Theory, simulation, and experimental test of turbulence spreading

We study turbulence spreading corresponding to the spatio-temporal propagation of a patch of turbulence from a region where it is locally excited to a region of weaker excitation, or even local damping. A single model equation for the local turbulence intensity I(x, t) includes the effects of local linear growth and damping, spatially local nonlinear coupling to dissipation and spatial scattering of turbulence energy induced by nonlinear coupling. In the absence of dissipation, the front propagation through the linearly unstable zone is ballistic with a characteristic speed given by a geometric mean of the linear growth rate and turbulent diffusion at nonlinear local saturation. (author)