary vortex region is affected by multiple modes of the fluctuating wind field. In addition, the primary vortex structures of the AMBS-generated flow present apparent evolutionary features. During the downward flow impinging on the ground, the nose-shaped wind velocity profile gradually forms at the turntable center and is well developed when the maximum velocity is reached. This observation is in accordance with the findings in available full-scale measurement campaigns.

Investigation into the Vortex Evolution characteristics of the Francis Turbine During the Runaway Process

Abstract The runaway process is a dynamic transition process that gradually deviates from the optimal operating condition. The flow passage components dissipate all of the energy corresponding to the hydraulic turbine’s head, causing the internal flow of the hydraulic turbine to gradually deteriorate and produce a large-scale vortex structure during the runaway process. A model Francis turbine is used as the research object in this paper to understand the evolution characteristics of an unsteady vortex during a runaway. The runaway process is investigated using high-precision simulation technology of gas-liquid two-phase flow, and the runaway speed is obtained, consistent with the experimental results. The results show that the cavitation volume increases as the rotating speed increases. The large incidence angle between the inlet flow and the blade causes large-scale flow separation in the runner, which leads to forming of a sheet vortex band near the hub of the runner hub and a columnar vortex in the blade passage of the runner blade suction surface. As the runaway process continues, the flow separation in the runner intensifies and the induced vortex size increases, obstructing the runner’s passage. In addition, pressure signal analysis reveals that the pressure fluctuation caused by the sheet vortex band is low-frequency. On the other hand, the pressure fluctuation caused by the columnar vortex is frequent. These two types of vortex structures with high-energy pressure fields degrade the flow in the runner and reduce the hydraulic performance of the turbine.

Aerodynamic performance and wake development of NACA 0018 airfoil with serrated gurney flaps

Improving the aerodynamic performance of the airfoil is important for optimising the rotor efficiency of the vertical axis wind turbines. As a simple passive control method, the Gurney flap is widely used to improve the aerodynamic performance of airfoils. In this paper, we study the impact of applying a novel serrated gurney flap with different heights on the NACA 0018 airfoil. An improved delayed detached eddy simulation method is adopted to investigate the lift-enhancing mechanism of the serrated gurney flap and the evolution of the downstream vortex system. The results show that the serrated gurney flap can effectively increase the airfoil lift coefficient and the lift-to-drag ratio. The improvement of the serrated gurney flap on the aerodynamic performance of the airfoil is more pronounced at moderate angles of attack. Further analysis of the downstream wake shows that a pair of vortices wraps over both sides of the airfoil and rotates perpendicular to the wake flow, which is produced by the columnar vortex upstream of the flap. These vortices mixed with the wake and accelerated the dissipation of the separated vortex on the suction surface of the airfoil.

Taylor Nearly Columnar Vortices in the Couette–Taylor System: Transition to Turbulence

Taylor Nearly Columnar Vortices in the Couette–Taylor System: Transition to Turbulence

Saltation‐Induced Dust Emission of Dust Devils in the Convective Boundary Layer—An LES Study on the Meter Scale

AbstractDust devils are vertically oriented, columnar vortices that form within the atmospheric convective boundary layer (CBL) of dry regions. They are able to lift a sufficient amount of soil particles including dust to become visible and are considered as a potentially important dust source for the atmosphere. Mineral dust, a key component of atmospheric aerosols, influences the climate by affecting the radiation budget and cloud formation. Current estimates of the contribution of dust devils to the global, regional, and local dust release vary considerably from less than 1% to more than 50%. To address this uncertainty, we perform the highest resolved large‐eddy simulation (LES) study on dust emission in the CBL to date, using the PALM model system and the saltation‐based Air Force Weather Agency (AFWA) dust emission scheme. Our results show that under desert‐like conditions, dust devils are responsible for an average of 5% of regional dust emissions, with temporary maxima of up to 15%. This contrasts with previous measurement‐based (>35%) and LES‐based estimates (∼0.1%). Local emissions of dust devils (up to 10 mg m−2 s−1) are 1–3 orders of magnitude higher than the emission in the surroundings. This makes dust devils important for air quality and visibility. Additionally, our study reveals previously unknown large‐scale convective dust emission patterns. These patterns are tied to the CBL's cellular flow structure and are the main cause of dust release. Contrary to other studies, our findings clarify the important role of saltation‐induced dust emission.

Evidence for layered anisotropic stratified turbulence in a freely evolving horizontal shear flow

We present evidence for layered anisotropic stratified turbulence (LAST) and mixing produced in a freely evolving uniformly stratified shear layer where the direction of shear is orthogonal to gravity. As originally reported by Basak & Sarkar (J. Fluid. Mech., vol. 568, 2006, pp. 19–54), such a flow develops a rich three-dimensional structure in the form of interlocking columnar vortices formed by horizontal shear instability that remain coherent at large scales due to the stabilising vertical stratification. Here, we modify the initial velocity field by introducing additional small-amplitude vertical perturbations designed to be representative of pre-existing horizontal layers often observed in strongly stratified ocean environments. This reveals a novel finite amplitude, non-normal mode growth mechanism through which the vertical shear between layers may be rapidly amplified by its interaction with the horizontal shear layer prior to the growth of shear instability, leading to a rapid turbulent transition instigated by the subsequent interaction of the layers with the emerging columnar vortices. Through a consideration of relevant flow statistics and associated dimensionless parameters, we demonstrate that turbulence can enter the LAST regime, thereby indicating a generic mechanism leading to the transient development of regions of strongly stratified turbulence in the ocean. We discuss the properties of mixing and the parameterisation of mixing efficiency in terms of the relationship between turbulent length scales in the flow, in particular highlighting links to models based on the classical vertical shear instability paradigm typically associated with more weakly stratified flows that produce isolated turbulent ‘patches’.

The nonlinear evolution of two surface quasi-geostrophic vortices

We investigate numerically the evolution of a baroclinic vortex in a two-level surface quasi-geostrophic model. The vortex is composed of two circular patches of uniform buoyancy, one located at each level. We vary the vortex radii, the magnitude of buoyancy, and the vertical distance between the two levels. We also study different radial profiles of buoyancy for each vortex. This article considers two main situations: firstly, initially columnar vortices with like-signed buoyancies. These vortices are contra-rotating, are linearly unstable and may break. Secondly, we consider initially tilted vortices with opposite-signed buoyancies, which may align vertically. Numerical experiments show that (1) identical contra-rotating vortices break into hetons when initially perturbed by low azimuthal modes; (2) unstable, vertically asymmetric, contra-rotating vortices can stabilise nonlinearly more often than vertically symmetric ones, and can form quasi-steady baroclinic tripoles; (3) co-rotating vortices can align when the two levels are close to each other vertically, and when the vortices are initially horizontally distant from each other by less than three radii; (4) for initially more distant vortices, two such vortices rotate around the plane center; and(5) in all cases, the vortex boundaries are disturbed by Rossby waves. These results compare favorably to earlier results with internal quasi-geostrophic vortices. Further modelling efforts may extend the present study to fully three-dimensional ocean dynamics.

Characteristics of columnar vortex airflow and dust escape prevention based on artificial tornado

Dust pollution resulting from industrial production posed a significant risk to worker health. To address this issue, the coupling model of airflow and dust was established using the Eulerian-Lagrange method and the effectiveness of this model was via experimental data. Then, the airflow-dust coupling diffusion results using the columnar vortex ventilation system from different jet angle θ, jet velocity vJ and suction-blowing flow ratio γ were simulated. The results indicated that the columnar vortex ventilation system generated “columnar vortex airflow” by synergistically combining the surrounding air supply jets and upper exhaust airflow. Firstly, as vJ increased, θ and γ decreased, the mutual impact between the jets would be intensified, resulting in decreased vortex stability, which in turn caused a decrease in the dust removal rate. Simultaneously, excessive θ and vJ resulted in vortex range expansion, increasing dust pollution area in the near-floor, and decreasing the dust removal efficiency. Finally, the optimal parameters were θ = 65°, vJ = 2.25 m/s, and γ = 1.06. Under the optimal parameter scheme, the dust removal efficiency of the column vortex ventilation system reached 86%. This study provided a theoretical reference for the application of the column vortex ventilation system and the improvement of the environment in industrial plants.

DEVELOPMENT OF ANALYTICAL MODEL FOR DESCRIPTION OF SWIRLING FLOWS

Swirling flows are widely used in engineering. Various types of separators, burners, dryers, coolers, etc. work due to flow swirling. To study swirling flows, methods of experi- mental modeling and computational hydrodynamics are used. The most productive approach to the description of swirling flows can be realized when it becomes possible to analytically describe the characteristics of the flow. In the present work, models of helical vortices are generalized to the case of a composition of a columnar vortex and a pair of coaxial helical vortices, which makes it possible to describe the recently obtained experimental data.

High-frequency vortex dynamics and losses in the mixed state of nanostructured superconductors with columnar defects

We suggest a theoretical model for the microwave response of Abrikosov vortices in nanostructured superconductors with columnar defects, which serve as effective pinning sites for vortices that are considered elastic strings. For this case, we suppose that microwave losses related to the vortex oscillations in a microwave field arise caused by viscous oscillations of vortex kinks, which connect different parts of the vortex line pinned on adjacent columnar defects and vortex segments pinned on these defects. We consider the case of inclined magnetic field H, which creates vortex staircases consisting of pinned vortex segments and vortex kinks inside the superconductor. In this case, rf losses due to viscous kinks oscillations under the microwave current action arise even at T = 0 if the inclination angle of the magnetic field θΗ, concerning the columnar pinning site axis (z axis), exceeds some locking angle value θL.

Two Dynamical Regimes of Coherent Columnar Vortices in a Rotating Fluid

Vortex flow generation in an incompressible fluid was investigated experimentally inside a rotating closed cubic aquarium. The flow was excited by producing small-scale eddies near the side edges of the cube. Coherent columnar vortices-cyclones extending from the bottom to the lid of the cube were observed in the liquid volume. The lifetime of the cyclones was much longer than the attenuation time due to the viscous friction on the bottom and the lid. It was found that there are two regimes of quasi-two-dimensional turbulence, which are characterized by different ways of interaction between quasi two-dimensional flow and inertial waves. The radial profiles of the time- averaged azimuth velocity in the coherent vortices in these two regimes are investigated. It is shown that the vortices differ in size and in vorticity distribution along the radius.

CFD informed design of bench-scale experiments to characterize air entrainment into fuel beds induced by columnar vortices

Recent experiments show that strong vortices, similar to fire whirls, can form far from a fire front in the region of smoldering fuel. These buoyancy-induced columnar vortices, visualized by entrained smolder smoke, were observed lofting hot embers into the air and in some cases lead to spot ignitions at the base of the vortex. Gaining insight on how the flow field of a buoyancy-induced columnar vortex could impact surrounding smoldering fuel is the focus of this study. Specifically, the potential air entrainment into a fuel substrate beneath the vortex. The flow field of such columnar vortices has been shown to drive air flow downward under certain conditions and, in the context of combustion, drive air deeper than typical entrainment, inducing spot ignitions and increasing burning and smolder rates. NIST's Fire Dynamics Simulator is utilized to successfully model buoyancy-induced columnar vortices. Then it is utilized to study the behavior of vortices as temperature and vorticity boundary conditions are changed. The flow field and fresh air entrainment potential are analyzed. The simulation results inform the experiment design and preliminary experimental results are presented. Understanding these high-risk phenomena will lead to better risk mitigation and more resilient Wildland-Urban interface communities.

Vortex analysis and flow field optimization for the particle classifier with three rotary cages

The particle classifiers with three rotary cages have significant advantage in powder handling capacity. The flow field inside the classifiers were investigated from the perspective of vortex using Q criterion. Formation process and distribution of the vortices were intuitively exhibited. Structure of the guide cone was further optimized, and the classifying performance of the classifiers was evaluated. The results show that complex vortex structures were formed inside the classifier. The large-scale columnar vortex under the guide cone oscillates irregularly. This unwanted vortex is eliminated by extending the guide cone. The structure of the guide cone has little effect on the cut size, but the optimized guide cone with the long cylinder and cone significantly enhances the separation degree of the fine and coarse particles. The classifier obtains finer silica powder with a median size of 2.5 μm and higher Newton efficiency about 71.5%.

Investigations of Spoilers to Mitigate Columnar Vortices in Propeller Turbines at Speed-No-Load Based On Steady and Unsteady Flow Simulations

Abstract With the introduction of an ever-larger share of renewable but intermittent energy sources on electrical grids, hydraulic turbines are more often used as network stabilizers. In such a role, they are generally operated in off-design operations like speed-no-load (SNL). No energy is extracted from the flow at SNL operation, but the runner rotates at the synchronous speed linked to the electrical grid. The flow inside the runner of low-head turbines operating at SNL is often dominated by a columnar vortex array that may induce damaging pressure fluctuations. This paper presents the study of a control device to mitigate those vortices. At SNL, the small guide vane opening leads to a high swirl in the runner generating secondary flows such as columnar vortices and backflows. The proposed concept is to move SNL operation toward a higher guide vane opening and hence lower swirl, preventing the formation of a columnar vortex array. Lowering the input swirl of SNL is accomplished by opening up the guide vanes while using a control device to limit the discharge. The control device, like a spoiler on an aircraft wing, is introduced on the guide vanes to generate added head losses, significantly decreasing the discharge. This paper compares the hydrodynamics of the flow in a propeller turbine with different spoiler geometries. The study is based on both RANS and URANS flow simulations. It highlights how such devices can successfully mitigate columnar vortices and their associated pressure fluctuations on runner blades.

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.

Flow regimes in a very wide-gap Taylor-Couette flow with counter-rotating cylinders.

In this study, we discuss the observed flow regimes in Taylor-Couette flow of radius ratio [Formula: see text] for various Reynolds numbers up to [Formula: see text]. We investigate the flow using a visualization method. The flow states in the centrifugally unstable flow are investigated in the case of counter-rotating cylinders and pure inner cylinder rotation. Beside classical known flow states as Taylor-vortex flow and wavy vortex flow, we observe a variety of new flow structures in the cylindrical annulus, especially for the transition to turbulence. Coexisting turbulent and laminar regions inside the system are observed. Turbulent spots and turbulent bursts are observed, as well as an irregular Taylor-vortex flow and non-stationary turbulent vortices. Especially, a single axially aligned columnar vortex between the inner and outer cylinder is found. The principal regimes observed in flow between independently rotating cylinders are summarized in a flow-regime diagram. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (Part 2)'.

Weakly nonlinear versus semi-linear models of the nonlinear evolution of the centrifugal instability

We carry out a weakly nonlinear analysis of the centrifugal instability for a columnar vortex in a rotating fluid, and compare the results to those of the semi-linear model derived empirically by Yim et al. (J. Fluid Mech., vol. 897, 2020, A34). The asymptotic analysis assumes that the Reynolds number is close to the instability threshold so that the perturbation is only marginally unstable. This leads to two coupled equations that govern the evolutions of the amplitude of the perturbation and of the mean flow under the effect of the Reynolds stresses due to the perturbation. These equations differ from the Stuart–Landau amplitude equation or coupled amplitude equations involving a mean field that have been derived previously. In particular, the amplitude does not saturate to a constant as in the supercritical Stuart–Landau equation, but decays afterwards reflecting the instability disappearance when the mean flow tends toward a neutrally stable profile in the direct numerical simulations (DNS). These equations resemble those of the semi-linear model except that the perturbation in the weakly nonlinear model keeps at leading order the structure of the eigenmode of the unperturbed base flow. The predictions of the weakly nonlinear equations are compared to those of the semi-linear model and to DNS for the Rossby number $Ro=-4$ and various Reynolds numbers and wavenumbers. They are in good agreement with the DNS when the growth rate is sufficiently small. However, the agreement deteriorates and becomes only qualitative for parameters away from the marginal values, whereas the semi-linear model continues to be in better agreement with the DNS.

Interplay between geostrophic vortices and inertial waves in precession-driven turbulence

The properties of rotating turbulence driven by precession are studied using direct numerical simulations and analysis of the underlying dynamical processes in Fourier space. The study is carried out in the local rotating coordinate frame, where precession gives rise to a background shear flow, which becomes linearly unstable and breaks down into turbulence. We observe that this precession-driven turbulence is in general characterized by coexisting two-dimensional (2D) columnar vortices and three-dimensional (3D) inertial waves, whose relative energies depend on the precession parameter Po. The vortices resemble the typical condensates of geostrophic turbulence, are aligned along the rotation axis (with zero wavenumber in this direction, kz = 0), and are fed by the 3D waves through nonlinear transfer of energy, while the waves (with kz≠0) in turn are directly fed by the precessional instability of the background flow. The vortices themselves undergo inverse cascade of energy and exhibit anisotropy in Fourier space. For small Po < 0.1 and sufficiently high Reynolds numbers, the typical regime for most geo- and astrophysical applications, the flow exhibits strongly oscillatory (bursty) evolution due to the alternation of vortices and small-scale waves. On the other hand, at larger Po > 0.1 turbulence is quasi-steady with only mild fluctuations, the coexisting columnar vortices and waves in this state give rise to a split (simultaneous inverse and forward) cascade. Increasing the precession magnitude causes a reinforcement of waves relative to vortices with the energy spectra approaching the Kolmogorov scaling, and therefore, the precession mechanism counteracts the effects of the rotation.

A three-dimensional numerical study of laminar buoyancy-driven columnar vortices

This study deals with the development of a low momentum whirling laminar plume that results from the interaction between a plume and a circulating flow imposed by a cylindrical rotating screen. Numerical simulations were performed by direct simulation of the fully three-dimensional flow governed by the compressible Navier-Stokes and energy equations. The rotating thermal plume forms a columnar vortex governed by the Grashof (GR) and nondimensional rotation velocity (Reynolds number (ReR)), based on the cylindrical domain’s radius and the angular frequency of rotation of the permeable lateral surface. Numerical solutions were obtained in the range 102<Gr<106 and 10<ReR<150. The vortex generated bifurcates into a pair of steady co-rotating vortices within the range 104<Gr<2×105 and 30<ReR<100. The steady state configuration of the flow is described and analysed, showing the evolution of the vorticity contours in cross-sections that start out as circlets, gradually turn into an ellipse and end up bifurcating into a pair of co-rotating vortices. The role of the boundary layer is also described, showing an increase in the distance between the co-rotating vortex pair for a given Gr and ReR numbers but in the absence of a boundary layer. At last, the temporal evolution of the vortex splitting process is compared to measurements of merging of two co-rotating vortices available in the literature, uncovering that the two phenomena take similar timescales to occur.

Validation of deep part load dynamic stresses for axial runners

To accommodate renewable energy production and load demand variability, hydropower plant owners need to increase their operating range regardless of their units’ original design load envelop. When this increased operating range is an issue for the fatigue life of the old runners, solutions need to be found with the design of a new runner to sustain those new challenging loads of the increased operating range. In recent years, many papers have been published to show the challenging loads on Francis runners at speed-no-load and deep part load conditions. Andritz demonstrated a good numerical prediction capability for stress levels at deep part load conditions for Francis runners. However, for axial units, very little has been published. Very recently, some papers showed good predictability by CFD of the flow behavior at deep part load including the vortices present at those conditions. This paper demonstrates the prediction capability of the numerical tools by comparing strain gage measurements on an axial runner to CFD-FEA stress predictions. The measurement campaign was conducted conjointly by the unit owner and the manufacturer for research purposes. In the deep part load operating zone under the effect of columnar vortices, frequency analysis of the measured vibrations and strain gage signals confirmed the flow behavior predicted by CFD, and the measured dynamic strain amplitudes were well predicted. Numerical prediction of dynamic stress in the complete range from 0-100% power of the measured unit as well as detection of high vibration zones was successful.