Single streamers are currently well simulated using detailed computational models. Most of these models are inhibitively complex to use for modelling many-streamer interactions in a streamer corona. This work develops reduced order models of single positive streamers in atmospheric pressure air that replicate the core macroscopic behaviour of detailed models while using a simpler physics representation. Models are developed using the 1.5D framework, with emphasis placed on solving the equations of motion in the streamer reference frame. The solution in this quasi-steady frame is shown to be a good representation of the instantaneous state of the streamer. Finally, a method of uniquely characterizing the instantaneous state of a streamer using its macroscopic parameters (velocity, radius, tip electric field and channel electric field) is developed. This characterization is interpreted graphically, with streamers treated as quasi-steady structures which evolve in time at a rate much slower than the time scale of electron transport. Previous work in the literature is shown to be well captured by this interpretation.

This paper details the use of nanosecond repetitively pulsed discharges to attenuate combustion instabilities in a 14 kW swirl-stabilized methane/air combustor. The combustor exhibits large-amplitude pressure oscillations ranging from 1 to 4% of the mean pressure during which the flame exhibits bulk motion in each instability cycle, upstream and downstream, as revealed by high-speed chemiluminescence. Control is accomplished with an electrode comprising a pin anode at the centerline of the combustor, allowing a nanosecond spark to be generated in a region spanning close to the flame base, through the shear layers of the swirling flow and ending at the metallic combustor wall. The discharges are generated using 20 kV, 9 kHz pulses; and they correspond to about 120 W of mean power. This results in a suppression of the peak amplitude of the pressure oscillations by a factor of two to four, and 5 dB in the rms value. Using phase-averaged visualizations of the flame with and without plasma, we detail the sequence of flame motion in the course of the instability. With the plasma active, this reveals significant interactions between the flame and the plasma during the suppression. Finally, we present a state-space model of the thermoacoustic system, and we demonstrate open-loop control of the instabilities.

This paper reports on a laboratory experiment to study the effect of vehicle net charge on the inception of a positive leader from an aircraft exposed to high atmospheric electric fields. The experiment models the first stage of aircraft-triggered lightning in which a positive leader typically develops from the vehicle and is shortly afterwards followed by a negative leader. This mechanism of lightning initiation amounts to around 90% of strikes to aircraft. Aircraft can acquire net charge levels of the order of a millicoulomb from a number of sources including corona emission, charged particles in the engine exhaust, and charge transfer by collisions with particles in the atmosphere. In addition, aircraft could potentially be artificially charged through controlled charge emission from the surface. Experiments were performed on a model aircraft with a 1m wingspan, which was suspended between two parallel electrodes in a 1.45m gap with voltage difference of a few hundred kilovolts applied across it. In this configuration, it is found that the breakdown field can vary by as much as 30% for the range of charging levels tested. The experimental results show agreement with an electrostatic model of leader initiation from aircraft, and the model indicates that the effect can be substantially stronger if additional negative charge is added to the aircraft. The results from this work suggest that flying uncharged is not optimal in terms of lightning avoidance and open up the possibility of developing risk-reduction strategies based on net charge control. Plain Language Summary Commercial aircraft are typically struck by lightning around onceper year, and the vast majority of these events are triggered by the aircraft itself. The lightning discharge originates on the surface of the aircraft in areas with sharp edges. Whether a discharge develops is in part due to the net electric charge of the aircraft, which can be acquired both naturally or artificially. Previous work has shown that it is theoretically possible to reduce the likelihood of a lightning strike occurring by manipulating the net charge of the aircraft. In this paper, the authors perform laboratory experiments to validate this hypothesis. These experiments demonstrate that the threshold for lightning could be increased by 30% by charging the aircraft negatively, which means that an aircraft could fly safely through ambient electric fields that are around 30% higher than those of an uncharged baseline. Theoretical estimates suggest that further improvement may be possible if the aircraft were charged to a more negative state than those tested. This work gives laboratory scale experimental evidence that it is possible to reduce the frequency of lightning strikes on aircraft by manipulating their charge and encourages further investigation of the proposed lightning strike risk reduction strategy.

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