Is the assumption of straight vortices valid for encounter hazard assessment?

Transcription

1 Is the assumption of straight vortices valid for encounter hazard assessment? Dennis Vechtel DLR Institute of Flight Systems Brétigny-sur-Orge, May 13 th, 2014 WakeNet-Europe 2014 Workshop Technology, Deployment, Safety Case, Long Term Perspective

2 Chart 2 D. Vechtel, WakeNet-EU, Overview Introduction Objective Encounter Analysis Subjective Hazard Assessment Conclusions

3 Chart 3 D. Vechtel, WakeNet-EU, Overview Introduction Objective Encounter Analysis Subjective Hazard Assessment Conclusions

4 Chart 5 D. Vechtel, WakeNet-EU, Introduction Wake Encounter Hazard Assessment Different approaches to wake encounter hazard rating: a-posteriori: objective: aircraft reaction (bank angle, accelerations etc.) subjective: e.g. by means of pilot questionnaires a-priori: prognosis of the encounter hazard by means of predictable values Aim: derive boundaries from in-situ-analyses (a-posteriori) to serve for a-priori-hazard assessment Metric: Roll Control Ratio (RCR) RRRRRR = CC ll,iiiiii CC ll,ξξmmmmmm

5 Chart 6 D. Vechtel, WakeNet-EU, Introduction Wake Encounter Hazard Assessment Real flight test most realistic, but mostly vortex parameters (e.g. circulation) are unknown specific encounter not repeatable Flight simulation accurate vortex model necessary, which represents the vortex decay realistically! In the past usually very simple vortex models assuming the wake to consist of straight vortices were used for encounter hazard assessment

6 Chart 7 D. Vechtel, WakeNet-EU, S. C. Crow, 1970 Introduction Vortex Deformation During the decay wake vortices tend to deform first by atmospheric disturbances, and with increasing age more and more selfinduced Typical stages of deformation sinusoidal shape with increasing amplitude (so-called wavy vortices ) linking of the vortices vortex rings with increasing lateral spread Phenomenon discovered in 1950s First detailed analysis by S.C. Crow in 1970 Effect mostly neglected for encounter hazard assessment Few studies showed an influence on aircraft reaction and pilot perception Smith & Beesmer, 1959

7 Chart 8 D. Vechtel, WakeNet-EU, Introduction Vortex Deformation These photos clearly show that the assumption of wake vortices being always straight is a strong simplification! But is this assumption nevertheless valid? [DLR] [Mila Zinkova] [Dietrich Fischenberg]

8 Chart 9 D. Vechtel, WakeNet-EU, Introduction Large Eddy Simulations Realistic representation vortex deformation and decay by LES Consideration of specific atmospheric conditions All simulations mentioned in the following based on the same set of LES generated by DLR Institute of Atmospheric Physics Moderate turbulence (ε* = 0.23) and neutral t age = 16 s t age = 80 s thermal stratification (N* = 0.00) Considered to be representative for lower altitudes (approach phase) Generator aircraft HEAVY in approach configuration t age = 108 s t age = 136 s 60.3 m wing span 190 t gross weight 70 m/s TAS

9 Chart 10 D. Vechtel, WakeNet-EU, Overview Introduction Objective Encounter Analysis Subjective Hazard Assessment Conclusions

10 Chart 11 D. Vechtel, WakeNet-EU, Objective Encounter Analysis Overview Parameter variation study Space-fixed encounter simulation LES-vortices of nine different vortex ages ( continuously increase of deformation) and straight vortices of same strength Simulation model of DLR research aircraft A320 ATRA with autopilot engaged in landing configuration Variation of encounter angles and positions Altogether almost 500,000 encounters simulated

11 Chart 12 D. Vechtel, WakeNet-EU, Objective Encounter Analysis Results Similar aircraft reaction with very young vortices (no deformation)

12 Chart 13 D. Vechtel, WakeNet-EU, Objective Encounter Analysis Results Generally, less aircraft reaction with deformed vortices (except n z )

13 Chart 14 D. Vechtel, WakeNet-EU, Objective Encounter Analysis Results Temporary increase of aircraft reaction with wavy vortices shortly prior links

14 Chart 15 D. Vechtel, WakeNet-EU, Objective Encounter Analysis Results Significant decrease in aircraft reaction after linking

15 Chart 16 D. Vechtel, WakeNet-EU, Objective Encounter Analysis Results Strong encounters occurred more often with straight vortices Much smaller probability of strong encounters with deformed vortices

16 Chart 17 D. Vechtel, WakeNet-EU, Objective Encounter Analysis Results Comparison of the both strongest encounters with straight and deformed vortices at age of 108 s (wavy vortices shortly prior linking) Encounter with deformed vortices results in slightly stronger aircraft reaction Similar encounter azimuth Different encounter inclination angle Straight: slightly from below Deformed: steep descent Relatively long flyby along vortex line in case of deformed vortices

17 Chart 19 D. Vechtel, WakeNet-EU, Overview Introduction Objective Encounter Analysis Subjective Hazard Assessment Conclusions

18 Chart 20 D. Vechtel, WakeNet-EU, Subjective Hazard Assessment Overview Pilots are probably the best source to judge what is acceptable and what is not in terms of threats posed to an aircraft Subjective perception differs between pilots but can be correlated with objective measures Dedicated wake encounter rating scale: aircraft control, demands on the pilot, excursions from flight path, hazard Definitions: acceptability boundary: everything below is harmless unacceptability boundary: everything above is unacceptable

19 Chart 21 D. Vechtel, WakeNet-EU, A330 Simulator Campaign wavy vortices shortly prior linking vs. straight ones ~ 0.4 ~ 0.8 ~ 0.3 ~ 0.35 Lower acceptance of the pilots with deformed vortices than with straight ones Vechtel, D. Flight simulator study on the influence of vortex curvature on wake encounter hazard using LES wind fields, The Aeronautical Journal, Volume 116, No. 1177, March 2012

20 Chart 22 D. Vechtel, WakeNet-EU, In-Flight Simulation with ATTAS wavy vortices vs. vortex rings ~ 0.25 ~ 0.5 ~ 0.27 ~ 1.02 Higher acceptance of the pilots with vortex rings than with wavy vortices Vechtel, D. In-flight simulation of wake encounters using deformed vortices, The Aeronautical Journal, Volume 117, No. 1196, October 2013

21 Chart 23 D. Vechtel, WakeNet-EU, AVES-A320 Simulator Campaign deformed vortices of different ages Preliminary results (campaign in February 2014) Largest aircraft response with vortices during linking All unaccepted encounters shortly prior or during linking Astonishingly, the maximum Linking RCR did not occur with the strongest vortices Linking Pacho, P. Preparation and conduction of a simulator study for analysis of wake encounter hazard, master thesis, DLR internal report IB /16, Braunschweig, 2013

22 Chart 24 D. Vechtel, WakeNet-EU, Overview Introduction Objective Parameter Analysis Subjective Hazard Assessment Conclusions

23 Chart 25 D. Vechtel, WakeNet-EU, Conclusions Vortex deformation seems to weaken the encounter impact for young and only slightly deformed vortices and for old vortices in ring stage Stronger aircraft reaction and worse pilot acceptance at vortex ages shortly prior or during linking than with straight vortices of same strength Strong encounters less probable with deformed vortices Acceptability boundary appears to be less dependent from vortex deformation than unacceptability boundary Below RCR ~ vortex deformation does not seem to be a factor Any revision of separations that shifts possible encounters from rings to wavy vortices might result in stronger encounters than expected

24 Chart 26 D. Vechtel, WakeNet-EU, Conclusions Coming back to the original question Is it valid to assume vortices as straight for hazard assessment? Well, yes and no For young vortices such assumption would result in a conservative representation (not relevant for operational application) For old vortices the same Disregarding deformation effects might underestimate the hazard of encounters with strongly deformed wavy vortices shortly prior or during linking In case that only weak encounters are analysed (RCR< ) deformation can be disregarded It depends on the purpose of the hazard assessment but vortex deformation should not be generally disregarded

25 Thank you for your attention German Aerospace Center (DLR) Institute of Flight Systems Department of Flight Dynamics & Simulation Lilienthalplatz Braunschweig Dipl.-Ing. Dennis Vechtel Telephone Telefax dennis.vechtel@dlr.de