Universal particle acceleration

An approximate theory, in the spirit of the mixing-length theory, of the acceleration of particles in low-density shear flows is presented. The energy required to accelerate the particles is drawn from the velocity field by means of a particle acceleration-induced viscosity. This theory is applied to a number of astrophysical flows, including accretion disks, radial accretion and convective turbulence. The most promising application is to disks, where the theory suggests a mechanism by which the bulk of the accretion power may be converted to particle acceleration.