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Wake Vortices of Landing Aircraft

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Wake Vortices of Landing Aircraft Wake Vortices of Landing Aircraft Anton Stephan Munchen¨ 2013 Wake Vortices of Landing Aircraft Anton Stephan Dissertation an der Fakult¨at fur¨ Physik der Ludwig–Maximilians–Universit¨at Munchen¨ vorgelegt von Anton Stephan aus Charkow (Ukraine) Munchen,¨ Dezember 2013 Erstgutachter: Prof. Dr. George Craig Zweitgutachter: Prof. Dr. Ulrich Schumann Tag der mundlichen¨ Prufung:¨ 26.02.2014 F¨ur meine Familie Abstract As a consequence of lift generation, flying aircraft generate a complex turbulent wake consisting of strong coherent flow structures. The evolution of the aircraft’s wake prior to and directly after touchdown still poses many open questions. The dynamics as well as the numerical treatment of the wake flow are very challenging, as multiple scales determine the flow related to the complex geometry of an aircraft, the interaction with the ground surface, and the environmental conditions. The evolution of the aircraft’s wake is an example of a complex turbulent flow challenging modern physics, because it is composed of strong coherent flow structures that exhibit a range of length scales spanning several orders of magnitude all interacting with one another. Fully rolled-up, two counter rotating wake vortices emerge, possessing a high amount of kinetic energy and therefore posing a potential hazard to following aircraft. To avoid wake-vortex encounters, aircraft regulatory separation distances have been introduced. In a growing economy the capacity problems of congested airports also get worse. Therefore, a process has been initiated by aviation authorities aiming at the recategorization of the static wake-vortex separations. Obviously, the safety of the passengers must not be compromised by any capacity increasing measures. Artificial devices for the alleviation of wake vortices could not be established so far. A novel method for artificial wake-vortex decay enhancement, consisting of obstacles in- stalled at the runway tails, is investigated in detail, with temporal large-eddy simulations (LES). First the mechanisms of wake-vortex decay of fully rolled-up vortices, interacting with a flat ground surface subjected to a turbulent crosswind, are studied. The simulations reveal that so-called crosswind streaks perturb the secondary vorticity layer at the ground surface, such that hairpin-shaped secondary vortices detach from the ground and trigger turbulent vortex decay. Obstacles at the ground surface trigger the rapid generation of powerful secondary vortices that interact vigorously with the primary vortices. Analyzing the vortex dynamics, it is found, that the obstacles trigger both, pressure disturbances propagating within the vortex cores and helical secondary vortex structures, propagating exteriorly along the primary vortices. A model for the propagation speed of the disturban- ces is elaborated. Plate lines are introduced as efficient alternative to massive obstacles that could not be installed in the vicinity of runways. Simulation results are compared with par- ticle image velocimetry measurements accomplished in a water towing tank. Qualitatively the evolutions of vortex position and decay agree well. In this thesis a method is developed to investigate the wake evolution during the complete landing phase, including final approach, flare, touchdown, and deceleration. Therefore a recently developed method to investigate the entire wake evolution from roll-up to decay is extended for downward sloping trajectories. For this purpose, the flow around an aircraft model in high-lift configuration, with deployed flaps and slats, obtained from a high-fidelity viii Reynolds-averaged Navier-Stokes (RANS) simulation, is swept through the ground fixed LES domain. This way the peculiarities of the flow details around the complex aircraft geometry can be transferred to the vortex characteristics, developing during and after roll- up. The simulations reveal that so-called end effects, appearing shortly after touchdown, propagate along the wake vortices, leading to rapid decay. Disturbances generated by a plate line interact with the end effects and further accelerate vortex decay. During the decay phase, the vortices are subjected to complex deformations, strong divergence as well as ground linking. Flight measurement data gathered during the WakeMUC campaign at Munich airport are used to compare flow characteristics. Good agreement is found concerning the surprisingly complex vortex trajectories. Deviations from the observed decay rates can be attributed to the lack of wake turbulence initiation during the RANS-LES transition step, motivating further work. Zusammenfassung Als Folge des Auftriebs entsteht bei einem fliegenden Flugzeug eine komplexe turbulente Nachlaufstr¨omung bestehend aus energiereichen koh¨arenten Strukturen. Die Entwicklung der Nachlaufwirbel vor und unmittelbar nach der Landung wirft viele ungekl¨arte Fragen auf. Sowohl die Dynamik, als auch die numerische Behandlung der Nachlaufstr¨omung stel- len eine große Herausforderung dar. Aufgrund der komplexen Flugzeuggeometrie, des Ein- flusses des Bodens und der meteorologischen Bedingungen bestimmen sehr unterschiedliche Skalen die Str¨omung. Somit stellt die Nachlaufentwicklung von Flugzeugen ein hervorra- gendes Beispiel fur¨ komplexe turbulente Str¨omungen und damit eine große Herausforderung der modernen Physik dar. Die entstehenden Wirbelstr¨omungen sind besonders gekennzeich- net durch koh¨arente Strukturen, die mehrere Gr¨oßenordnungen an L¨angenskalen uberspan-¨ nen und nichtlinear miteinander interagieren. Voll aufgerollt bilden sich zwei energiereiche, gegensinnig rotierende Nachlaufwirbel, die sogenannte Wirbelschleppe, die ein Risiko fur¨ nachfolgende Flugzeuge darstellt. Um Wirbelschleppenbegegnungen zu vermeiden, wurden an Flugh¨afen Staffelungsabst¨ande zwischen aufeinander folgenden Flugzeugen eingefuhrt.¨ In einer prosperierenden Wirtschaft werden Kapazit¨atsprobleme verkehrsreicher Flugh¨afen uber¨ die Jahre versch¨arft. Daraufhin haben Luftfahrtbeh¨orden eine Uberpr¨ ufung¨ in die We- ge geleitet, der die statische Flugzeugstaffelung neu gestalten soll. Dabei darf die Sicherheit der Flugg¨aste nicht durch kapazit¨atserweiternde Maßnahmen eingeschr¨ankt werden. Effek- tive technische Vorrichtungen, die den Wirbelzerfall beschleunigen, wurden bisher noch nicht etabliert. Eine neuartige Methode zur kunstlichen¨ Beschleunigung des Wirbelschleppenzerfalls, bestehend aus vor der Rollbahn aufgestellten Hindernissen, wird in dieser Arbeit weiter entwickelt und detailliert mit Large Eddy Simulationen (LES) untersucht. Dazu werden zun¨achst die Zerfallsmechanismen von voll-aufgerollten, mit dem Boden interagierenden Wirbeln, unter dem Einfluss eines Seitenwindes, betrachtet. Die Simulationen zeigen, dass streifenf¨ormige Wirbel (Streaks) die induzierte Wirbelschicht am Boden st¨oren, so dass sich Haarnadelwirbel vom Boden abl¨osen und turbulente Zerfallsprozesse der Wirbel ausl¨osen. Die Hindernisse l¨osen gezielt die Entstehung von starken Sekund¨arwirbeln aus, die aktiv mit den Prim¨arwirbeln interagieren. Es zeigt sich, dass die Hindernisse sowohl Druckst¨orun- gen im Wirbelkern, als auch Helix-artige Sekund¨arwirbel ausl¨osen, die sich beide schnell entlang der Wirbel ausbreiten. Ein Modell fur¨ die Ausbreitungsgeschwindigkeit der Wirbel wird entwickelt. Plattenreihen (plate lines) stellen eine praktikable Alternative fur¨ mas- sive Hindernisse in Landebahnn¨ahe dar. Die Simulationen werden mit Messergebnissen von Str¨omungsfeldern (Particle Image Velocimetry) in Wasserschleppkanalexperimenten verglichen. Qualitativ stimmen die Entwicklung der Wirbelposition und des -zerfalls in Simulation und Experiment gut uberein.¨ x Abstract Ferner wird eine Methode entwickelt, die die Untersuchung der Nachlaufentwicklung w¨ahrend der kompletten Landung, einschließlich Anflug, Abfangen, Aufsetzen und Ab- bremsen, erm¨oglicht. Dafur¨ wird eine kurzlich¨ entwickelte Methode, die die vollst¨andige Nachlaufentwicklung, vom Aufrollen bis zum Zerfall der Wirbel abdeckt, auf geneigte Tra- jektorien erweitert. Dazu wird die Umstr¨omung eines Flugzeugmodells in Hochauftriebs- konfiguration, mit ausgefahrenen Klappen und Vorflugeln,¨ die vorab mit einer Reynolds- gemittelten Navier-Stokes (RANS) Simulation berechnet wurde, durch ein LES Rechenge- biet gezogen. Dadurch werden Besonderheiten und Str¨omungsdetails der komplexen Flug- zeugumstr¨omung uber¨ den Aufrollvorgang auf die Wirbeleigenschaften ubertragen.¨ Die Simulationen zeigen, dass sogenannte Endeffekte, die kurz nach dem Aufsetzen entstehen, sich entlang der Wirbel ausbreiten und zu schnellem Zerfall fuhren.¨ Die St¨orungen der Plat- tenreihen kollabieren mit den Endeffekten und verst¨arken den Zerfall zus¨atzlich. W¨ahrend der Zerfallsphase werden die Wirbel auf komplexe Weise stark deformiert, sie divergieren und verbinden sich mit dem Boden. Simulationsergebnisse werden mit Messdaten vergli- chen, die w¨ahrend der WakeMUC Kampagne am Flughafen Munchen¨ gesammelt wurden. Gute Ubereinstimmung¨ kann bei den uberraschend¨ komplexen Wirbeltrajektorien beob- achtet werden. Abweichungen bezuglich¨ der Zerfallsraten k¨onnen auf eine unvollst¨andi- ge Initialisierung der Turbulenz w¨ahrend der Assimilierung der RANS Str¨omungsfelder zuruckgef¨ uhrt¨ werden und motivieren zukunftige¨ Arbeiten. Inhaltsverzeichnis Abstract vi Zusammenfassung ix 1. Introduction 1 1.1. Motivation.................................... 1 1.2. AimsoftheThesis ............................... 2 1.3. State-of-the-art-Recent Developments . ...... 7 1.4. OverviewovertheThesis...........................
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