Terms Of Vortices Research Articles

Laminar flow over a rectangular cylinder experiencing torsional flutter: Dynamic response, forces and coherence modes

This study investigates the flutter response of a rectangular cylinder model with an aspect ratio of 5 at the Reynolds number Re = 100 via direct numerical simulation. The effects of two key parameters, i.e., the moment of inertia and reduced flow velocity, on the aerodynamic performance and dynamic responses of the cylinder in the state of torsional flutter are investigated. To reveal the flutter mechanism, the high-order dynamic mode decomposition (HODMD) analysis is conducted to decompose the flow field. The results show that both an increase in the moment of inertia and a higher reduced flow velocity lead to a larger torsional amplitude and a corresponding decrease in torque. At the same time, the primary frequency decreases and the size of the shedding vortex gradually enlarges. The vortices shed from the leading edge and the trailing edge of the model form a 2P wake pattern. The leading-edge vortex is significantly larger than the trailing-edge vortex in terms of strength and size. The leading edge plays a dominant role and only contributes to the odd-order HODMD modes while the even-order modes are deemed inconsequential. As the moment of inertia increases, the total energy of the higher-order modes increases, which has the same results as the power spectral density of torque, reflecting increased nonlinearity and complexity of the system. Similarly, increasing the reduced flow velocity at the same moment of inertia has similar excitation effects.

The [formula omitted]-vortex problem in a doubly periodic rectangular domain with constant background vorticity

We formulate the N point vortex problem in a doubly-periodic rectangle with a constant background vorticity using the hydrodynamic Green’s function. We derive an explicit formula for the hydrodynamic Green’s function and the velocities of the point vortices in terms of the Schottky–Klein prime function using a conformal mapping approach. The sum of vortex strengths can be arbitrary, and when it is zero we recover previous known results including the integrability of the two and three-vortex problems. A non-zero sum of vortex strengths leads to a constant background vorticity. We derive a Hamiltonian structure for the equations and show that the two-vortex problem is integrable, and classify all possible vortex motions. In addition, for general N, we obtain several fixed lattice equilibrium configurations including single-layered and double-layered equilibria. In the latter case, we also obtain lattice configurations with defects consisting of point vortices with inhomogeneous strengths. We find equilibria arranged in doubly-periodic rectangular and parallelogram lattices consisting of N=1,2,3,4 vortices per fundamental lattice.

Spin-flux attachment by dimensional reduction of vortices

The description of a system of vortices in terms of dual fields provides a window to new phases of the system. It was found recently that dualizing a 3+1-d boson-fermion system leads to a system of fermions and vortices interacting via a 2-form field through a non-local term. Here we explore some consequences of such an interaction when the degrees of freedom of the system are confined to a 2+1-d space-time. In particular, we show that the vortices in the 2+1-d system are attached to the fermions via their non-zero spin magnetic moment in a way similar to the phenomenon of flux attachment in Chern-Simons gauge theory coupled to matter. We also show that such flux attached particles exhibit fractional statistical behaviour like anyons. Thus our model provides a realization of anyons without Chern-Simons theory.

Development of Vortex Asymmetry on a Generic Projectile Configuration

A combined experimental and numerical study was carried out to understand the development of vortices on the U.S. Army/Navy basic finner configuration at high angles of incidence. The experiments were carried out at the Florida A&M University and Florida State University low-speed wind tunnel, and numerical simulations were performed at U.S. Army Research Laboratory using the U.S. Department of Defense Kestrel solver. The results show a good agreement of basic aerodynamic characteristics between the experimental data and numerical simulations at low angles of incidence over the range of flow conditions; however, at high angles of incidence, the predicted aerodynamic coefficients from simulations were relatively lower. Both experimental and numerical simulation results show that asymmetric vortices develop on the cone cylinder and interact with the fins downstream at high angles of incidence. The primary vortices eventually lift off from the body, resulting in multiple secondary shear-layer vortices. The vortex liftoff location moves upstream with an increase in angles of incidence. The liftoff location of the two primary vortices can result in a change of side-force magnitude and direction. At supersonic speeds, the characteristics of the two primary vortices in terms of their size, strength, and the location on the body are very different as compared to those at subsonic speeds.

Hairpin vortices in the largest scale of turbulent boundary layers

We have conducted direct numerical simulations of a turbulent boundary layer for the momentum-thickness-based Reynolds number Reθ = 180–4600. To extract the largest-scale vortices, we coarse-grain the fluctuating velocity fields by using a Gaussian filter with the filter width comparable to the boundary layer thickness. Most of the largest-scale vortices identified by isosurfaces of the second invariant of the coarse-grained velocity gradient tensor are similar to coherent vortices observed in low-Reynolds-number regions, that is, hairpin vortices or quasi-streamwise vortices inclined to the wall. We also develop a percolation analysis to investigate the threshold-dependence of the isosurfaces and objectively identify the largest-scale hairpin vortices in terms of the coarse-grained vorticity, which leads to the quantitative evidence that they never disappear even in fully developed turbulent regions. Hence, we conclude that hairpin vortices exist in the largest-scale structures irrespective of the Reynolds number.

Hairpin vortices in the largest scale of turbulent boundary layers

Abstract We have conducted direct numerical simulations of a turbulent boundary layer for the momentum-thickness-based Reynolds number Re θ = 180–4600. To extract the largest-scale vortices, we coarse-grain the fluctuating velocity fields by using a Gaussian filter with the filter width comparable to the boundary layer thickness. Most of the largest-scale vortices identified by isosurfaces of the second invariant of the coarse-grained velocity gradient tensor are similar to coherent vortices observed in low-Reynolds-number regions, that is, hairpin vortices or quasi-streamwise vortices inclined to the wall. We also develop a percolation analysis to investigate the threshold-dependence of the isosurfaces and objectively identify the largest-scale hairpin vortices in terms of the coarse-grained vorticity, which leads to the quantitative evidence that they never disappear even in fully developed turbulent regions. Hence, we conclude that hairpin vortices exist in the largest-scale structures irrespective of the Reynolds number.

Experimental investigation of Görtler vortices in hypersonic ramp flows

A sharp leading-edge ramp model with 15°, 20°, and 25° ramp angles is experimentally investigated at a freestream Mach number 7.7 and a unit Reynolds number 4.2 × 106 m−1 in the Aachen Shock Tunnel TH2. The objective of this paper is to analyze Gortler vortices in terms of shear layer length, flow curvature, and vortex related parameters. The spanwise heat flux variation caused by it and the streamwise heat flux enhancement during its tenure are also studied. Thermocouples, pressure sensors, schlieren imagery, and infrared imaging are used for the investigation. The boundary layer on the flat plate until separation is laminar. An important outcome is that the arc length at reattachment is constant irrespective of the ramp angle. Characteristic boundary layer thicknesses at different Mach number show that the momentum thickness is insensitive to compressibility and is used to determine the Gortler number. A model is presented to determine the Gortler number in terms of ramp angle and a constant based on separation angle, arc length, momentum thickness, and Reynolds number. The half of the vortex wavelength is equal to the boundary layer thickness just before reattachment. The length scale required for breakdown of Gortler vortices decreases with rising ramp angle and is analogous to peak heating length. The streamwise heat flux enhancement occurs during the tenure of Gortler vortices and the enhancement rises with ramp angle. Although the visibility of Gortler vortices through temperature variation is distinct, the spanwise heat flux variation is not too high. Moreover, the spanwise heat flux variation rises marginally with ramp angle.

Observations of Misovortices within a Long-Lake-Axis-Parallel Lake-Effect Snowband during the OWLeS Project

AbstractRecent lake-effect snow field projects in the eastern Great Lakes region have revealed the presence of misovortices with diameters between 40 and 4000 m along cyclonic horizontal shear zones within long-lake-axis-parallel bands. One particular band in which an abundance of misovortices developed occurred on 7 January 2014. The leading hypothesis for lake-effect misovortexgenesis is the release of horizontal shearing instability (HSI). An analysis of three-dimensional dual-Doppler wind syntheses reveals that two criteria for HSI are satisfied along the horizontal shear zone, strongly suggesting that HSI was the likely cause of the misovortices in this case. Furthermore, the general lack of anticyclonic–cyclonic vortex couplets throughout the event reveal that tilting of horizontal vorticity into the vertical is of less importance compared to the release of HSI and subsequent strengthening via vortex stretching. A WRF simulation depicts misovortices along the horizontal shear zone within the simulated band. The simulated vortices display remarkable similarities to the observed vortices in terms of intensity, depth, spacing, and size. The simulated vortices persist over the eastern end of the lake; however, once the vortices move inland, they quickly dissipate. HSI criteria are also calculated from the WRF simulation and are satisfied along the shear zone. Competing hypotheses of misovortexgenesis are presented, with results indicating that the release of HSI is the likely mechanism of vortex formation.

The Relationship between Polar Vortex and Ozone Depletion in the Antarctic Stratosphere during the Period 1979–2016

As the most prominent feature of the polar stratosphere, polar vortex results in widespread changes in the climate system, especially in the ozone variation. In this study, the linkage between polar vortex and ozone depletion in Antarctic stratosphere during the period 1979–2016 is investigated; we calculated the averaged total column ozone within the polar vortex based on the vortex edge (−28.8 PVU PV contour) instead of the geographical region defined by latitude and longitude. Results from the spatial patterns of ozone and polar vortex suggest that the morphological changes of polar vortex can impact the horizontal distribution of ozone and the ozone within the polar vortex experiences a severe depletion in spring. The negative relationship between ozone and polar vortex in terms of vortex area, strength, and breakup time is significant with the correlation coefficients of −0.57, −0.68, and −0.76, respectively. The breakup time of polar vortex plays an important role in the relation between polar vortex and ozone depletion with the highest-value correlation coefficient among three polar vortex parameters. Furthermore, the possible mechanism for this relationship is also discussed in this article.

A planetary wave model for Saturn’s 10.7-h periodicities

A proposed resolution of the unexplained 10.7-h periodicities in Saturn’s magnetosphere is a system of atmospheric vortices in the polar regions of the planet. We investigate a description of such vortices in terms of planetary-scale waves. Approximating the polar regions as flat, we use theory developed originally by Haurwitz (Haurwitz, B. [1975]. Geophys. Bioklimatol. 24, 1–18) to find circumpolar Rossby wave solutions for Saturn’s upper stratosphere and lower thermosphere. We find vertically propagating twin vortex solutions that drift slowly westwards at <1% of the deep planetary angular velocity and are thus ideal candidates for explaining the observed periodicities. To produce integrated field-aligned currents of the order of 1MA we require wind velocities of ∼70ms-1. A particular class of vertically propagating solutions are potentially consistent with wave energy being ‘trapped’ between the deep atmosphere and lower thermosphere, at altitudes suited to the production of the necessary field-aligned current systems.

Impurity Effects on Bound States in Vortex Core of Topological s-Wave Superconductor

We study the impurity effects on the Caroli-de Gennes-Matricon (CdGM) states, particularly on the level spacings in a vortex core in topological s-wave superconductor (SC) by two means, numerically and analytically. The topological s-wave SC belongs to the same class as a chiral p-wave SC and thus there are two inequivalent vortices in terms of any symmetry operation. We take into account this inequivalence and numerically calculate the scattering rates based on an improved version of Kopnin-Kravtsov (iKK) scheme, which enables us to treat the discrete levels in the presence of white-noise disorder. We also construct the Andreev equation for the topological s-wave SC and obtain the Andreev bound states analytically. We use a correspondence between the wave functions for the Bogoliubov-de Gennes equation and the Andreev equation in the iKK scheme and deduce the formula of scattering rates described by the wave function for the Andreev equation. With this formula, we discuss the origin of impurity scattering rates for CdGM states of topological s-wave SC and the dependence on the types of vortices related to the inequivalence.

Vortex Preconditioning due to Planetary and Gravity Waves prior to Sudden Stratospheric Warmings

Abstract Reanalysis data are used to evaluate the evolution of polar vortex geometry, planetary wave drag, and gravity wave drag prior to split versus displacement sudden stratospheric warmings (SSWs). A composite analysis that extends upward to the lower mesosphere reveals that split SSWs are characterized by a transition from a wide, funnel-shaped vortex that is anomalously strong to a vortex that is constrained about the pole and has little vertical tilt. In contrast, displacement SSWs are characterized by a wide, funnel-shaped vortex that is anomalously weak throughout the prewarming period. Moreover, during split SSWs, gravity wave drag is enhanced in the polar night jet, while planetary wave drag is enhanced within the extratropical surf zone. During displacement SSWs, gravity wave drag is anomalously weak throughout the extratropical stratosphere. Using the composite analysis as a guide, a case study of the 2009 SSW is conducted in order to evaluate the roles of planetary and gravity waves for preconditioning the polar vortex in terms of two SSW-triggering scenarios: anomalous planetary wave forcing from the troposphere and resonance due to either internal or external Rossby waves. The results support the view that split SSWs are caused by resonance rather than anomalously large wave forcing. Given these findings, it is suggested that vortex preconditioning, which is traditionally defined in terms of vortex geometries that increase poleward wave focusing, may be better described by wave events (planetary and/or gravity) that “tune” the geometry of the vortex toward its resonant excitation points.

Higher order optical vortices and formation of speckles.

We have experimentally generated higher order optical vortices and scattered them through a ground glass plate that results in speckle formation. Intensity autocorrelation measurements of speckles show that their size decreases with an increase in the order of the vortex. It implies an increase in the angular diameter of the vortices with their order. The characterization of vortices in terms of their annular bright ring also helps us to understand these observations. The results may find applications in stellar intensity interferometry and thermal ghost imaging.

Stochastic resonance of vortices in a washboard pinning potential

In a bistable potential at low temperatures, stochastic resonance can be characterized as a synchronization effect of the hopping mechanism induced by an external periodic stimulus, where synchronization attains a maximum by fine-tuning the forcing frequency close to the relevant switching rate. In this work, we theoretically investigate the nonlinear single-vortex dynamics in a tilted cosine (multistable) washboard pinning potential at nonzero temperature in the presence of dc and ac currents of arbitrary amplitudes and frequency. The conditions for stochastic resonance to appear are derived on the basis of the exact solution of the corresponding Langevin equation for non-interacting vortices in terms of a matrix continued fraction. The nonlinear ac voltage response is analyzed as a function of temperature, dc bias, ac amplitude and frequency, with particular focus on the amplification of the external harmonic signal and its conversion to the third harmonics of the input frequency.

Assimilating Airborne Doppler Radar Observations with an Ensemble Kalman Filter for Convection-Permitting Hurricane Initialization and Prediction: Katrina (2005)

Abstract Through a Weather Research and Forecasting model (WRF)-based ensemble Kalman filter (EnKF) data assimilation system, the impact of assimilating airborne radar observations for the convection-permitting analysis and prediction of Hurricane Katrina (2005) is examined in this study. A forecast initialized from EnKF analyses of airborne radar observations had substantially smaller hurricane track forecast errors than NOAA’s operational forecasts and a control forecast initialized from NCEP analysis data for lead times up to 120 h. Verifications against independent in situ and remotely sensed observations show that EnKF analyses successfully depict the inner-core structure of the hurricane vortex in terms of both dynamic (wind) and thermodynamic (temperature and moisture) fields. In addition to the improved analyses and deterministic forecast, an ensemble of forecasts initiated from the EnKF analyses also provided forecast uncertainty estimates for the hurricane track and intensity. Also documented here are the details of a series of data thinning and quality control procedures that were developed to generate superobservations from large volumes of airborne radial velocity measurements. These procedures have since been implemented operationally on the NOAA hurricane reconnaissance aircraft that allows for more efficient real-time transmission of airborne radar observations to the ground.

Phase Vortex: A Dynamical System Approach

In this paper we study the flow lines defined by integral curves of the current field Im ψ* ∇ ψ associated to a complex scalar field ψ near a general phase vortex. This study naturally leads to the center–focus problem. Sufficient conditions for spiral behavior of the flow lines near a vortex in terms of the Lyapunov numbers are given.

Magnetic vortex dynamics induced by spin-transfer torque

We theoretically study the dynamics of a magnetic vortex under spin-polarized electric current in ferromagnets. The equation of motion of the vortex in terms of collective coordinates is derived. We compare our theory with recent experiments for current-induced vortex displacement and resonance motion in a ferromagnetic nanodot. Our estimate for the displacement and the resonance frequency shows a good agreement with the experiment. We also study the current-induced motion of a vortex wall in a ferromagnetic thin wire.

THE PHYSICS OF TROPICAL CYCLONE MOTION

▪ Abstract This article reviews our current understanding of the physical mechanisms governing the movement of a tropical cyclone. In a barotropic framework, a tropical cyclone is basically “steered” by the surrounding flow but its movement is modified by the Coriolis force (referred to as the beta effect) and the horizontal vorticity gradient of the surrounding flow. In the presence of vertical wind shear and latent heat release, a tropical cyclone tends to move toward an area with a maximum in the time tendency of potential vorticity, which is mainly contributed by two processes: (a) advection that depends on the structures of the vortex and the environment surrounding the vortex in terms of their flow speed and vorticity gradient (including the beta effect), and (b) heating that results from a coupling between the latent heat released in the clouds and the vertical wind shear.

Coupling of noncanonical optical vortices

When canonical vortices with different topological charges coexist in an optical beam, they inevitably become noncanonical during propagation. The noncanonical nature of optical vortices can be expressed in terms of morphological parameters, which can be represented in terms of a spinor, similar to the Jones vectors for polarization. This allows one to express the prefactor for a Gaussian beam containing two arbitrary vortices in terms of two terms. One is a product of two linearly propagating vortices. The other one, which is called the coupling term, represents the interaction between the vortices. The coupling term is independent of the vortex positions but depends on their initial morphological parameters. There are two situations where the interaction is zero. One is when the vortices are canonical with the same topological charge, and the other is when the vortices have the same anisotropy but with orthogonal orientations. The former situation is well known. The noninteraction of the latter situation is confirmed by a numerical simulation.

The mean characteristics of horseshoe vortex at a cylindrical pier

The primary horseshoe vortex formed in front of a cylindrical pier and believed to be the prime agent responsible for scour has been investigated experimentally during the entire process of scouring. A simple and effective method was employed to obtain the time-averaged characteristics of the vortex in terms of parameters relating variables of flow, pier and the channel bed. An expression for the maximum equilibrium scour depth has also been developed from the vortex velocity distribution inside scour hole. The resulting scour prediction equation has been found to give better results compared to the results of well-known predictor models when applied to model scour data.