We study a continuum model of an extensile active nematic to show that mesoscale turbulence develops in two stages: i) ordered regions undergo an intrinsic hydrodynamic instability generating walls, lines of strong bend deformations; ii) the walls relax by forming oppositely charged pairs of defects. Both creation and annihilation of defect pairs reinstate nematic regions which undergo further instabilities, leading to a dynamic steady state. We compare this with the development of active turbulence in a contractile active nematic.
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