Effects of numerical and physical anisotropic diffusion on branching phenomena of negative-streamer dynamics

This paper is a contribution to the fluid modelling and simulation of the spontaneous branching of an initial mono-filamentary negative streamer propagating in molecular nitrogen at atmospheric pressure. The effects of both numerical diffusion and physical anisotropic diffusion on the branching structure are studied. We used MUSCL-type flux limiters where an artificial amount of numerical diffusion can be introduced through the choice of the value of a characteristic slope parameter. It was shown that a small amount of numerical diffusion can inhibit the spontaneous streamer branching. This means that the use of a high-order numerical scheme preventing the numerical diffusion and dispersion is a major parameter that must be taken into account in the interpretation of the simulated streamer development and splitting. This paper also clearly shows that the consideration of the anisotropy of electron diffusion affects the streamer head structure in comparison with the isotropic diffusion case. This especially occurs for electrons in gases presenting a large difference between the longitudinal and transversal diffusion coefficients as in N-2 or in air.