Impact of a Formation Flight Wake-Tracking Strategy on the Dynamics of the Downstream Wake
Creators: name:  I. RANSQUIN 
name:   M. DUPONCHEEL 
name:   D. CAPRACE 
name:   P. CHATELAIN 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLIGHT PHYSICS
SEPARATED FLOWS
SHEAR LAYERS
WAKES

description: Formation flight is known to improve the efficiency of a two-aircraft formation (leader+follower). Indeed, the follower benefits from substantial apparent drag reduction when flying in the upwash region of the leader wake. This bio-inspired technique can therefore lead to significant fuel savings as recently showed in commercial aviation \\cite{Flanzer:2020,Airbus:2021}. For civilian applications, the concept of extended formation flight \\cite{Ning:2011a} is more appropriate than close formation flight as it better adresses safety concerns. In that case, the follower is flying several tens of wingspans downstream of the leader and still benefits from a positive interaction. However, an accurate tracking of the wake position is thus mandatory in order to perform efficient formation flight, as it has been shown that 50\\% of the benefit is lost if the follower cannot maintain a lateral and vertical relative position within 10\\% span of the leader. This energy-efficient formation flight strategy leads however to different wake dynamics compared to those of the wake of an isolated aircraft. Even more, the uncertainty on the relative position between the follower and the wake of the leader that is inherent to any tracking strategy, further increases the complexity of the underlying vortex dynamics. In this work, we want to quantify the impact of that uncertainty on the propagation of the formation wake through the use of Large Eddy Simulations (LES). We use the state-of-the-art LES flow solver, called Vortex-Particle-Mesh method (VPM) \\cite{Chatelain:2008}, and a calibrated combination of 3D Space Developing and 3D Time Developing (TD) simulations to capture the roll-up of the vortex sheets originating from the leader and the follower, their mutual interactions and their long term evolution in a turbulent environment. In order to model as realistically as possible the uncertainty related to wake tracking, we also use a real time wake tracking tool based on aircraft dynamics measurements \\cite{Ransquin:2021a}. Indeed, the 6 degrees-of-freedom dynamics of the leader and the follower are accounted through in the VPM simulations. By measuring its dynamics, the follower estimates the position of the wake of the leader, and tracks it in order to maintain a constant relative position. The motion of the follower induced by the uncertainty of the estimated wake position leads to oscillations of the follower\'s wake and therefore significantly different formation wake dynamics. We then study the consequences and constraints induced by the uncertain wake for the air traffic management, e.g., in terms of Rolling Moment Coefficient sensed in the wake of the formation. In this paper, we will first briefly review the VPM method and the associated wake tracking tool. We then present the methodology followed to perform either the SD and TD simulations and we will describe the formation flight configuration studied. We will finally describe the wake vortices evolution, and the wake vortex characteristics observed in the far wake behind two aircraft flying in formation and equipped with the tracking tool that leads to uncertain relative position. We will draw the resulting conclusions in terms of wake hazard and air traffic management.