Littérature scientifique sur le sujet « Vortice polare »

Abstract Tropopause polar vortices are coherent circulation features based on the tropopause in polar regions. They are a common feature of the Arctic, with typical radii less than 1500 km and lifetimes that may exceed 1 month. The Arctic is a particularly favorable region for these features due to isolation from the horizontal wind shear associated with the midlatitude jet stream, which may destroy the vortical circulation. Intensification of cyclonic tropopause polar vortices is examined here using an Ertel potential vorticity framework to test the hypothesis that there is an average tendency for diabatic effects to intensify the vortices due to enhanced upper-tropospheric radiative cooling within the vortices. Data for the analysis are derived from numerical simulations of a large sample of observed cyclonic vortices over the Canadian Arctic. Results show that there is on average a net tendency to create potential vorticity in the vortex, and hence intensify cyclones, and that the tendency is radiatively driven. While the effects of latent heating are considerable, they are smaller in magnitude, and all other diabatic processes have a negligible effect on vortex intensity.

Abstract Polar vortices are common planetary-scale flows that encircle the pole in the middle or high latitudes and are observed in most of the solar system’s planetary atmospheres. The polar vortices on Earth, Mars, and Titan are dynamically related to the mean meridional circulation and exhibit a significant seasonal cycle. However, the polar vortex’s characteristics vary between the three planets. To understand the mechanisms that influence the polar vortex’s dynamics and dependence on planetary parameters, we use an idealized general circulation model with a seasonal cycle in which we vary the obliquity, rotation rate, and orbital period. We find that there are distinct regimes for the polar vortex seasonal cycle across the parameter space. Some regimes have similarities to the observed polar vortices, including a weakening of the polar vortex during midwinter at slow rotation rates, similar to Titan’s polar vortex. Other regimes found within the parameter space have no counterpart in the solar system. In addition, we show that for a significant fraction of the parameter space, the vortex’s potential vorticity latitudinal structure is annular, similar to the observed structure of the polar vortices on Mars and Titan. We also find a suppression of storm activity during midwinter that resembles the suppression observed on Mars and Earth, which occurs in simulations where the jet velocity is particularly strong. This wide variety of polar vortex dynamical regimes that shares similarities with observed polar vortices, suggests that among exoplanets there can be a wide variability of polar vortices.

Abstract Polar vortices are common planetary-scale flows that encircle the pole in the middle or high latitudes and are observed in most of the solar system’s planetary atmospheres. The polar vortices on Earth, Mars, and Titan are dynamically related to the mean meridional circulation and exhibit a significant seasonal cycle. However, the polar vortex’s characteristics vary between the three planets. To understand the mechanisms that influence the polar vortex’s dynamics and dependence on planetary parameters, we use an idealized general circulation model with a seasonal cycle in which we vary the obliquity, rotation rate, and orbital period. We find that there are distinct regimes for the polar vortex seasonal cycle across the parameter space. Some regimes have similarities to the observed polar vortices, including a weakening of the polar vortex during midwinter at slow rotation rates, similar to Titan’s polar vortex. Other regimes found within the parameter space have no counterpart in the solar system. In addition, we show that for a significant fraction of the parameter space, the vortex’s potential vorticity latitudinal structure is annular, similar to the observed structure of the polar vortices on Mars and Titan. We also find a suppression of storm activity during midwinter that resembles the suppression observed on Mars and Earth, which occurs in simulations where the jet velocity is particularly strong. This wide variety of polar vortex dynamical regimes that shares similarities with observed polar vortices, suggests that among exoplanets there can be a wide variability of polar vortices.

Abstract The term polar vortex has become part of the everyday vocabulary, but there is some confusion in the media, general public, and science community regarding what polar vortices are and how they are related to various weather events. Here, we clarify what is meant by polar vortices in the atmospheric science literature. It is important to recognize the existence of two separate planetary-scale circumpolar vortices: one in the stratosphere and the other in the troposphere. These vortices have different structures, seasonality, dynamics, and impacts on extreme weather. The tropospheric vortex is much larger than its stratospheric counterpart and exists year-round, whereas the stratospheric polar vortex forms in fall but disappears in the spring of each year. Both vortices can, in some circumstances, play a role in extreme weather events at the surface, such as cold-air outbreaks, but these events are not the consequence of either the existence or gross properties of these two vortices. Rather, cold-air outbreaks are most directly related to transient, localized displacements of the edge of the tropospheric polar vortex that may, in some circumstances, be related to the stratospheric polar vortex, but there is no known one-to-one connection between these phenomena.

Abstract Polar vortices are a prominent feature in Titan's stratosphere. The Cassini mission has provided a detailed view of the breakdown of the northern polar vortex and formation of the southern vortex, but the mission did not observe the full annual cycle of the evolution of the vortices. Here we use a TitanWRF general circulation model simulation of an entire Titan year to examine the full annual cycle of the polar vortices. The simulation reveals a winter weakening of the vortices, with a clear minimum in polar potential vorticity and midlatitude zonal winds between winter solstice and spring equinox. The simulation also produces the observed postfall equinox cooling followed by rapid warming in the upper stratosphere. This warming is due to strong descent and adiabatic heating, which also leads to the formation of an annular potential vorticity structure. The seasonal evolution of the polar vortices is very similar in the two hemispheres, with only small quantitative differences that are much smaller than the seasonal variations, which can be related to Titan's orbital eccentricity. This suggests that any differences between observations of the northern hemisphere vortex in late northern winter and the southern hemisphere vortex in early winter are likely due to the different observation times with respect to solstice, rather than fundamental differences in the polar vortices.

We present the results of an experimental investigation of the generation of coherent vortical structures by buoyant line plumes in rotating fluids. Both uniform and stratified ambients are considered. By combining the scalings describing turbulent plumes and geostrophically balanced vortices, we develop a simple model which predicts the scale of the coherent vortical structures in excellent accord with laboratory experiments.We examine the motion induced by a constant buoyancy flux per unit length B, released for a finite time ts, from a source of length L into a fluid rotating with angular speed Ω = f/2. When the plume discharges into a uniformly stratified environment characterized by a constant Brunt–Väisälä frequency, N>f, the fluid rises to its level of neutral buoyancy unaffected by the system rotation before intruding as a gravity current. Rotation has a strong impact on the subsequent dynamics: shear develops across the spreading neutral cloud which eventually goes unstable, breaking into a chain of anticyclonic lenticular vortices. The number of vortices n emerging from the instability of the neutral cloud, n = (0.65±0.1)Lf1/2/ (t1/2sB1/3), is independent of the ambient stratification, which serves only to prescribe the intrusion height and aspect ratio of the resulting vortex structures. The experiments indicate that the Prandtl ratio characterizing the geostrophic vortices is given by P = Nh/(fR) = 0.47±0.12; where h and R are, respectively, the half-height and radius of the vortices. The lenticular vortices may merge soon after formation, but are generally stable and persist until they are spun-down by viscous effects.When the fluid is homogeneous, the plume fluid rises until it impinges on a free surface. The nature of the flow depends critically on the relative magnitudes of the layer depth H and the rotational lengthscale Lf = B1/3/f. For H>10Lf, the ascent phase of the plume is influenced by the system rotation and the line plume breaks into a series of unstable anticylonic columns of characteristic radius (5.3±1.0)B1/3/f which typically interact and lose their coherence before surfacing. When H<10Lf, the system rotation does not influence the plume ascent, but does control the spreading of the gravity current at the free surface. In a manner analogous to that observed in the stratified ambient, shear develops across the surface current, which eventually becomes unstable and generates a series of anticyclonic surface eddies with characteristic radius (1.6±0.2)B1/3t1/3s /f2/3. These surface eddies are significantly more stable than their columnar counterparts, but less so than the lenticular eddies arising in the uniformly stratified ambient.The relevance of the study to the formation of coherent vortical structures by leads in the polar ocean and hydrothermal venting is discussed.

ABSTRACT The Cassini and Juno probes have revealed large coherent cyclonic vortices in the polar regions of Saturn and Jupiter, a dramatic contrast from the east–west banded jet structure seen at lower latitudes. Debate has centred on whether the jets are shallow, or extend to greater depths in the planetary envelope. Recent experiments and observations have demonstrated the relevance of deep convection models to a successful explanation of jet structure, and cyclonic coherent vortices away from the polar regions have been simulated recently including an additional stratified shallow layer. Here we present new convective models able to produce long-lived polar vortices. Using simulation parameters relevant for giant planet atmospheres we find flow regimes of geostrophic turbulence (GT) in agreement with rotating convection theory. The formation of large-scale coherent structures occurs via 3D upscale energy transfers. Our simulations generate polar characteristics qualitatively similar to those seen by Juno and Cassini: They match the structure of cyclonic vortices seen on Jupiter; or can account for the existence of a strong polar vortex extending downwards to lower latitudes with a marked spiral morphology, and the hexagonal pattern seen on Saturn. Our findings indicate that these vortices can be generated deep in the planetary interior. A transition differentiating these two polar flows regimes is described, interpreted in terms of force balances and compared with shallow atmospheric models characterizing polar vortex dynamics in giant planets. In addition, heat transport properties are investigated, confirming recent scaling laws obtained with reduced models of GT.

L’Indian Ocean Dipole (IOD) è un modo di variabilità dell’Oceano Indiano, caratterizzato dalla fluttuazione interannuale dell’intensità della componente zonale del gradiente di temperatura superficiale dell’oceano. Questo modo di variabilità è intimamente connesso con El Niño Southern Oscillation (ENSO) fenomeno noto per i suoi impatti globali sull’atmosfera e sul clima di superficie. IOD e ENSO generano teleconnessioni con effetti diversi in atmosfera. Alcune di queste possono raggiungere anche la stratosfera terrestre, alterando, all’inizio della stagione fredda, l’intensità del vortice polare stratosferico. Nel corso dell’autunno-inverno 2019/20 sono stati osservati sia uno stato eccezionale del vortice polare stratosferico, intenso e persistente fino a primavera inoltrata, sia una marcata fase positiva dell'Indian Ocean Dipole, mentre El Niño Southern Oscillation si trovava in una fase neutrale. Questa tesi analizza l'autunno-inverno 2019/20 nel contesto della climatologia del periodo 1979-2020. Partendo dalle osservazioni, in particolare dall’andamento nel corso della stagione invernale del flusso di onde a 100 hPa e della velocità del vento zonale a 60° N a 10 e 30 hPa, si formula un’ipotesi di lavoro per l’approfondimento della relazione tra presenza di un intenso evento di dipolo in autunno e stato del vortice polare stratosferico nell’inverno successivo. La conoscenza di questo legame, se confermato, aiuterebbe a migliorare la qualità delle previsioni stagionali e multiannuali anche dell’inverno europeo.

È noto che le precipitazioni invernali europee sono ben stimate nei dati di rianalisi e nelle simulazioni dai modelli dato che sono altamente correlate con modi con variabilità su larga scala e a bassa frequenza come l'oscillazione del Nord Atlantico (NAO) e l'oscillazione artica (AO). Dal momento che NAO e AO sono principalmente modi di variabilità invernale, la capacità di stimare le precipitazioni è più limitata nelle altre stagioni, soprattutto in estate, in cui (quando) le precipitazioni sono principalmente dovute alla convezione a mesoscala. La prima parte dello studio utilizza dati osservativi, dati di rianalisi e l'output del modello WRF (Weather Research and Forecast) per studiare i cambiamenti recenti negli eventi estremi di precipitazione giornaliera in Europa. Si è stato riscontrato che nelle stagioni estive e di transizione, sono più le regioni che hanno registrato un aumento di eventi di precipitazioni estreme rispetto a quelle che hanno registrato una tendenza al ribasso. Questo è coerente con le tendenze del riscaldamento globale con per via della relazione di Clausius-Clapeyron. è stato studiato anche il valore aggiunto dell'utilizzo di un modello ad alta risoluzione che consenta la convezione per stimare gli estremi delle precipitazioni. I risultati mostrano che WRF riesca a correggere il fallimento della rianalisi ERA-Interim per I risultati mostrano che WRF riesce, al contrario di ERA-Interim, a catturare le tendenze positive delle precipitazioni estreme europee sia in estate che nelle stagioni di transizione indicate dai dati osservativi (EOBS) e dalla letteratura precedente. D'altra parte, sono di più le aree in Europa che hanno registrato tendenze negative nelle precipitazioni estreme rispetto alle aree con tendenze positive. Questo si tratta di una conseguenza del recente trend positivo)della NAO negli ultimi decenni, che ha causato eventi NAO positivi più frequenti, riducendo le precipitazioni estreme in più regioni in Europa. Dato che sia la NAO che l'AO, che sono altamente correlate, stanno cambiando, sono state svolte ulteriori indagini sulla natura delle due oscillazioni. I dati di rianalisi e le simulazioni del modello climatico, (sia per run climatiche che rappresentative di condizioni a temperatura più elevata), dei climi storici e caldi sono stati utilizzati per mostrare che la relazione tra le due oscillazioni cambia con il riscaldamento climatico. Le due modalità sono attualmente altamente correlate, poiché entrambe sono fortemente influenzate dalla propagazione verso il basso delle anomalie del vortice polare stratosferico nella troposfera. Tuttavia, se si considera uno scenario climatico molto caldo, il modello AO definito emisfericamente si sposta per riflettere la variabilità della rotta delle tempeste del Pacifico settentrionale, mentre il modello NAO definito a livello regionale rimane stabile. La stratosfera rimane un importante precursore della NAO e le anomalie di pressione superficiale eurasiatica e aleutiana precedono le anomalie stratosferiche. Simulazioni con modelli idealizzati di circolazione generale suggeriscono che queste modifiche sono legate al riscaldamento più forte del Pacifico rispetto al riscaldamento più lento dell'Oceano Atlantico, dovuto al rallentamento della Overturning Meridional Circulation (AMOC).<br>European wintertime precipitation is known to be skilfully estimated in reanalysis data and model simulations since it is highly correlated with large scale, low frequency modes of variability, namely the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO). Since the NAO and AO are mainly a wintertime mode of variability, the skill of estimating precipitation becomes more limited in the other seasons, most importantly in the summer, in which precipitation is mainly a result of mesoscale convection. The first part of the study uses observational data, reanalysis data, and the output of Weather Research and Forecast (WRF) model to study the recent changes of extreme daily precipitation events over Europe. It is found that in summer and transition seasons, more regions recorded an increase of extreme precipitation events than regions that recorded a decreasing trend. This is consistent with the global warming trends with Clausius-Clapeyron relation. The added value of using a high resolution, convection-permitting model to estimate precipitation extremes is deduced. The results show that WRF succeeds to correct the failure of ERA-Interim reanalysis to capture the positive trends of European extreme precipitation in summer and transition seasons that are indicated by the observational data (EOBS) and previous literature. On the other hand, more regions in Europe recorded negative extreme precipitation trends than regions with positive trends. This is found to be a consequence of the recent positive trend of the NAO over the past decades, causing more frequent positive NAO events, reducing extreme precipitation outbreaks to more regions in Europe. As the NAO and the highly correlated AO are changing, further investigations to the nature of the two oscillations are carried out. Reanalysis data and climate model simulations of historical and warm climates are used to show that the relation between the two oscillations changes with climate warming. The two modes are currently highly correlated, as both are strongly influenced by the downward propagation of stratospheric polar vortex anomalies into the troposphere. However, when considering a very warm climate scenario, the hemispherically defined AO pattern shifts to reflect variability of the North Pacific storm track, while the regionally defined NAO pattern remains stable. The stratosphere remains an important precursor for NAO, and surface Eurasian and Aleutian pressure anomalies precede stratospheric anomalies. Idealized general circulation model simulations suggest that these modifications are linked to the stronger warming of the Pacific compared to the slower warming of the Atlantic Ocean, that is due to the slowdown of the Atlantic Meridional Overturning Circulation (AMOC).

The mean strengths of the wintertime stratospheric polar vortices are known to be related to the phase of the quasi-biennial oscillation (QBO) in the tropical stratosphere from circulation statistics - the "Holton-Tan relationship". The principal topic of this thesis is improving understanding of the mechanism behind the QBO's influence. Following the example of previous studies, the QBO influence on the Northern Hemisphere (NH) extratropics on monthly time scales in an observational reanalysis is examined, and is shown to closely resemble the stratospheric Northern annular mode (NAM). It is argued that this may not be informative about the mechanism, as the response could be NAM-like for many different mechanisms. It is suggested that examining the transient response of the NH extratropics to forcing by the QBO would be much more informative, particularly on time scales of a few days. In a primitive equation model of the middle atmosphere, the long-term stratospheric NH response to imposed zonal torques is often found to be NAM-like under perpetual January conditions, with wave feedbacks making a very important contribution. However, the response in runs with a seasonal cycle is not NAM-like. Investigation of the transient responses indicates the wave feedbacks are qualitatively similar in each case but only strong enough under perpetual January conditions to make the long-term response NAM-like. This supports the hypothesis that feedbacks from large-scale dynamics tend to make the stratospheric response to arbitrary forcings NAM-like, and therefore indicates that the long-term response is not generally useful for understanding forcing mechanisms. Examining the short-term transient response to known torques is found to be more successful at inferring information about the torques than several other previously proposed methods. Finally, the short-term transient response of the NH extratropics to forcing by the easterly QBO phase in a general circulation model is found to be consistent with the proposed mechanism of Holton and Tan (1980), indicating that this mechanism plays a role in the Holton-Tan relationship.

Thesis (M.S.)--University of Wisconsin--Madison, 1997.<br>Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 124-127).

Numerical simulations of evolving atmospheric phenomena are considered. The height of the vortices is small with respect to their width and depending on the atmospheric phenomenon being considered can have a diameter of hundreds if not thousands of kilometres. They can therefore be thought of as large flat structures in a shallow atmosphere. A weakly compressible atmosphere is assumed for both Earth-bound and Saturn simulations. The atmospheric fluid motion is subject to the Coriolis pseudo-force, due to atmospheres being in a non-inertial rotating reference frame. The simulations involve reducing the fully spherical nature of the atmosphere to a localised region, so that the commonly used ‘tangent plane’ approximations apply. The advantage of using ‘tangent plane’ approximations is that necessary spheroidal effects can be studied using Cartesian based equations. Three types of ‘tangent plane’ approximations are used, (i) the f-plane, where the Coriolis parameter is assumed constant over the entire region; (ii) the β-plane, where the Coriolis parameter varies linearly with latitude and (iii) the δ-plane, which is a high latitude approximation where quadratic effects of the Coriolis term are accounted for. Large-scale low-pressure systems in the atmosphere are occasionally observed to possess Kelvin-Helmholtz fingers spiralling outwards, and an example is shown in this thesis. However, these structures are hundreds of kilometres long, so that they are necessarily affected strongly by non-linearity. They are evidently unstable and are commonly observed to dissipate after a few hours, and in rare cases may last for days. A model for this phenomenon is presented in this thesis, based on the usual f-plane equations of meteorology, assuming an atmosphere governed by the ideal gas law. Large-amplitude perturbations are accounted for, by retaining the equations in their non-linear forms, and these are then solved numerically using a spectral method. Finger formation is modelled as an initial perturbation to the nth Fourier mode, and the numerical results show that the fingers grow in time, developing structures that depend on the particular mode. Results are compared with predictions of the β-plane theory and there is close alignment with f-plane results at mid-latitudes. An idealized vortex in the northern hemisphere is considered, but the results are at least in qualitative agreement with an observation of a system in the southern hemisphere. Vortices in the atmosphere are rarely observed to be singular entities. Thus the non-linear behaviour of interacting mid-latitude vortices is also investigated. The vortices studied are coupled binary systems and the high- or low-pressure in each vortex is modelled initially using an exponential function. Non-linear results in the f-plane approximation are discussed at mid-latitudes. It is found that the vortices do or do not interact, depending on their initial radii and the location of their centres. A scaling law is found numerically for the ratio of these two quantities, which determines whether interaction does occur at the approximate mid-latitude 43°N. An approximate rule has been developed, to generalize the scaling law to other latitudes. Atmospheric vortices are rarely circular structures and have been observed to have a definite polygonal form. Saturn’s North Polar Hexagon is an example of such a vortex, and was discovered by Godfrey [31] who pieced together map projections of images captured by the Voyager mission to unveil a hexagonal structure over the north pole of Saturn. This thesis attempts to answer whether or not a hexagonal structure can be formed through anti-cyclones impinging on the dominant eastward circumpolar flow and is in part based upon the proposed theory by Allison et al. [1] that the Hexagon may be the result of at least one impinging anti-cyclone perturbing a circumpolar jet centrally located around the 76°N latitude. A high-latitude δ-plane approximation is used to simulate the interaction between an initially circular circumpolar jet and at least one perturbing anti-cyclone. The simulations with one perturbing anti-cyclone failed to form a hexagonal structure; yet by including an additional anti-cyclone it was found that depending on the strength, location and radius of the perturbing anti-cyclones a hexagonal feature could develop. However, the longevity and drift rate of the actual Hexagon must be attributed to other factors not considered in this thesis.

This chapter introduces research on functional domain boundaries. Ever since the discovery of superconducting twin boundaries in the 1990s, highly conducting, polar, photovoltaic, magnetic, and so on, domain boundaries have been discovered while the same bulk material displays none of these properties. Domain boundaries constitute planar templates for device applications with thicknesses of ca. 1 nm. Domains within domains are then the next step in miniaturization with Bloch lines within domain walls and Bloch points between Bloch lines. In the overwhelming majority of cases, the geometrical template for the functional domain boundaries stems from the ferroelastic domain structure, while antiphase boundaries are equally potential template providers. Complex structures are a particular case because they add vortices and skyrmions to the template topology. Correlations between such sub-structures maintain features like polarity and piezoelectricity in randomized samples where structural averages would not allow macroscopic polar effects. The dynamics of the change of functionality is often much faster than the speed with which twin boundaries move. The novel information carrier is the kink inside twin walls, which moves with supersonic speed.

This chapter focuses on the spectroscopy of the superconducting state. Various manifestations of macroscopic quantisation are described, including flux quantisation, the Josephson effect, vortices, and the Little–Parks effect. The Ginzburg–Landau theory and its microscopic derivation are presented. An interesting new direction, the search for the lossless ground current state, undergoes an intensive development. If the electronic density of states contains several peaks, it manifests as a multigap structure. Impurity scattering and, especially, the pair-breaking effect can drastically affect the spectrum and lead to gapless superconductivity. Pairing can be induced by the proximity effect (S–N contact). The isotope effect is the signature of the pairing mechanism, but it can be affected by Coulomb terms, magnetic impurities, and polaron formation. The study of fluctuations forms a large area of research. Fluctuations affect the behaviour of heat capacity and nuclear magnetic resonance relaxation, lead to peculiar paraconductivity, and so on.

Probability distributions plotted to date from large volumes of high quality atmospheric observations invariably have ‘long tails’: relatively rare but intense events make significant contributions to the mean. Atmospheric fields are intermittent. Gaussian distributions, which are assumed to accompany second moment statistics and power spectra, are not seen. An inherently stochastic approach, that of statistical multifractals, was developed as generalized scale invariance by Schertzer and Lovejoy (1985, 1987, 1991); it incorporates intermittency and anisotropy in a way Kolmogorov theory does not. Generalized scale invariance demands in application to the atmosphere large volumes of high quality data, obtained in simple and representative coordinate systems in a way that is as extensive as possible in both space and time. In theory, these could be obtained for the whole globe by satellites from orbit, but in practice their high velocities and low spatial resolution have to date restricted them to an insufficient range of scales, particularly if averaging over scale height-like depths in the vertical is to be avoided; analysis has been successfully performed on cloud images, however (Lovejoy et al. 2001). Some suitable data were obtained as an accidental by-product of the systematic exploration of the rapid (1–4% per day) ozone loss in the Antarctic and Arctic lower stratospheric vortices during winter and spring by the high-flying ER-2 research aircraft in the late 1980s through to 2000. Data initially at 1Hz and later at 5Hz allowed horizontal resolution of wind speed, temperature, and pressure at approximately 200 m and later at 40 m, with ozone available at 1 Hz, over the long, direct flight tracks necessitated by the distances involved between the airfield and the vortex. Some later flights also had data from other chemical instruments, such as nitrous oxide, N2O, reactive nitrogen, NOy, and chlorine monoxide, ClO, which could sustain at least an analysis for H1, the most robust of the three scaling exponents. Better than four decades of horizontal scale were available for 1Hz and 5Hz data. Since then, a lesser volume of adequate data has been obtained away from the polar regions by the WB57F.

The pressure field of deep rotating stall of a centrifugal compressor with two stall cells is analysed by means of the two-dimensional pressure pattern in the impeller determined by Chen et al. (1993). These authors transferred the pressure pattern measured on the shroud surface (i.e. in the absolute frame) to that related to the rotating blade channels. The transferred pressure pattern is thus a two-dimensional one. The existence of the low and high pressure vortices according to the Rossby wave theory is confirmed by this experiment. The development stages of the two vortices, in combination with the Rossby wave that steers the rotating stall, can be evaluated very well. The vortex low is developed from the front between the reverse flow (with high temperature and entropy) and the forward flow (with low temperature and entropy) due to baroclinic instability. Its center is situated within the channel of the splitter blade. This front is accompanied by a squall line of small-scaled eddies. This is the same phenomenon as can be observed on the meteorological polar front. The vortex high is induced by the vortex low. Its embryo starts on the pressure surface. Its center is situated behind the inlet edge of the splitter blade. It can be further verified that the stall cell is caused by the backflows of the induction fields of the two vortices (low and high).

Front rotor tip vortices impinging on a downstream blade row of an unducted propulsor induce distinct unsteadiness to blade loads with associated sound emissions. Since the region of unsteadiness is concentrated near the blade tips, reducing the rear rotor tip diameter represents a potential means for minimising interaction noise. A survey on the aeroacoustic effects resulting from a cropped rear rotor in combination with a front rotor blade tip modification is therefore presented. Analyses are based on data from computational fluid dynamics solutions with the Reynolds-averaged Navier-Stokes equations and direct acoustic predictions. The evaluation of polar directivities, blade surface pressure disturbances and details of the unsteady flow field provide insight into the underlying phenomena. Results show that an arbitrary reduction of the rear rotor tip diameter does not necessarily decrease noise radiation and that winglet-like structures applied to the front rotor blade tips are capable of reducing acoustic emissions due to tip vortex-rotor interactions.

We study arrangements of point vortices on a sphere in the form of a single von Ka´rma´n vortex street, with and without the addition of pole vortices using the configuration matrix approach recently introduced by (1) and (2) on the sphere. We derive the general form of the von Ka´rma´n matrix thereby obtaining all possible vortex strengths for which the configuration rotates rigidly, perpendicular to the axis of rotation of the sphere. The distribution of normalized singular values of the von Ka´rma´n matrices allows us to calculate their Shannon entropy (as a function of the vortex street parameters), which we interpret as a scalar measure of ‘disorder’ and robustness of the vortex street. We also study the streamline topologies associated with the vortex street and identify a number of distinct topologies which support a jet-stream passing through the street capable of transporting particles globally.

The evolution of the wake of a wind turbine contributes significantly to its operation and performance, as well as to those of machines installed in the vicinity. The inherent unsteady and three-dimensional aerodynamics of Vertical Axis Wind Turbines (VAWT) have hitherto limited the research on wake evolution. In this paper the wakes of both a troposkien and a H-type VAWT rotor are investigated by comparing experiments and calculations. Experiments were carried out in the large-scale wind tunnel of the Politecnico di Milano, where unsteady velocity measurements in the wake were performed by means of hot wire anemometry. The geometry of the rotors was reconstructed in the open-source wind-turbine software QBlade, developed at the TU Berlin. The aerodynamic model makes use of a lifting line free-vortex wake (LLFVW) formulation, including an adapted Beddoes-Leishman unsteady aerodynamic model; airfoil polars are introduced to assign sectional lift and drag coefficients. A wake sensitivity analysis was carried out to maximize the reliability of wake predictions. The calculations are shown to reproduce several wake features observed in the experiments, including blade-tip vortex, dominant and submissive vortical structures, and periodic unsteadiness caused by sectional dynamic stall. The experimental assessment of the simulations illustrates that the LLFVW model is capable of predicting the unsteady wake development with very limited computational cost, thus making the model ideal for the design and optimization of VAWTs.

Due to their inherent noise challenge and potential for significant reductions in fuel burn, counter-rotating propfans (CRPs) are currently being investigated as potential alternatives to high-bypass turbofan engines. This paper introduces an integrated noise and performance assessment methodology for advanced propfan powered aircraft configurations. The approach is based on first principles and combines a coupled aircraft and propulsion system mission and performance analysis tool with 3-D unsteady, full wheel CRP CFD computations and aero-acoustic simulations. Special emphasis is put on computing CRP noise due to interaction tones. The method is capable of dealing with parametric studies and exploring noise reduction technologies. An aircraft performance, weight and balance and mission analysis was first conducted on a candidate CRP powered aircraft configuration. Guided by data available in the literature, a detailed aerodynamic design of a pusher CRP was carried out. Full wheel unsteady 3-D RANS simulations were then used to determine the time varying blade surface pressures and unsteady flow features necessary to define the acoustic source terms. A frequency domain approach based on Goldstein’s formulation of the acoustic analogy for moving media and Hanson’s single rotor noise method were extended to counter-rotating configurations. The far field noise predictions were compared to measured data of a similar CRP configuration and demonstrated good agreement between the computed and measured interaction tones. The underlying noise mechanisms have previously been described in the literature but, to the authors’ knowledge, this is the first time that the individual contributions of front-rotor wake interaction, aft-rotor upstream influence, hub-endwall secondary flows and front-rotor tip-vortices to interaction tone noise are dissected and quantified. Based on this investigation, the CRP was re-designed for reduced noise incorporating a clipped rear-rotor and increased rotor-rotor spacing to reduce upstream influence, tip-vortex, and wake interaction effects. Maintaining the thrust and propulsive efficiency at takeoff conditions, the noise was calculated for both designs. At the interaction tone frequencies, the re-designed CRP demonstrated an average reduction of 7.25 dB in mean SPL computed over the forward and aft polar angle arcs. On the engine/aircraft system level, the re-designed CRP demonstrated a reduction of 9.2 EPNdB and 8.6 EPNdB at the FAR 36 flyover and sideline observer locations, respectively. The results suggest that advanced open rotor designs can possibly meet Stage 4 noise requirements.

The notched nozzle as a new concept has been investigated for conventional nozzle design together with the Chevron nozzle and Micro-jets, through feasibility studies. The notched nozzle has a plurality of triangular pyramid-shaped dent positioned in a circumferential direction along the nozzle exit. These studies include acoustic experiments that utilize a lab-scale simple model in an anechoic chamber and numerical approaches. The results of the Large Eddy Simulation are compared with the results of either acoustic or aerodynamic experiments. The objective of these investigations is to verify the effects of noise mitigation and to gain understanding of the physics of fluid dynamics around the nozzle exit, especially within the shear layer between high velocity jet flow and external flow/or ambient air. One concept of conventional noise mitigation devices involves mixing enhancements in the shear layer, but this sometimes produces high frequency self noise. Moreover it will result in a penalty in terms of thrust loss, additional weight and extra manufacturing cost due to the complicated shapes around the nozzle exit. It is difficult to produce a nozzle design without affecting high frequency self-noise and decreasing low-frequency noise towards to down stream of the jet engines even though there is no thrust loss. Most of this study, the experimental data were physically validated by three kinds of nozzle concepts designed to be equal to the conventional model in terms of size of nozzle exit diameter and Mach number. Essentially far-fields noise measurements and pressure measurements are conducted by polar angle microphones and arch-shaped pitot tubes are located downstream of the jet. The noise benefit which is produced by the notched nozzle as a lab-scale in far-fields noise measurements is up to 1.3dB at the side of the jet and 0.5dB at downstream, in terms of size of small-engine. Furthermore this provided an advantage over the chevron nozzle due to the decreasing self-noise production when the Mach number of the jet was lower than 0.9. Moreover, numerical predictions which are provided by the Large Eddy Simulation were used to estimate the noise mitigation by performing turbulence statistical analysis. Numerical results which refer to the turbulent statistics are discussed in order to define how they can be affected to the acoustic results at the side of the jet. This shows how each device can deform the shear layer without producing additional streamwise and small scale vortices.