Ionic wind produced by positive and negative corona discharges in air

In this experimental study, we aimed at better understanding the electrohydrodynamic phenomena occurring inside positive and negative point-to-plate corona discharges in atmospheric air. First, electrical and optical measurements allowed us to observe the different discharge regimes according to the voltage polarity; the Hermstein's glow regime and then the breakdown streamer one in the case of a positive corona, and the Trichel pulse regime and then the pulseless one for the negative corona. More, we highlighted that the discharge current always follows the theoretical Townsend's expression, except in one case. Indeed, for the positive corona, the discharge current starts to evolve linearly with the voltage when the streamer regime appears. Secondly, the time-averaged and time-resolved velocity of the ionic wind has been characterized by high speed particle imaging velocimetry. We observed that when the high voltage is switched on, a jet starts from the needle and then its moves toward the plate, resulting in a wall-impinging jet with a vortex ring. Moreover, we highlighted that a negative corona discharge produces a steady ionic wind with weak velocity fluctuations. On the contrary, the positive discharge induces a faster ionic wind, showing that it is more efficient than the negative discharge in ionic wind production, more especially when the breakdown streamer regime appears. Indeed, in this case, the mean velocity becomes constant along the x axis, meaning that there is still a EHD force in the inter-electrode gap, which is able to counter the viscous effects.