Vortex Asymmetry Research Articles

An asymmetric optical vortex generated by a spiral refractive plate

Using a revised Kirchhoff diffraction integral and the finite-difference time-domain method we show that an optical vortex generated by a refractive spiral plate with a relief step has an asymmetric profile. The annular diffraction pattern in the optical vortex beam cross-section is found to be disturbed not only for near-field diffraction but also for the Fresnel zone. For a spiral phase plate with topological charge 3, fabricated on a resist, optical vortex asymmetry has been shown experimentally by near-field scanning optical microscopy.

Bifurcating flows of plunging aerofoils at high Strouhal numbers

AbstractForce and particle image velocimetry measurements were conducted on a NACA 0012 aerofoil undergoing small-amplitude high-frequency plunging oscillation at low Reynolds numbers and angles of attack in the range 0–$2{0}^{\ensuremath{\circ} } $. For angles of attack less than or equal to the stall angle, at high Strouhal numbers, significant bifurcations are observed in the time-averaged lift coefficient resulting in two lift-coefficient branches. The upper branch is associated with an upwards deflected jet, and the lower branch is associated with a downwards deflected jet. These branches are stable and highly repeatable, and are achieved by increasing or decreasing the frequency in the experiments. Increasing frequency refers to starting from stationary and increasing the frequency very slowly (while waiting for the flow to reach an asymptotic state after each change in frequency); decreasing frequency refers to impulsively starting at the maximum frequency and decreasing the frequency very slowly. For the latter case, angle of attack, starting position and initial acceleration rate are also parameters in determining which branch is selected. The bifurcation behaviour is closely related to the properties of the trailing-edge vortices. The bifurcation was therefore not observed for very small plunge amplitudes or frequencies due to insufficient trailing-edge vortex strength, nor at larger angles of attack due to greater asymmetry in the strength of the trailing-edge vortices, which creates a preference for a downward deflected jet. Vortex strength and asymmetry parameters are derived from the circulation measurements. It is shown that the most appropriate strength parameter in determining the onset of deflected jets is the circulation normalized by the plunge velocity.

Magnus Effect: Physical Origins and Numerical Prediction

An overview of the Magnus effect of projectiles and missiles is presented. The first part of the paper is devoted to the description of the physical mechanisms governing the Magnus effect. For yawing and spinning projectiles, at small incidences, the spin induces a weak asymmetry of the boundary layer profiles. At high incidences, increased spin causes the separated vortex sheets to be altered. Vortex asymmetry generates an additional lateral force which gives a vortex contribution to the total Magnus effect. For finned projectiles or missiles, the origin of the Magnus effect on fins is the main issue. There are two principal sources contributing to the Magnus effect. Firstly, the interaction between the asymmetric boundary layer-wake of the body and the fins, and secondly, the spin induced modifications of the local incidences and of the flow topology around the fins. The second part of the paper is devoted to the numerical prediction and validation of these flow phenomena. A state of the art is presented including classical CFD methods based on Reynolds-averaged Navier–Stokes (RANS) and unsteady rans (URANS) equations, and also hybrid RANS/LES approach called ZDES. This last method is a recent advance in turbulence modeling methodologies that allows to take into account the unsteadiness of the flow in the base region. For validation purposes computational results were compared with wind tunnel tests. A wide range of angles of attack, spin rates, Reynolds and Mach numbers (subsonic, transonic and supersonic) have been investigated.

Modeling and simulation of multiple bubble entrainment and interactions with two dimensional vortical flows

Simulations of bubble entrainment and interactions with two dimensional vortical flows are preformed using a discrete element model. In this Eulerian-Lagrangian approach, solution to the carrier phase is obtained using direct numerical simulation whereas motion of subgrid bubbles is modeled using Lagrangian tracking. The volumetric displacement of the fluid by the finite size of the bubbles is modeled along with interphase momentum-exchange for a realistic coupling of the bubbles to the carrier phase. In order to assess the importance of this volumetric coupling effect, even at low overall volume loading, simulations of a small number of microbubbles entrained in a traveling vortex tube are studied in detail. The test case resembles the experiments conducted by Sridhar and Katz [JFM, 1999] on bubble entrainment in vortex rings. It is shown that under some conditions, the entrainment of eight small bubbles, 1100 μm or less in diameter, result in significant levels of vortex distortion when modeled using the volumetric coupling effect. Neglecting these effects, however, does not result in any vortex distortion due to entrained bubbles. The nondimensionalized vortex strength versus bubble settling locations are compared with experimental data to show collapse of the data along the trends observed in experiments only when the volumetric effects are modeled. Qualitative and quantitative assessments of this distortion observed with volumetric coupling are made using three methods; bubble induced vortex asymmetry, relative change in the decay of angular momentum, and relative change in the peak vorticity. It is found that in all cases the volumetric effects result in a relative increase of the vortex decay rate. The concept of a relative reaction force, defined as the ratio of net bubble to fluid reaction to the local driving force of the vortex, is introduced to analyze this effect. It is shown that the global increases in vortex decay rate are directly proportional to the magnitude of this highly local relative reaction force.

Design of an Optimal Wing-Body Configuration to Delay Onset of Vortex Asymmetry

O NE important phenomenon associated with high-angle-ofattack aerodynamics is the sudden onset of vortex asymmetry on the forebody of an air vehicle in symmetric flight. One of the first observations of the onset of vortex asymmetry was reported in 1951 by Allen and Perkins [1]. Interest in the phenomenon has been intensified since the late 1970s as concepts for highly maneuverable aircraft have been developed. These high-performance aircraft are expected to operate routinely at angles of attack at which vortex asymmetry is likely to occur. When vortex asymmetry occurs, the aerodynamic, stability, and control characteristics of the vehicle change dramatically. In themeantime, the conventional aerodynamic controls become ineffective due to the vortex wakes generated by the forebody. The subject was reviewed by Hunt [2], Ericsson and Reding [3], and Champigny [4]. High-angle-of-attack flow control is most effective when applied in the region close to the tip of the forebody. The presence of two closely spaced vortices around the pointed forebody at high angles of attack enhances the effectiveness. Compared with that on wings, control on the forebody is required over a much smaller area; thus, physical requirements such as size and weight should be much smaller. The lengthy forebody of a modern fighter further enhances the control effectiveness by providing a long moment arm. Excellent reviews of this activity can be found in papers by Malcolm [5,6] and Williams [7]. Recently, Liu et al. [8] successfully demonstrated linear proportional control of the side forces over a slender circular cone using duty-cycled single-dielectric-barrier-discharge plasma actuators. One of the devices to suppress the flow asymmetry are the horizontal nose strakes. Coe et al. [9] showed, by wind-tunnel tests, the alleviating effects of horizontal and symmetrical strakes placed close to the apex of a slender ogive forebody and a slender circular cone. It is observed that the effect of the strakes is to produce a welldefined point of separation at the leading edges of the strakes, which results in a symmetrical flowfield over a wide range of high angles of attack. Two sharp side edges on an otherwise smooth body can also serve to define the point of separation. Siclari [10] used a Navier– Stokes solver to study the natural occurrence of separated flows over biparabolic and biwedge cones at a freestream Mach number of 1.8 and an angle of attack of 20 . As the thickness ratiowas increased, the originally symmetric vortex pair separated from the sharp side edges, becoming asymmetric at thickness ratios of 0.5 and 0.6 for the biparabolic and biwedge cones, respectively. Cai et al. developed a stability analysis method [11] and used it to study high angle-of-attack flow about slender conical wing and wing–body combinations [12,13]. The analytic method is based on an eigenvalue analysis on the motion of the vortices under small temporal perturbations. The theoretical results agree well with available experimental observations and are corroborated by numerical computations [14]. The aim of the present paper is to manipulate the aerodynamic configuration for the purpose of keeping the stationary symmetric vortex pair stable at high angles of attack. In the following sections, the analyticmethod in [11] is summarized and a numerical conformal mapping technique is described. A series of configurations are analyzed for the stability of the symmetric vortex pair. A most effective configuration of thewing–body combination is obtained by the analytic method and validated by an Euler solver. In the last section, conclusions are drawn.

ENSO influences on Southern Hemisphere column ozone during the winter to spring transition

This study investigates how the El Nino Southern Oscillation (ENSO) modulates the distribution of column ozone during the Southern Hemisphere (SH) winter to spring transition, when a zonal asymmetry in column ozone develops near 55°S, with a maximum south of Australia. ENSO differences in column ozone and planetary wave structure are explored for the average of seven El Nino (EN) versus six La Nina (LN) events during 1982–2004. A westward shift in convection during LN corresponds to a 30–50° westward shift of SH planetary wave and column ozone patterns. The extratropical lower stratospheric temperature anomalies are inversely correlated with tropical anomalies in similar longitude bands and are highly correlated with local column ozone anomalies. The tropical signal is communicated to higher latitudes by modulating subtropical anticyclones, with distinctive responses for EN and LN years for each month. During August and EN a subtropical anticyclone extends from South America to South Africa, and the wave one asymmetry of the Antarctic vortex is more pronounced and shifted eastward, along with column ozone. In September and EN a strong anticyclone exists over South Africa, with a corresponding cold, ozone‐poor anomaly in the stratosphere, diminishing the western edge of the ozone maximum. In October and LN a strong anticyclone exists over Australia, with an amplified trough on its poleward flank. This trough is overlain by an ozone‐rich warm anomaly, which expands the western edge of the ozone maximum, and the polar vortex asymmetry is amplified and rotated westward.

Computation of Unsteady Low Reynolds Number Free-Flight Aerodynamics of Flapping Wings

The observation that thrust is produced when a wing flaps above a particular reduced frequency at low Reynolds numbers has been associated with the formation of a reverse Karman vortex street. Through experiments, numerical computations, and analytical methods, scientists have been able to explain thrust generation in birds and insects. This has increased the interest in the development of micro aerial vehicles. There is, however, a paucity of experimental or computational studies that explain the basic mechanism of thrust generation from the viewpoint of the surrounding vortical structures and the effect of these vortices on free flight when the wing is accelerating in the direction of thrust. In the present work, three-dimensional free-flight computations are carried out on flapping wings that have rectangular planforms composed of NACA 0012 and an elliptical airfoil section, respectively, to demonstrate that thrust is generated as a result of vortex asymmetry. Based on the computed vortex dynamics, it is also shown that the relative convection rates of positive and negative vorticity are a key parameter involved in thrust generation. Numerical computations are performed using the unsteady three-dimensional Navier―Stokes solver composite grid incompressible Navier―Stokes, which is based on the Overture framework for overlapping grids. Comparative analysis with existing experimental/computational data is also presented.

Experiments of Hurricane Initialization with Airborne Doppler Radar Data for the Advanced Research Hurricane WRF (AHW) Model

Abstract Initialization of the hurricane vortex in weather prediction models is vital to intensity forecasts out to at least 48 h. Airborne Doppler radar (ADR) data have sufficiently high horizontal and vertical resolution to resolve the hurricane vortex and its imbedded structures but have not been extensively used in hurricane initialization. Using the Weather Research and Forecasting (WRF) three-dimensional variational data assimilation (3DVAR) system, the ADR data are assimilated to recover the hurricane vortex dynamic and thermodynamic structures at the WRF model initial time. The impact of the ADR data on three hurricanes, Jeanne (2004), Katrina (2005) and Rita (2005), are examined during their rapid intensification and subsequent weakening periods before landfall. With the ADR wind data assimilated, the three-dimensional winds in the hurricane vortex become stronger and the maximum 10-m winds agree better with independent estimates from best-track data than without ADR data assimilation. Through the multivariate incremental structure in WRF 3DVAR analysis, the central sea level pressures (CSLPs) for the three hurricanes are lower in response to the stronger vortex at initialization. The size and inner-core structure of each vortex are adjusted closer to observations of these attributes. Addition of reflectivity data in assimilation produces cloud water and rainwater analyses in the initial vortex. The temperature and moisture are also better represented in the hurricane initialization. Forty-eight-hour forecasts are conducted to evaluate the impact of ADR data using the Advanced Research Hurricane WRF (AHW), a derivative of the Advanced Research WRF (ARW) model. Assimilation of ADR data improves the hurricane-intensity forecasts. Vortex asymmetries, size, and rainbands are also simulated better. Hurricane initialization with ADR data is quite promising toward reducing intensity forecast errors at modest computational expense.

Nose Blowing for Side Force Control on Slender Cones at High Incidence

Vortex asymmetry on axisymmetric pointed forebodies at high angles of incidence results in side forces and adverse yawing moments even in symmetric flight. An experimental investigation has been carried out at low speeds to study the effectiveness of axial nose blowing against the oncoming flow for side force control on two slender cones. Tests were made on the 8 and 12 deg cone models with (circular)jet flow over a Reynolds number range of $0.4\times10^6$ to $5.4\times10^6$. Jet-to-freestream velocity ratio was varied up to 2.0, and the blowing diameter was varied as well. Force measurements were carried over the incidence range of 0 to 45 deg, and limited surface pressure distributions were obtained on the 12 deg cone model. The results explicitly show the effectiveness of nose blowing for side force control over a wide range of Reynolds numbers, and minimum side force was reached around a jet velocity ratio of 1.0. An important result is that the side force reduction correlates with jet velocity ratio in the range of parameters studied. Broad similarities in the trend of side force reduction with the nose blowing and the nose bluntness suggest that the interaction of axial jet with freestream flow results, possibly, in what we may call fluid dynamic blunting.

Hurricane Vortex Dynamics during Atlantic Extratropical Transition

AbstractSimulations of six Atlantic hurricanes are diagnosed to understand the behavior of realistic vortices in varying environments during the process of extratropical transition (ET). The simulations were performed in real time using the Advanced Research Weather Research and Forecasting (WRF) model (ARW), using a moving, storm-centered nest of either 4- or 1.33-km grid spacing. The six simulations, ranging from 45 to 96 h in length, provide realistic evolution of asymmetric precipitation structures, implying control by the synoptic scale, primarily through the vertical wind shear.The authors find that, as expected, the magnitude of the vortex tilt increases with increasing shear, but it is not until the shear approaches 20 m s−1 that the total vortex circulation decreases. Furthermore, the total vertical mass flux is proportional to the shear for shears less than about 20–25 m s−1, and therefore maximizes, not in the tropical phase, but rather during ET. This has important implications for predicting hurricane-induced perturbations of the midlatitude jet and its consequences on downstream predictability.Hurricane vortices in the sample resist shear by either adjusting their vertical structure through precession (Helene 2006), forming an entirely new center (Irene 2005), or rapidly developing into a baroclinic cyclone in the presence of a favorable upper-tropospheric disturbance (Maria 2005). Vortex resiliency is found to have a substantial diabatic contribution whereby vertical tilt is reduced through reduction of the primary vortex asymmetry induced by the shear. If the shear and tilt are so large that upshear subsidence overwhelms the symmetric vertical circulation of the hurricane, latent heating and precipitation will occur to the left of the tilt vector and slow precession. Such was apparent during Wilma (2005).

Effects of Forced Asymmetric Transition on Vortex Asymmetry Around Slender Bodies

A wind-tunnel experiment was conducted to study the effects of forced asymmetric transition on the asymmetric vortex system of a slender body by adding a grit strip on either side of the body. The results showed that the effect is very different depending on whether transition of the boundary layer occurs first on the higher vortex side or on the lower vortex side. If the transition first occurs on the higher vortex side, the transition will strongly influence the asymmetric vortex behavior and the side forces induced. The asymmetric vortex system will abruptly reverse its orientation when transition occurs and then gradually recover to its original position with increasing Reynolds number. However, if the transition first occurs on the lower vortex side, the trend of the asymmetric vortex system with increasing Reynolds number will be very similar to the case of natural transition. In addition, the asymmetric transition of the boundary layers on the forebody seems to have a dominant effect on the behavior of the asymmetric vortex system. The associated separation angles of the boundary layers appear to play a pronounced role in the development of the asymmetric vortex system.

Nose Bluntness for Side-Force Control on Circular Cones at High Incidence

An experimental investigation has been car ked out to study the effect of nose bluntness on the characteristics of vortex asymmetry and induced side forces on slender cone models at low speeds. Two cone models with semi-apex angles of 8 and 12 deg were utilized, and on each model nose bluntness was varied up to 20% (based on base diameter).Measurements made consisted of six-component balance loads over a Widerange of Reynolds number from 0.4 x 10(6) to 5.4 x 10(6). The results show that the onset of vortex asymmetry with nose blunting correlates with geometrical parameters and is practically independent of Reynolds number. The (maximum) side-force levels decrease initially up to a certain bluntness ratio, then increase, followed by a gradual decrease for large values of bluntness ratios. This decrease or increase inside-force levels with nose blunting is associated with a corresponding increase or decrease, respectively, in the onset of vortex asymmetry.Whereas the increase in side-force level beyond the first minimum is qualitatively similar to that observed in earlier studies, the present data definitely indicate that there is a progressive decrease inside-force levels to relatively low values for large bluntness ratios,which could form an important design input. The bluntness ratio corresponding to the occurrence of the first minimum in (maximum) side force depends on the cone angle and is nearly independent of Reynolds number.

Observed Cyclone–Anticyclone Tropopause Vortex Asymmetries

Abstract Relatively little is known about coherent vortices near the extratropical tropopause, even with regard to basic facts about their frequency of occurrence, longevity, and structure. This study addresses these issues through an objective census of observed tropopause vortices. The authors test a hypothesis regarding vortex-merger asymmetry where cyclone pairs are repelled and anticyclone pairs are attracted by divergent flow due to frontogenesis. Emphasis is placed on arctic vortices, where jet stream influences are weaker, in order to facilitate comparisons with earlier idealized numerical simulations. Results show that arctic cyclones are more numerous, persistent, and stronger than arctic anticyclones. An average of 15 cyclonic vortices and 11 anticyclonic vortices are observed per month, with maximum frequency of occurrence for cyclones (anticyclones) during winter (summer). There are are about 47% more cyclones than anticyclones that survive at least 4 days, and for longer lifetimes, 1-day survival probabilities are nearly constant at 65% for cyclones, and 55% for anticyclones. Mean tropopause potential-temperature amplitude is 13 K for cyclones and 11 K for anticyclones, with cyclones exhibiting a greater tail toward larger values. An analysis of close-proximity vortex pairs reveals divergence between cyclones and convergence between anticyclones. This result agrees qualitatively with previous idealized numerical simulations, although it is unclear to what extent the divergent circulations regulate vortex asymmetries.

Airflow pressure and shear forces on a rotating, deformed disk in an open shroud

The static pressure and surface shear forces on a rotating, umbrella-deformed disk in an open shroud are investigated numerically to develop a proper dynamical model for investigation of the self-excited vibration of the disk. The fluid force distributions along the radial direction of the rotating disk are obtained by use of a two-dimensional numerical model of the axisymmetric deformation of disk. It is observed that the effects of Reynolds number and deformation amplitude are very significant on the fluid forces. Once the rotating disk is deformed, the vortex asymmetry in the airflow patterns appears in the narrow and wide gaps between the rotating disk and the two fixed walls. With increasing deformation amplitude, the asymmetry becomes more obvious. The vortex becomes strong as Reynolds number increases. In addition, it is observed that the transition flow between the laminar and turbulent flows exists in the narrow-gap side. Such a transition flow causes the fluid forces to be non-smooth in the corresponding region, and the force discontinuity will lead to the complicated vibration of rotating disk. From this investigation, the fluid forces for different modes of the rotating disk should be further investigated analytically and numerically for a better understanding of the mechanism of self-excited vibration in hard-disk drive system.

Effects of vertical differencing in a minimal hurricane model

AbstractThe minimal three‐layer hurricane model developed by the authors is reformulated with a Charney–Phillips grid (CP‐grid) in the vertical instead of the Lorenz grid (L‐grid) that was used in the original model. It has been shown by others that the popular L‐grid supports a computational mode in the temperature field that can lead to inaccuracies, especially when moist processes are involved. Here we compare calculations of the hurricane model using the L‐grid and CP‐grid. We present evidence that the computational mode is excited in the L‐grid model during the period of rapid vortex intensification, which begins when grid‐scale latent‐heat release occurs in the core region. Thus it would appear that the solution in the mature stage of evolution is contaminated by this mode. We show also that the vortex asymmetries that develop during the mature stage are sensitive to the choice of vertical grid and argue that those that occur in the CP‐grid formulation are more realistic. It is possible that the computational mode is a spurious feature of many earlier studies of asymmetries in hurricane models in which moist processes are represented. Copyright © 2003 Royal Meteorological Society

Microblowing for High-Angle-of-Attack Vortex Flow Control on a Fighter Aircraft

Effectiveness of the microblowing technique for controlling forebody vortex asymmetry at high angles of attack has been investigated in low-speed wind-tunnel experiments with the F-15E fighter aircraft configuration. One blowing-port installation, based on previous experiments with generic forebody shapes, was evaluated. At very high angles of attack (a > 50 deg), microblowing was effective in generating and controlling yawing moment levels comparable to those demonstrated by high-pressure jet nozzle blowing, while requiring only 1/100 as much mass flow, demonstrating the leveraging available through controlling the vortex flowfield at the point of instability. The microblowing port configuration employed was unable to generate usable yawing moments at moderate angles of attack (α≤35 deg). Control-port optimization promises to improve microblowing effectiveness in the intermediate α range

Role of Tip and Edge Geometry on Vortex Asymmetry

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Centrifugal-dissipative instability of Rossby vortices and their cyclonic-anticyclonic asymmetry

A new linear centrifugal-dissipative mechanism is proposed that explains the vortex asymmetry observed, in particular, in the structure of low-frequency anticyclonic Rossby vortices. It is shown that the relevant centrifugal-dissipative instability, which spontaneously breaks the chiral symmetry of the vortices, takes place only in the range ω<Ω, where ω is the frequency of small oscillations corresponding to the effective solid-body rotation of a vortex and Ω is the rotation rate of a noninertial frame of reference. The onset of the instability is associated with the existence of an optimum magnitude of the frictional force. In the vortex model based on a two-dimensional oscillator with the natural frequency ω in a noninertial reference frame rotating at the rate Ω, the instability shows up as an exponential increase in the total angular momentum. It is noted that the centrifugal dissipative instability may also manifest itself in the seismically active regions of the world.

Role of tip and edge geometry on vortex asymmetry

Role of tip and edge geometry on vortex asymmetry

The BMRC High-Resolution Tropical Cyclone Prediction System: TC-LAPS

Abstract A new Tropical Cyclone Limited Area Prediction System has been developed at the Australian Bureau of Meteorology Research Centre. The features of the new prediction system can be summarized as follows: first, a 12-h, coarse-resolution data assimilation is used to define the outer structure and environment of the storm and to provide initial conditions for coarse mesh prediction. Then, synthetic data are generated to define a storm’s circulation, consistent with observed location, size, intensity, and past motion. The method involves a definition of the environmental flow by filtering of the misplaced tropical cyclone circulation in the (old) objective analysis, the generation of a new, correctly located and intense symmetric vortex, and the construction of vortex asymmetries by requiring that the observed motion be the vector sum of the environmental flow and the asymmetric flow. The subsequent initialization for fine-mesh prediction is carried out using 24 h of diabatic, dynamical nudging throug...