Wind Tunnel Experiments of Long Arcs in Crossflow

Lightning attachment to aircraft follows a series of distinct phases occurring at disparate timescales. Whereas the stages of leader inception and initial attachment are fast compared to the fluid timescales, once the bidirectional discharge is established, the current flow phase can last on the order of a second: including, current pulses of amplitude ~1kA associated with the stepped propagation of the negative leader, a steady current component (of a few hundred A) and a series of intracloud, cloud-to-ground or cloud-to-cloud process pulses (<100kA). During this time, which corresponds to the most damaging phases of the arc, the aircraft can travel several times its length. At the local level, this swept stroke phase involves the complex interaction between a long arc and a fluid boundary layer. Improved understanding of this local problem can lead to revised modelling approaches of the swept stroke phase. In this contribution we perform wind tunnel experiments of a long arc in cross flow and perform synchronized diagnostics of both the fluid dynamics and the arc dynamics, as a first step to tackling the local problem of lightning attachment associated with the swept stroke. Although the swept stroke phase is most closely represented by a moving electrode setup (the arc column is stagnant with respect to the ambient air and the segment attached to the surface experiences the relative velocity), wind tunnel tests can be useful to isolate the effects of different flow fields on the arc dynamics, as the environment can be easily controlled. In this joint effort between the Massachusetts Institute of Technology, the Universitat Politècnica de Catalunya, and ONERA, we have initiated wind tunnel studies of long arcs, starting from an arc column of length around 20-80cm and currents of ~3A. The experimental setup is equipped to diagnose both the arc properties and the fluid flow. The arc is characterized by its electrical signature (time-resolved current and voltage waveforms) as well as high-speed videography to resolve the elongation of the arc in response to the flow and possible reconnection and reattachment events. The fluid flow field is visualized in 2D using particle image velocimetry (PIV). This setup allows to isolate the influence of the flow dynamics (laminar versus turbulent, separation, etc.) on the arc dynamics.