Plasmas in aerospace

Aircraft Lightning Strike Research

Introduction

A civil aircraft gets struck by lightning at least once per year. Lightning strikes are considered one of the most dangerous atmospheric hazards to aviation, and cause costly damages and service disruptions. Despite its practical importance in terms of safety and economic impact in aviation, the interaction of an aircraft in flight with lightning is still at an early stage of research.

Project description

This research program, in conjunction with the MIT Aerospace Computational Design Lab, targets an improved understanding of the physics of aircraft lightning strikes, leading to the conception of preventive countermeasures. The research focus is on the development of physics-based models, of the lightning inception and swept stroke phases, that combine detailed numerical simulation to analytical or semi-empirical sub-models.

Lightning inception: One of the main differences between lightning to aircraft, as compared to ground-based structures, is that the airborne vehicle usually acts as the trigger of lightning. In the presence of an ambient electric field, an aircraft becomes polarized: one end becomes positively charged and the opposite becomes negatively charged. Through this mechanism, the electric field on the aircraft’s surface is significantly enhanced (Figure 1) and may initiate a bidirectional leader from the body that precedes the lightning arc [1,2]. 

Figure 1: Electric field amplification on the surface of a Falcon aircraft for several ambient electric field orientations. Results of electrostatic simulation.

Swept stroke: The lightning arc is established between a cloud or the ground (stationary electrode) and the aircraft (moving electrode) [3]. Therefore, the majority of the arc’s length is stationary with respect to the air except for the segment in close proximity to the aircraft’s surface. This segment is typically elongated (Figure 2) and can lead to a reattachment of the arc to a new location along the aircraft’s surface [3,4]. Insight into where these reattachment points may occur is crucial in terms of ensuring that adequate protection measures are embedded in the vehicle.

Figure 2: Model of lightning arc convected by turbulent flow over an airfoil: (a) mean velocity field, (b) instantaneous velocity field [5].

References

[1] Mazur V 1989 Journal of Geophysical research 94 3311-3325

[2] Cooray V 2015 An introduction to lightning (Springer)

[3] Plumer J A 2012 International Conference on Lightning Protection (ICLP)

[4] Zaepfel K P, Fisher B D and Ott M S 1985 NASA Technical Memorandum NASA-TM-86347 19850013567

[5] Guerra-Garcia C, Nyguyen N C, Peraire, J, Martinez-Sanchez M 2016 J. Phys. D: Appl. Phys., 49(37): 375204

 

Project sponsor

The Boeing Company

 

Plasma-assisted combustion and electrical control of flames

Introduction

If we are to reduce the climate impact of aviation while growing at the projected rates, that double the air travel demand by mid-century, disruptive technologies might be required. In particular, igniters and actuators based on plasma can introduce additional authority to control combustion. This control can range from improving the stability of lean combustion, which is less prone to NOx emissions, to speeding up the ignition process in a supersonic combustion chamber.

Our efforts in this field have involved different fundamental aspects of this problem ranging from the response of nonpremixed flames to AC electric fields to a study of electrical breakdown in the presence of a flame when using pulsed voltage.

Work on electrical control of flames

Most flames can be considered weakly ionized plasmas and so electric fields below the electrical breakdown threshold can be used to manipulate their properties. If the timescale of the electric field variation is slow as compared to the time response of the flame ions, the ions will be accelerated by the field and along their motion transfer momentum to the gas creating a so-called ion-driven wind [1,2]. In this project we explored the impact of ion-driven winds induced by an AC electric field on a counterflow nonpremixed flame (Figure 3) and proposed an analytical model based on an oscillating porous disk to explain the readjustment of the flame position.

 

Figure 3: Effect of sub-breakdown AC voltage on a counterflow nonpremixed flame, the electric field is applied orthogonal to the flame. Cyan line corresponds to baseline flame (no electric field) and red line to instantaneous flame position.

Work on pulsed nanosecond discharges

Non-thermal plasmas used to assist or stabilize flames are subject to gradients in temperature, composition and pre-ionization, which impact the electrical breakdown characteristics and the energy coupling to the flame and its flow field. In this work, we explored the impact of such inhomogeneities in the breakdown modes of repetitive pulsed nanosecond discharges [3], one of the main strategies used for plasma creation in the field of plasma assisted combustion [4], using experimental (Figure 4) and theoretical models.

Figure 4: Repetitive pulsed nanosecond discharge development across a nonuniform medium of N2-He-N2 (15kV amplitude, 20ns duration, 3kHz). The plasma is localized to the helium stream and the nitrogen layers remain non-ionizing [5].

 

For further information, please contact Carmen Guerra

 

References

[1] Calcote H F 1961 Symp. (Int.) Combust. 8(1): 184-199

[2] Lawton J, Mayo P J, Weinberg F J 1968 Proc. Soc. Lond. Ser. A 303(1474): 275-298

[3] Pai D, Stancu G D, Lacoste D A and Laux C O 2009 Plasma Sources Sci. Technol. 18: 045030

[4] Starikovskaia S M 2014 J. Phys. D: Appl. Phys. 47: 353001

[5] Guerra-Garcia C, Martinez-Sanchez M 2013 J. Phys. D: Appl. Phys., 46(34): 345204