We investigate the evolution equation for the average vortex length per unit volumeudL of superfluid turbulence in inhomogeneous flows. Inhomogeneities in line density Ludand in counterflow velocity V may contribute to vortex diffusion, vortex formation andudvortex destruction. We explore two different families of contributions: those arising fromuda second order expansion of the Vinen equation itself, and those which are not relatedudto the original Vinen equation but must be stated by adding to it second-order termsudobtained from dimensional analysis or other physical arguments.
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