Dynamical Similarity of Streamer Propagation in Geometrically Similar Combined Air Gaps

Similarity methods are useful tools for correlating gas discharge characteristics at different dimensional scales, which aid in designing tests using reduced models for extrapolating original full-dimensional scale discharges that are either experimentally expensive or practically unachievable. In this article, based on a 2-D fluid model, we investigated the validity of similarity laws for streamer propagation in combined air gaps, which have a floating metal structure between the high voltage and ground electrodes. The dynamic similarities of high-pressure streamer propagation inside combined air gaps are demonstrated by keeping the product of geometrical length and gas pressure constant in the original and downscaled gaps. The streamer propagation represented by the electron source term and the streamer velocity in single and combined gaps are obtained and compared, which also confirms the influence of floating metal on streamer characteristics. Temporal behaviors of the streamer propagation represented by the electron density distributions are found to be rather the same, which validates the dynamic streamer similarity. In addition, the simulation results show that similarity properties could be undermined by the photoionization effect whereas its impact can be excluded if the quenching pressure of the photoionization source term is linearized. By further excluding the nonlinear processes, such as the three-body electron attachment process, similarity laws are more rigorously verified. The results from this study confirm the applicability of similarity laws for streamer discharge in combined air gaps under designed conditions, which are beneficial for the exploration and conduction of downscaled aircraft ground tests for simulating lightning strike effects.