Wake Vortex Evolution Research Articles

Large-eddy simulation of aircraft wake vortex deformation and topology

On the one hand, it is known from visual observations, Lidar measurements, and numerical simulations that aircraft wake vortices may live significantly longer than anticipated by todays standard regulations of air traffic control. On the other hand, the initially counter-rotating, parallel vortex pair deforms quickly, which reduces the impact time of adverse forces and moments on encountering aircraft. Therefore, large-eddy simulations of wake vortex evolution in different meteorological conditions are conducted in order to analyse the physics of vortex deformation. A new post-processing algorithm has been developed that at first determines the three-dimensional path of the vortex core line and then computes piecewise curvature radii as a measure of vortex deformation. It is found that larger turbulence levels and neutral to weakly stable temperature stratification particularly support vortex ring formation. The vortex ring regime is characterized by a reduced descent rate and by a surprising variability of its core radius. It is shown that the varying core size directly affects the evolution of radii-averaged circulation. Comparisons with field experiments indicate good agreement with the considerable life time of the vortex rings and topology.

Assessment of Wake-Vortex Encounter Probabilities for Crosswind Departure Scenarios

The WakeScene-D (Wake Vortex Scenarios Simulation Package for Departure) software package has been developed for comprehensive airspace simulations of takeoff and departure. WakeScene-D consists of modules that model traffic mix, aircraft trajectories, meteorological conditions, wake-vortex evolution, and potential hazard area. The software package estimates the probability to encounter wake vortices in different traffic and crosswind scenarios using Monte Carlo simulation in a domain ranging from the runway to an altitude of 3000 ft above ground. A comparison with measured vortex tracks of about 10,000 departures from runway 25R of Frankfurt airport indicates good agreement of global wake-vortex transport characteristics in ground proximity. The standard departure situation employing a 2 min aircraft separation is compared with scenarios with reduced departure separations and various crosswind conditions. Comprehensive sensitivity analyses have been conducted that are briefly recapitulated. Effects related to departure-route combinations and wind-direction sectors are reported in more detail. Finally, an advanced scenario with an asymmetric crosswind criterion is introduced.

Aircraft Wake Vortex Scenarios Simulation Package for Takeoff and Departure

The WakeScene-D software package (Wake Vortex Scenarios Simulation Package for Departure) has been developed for comprehensive airspace simulations of take-off and departure. WakeScene-D consists of modules that model traffic mix, aircraft trajectories, meteorological conditions, wake vortex evolution, and potential hazard area. The software package estimates the probability to encounter wake vortices in different traffic and crosswind scenarios using Monte Carlo simulation in a domain ranging from the runway to an altitude of 3000 ft above ground. The operating sequence of WakeScene-D, the employed sub-models and data bases, the simulation environment and evaluation tools together with the so far accomplished validation work are introduced.

Landing Gear Influence on the Wake Vortex of a Large Transport Aircraft

An experimental investigation on the wake vortex formation and evolution of a four-engine large transport aircraft model in the near field and extended near field has been conducted by means of hot-wire anemometry in a wind tunnel. The half-model used has a scale of 1:19.25 and the tests focus on the high-lift case of a typical landing configuration at a Reynolds number of 0.52 x 10 6 based on the wing mean aerodynamic chord. The flowfield is investigated up to 4.7 spans downstream of the model, which is investigated with and without landing gear. Based on the measured time-dependent velocity components, the wake flowfield is analyzed by distributions of vorticity, turbulence intensities, and spectral densities. The landing gear consists of a stay and four wheels, which create a bluff-body wake that interferes with the shear layer caused by the wing and wing junction. The landing gear creates a low-energy vortex system and a highly turbulent wake. Due to the landing gear, the main vortex centers are located slightly further outboard and the roll-up process is retarded, especially the inboard movement of the horizontal tail plane vortex. The velocity spectra of the landing gear wake show broadband characteristics with some moderate narrowband energy concentrations in the close near field, which do not enhance inherent wake instabilities. Although the turbulence caused by the landing gear is still present in the extended near field, only a small influence on the overall vortex system of the entire aircraft can be found. The landing gear influence is therefore negligible in the context of reducing minimum aircraft separation distances.

Experimental investigation of counter-rotating four vortex aircraft wake

Abstract An experimental investigation on the wake vortex formation and evolution of a four vortex system of a generic model in the near field and extended near field as well as the behaviour and decay in the far field region has been conducted by means of hot-wire anemometry in a wind tunnel. The results were obtained during an experimental campaign as part of the EC project “FAR-Wake”. The model used consists of a wing–tail plane configuration with the wing producing positive lift and the tail plane negative lift. The circulation ratio of tail plane to wing is −0.3 and the span ratio is 0.3. Thus, a four vortex system with counter-rotating neighboured vortices exists. The model set-up was chosen on the condition to create a most promising four vortex system with respect to accelerate wake vortex decay by optimal perturbations enhancing inherent instability mechanisms. The flow field has been investigated for a half plane of the entire wake up to a distance of 48 span dimensions downstream of the model. The results obtained at 1, 12, 24 and 48 span distances are shown as non-dimensional axial vorticity and vertical turbulence intensities. A significant decay in peak vorticity, swirl velocity and circulation is observable during the downward motion of the vortices. Spectral analysis of the unsteady velocity data reveals a peak in the power spectral density distributions indicating the presence of a dominating instability. Using two hot-wire probes cross spectral density distributions have also been evaluated, which highlight the co-operative instability leading to a rapid wake vortex decay within 30 span dimensions downstream.

Aircraft wake vortex scenarios simulation package – WakeScene

Wake-vortex advisory systems and modifications of ATC procedures that aim at increasing airport capacity without compromising safety have been developed in recent years. Prior to the introduction of such systems the associated risks must be assessed. The WakeScene (Wake Vortex Scenarios Simulation) Package allows to assess the encounter probability behind different wake-vortex generating aircraft during approach and landing. WakeScene consists of modules that model traffic mix, aircraft trajectories, meteorological conditions, wake vortex evolution, and potential hazard area. This manuscript introduces the operating sequence of WakeScene, the employed sub-models and data bases, the simulation environment and evaluation tools together with the so far accomplished validation work. Examples of WakeScene applications and an outlook on further developments of the software package conclude the report. Konzeptionen von Wirbelschleppen-Warnsystemen und modifizierten Anflugverfahren zur Kapazitätssteigerung großer Flughäfen haben mittlererweile einen beachtlichen Reifegrad erreicht. Bevor derartige Systeme jedoch operationell genutzt werden können, müssen die mit ihrer Einführung verbundenen Risiken umfassend untersucht und bewertet werden. Das vorgestellte WakeScene (Wake Vortex Scenarios Simulation) Software Paket dient der Bestimmung der Wahrscheinlichkeit einer Wirbelschleppen Begegnung im Landeanflug für unterschiedliche Szenarien. WakeScene besteht aus mehreren Modulen, die den Verkehrsmix, die Flugzeugtrajektorien, die meteorologischen Bedingungen, das Wirbelschleppen Verhalten sowie die potenziellen Gefährdungsräume simulieren. Das vorliegende Manuskript beschreibt die verwendeten Modelle und Datenbasen, das Zusammenspiel der einzelnen Module, die Simulationsumgebung und Auswertewerkzeuge, sowie den Stand der Validierung der einzelnen Komponenten. Anwendungsbeispiele und ein Ausblick auf die geplanten Weiterentwicklungen des Software Pakets beschließen den Bericht.

Transport Aircraft Wake Influenced by a Large Winglet and Winglet Flaps

Detailed flowfields of a wind-tunnel investigation are discussed presenting the wake vortex development and evolution in the near field and extended near field behind a four-engined large transport aircraft model fitted with a large winglet. The tests use a half-model of 1:32 scale focusing on the high-lift case of a typical approach configuration at a Reynolds number of 0.5 x 10 6 based on the wing mean aerodynamic chord and at an angle of attack of 7 deg. Flowfields are carefully inspected by advanced hot-wire anemometry at seven crossflow planes up to 5.6 spans downstream of the model. Based on the measured time-dependent velocity components, the wake flowfield is analyzed by distributions of mean velocity and vorticity, turbulence intensities, and spectral densities. The near-field wing vortex sheet is dominated by seven main vortical structures, namely the winglet vortex, the wing tip vortex, the outboard flap vortex, the outboard and inboard nacelle vortices, and the vortices shed at the wing-body junction and the horizontal tail plane. In the extended near field, these vortices roll up and merge to form the remaining rolled-up vortex. The deflection of winglet flaps produce additional vortices influencing the wing tip near field and enhancing the overall merging process. Especially for the cases with asymmetrical flap deflection, the remaining rolled-up vortex shows a significant narrowband concentration of turbulent kinetic energy which may result in an amplification of inherent far-field instabilities.

Aircraft Wake-Vortex Evolution in Ground Proximity:Analysis and Parameterization

Field measurement data of 282 wake vortex pairs and respective environmental conditions acquired at Frankfurt Airport by means of lidar, Sodar/RASS, and ultrasonic anemometer are used to analyze wake vortex behavior in ground proximity. Exceptional cases with strong rebounds caused by detached shear layers and obstacles are introduced and estimates of the time needed to clear the runway from wake vortices by advection are provided. The impact of turbulence and crosswind on wake vortex decay proves to be weak, whereas already light crosswind turns out to be sufficient to cause pronounced asymmetric rebound characteristics. Based on the analyses vortex decay and rebound characteristics are parameterized and implemented into the probabilistic two-phase aircraft wake-vortex model. Deterministic and probabilistic prediction skill of the enhanced vortex model are assessed. Comparison to wake predictions out of ground effect indicates that in ground effect (i) the rapid-decay phase progresses slower, (ii) wake vortex evolution can be predicted with improved accuracy, and (iii) fair prediction skill requires only limited environmental data.

Concept of Wake Vortex Behavior Classes

The concept of wake vortex behavior classes is introduced. Rather than consider the individual wake vortex evolution, diagnosis of the meteorological situation causing a characteristic wake vortex behavior is proposed. We define four wake vortex behavior classes that explicitly refer to both the wake vortex transport and the decay behavior. The statistical analysis of the Memphis wake vortex database supports this new concept and also shows the distinct influence of weather on wake vortex behavior. The analysis indicates that the high-level safety of International Civil Aviation Organization (ICAO) separations between consecutive aircraft is primarily due to the transport of wake vortices out of a flight corridor. It is found that, depending on atmospheric conditions, a significant fraction of wake vortices does not decay enough to satisfy the existing ICAO separations. Of these wake vortices, a small fraction is within a safety corridor and therefore poses a potential risk to a following aircraft. A simple crosswind criterion with a threshold of 2 m/s provides a safety corridor that is free of wake vortices under radar separation. This demonstrates the large potential for safe reduction of aircraft separation under crosswind conditions. This is supported by an analysis of a wind climatology representative for Frankfurt airport.

Analysis of wake vortex decay mechanisms in the atmosphere

Results of high-resolution numerical simulations of aircraft wake vortex evolution and decay in different regimes and atmospheric conditions are presented. The different cases comprise (i) the near field interaction of a trailing vortex with an exhaust jet, (ii) the evolution of single vortices and counter-rotating vortex pairs in homogeneous isotropic turbulence, as well as (iii) the decay of wake vortices in a turbulent stably stratified atmosphere, and (iv) in a weakly turbulent sheared environment. The different cases are used to analyse common aspects of vortex dynamics and decay mechanisms. In all scenarios the formation of coherent secondary vorticity structures that enclose the primary vortices is observed. These secondary vorticity structures deform and weaken the primary vortices and in some cases lead to rapid vortex decay. It is shown that the mean swirling flow effectively rearranges and intensifies any secondary vorticity by tilting and stretching. The secondary vorticity may either originate from the turbine jet, ambient turbulence or may be produced baroclinically. Based on the observed phenomena, eleven postulates are established that pinpoint fundamental aspects of the observed decay mechanisms.

A Probabilistic Prediction Scheme for Wake Vortex Evolution in a Convective Boundary Layer

A simple probabilistic prediction scheme for wake vortex behaviour in a convective boundary layer is proposed. Large eddy simulations (LES) of wake vortices (WVs) in an evolving convective boundary layer serve as a test bed to investigate the performance of an engineering WV model which is forced by virtual measurement systems placed in the LES domain. The basic principles of the WV model are taken from literature. Considered are a meteorological tower and a profiler, from which the input data for the WV model are computed. The stochastic nature of turbulence requires both time and space averaging of wind and temperature data which in consequence leads to statistical WV forecasts. It is shown that both virtual measurement systems yield conservative predictions of WV trajectories. The predicted circulation is within the range analyzed from LES data. Operational aspects of these simple approaches are discussed.

Wake vortex evolution and ENcounter (WAVENC)

WAVENC was a short research project to improve understanding of the evolution of aircraft wake vortices and of their impact on the safety of flight of following aircraft. The work included numerical modelling of vortex behaviour, flight simulation and analysis of measured encounter data. This article describes some of the main results.