Thomas Gerz

Commercial aircraft wake vortices

Abstract This paper discusses the problem of wake vortices shed by commercial aircraft. It presents a consolidated European view on the current status of knowledge of the nature and characteristics of aircraft wakes and of technical and operational procedures of minimizing and predicting the vortex strength and avoiding wake encounters. Methodological aspects of data evaluation and interpretation, like the description of wake ages, the characterization of wake vortices, and the proper evaluation of wake data from measurement and simulation, are addressed in the first part. In the second part an inventory of our knowledge is given on vortex characterization and control, prediction and monitoring of vortex decay, vortex detection and warning, vortex encounter models, and wake-vortex safety assessment. Each section is concluded by a list of questions and required actions which may help to guide further research activities. The primary objective of the joint international efforts in wake-vortex research is to avoid potentially hazardous wake encounters for aircraft. Shortened aircraft separations under appropriate meteorological conditions, whilst keeping or even increasing the safety level, is the ultimate goal. Reduced time delays on the tactical side and increased airport capacities on the strategic side will be the benefits of these ambitious ventures for the air transportation industry and services.

The turbulent decay of trailing vortex pairs in stably stratified environments

The decay of trailing vortex pairs in thermally stably stratified environments is investigated by means of large eddy simulations. Results of in-situ measurements in the wakes of different aircraft are used to find appropriate intitializations for the simulation of wake turbulence in the quiescent atmosphere. Furthermore, cases with weak atmospheric turbulence are investigated. It is shown that the early development of the vortices is not affected by turbulence and develops almost identically as in 2D simulations of wake vortices in stably stratified environments. In a quiescent atmosphere the subsequent vortex decay is controlled by the interaction of short-wave disturbances, owing to the aircraft induced turbulence, and baroclinic vorticity, owing to stable stratification. As a consequence, vertical vorticity streaks between the vortices are induced which are substantially intensified by vortex stretching and finally lead to rapid turbulent wake-vortex decay. When in addition atmospheric turbulence is also present, the long-wave instability is dominantly promoted. For very strong stratification ( Fr < 1) it is observed that wake vortices may rebound but lose most of their strength before reaching the flight level. Finally, the simulation results are compared to the predictive capabilities of Greene’s approximate model.  2001 Editions scientifiques et medicales Elsevier SAS

Wing-tip vortices, turbulence, and the distribution of emissions

The decay of wing-tip vortices under the Influence of turbulence in a stably stratified atmosphere is discussed by means of large-eddy simulations. The vortices originate from a B-747 aircraft in cruise. Atmospheric turbulence and turbulence originating from the boundary layer around the aircraft are distinguished. The former is weak and anisotropic with eddy sizes in the order of the wing span, whereas the latter is wrapped around the vortices with the maximum intensity at the core diameter. During their descent, the parallel vortex tubes approach each other because stratification and turbulence detrain mass into the ambient air. The atmospheric eddies deform the trailing vortices such that their spacing varies. This, in turn, yields different mutually induced velocities that amplify the deformation quickly according to Crows instability theorem. The bent vortex tubes link after about 1.5 min and form rings. The continuous trail of turbine exhaust is reorganized in a row of single puffs

Wake Vortices in Convective Boundary Layer and Their Influence on Following Aircraft

The decay of three wake vortex pairs of a B-747 aircraft in an evolving and convectively driven atmospheric boundary layer is investigated by means of large-eddy simulations (LES). Convective boundary layers are considered hazardousbecausetheupdraft velocitiesofa thermalmay compensatetheinduceddescent speed ofthevortex pair such that the vortices stall in the e ight path. The LES results illustrate that 1 )the primary rectilinear vortices are rapidly deformed on the scale of alternating updraft and downdraft regions; 2 ) parts of the vortices stay on e ight level but are quickly eroded by the turbulence of the updraft; 3 )the longest living sections of the vortices are foundinregionsofrelativelycalmdownwarde ow,which augmentstheirdescent.Striptheory calculationsareused to illustrate the temporal and spatial development of lift and rolling moments experienced by a following medium weight class B-737 aircraft. Characteristics of the respective distributions are analyzed. Initially, the maximum rolling moments slightly exceed theavailableroll controloftheB-737. After60 sthe probability ofrolling moments exceeding 50% of the roll control has decreased to 0.009% in a safety corridor around the glide path. Nomenclature b = aircraft span b0 = initial vortex spacing c = section chord cl = section lift coefe cient dP = probability difference g = gravitational acceleration k = wave number

Strategies for Circulation Evaluation of Aircraft Wake Vortices Measured by Lidar

An assessment of different methods for circulation evaluation from lidar measurement data of aircraft wake vortices is performed. The surface integral of vorticity serves as baseline case that is compared to a method that evaluates the lidar line-of-sight velocity midway between the vortices and to another method that calculates radii averages of circulations derived from tangential velocities. Systematic deviations from nominal circulation are discussed based on analytical vortices. High-resolution numerical simulation data are applied to perform virtual lidar measurements that reproduce, explain, and quantify (i) the frequently observed initial overestimation of circulation and (ii) the scatter of circulation data caused by the genuine variability of wake vortices in the atmospheric boundary layer. The theoretically derived characteristics of the different evaluation methods are verified against lidar data recorded by several lidar teams during the Wake Vortex Forecasting and Measuring Campaign at Oberpfaffenhofen (WakeOP), performed in spring 2001 at Fairchild Dornier Airport in Oberpfaffenhofen, Germany.

Vortex bursting and tracer transport of a counter-rotating vortex pair

Large-eddy simulations of a coherent counter-rotating vortex pair in different environments are performed. The environmental background is characterized by varying turbulence intensities and stable temperature stratifications. Turbulent exchange processes between the vortices, the vortex oval, and the environment, as well as the material redistribution processes along the vortex tubes are investigated employing passive tracers that are superimposed to the initial vortex flow field. It is revealed that the vortex bursting phenomenon, known from photos of aircraft contrails or smoke visualization, is caused by collisions of secondary vortical structures traveling along the vortex tube which expel material from the vortex but do not result in a sudden decay of circulation or an abrupt change of vortex core structure. In neutrally stratified and weakly turbulent conditions, vortex reconnection triggers traveling helical vorticity structures which is followed by their collision. A long-lived vortex ring links ...

High-Resolution Weather Database for the Terminal Area of Frankfurt Airport

Abstract A 1-yr meteorological dataset for the terminal area of Frankfurt Airport in Germany has been generated with a numerical weather prediction system to provide a synthetic though realistic database for the evaluation of new operational aircraft arrival procedures and their associated risks. The comparison of the 1-yr dataset with a local surface wind climatology indicates that the main climatological features are recovered. A subset of 40 days is validated against measurements from a sound detection and range/radio acoustic sounding system (SODAR/RASS) taken at Frankfurt Airport. The RMS errors of wind speed and direction are between 1.5 m s−1 at the surface and 2 m s−1 at 300 m and 40°, respectively. The frequency distribution of meteorological parameters, such as the wind component perpendicular to the glide path, shear, and thermal stratification, show good agreement with observations. The magnitude of the turbulent energy dissipation rate near the surface is systematically overestimated, whereas...

VORTEX STRUCTURES AND MICROFRONTS

This study addresses the relationship between thermal microfronts and coherent vortex structures in homogeneous turbulence. The turbulence is created by mean shear in a weakly stratified flow. The data set is generated by direct numerical simulation providing highly resolved instantaneous three‐dimensional fields of fluctuating velocity and temperature (1603 data points for each field). Vertically inclined large‐scale horseshoe vortices develop due to stretching and rotation by the mean shear rate, as would also occur in neutrally stratified flow. In a homogeneous shear flow, the structures on the tilted plane are oriented both upward and downward with equal probability, and are referred to as ‘‘head‐up’’ and ‘‘head‐down’’ horseshoe eddies. Vorticity structures are sampled in those regions of the flow where the strongest coherent local temperature gradients occur. The sampled fields are composited. It is found that the microfronts are caused by the local outflow between the legs of the horseshoe eddies. A...

Large-Eddy Simulation of Aircraft Wake Evolution from Roll-Up until Vortex Decay

The evolution of aircraft wake vortices from the roll-up until vortex decay is studied by large-eddy simulation. An aircraft model and the surrounding flow field obtained from high-fidelity Reynolds-averaged Navier–Stokes simulation are swept through a ground-fixed computational domain to initialize the wake. After the wake initialization a large-eddy simulation of the vortical wake is performed until vortex decay. In this paper the methodology is tested with a NACA 0012 wing and applied to the DLR-F6 wing-body model in cruise condition and a long-range aircraft model in high-lift configuration which was used in the European Aircraft Wing with Advanced Technology Operation project. The correlation between the detailed roll-up process of the vorticity sheet shedding from the main wing and the characteristics of the rolled-up vortex pair such as vortex circulation, core radius, and separation is investigated with and without ambient turbulence. In both the DLR-F6 wing-body and the long-range aircraft models...

Prediction of dynamic pairwise wake vortex separations for approach and landing

Design and performance of the Wake Vortex Prediction and Monitoring System WSVBS are described. The WSVBS has been developed to tactically increase airport capacity for approach and landing on single runways as well as closely-spaced parallel runways. It is thought to dynamically adjust aircraft separations dependent on weather conditions and the resulting wake vortex behavior without compromising safety. Dedicated meteorological instrumentation and short-term numerical terminal weather prediction provide the input to the prediction of wake-vortex behavior and respective safety areas. LIDAR monitors the correctness of WSVBS predictions in the most critical gates at low altitude. The WSVBS is integrated in the arrival manager AMAN of DLR. Performance tests of the WSVBS have been accomplished at Frankfurt airport in winter 2006/07 and at Munich Airport in summer 2010. Aircraft separations for landings on single runways have been compared employing the concepts of either heavy-medium weight class combinations or dynamic pairwise separations where individual aircraft type pairings are considered. For the very conservative baseline setup of the WSVBS the potential capacity gains of dynamic pairwise operations for single runways appear to be very small. On the other hand, the consideration of individual aircraft types and their respective wake characteristics may almost double the fraction of time when radar separation could be applied.