The goal of the present study is to better understand the electrohydrodynamic (EHD) phenomena occurring in 15 mm gap point-to-plate corona discharges supplied by positive and negative DC voltages, and to link them to the discharge regime. For that, discharge current measurements have been conducted and a 20 kHz Particle Image Velocimetry system has been used to characterize the ionic wind produced by the discharges. The main results are as follows: (a) the Townsend’s law can always correctly interpolate the experimental I–V characteristics of a negative corona, even in presence of Trichel pulses, (b) in the case of a positive streamer discharge, the current does not follow the Townsend’s law as it evolves in Vk with k  =  4 in the present study, (c) Indeed, the discharge current becomes higher than the Townsend’s law when the glow-to-streamer regime transition occurs, (d) the ionic wind is unsteady, more especially in the case of a positive corona for which it seems that its velocity is pulsed at the same frequency than the streamer one, (e) when the positive high voltage is switched on, a strong streamer occurs at the end of the voltage rising (t  =  25 µs for  +6 kV), resulting in an over-velocity region upstream the head of the ionic wind jet that progresses in quiescent air toward the plate, (f) the topology of the time-averaged ionic wind is fully different compared to the one observed in a previous study where the gap was equal to 25 mm, (g) in the case of a negative corona, the ionic wind velocity is nearly constant in the electrode gap, (h) in the case of the positive corona, the results are even more surprising since the velocity is minimum at the tip and increases when one approaches the grounded plate, indicating that there is a significant space charge remained by the streamers in the second half of the inter-electrode region, (i) generally speaking, this study highlights that the spatio-temporal characteristics of the EHD force and the resulting ionic wind depend on the distribution of the space charge between both electrodes, the latter being linked to the voltage polarity, the discharge regime and the electrode gap.

Résumé :Nous présentons des mesures originales de la relation de dispersion des vagues linéaires en présence d'un courant constant uniforme selon la direction verticale et spatialement homogène dans la direction de contre-propagation. Pour cela, nous utilisons un canal hydraulique SANS batteur à houle : en effet, le bruit spontanément généré par l'installation hydraulique permet de déduire la relation de dispersion des modes excités grâce à la détection sub-pixel de l'interface. Celle-ci est illuminée par une nappe laser qui est absorbée par un colorant fluorescent dans le liquide et est simplement observée à travers la paroi latérale transparente du canal. Nous montrons l'évolution de la relation de dispersion en fonction du débit et de la hauteur d'eau dans le canal avec une concordance excellente avec la théorie. Dans certaines conditions, les modes transverses du canal sont aussi observés. Nous concluons sur la généralisation de nos résultats pour un courant variant dans la direction de propagation longitudinale par exemple en immergeant un obstacle de fond.

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