Wake Vortex Transport Research Articles

A nudged hybrid analysis and modeling approach for realtime wake-vortex transport and decay prediction

We put forth a long short-term memory (LSTM) nudging framework for the enhancement of reduced order models (ROMs) of fluid flows utilizing noisy measurements for air traffic improvements. Toward emerging applications of digital twins in aviation, the proposed approach allows for constructing a realtime predictive tool for wake-vortex transport and decay systems. We build on the fact that in realistic application, there are uncertainties in initial and boundary conditions, model parameters, as well as measurements. Moreover, conventional nonlinear ROMs based on Galerkin projection (GROMs) suffer from imperfection and solution instabilities, especially for advection-dominated flows with slow decay in the Kolmogorov n-width. In the presented LSTM nudging (LSTM-N) approach, we fuse forecasts from a combination of imperfect GROM and uncertain state estimates, with sparse Eulerian sensor measurements to provide more reliable predictions in a dynamical data assimilation framework. We illustrate our concept by solving the two-dimensional vorticity transport equation. We investigate the effects of measurements noise and state estimate uncertainty on the performance of the LSTM-N behavior. We also demonstrate that it can sufficiently handle different levels of temporal and spatial measurement sparsity, and offer a huge potential in developing next-generation digital twin technologies for aerospace applications.

Multimodel Ensemble Methods for Prediction of Wake-Vortex Transport and Decay

Several multimodel ensemble methods are selected and further developed to improve the deterministic and probabilistic prediction skills of individual wake-vortex transport and decay models. The different multimodel ensemble methods are introduced, and their suitability for wake applications is demonstrated. The selected methods include direct ensemble averaging, Bayesian model averaging, and Monte Carlo simulation. The different methodologies are evaluated employing data from wake-vortex field measurement campaigns conducted in the United States and Germany.

Multi-model ensemble wake vortex prediction

Purpose Wake vortices that are generated by an aircraft as a consequence of lift constitute a potential danger to the following aircraft. To predict and avoid dangerous situations, wake vortex transport and decay models have been developed. Being based on different model physics, they can complement each other with their individual strengths. This paper investigates the skill of a Multi-Model Ensemble (MME) approach to improve prediction performance. Therefore, this paper aims to use wake vortex models developed by NASA (APA3.2, APA3.4, TDP2.1) and by DLR (P2P). Furthermore, this paper analyzes the possibility to use the ensemble spread to compute uncertainty envelopes.

Fast-Time Modeling of Ground Effects on Wake Vortex Transport and Decay

A fast-time model for wake vortex behavior prediction in ground proximity is presented. This model takes into account the combined effects of ground proximity and wind (both crosswind and headwind components) on the wake vortex transport and decay. It aims to mimic the whole flow using a limited set of vortex particles to model both the primary two-vortex system and the ground-generated secondary vorticity. A redistribution of the vortex particles is also included in the model to limit the number of used particles and hence, guarantee a high computational efficiency. The model is integrated in the deterministic wake vortex model, which is a software that predicts, also in real-time, the temporal evolution of the wake vortices generated by a given aircraft evolving in given ambient meteorological conditions. The new ground effect model capacity is assessed using data from a measurement campaign and from large-eddy simulations. It is found that the model is able to reproduce the wake vortex behavior (both for transport and decay) for the most relevant times required for air traffic management applications.

Aircraft Vortices in Stably Stratified and Weakly Turbulent Atmospheres: Simulation and Modeling

This study investigates the influence, on aircraft wake vortex behavior, of atmospheres that are stably stratified (neutral to very strong) and weakly turbulent, by means of large-eddy simulations at very high Reynolds number and on relatively fine grids. The atmospheric fields are first generated using large-eddy simulations of forced and stratified turbulence reaching a statistically stationary state. The obtained fields are shown to be realistic and consistent with the literature. A pair of counter-rotating vortices, with relatively tight cores, is then put in the obtained fields and evolved. The evolution of the wake vortex topology, transport, and decay is analyzed in depth by measuring the wake vortex characteristics in all cross planes. The vortex transport and deformation are related to the stratification and turbulence levels. Stratification combined with weak turbulence is seen to strongly affect the Crow instability development. Different decay mechanisms are identified, related to the interactions with the turbulence, the turbulent baroclinic vorticity, and/or between the vortices themselves. Finally, improved simplified models of vortex altitude evolution and of vortex decay (with two phases) are proposed and calibrated on the present results. They yield good agreement with the large-eddy simulation results and are usefully integrated in our operational models.

Crosswind thresholds supporting wake‐vortex‐free corridors for departing aircraft

AbstractDuring two field measurement campaigns aircraft wake vortex trajectory and wind measurement data have been collected at Frankfurt airport. Three different approaches have been used to analyse the data in order to estimate crosswind threshold values supporting vortex‐free corridors for departing aircraft. Although several competing effects such as wake vortex transport in and out of ground effect, temporal and spatial wind variability, and the spreading of aircraft trajectories after take‐off, complicate the analyses, all three approaches lead to similar crosswind thresholds. Employing standard instrumentation at 10 m, a minimum crosswind threshold of 3.5–4.6 m s−1 has been identified to clear a safety corridor of 150 m width from wake vortices with a 95% probability within 60 s. Alternative estimations of crosswind thresholds employing different instrumentation and different height ranges are reported. Crosswind thresholds can be reduced if the wind is measured close to the air mass in which the vortices evolve. A definite crosswind threshold for operational use cannot be deduced solely from this study since critical factors such as risk and safety assessment have not yet been taken into account. Copyright © 2011 Royal Meteorological Society

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.

Skill of an Aircraft Wake-Vortex Model Using Weather Prediction and Observation

The performance of the two-phase probabilistic wake-vortex transport and decay model using numerical weather prediction and weather observations as input is analyzed using data from a wake measurement campaign carried out at Frankfurt airport during fall 2004. The wake-vortex observations include wakes evolving in and out of ground effects. The best forecast quality is achieved for wakes evolving in ground effect. We include the simplified hazardarea prediction model to compute the time to clear the corridor from hazardous wakes and to assess the potential capacity gain for single-runway operations. The highest-capacity potential can be expected when weather observations are employed for wake predictions. The limiting factor for capacity are wakes evolving in ground effect. Out of ground effect, the self-induced vertical transport of the wake vortex proves to be a robust mechanism to clear the approach corridor in an efficient way. This is found independently from the source of meteorological input. In contrast, the consideration of lateral wake transport alone leads to marginal-capacity potential. The analysis also shows that weather data along the glide path can be provided by numerical weather prediction for safe wake prediction.

Research towards a wake-vortex advisory system for optimal aircraft spacing

Wake turbulence is a major concern for busy airports since it limits capacity. Solutions for new aircraft staggering procedures are sought which relax the current separations but keep safety on a high level. Systems which advice air-traffic control on wake-vortex behaviour under present and expected weather conditions will, hopefully, contribute to such a solution. Knowledge on transport and decay of wake vortices in the atmosphere is presented. Concepts and designs of wake-vortex advisory systems in Europe and the USA are outlined. European wake-vortex measurement and prediction campaigns are described where the components of such systems have been tested successfully. To cite this article: T. Gerz et al., C. R. Physique 6 (2005).

Probabilistic Two-Phase Aircraft Wake Vortex Model: Application and Assessment

Predictions of the parametric probabilistic two-phase aircraft wake-vortex transport and decay model P2P are compared with field observations. The two-phase decay model predicts probabilistic wake-vortex behavior as a function of aircraft and environmental parameters in real time. Observation data from field deployments accomplished at the International Airports Memphis and Dallas Fort Worth and from the WakeOP campaign performed at the airfield in Oberpfaffenhofen, Germany, are employed. In a scoring procedure, the predictive capabilities of a deterministic version of P2P are compared to Sarpkaya's model

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.

Probabilistic Two-Phase Wake Vortex Decay and Transport Model

A new parametric wake vortex transport and decay model is proposed that predicts probabilistic wake vortex behavior as a function of aircraft and environmental parameters in real time. The probabilistic two-phase wake vortex decay model (P2P) accounts for the effects of wind, turbulence, stable stratie cation, and ground proximity. The model equations are derived from the analytical solution of the spatiotemporal circulation evolution of the decaying potential vortex and are adapted to wake vortex behavior as observed in large-eddy simulations. Vortex decay progresses in two phases, a diffusion phase followed by rapid decay. Vortex descent is a nonlinear function of vortex strength. Probabilistic components account for deviations from deterministic vortex behavior inherently caused by the stochastic nature of turbulence, vortex instabilities, and deformations, as well as uncertainties and e uctuations that arise from environmental and aircraft parameters. The output of P2P consists of cone dence intervals for vortex position and strength. To assign a dee ned degree of probability to the predictions reliably, the model design allows for the continuous adjustment of decay parameters and uncertainty allowances, based on a growing amount of data. The application of a deterministic version of P2P to the Memphis wake vortex database yields favorable agreement with measurements.

Algorithm for Prediction of Trailing Vortex Evolution

We describe an algorithm that predicts the trajectories and circulation decay of aircraft trailing vortices. We e rst presentthemethodology used by the algorithm to simulatetrailing vortex behavior, including 1 )thedescent of the vortices through a realistic atmosphere dee ned by crosswind, turbulence, and stratie cation proe les and 2 ) the interaction of the vortices with the ground. Examples are presented to demonstrate the algorithm’ s capability. We then describea database approach forquantifying comparisons between algorithm predictions and measurements of wake vortex transport and decay. The use of databases to optimize the constant factor k in the term governing the effect of turbulence on vortex decay is reported, with the result that k appears to depend on the intensity of ambient turbulence. Using selected groupings of cases from the database, we have determined that vortices not interacting with the ground decay about a factor of two faster in high-intensity ambient turbulence than in low-intensity ambient turbulence. We also show that the circulation decay for vortices in the ground effect appears to be independent of the ambient turbulence level.

The Development of the Wake Vortices Warning System for Frankfurt Airport: Theory and Implementation

A Wake Vortices Warning System (WVWS) has been identified as a candidate means for increasing the capacity of Frankfurt Airport. For certain meteorological conditions, the WVWS is expected to allow for the suspension of required wake vortex separations between aircraft approaching the airport’s two parallel runways 25L and 25R. At present, the proximity of these parallel runways does not allow wake-vortex-independent operations as a standard procedure. The task of the WVWS is to model the propagation of wake vortices produced in the atmospheric boundary layer by aircraft approaching the runways, and to forecast the wake vortex transport, which is influenced mainly by the crosswind. This predicted propagation of the wake vortices is used to determine the risk for approaching aircraft that is produced by wake vortices generated in the vicinity of the two runways. On the basis of this hazard prediction, an appropriate landing/operational procedure is recommended to the controller.The development of the WVWS is based on the results of a long-term research program started in the 1980s. This program included an extensive measurement campaign at Frankfurt Airport and thorough analysis of the operational implications of a WVWS. The development of the basic algorithms for the WVWS prototype began in 1992. A project with the objective of implementing, installing, and evaluating a WVWS system at Frankfurt Airport was started in March 1995. The system integration was completed in the summer of 1996. Evaluation of the system is still in progress.The WVWS project at Frankfurt Airport will be reviewed in two papers. This paper presents the project history, the basic theory and algorithms of the WVWS, the landing procedures for operational use of the system, the WVWS design and implementation, and the testing and evaluation of the system. A future paper will present the methods used for risk analysis, costbenefit analysis, and the results obtained.

Empirical model of transport and decay of wake vortices between parallel runways

This article describes a simple empirical model for the transport and decay of aircraft trailing wake vortices between parallel runways in a turbulent atmosphere. It is suitable for applications to both single and closely spaced parallel runways. The model includes the influence of ambient turbulence on the decay rate of the wake vortices as well as the effect of cross wind shear. Well-known Monin-Obukh off similarity laws in the lower atmospheric boundary layer are used to compute the ambient shear and turbulence. Vortex bounce that occurs near the ground is also parameterized in the model. The results show that ambient shear plays an important role in vortex decay and transport. Atmospheric stability also affects the fate of these vortices, but with stable stratification having a much larger effect than convective heating of the atmospheric boundary layer. These results may prove useful for a better understanding of the factors that affect the transport and decay of aircraft wake vortices near the ground in the presence of crosswinds in a turbulent atmosphere.

Doppler lidar investigation of wake vortex transport between closely spaced parallel runways

The paper demonstrates the potential of the Doppler lidar technique for the investigation of aircraft wake vortices. During several extended field experiments at the Frankfurt/Main Airport, the DLR laser Doppler anemometer has measured the vortex properties of a large variety of landing aircraft. These data are used to increase the understanding of the behavior of wake vortices with special emphasis on the horizontal vortex transport in ground effect. In particular, the results of the vortex transport considerations are the basis for air traffic control to enhance the efficiency of closely spaced parallel runway systems with respect to wake vortices.

Effect of Wind Shear and Ground Plane on Aircraft Wake Vortices

A previous comparison of predicted wake vortex transport with experiment revealed two consistent sources of discrepancy, namely, lower predicted sink rates of the vortex pair relative to observed and a lagging of the predicted upwind vortex transport compared with the measured values. It was hypothesized that either a wake tilting or a decrease in circulation was occurring to account for this lagging. Accordingly, an analysis was performed to determine the streamlines associated with the presence of a vortex pair in a shear near the ground and the movement of the stagnation points in such a region. It was found that the upper and lower stagnation points rise or sink relative to their shearless location, depending upon the magnitude of nondimensional parameters defining ground proximity and wind shear to circulation.

Analysis of Predicted Aircraft Wake Vortex Transport and Comparison with Experiment

symmetrically over the surface becomes highly nonuniform at increasing body and angle of attack, generating a region of lower temperature on the leeward side and a region of higher temperature on the windward side. In practical applications a lower average temperature and then a higher average heat transfer rate are inevitable for the entire surface to maintain every point below the maximum permissible temperature for the material used. To minimize the severe deterioration of the cooling capability of slot injection due to three-dimensional effects, new designs are required for the slot system which give a nonuniform peripheral distribution of the coolant. At this purpose a very simple slot system scheme which counters the cross-flow effects has been investigated in the present study. The scheme consists of nonsymmetric slots, higher on the windward side and lower on the leeward side which, maintaining the average height Seq = (SL + Sw)/2, i.e., the injection coolant mass flow, and producing a nonuniform injection pattern, act to prevent and reduce the negative cross-flow effects on cooling. The experimental results (Fig. 5), compared with the previous symmetric slot results, generate a more uniform adiabatic wall temperature distribution and thus improve the cooling capability of the system. The results obtained with this primitive nonsymmetric slot scheme are only indicative, however they can suggest a possible direction of investigation.