AIRCRAFT WAKE VORTEX DEFORMATION IN TURBULENT ATMOSPHERE

The NRC has undertaken re-instrumentation of the NRC CT-133 ruggedised research aeroplane, for high altitude atmospheric research. In the process, a new air data system has been designed, and optimized for high data acquisition rates, in particular matching the capability of the aircraft’s FRL inertial reference and navigation system, to derive and acquire wind vector data at 600 Hz. The platform has been used to conduct transport aircraft wake vortex (WV) surveys, and acquire instability wake vortex flowfield data, most recently in comparative straight flight and turning flight manoeuvres, using a Falcon 20 wake generator, to 3 nm distance. Vortex core traverse analysis has highlighted a greater frequency of small-scale vortex segments with high (>150% exceedance of the primary vortex) tangential velocities in turning flight; also, a greater bandwidth in short-wave instability wavelengths, and higher amplitude in long-wave instability, also multi-wavelength, but centred at 3 to 5 generator wingspans, whether in straight or turning flight. Such observations and general considerations of possible tactical jet aircraft wake vortex flowfields suggests the need to establish from flight data, wake vortex flowfield environmental design requirements, in order that contemporary and new tactical aircraft types may be developed with robust flight control systems, able to handle the [frequency amplitude] environments experienced during wake vortex encounters.

PurposeIn this study, 12 potential wake vortex encounters that were reported at a major European airport have been investigated. Because almost all encounters occurred in ground proximity, most pilots conducted a go-around. The primary purpose of this study is to discriminate between incidents caused by wake vortices or rather by effects like wind shear or turbulence. Detailed knowledge of real-world encounter scenarios and identification of worst-case conditions during the final approach constitute highly relevant background information to assess the standard scenario used for the definition of revised wake turbulence separations.Design/methodology/approachWake vortex predictions using the probabilistic two-phase wake vortex model (P2P) are used to investigate the incidents in detail by using data from the flight data recorder, meteorological instrumentation at the airport and numerical weather prediction.FindingsIn the best documented cases, the flight tracks through the vortices could be reconstructed in good agreement with wake vortex predictions and recorded aircraft reactions. Out of the eight plausible wake vortex encounters, five were characterized by weak crosswinds below 1.5 m/s combined with tailwinds. This meteorological situation appears favourable for encounters because, on the one hand, weak crosswinds may compensate the self-induced lateral propagation of the upwind vortex, such that it may hover over the runway directly in the flight path of the following aircraft. On the other hand, tailwinds limit the propagation of the so-called end effects caused by the breakdown of lift during touchdown.Practical implicationsThe installation of plate lines beyond the runway tails may improve safety by reducing the number of wake vortex encounters.Originality/valueThe conducted investigations provide high originality and value for both science and operational application.

The current top safety risk concern for commercial air travel in Europe is known as “Flight Upset”. This term, also known as “Loss of Control in Flight”, entails the flight crew suddenly finding themselves in an unexpected, complex, and even confusing situation that if not resolved quickly can lead to a major accident. Accidents such as AF447 and the two B737 Max accidents fall into this category. An undesirable aspect of such events is known as the “startle response”, wherein one or both flight crew, finding themselves in dire and dangerous conditions, may experience ‘startle’, which temporarily affects their cognitive functioning. This may only last half a minute, but its effect can have a severe impact on the survivability of such events. A Horizon 2020 research project called SAFEMODE, which aims to integrate Human Factors techniques into a unified framework for designers in aviation and maritime domains, is exploring the use of state-of-the-art flight simulation facilities to measure pilot performance in severe wake turbulence events, which can induce the startle effect. This is part of a broader use case within SAFEMODE to validate the design of a new Wake Vortex Air Traffic Alert for the Cruise phase of flight. A tactical short-term alert to the Flight Crew, ahead of the wake encounter, is seen as beneficial to reduce the startle effect and support the appropriate management of these conflicts. The envisaged risk-alerting logic relies on a ground-based predictor, connected to the Air Traffic Control system, displaying an alert to the En-route Air Traffic Controllers, who can then provide a cautionary advisory to the Flight Crew so they can take appropriate actions.The cockpit flight simulations involve type-rated flight crews in realistic and representative cruise flight conditions, using a Type VI Boeing 737-800 full flight motion-based simulator (also used for Upset Prevention and Recovery training programs). During the simulation runs, pilots are exposed to simulated wake vortex encounters, corresponding to a strong wake-induced upset (between 30 and 40 degrees of bank), with or without prior ATC wake caution, and varying the initial direction of roll between left and right to limit the simulation training effect.Human Factors measurements include workload, situation awareness, trust, acceptability-based user feedback, as well as psychophysiological measures such as eye-tracking and Electro-Dermal Activity (EDA). In particular, eye-tracking is expected to support the refined determination of the sequence of actions before and after detection, and the reaction of flight crews to the en-route ATC Wake alert.A cockpit flight simulation, via combining the analyses of psychophysiological measures, flight parameters, expert observations and subjective pilot feedback, enables evaluation of Flight Crews performance in preparing for, managing or avoiding wake encounter upsets with the new ATC wake alerts, showing the net safety benefits. Early results indicate that the simulations can indeed induce startle effect, and that repeated exposure enables flight crew to overcome it and manage the situation in a more measured and controlled fashion.

To mitigate the risk of wake vortex encounters during final approach, so-called plate lines have been developed. Data collected during a six-month measurement campaign at Vienna International Airport are used to assess the potential for reducing minimum aircraft separations facilitated by plate lines during approach and landing following the re-categorisation (RECAT-EU) methodology for revised wake turbulence categorisation. To ensure that no other parameters controlling wake vortex decay bias the analysis, it is verified that wind speed, atmospheric turbulence, thermal stratification and flight altitude reside in similar ranges with and without the plates. The analysis follows the steps of the RECAT-EU method to generate non-dimensional so-called reasonable worst-case circulation decay curves; one as a reference for nominal operations without plates and one representing the accelerated wake vortex decay brought about by the plate lines. The difference between the two circulation decay curves corresponds to the non-dimensional time-based aircraft separation reduction potential that can be translated into distance-based separation gains. Depending on the particular RECAT-EU category combination, the attained aircraft separation reduction potential ranges between 12% and 15%. Constricting the analysis to wake vortices generated by one aircraft type representing the Upper Heavy RECAT-EU category, the separation reduction potential amounts up to 24%.

The wake vortices of an aircraft descend by self-induction that enables a pilot to identify approximate wake location based on the preceding aircraft path. A pilot taking off an aircraft can avoid a wake vortex encounter by performing early rotation and flying at or above the climb path of the preceding aircraft. Such operation results in a situation in which two wake vortices are present in the air simultaneously in close proximity. In this study, the transport and decay characteristic processes involving interactions between the wake vortex pairs formed by the preceding and the following aircraft have been analyzed. The high altitude of the following aircraft was included as initial conditions, and atmospheric turbulence conditions were applied to large eddy simulations to account for the vortex core instability and non-linear transport and decay. The obtained results were used to identify the decay processes that induce the rapid dissipation of wake vortices such as merging into a single counter-rotating pair, rapid dissipation, and formation of a vortex ring with or without deformation of the lower vortex pair. The third process could be effectively used to adjust the take-off separation time for increasing the capacity at airports.

Abstract. In the present study, blade-tip vortices have been experimentally identified in the wake of a commercial wind turbine using the Multi-purpose Airborne Sensor Carrier Mark 3 (MASC Mk 3) unmanned aircraft system (UAS) of the University of Tübingen. By evaluation of the wind components, detached blade-tip vortices were identified in the time series. From these measurements, the circulation and core radius of a pair of detached blade-tip vortices is calculated using the Burnham–Hallock (BH) wake vortex model. The presented data were captured under a dominating marine stratification about 2km from the North Sea coastline with northern wind direction. The measured vortices are also compared to the analytical solution of the BH model for two vortices spinning in opposite directions. The model has its origin in aviation, where it describes two aircraft wake vortices spinning in opposite directions. An evaluation method is presented to measure detached-tip vortices with a fixed-wing UAS. The BH model will be used to describe wake vortex properties behind a wind energy converter (WEC). The circulation and core radius of detached blade-tip vortices will be calculated. Also a proposition of the model for WEC wake evaluations will be made to describe two independent co-rotating vortices. Quantifying blade-tip vortices helps to understand the process of vortices detaching from a rotor blade of a wind turbine, their development in the wake until finally dissipating in the far wake and contributing to overall atmospheric turbulence. This is especially interesting for set-ups of numerical simulations when setting the spatial resolution of the simulation grid.

To mitigate the risk of wake vortex encounters during final approach, so-called plate lines have been developed. Wake vortices generated by landing aircraft induce secondary vortices at the plates’ surfaces that approach the primary vortices and trigger premature wake vortex decay. Each plate line consists of several upright plates being installed underneath the approach slope. While the plate line extends perpendicular to the flight direction, its individual plates are oriented in parallel to the runway centerline. To obtain the approval of the authorities for the installation of the plate lines at runway 16 of Vienna International Airport, the plate design had to comply with airport requirements like obstacle clearance, stability, and frangibility. During a six-month campaign, wake vortex behavior of about 9500 landings with and without plates was measured simultaneously by three lidars complemented by a comprehensive suite of meteorological instrumentation. The analysis of over 1000 measured wake vortex evolutions indicates that the plate lines reduce the lifetimes of the vortices in a safety corridor along the final approach by 22–37% depending on the aircraft type. This corresponds to a reduction of vortex circulation by about 50% for the most relevant International Civil Aviation Organization separation (Medium behind Heavy).

Enhancement of wake vortex decay by air blowing from the ground

Control of trailing vortex wakes is an important challenges for both military and civilian applications. This paper summarizes an assessment of the feasibility of mitigating adverse vortex wake effects using control surfaces actuated via Shape Memory Alloy (SMA) technology. The assessment involved a combined computational/design analysis that identified methods for introducing small secondary vortices to promote the deintensification of vortex wakes of submarines and aircraft. Computational analyses of wake breakup using this `vortex leveraging' strategy were undertaken, and showed dramatic increases in the dissipation rate of concentrated vortex wakes. This paper briefly summarizes these results and describes the preliminary design of actuation mechanisms for the deflectable surfaces that effect the required time-varying wake perturbations. These surfaces, which build on the high-force, high- deflection capabilities of SMA materials, are shown to be well suited for the very low frequency actuation requirements of the wake deintensification mission. The paper outlines the assessment of device performance capabilities and describes the sizing studies undertaken for full-scale Vortex Leveraging Tabs (VLTs) designed for use in hydrodynamic and aerodynamic applications. Results obtained to date indicate that the proposed VLTs can accelerate wake breakup by over a factor of three and can be implemented using deflectable surfaces actuated using SMAs.

The reproduction and survival of fish are often negatively affected by the construction of dams and other hydroelectric projects, which cut off their migratory routes. Building effective fish passage facilities that allow fish to pass through dams smoothly alleviates the negative impact of hydroelectric projects on the ecological environment, thus protecting the diversity of aquatic species and preventing the extinction of indigenous fish. Vertical slit fishways are highly effective, but turbulence inside the fishway pools directly affects fish passage. In this study, the large-eddy simulation framework is used to capture the vortex characteristics in the interior of vertical slit fishway pools, and the volume of fluid method is applied to simulate the free surface. The independence of the grid is assessed by the large-eddy simulation quality index, and the simulation results are compared with experimental acoustic Doppler velocimetry data. This work characterizes the vortex flow field inside the vertical slit fishway using the Q-criterion, Omega method, and Liutex vortex identification method. The results show that the vortex structure inside the fishway pool has obvious three-dimensional characteristics and vortex structure varies within the different fishway pool chambers. The analysis and comparison of the three different vortex identification methods show that the vortex structure captured by the Liutex method is more consistent with the actual motion pattern of the fishway water flow.

NASA has an on-going field experiment at Memphis International Airport to measure wake turbulence generated by aircraft in the near-ground effect region. This new data will be used in the validation and improvement of fast-time numerical wake vortex models, and provide an important data set to be used in the further development of probabilistic wake vortex estimation algorithms. An important component of this field program is to obtain high-quality estimates of important weather conditions in, or near, the region where the wake vortices are generated and tracked, and at a frequency such that the scales of variability important to the evolution of wake vortices is observed. In this paper we will describe the intercomparisons of the estimates derived from multiple sensors for the important weather parameters of atmospheric turbulence and wind speed and direction. Using the results of this intercomparison, we will provide some guidance on what uncertainties may be applicable and useful when utilizing different sources for the estimation of the meteorological condition in probabilistic fast-time wake vortex simulations.

An aircraft wake vortex retrieval method that estimates the horizontal and vertical positions and radii-averaged circulations of wake vortices from lidar-measured Doppler spectra on a lateral range–height indicator is proposed. The method formulates relationships between state variables and measured Doppler spectra for each individual wake vortex and iteratively optimizes the state variables so as to match the measured Doppler spectra. Because the state variables consist of a position, the parameters of an assumed two-dimensional vortex model, and a local background wind, high retrieval accuracy is achieved without adverse influences from errors in the preestimation of position or background wind. The separate formulation of each wake vortex enables the method to flexibly handle nonideal cases, for example where only one wake vortex of a pair remains or a long-persistent wake vortex and a newly generated pair of wake vortices coexist on a single range–height indicator. The retrieval results were evaluated against reference flight data from 864 flights collected during a wake vortex observation campaign conducted by the Japan Aerospace Exploration Agency at Narita International airport. The evaluation was performed by an approach using self- and cross-evaluation models, which allowed the evaluation of a large number of retrievals considering time variation of the wake vortex parameters. As a result, the proposed method was found to retrieve horizontal position with a bias error of and a standard error of 4.8 m; vertical position with bias and standard errors of and 3.5 m, respectively; and radii-averaged circulation with bias and standard errors of and , respectively.

Aircraft wake is a pair of counter-rotating vortices generated behind the aircraft, which can greatly impact the safety of fast takeoff and landing of aircraft and limit the improvement of airport capacity. The current wake parameter retrieval methods cannot locate the wake vortex's position and estimate its strength level in real time. To deal with this issue, a novel algorithm based on YOLOv5s deep learning network is proposed. The new algorithm establishes single vortex locating concept to adapt the wake vortex's evolution at complicate background wind field conditions, and proposes strength-based classification standard which can represent the real-time hazard of wake vortex to shorten the takeoff and landing intervals. Meanwhile, EIOU loss function is introduced to improve the precision of YOLOv5s network. Compared with the state-of-the-art object detection approaches, such as Cascade R-CNN, FCOS, and YOLOv5l, the superiority of new method is demonstrated in terms of accuracy and robustness by using the field detection data from Hong Kong International Airport.

Along with the rapid development of the air transportation industry, the impact of aircraft wake vortices on flight safety and airport capacity has become increasingly prominent. In this paper, we propose a transformer-based model to solve the problem of multiple LIDAR wake vortex detection and recognition in airports. By setting up multiple Doppler LIDARs in the near-Earth flight areas of different runways of Shenzhen Baoan Airport (SZX), a large amount of accurate wind field data is captured for wake vortex data collection. In the deep learning framework, the radial velocity sequence obtained from the LIDAR is used as the input of the transformer. Meanwhile, local meteorological information and LIDAR operating parameters are introduced into the model, providing prior knowledge at different observation points. The experimental results show that the model has unified modeling for different LIDAR wake vortex detection, and has obtained excellent recognition results.

The paper is devoted to the problem of safe flight in vortex hazard conditions. A vortex wake is always produced behind flying aircraft. This vortex wake is invisible to other aircraft following it. Therefore, getting into a vortex wake from a preceding aircraft often becomes unexpected and quite often leads to flight incidents. This is confirmed by the statistics of flight accidents. With the introduction of aircraft weighing more than 500 tons, the problem of vortex safety has only sharpened. The fact is that the old standards that define safe intervals between flying aircraft are still active. These standards are given in this paper. It is also shown that even if these standards are observed, the flight of a medium-class aircraft at the same level as a heavy-class aircraft is unsafe. To study the effect of the vortex wake from the preceding aircraft, a special computational software complex based on the discrete vortex method was developed. This complex has passed the necessary testing and state registration. A number of measures were carried out to validate and verify the developed complex, confirming the operability of the programs included in it and the reliability of the results obtained. On the basis of this computational and software complex studies on the effect of the A380 aircraft vortex wake on MS-21 class aircraft were performed. The studies were undertaken for various altitudes and speeds of the A380 aircraft and for its various flight configurations. As a criterion for the MS-21 aircraft safe flight, the roll moment that occurs due to falling into the vortex wave, in particular, its coefficient was chosen. If the coefficient of the MS-21 aircraft roll moment when it hits the vortex wake exceeds that from the ailerons, such a flight is considered dangerous. The distances behind the A380 that are unsafe for the MS-21 aircraft flight are given.