  The mechanism of turbulent viscosity is the central question in
investigations of turbulence. This is also the case in the accretion disk
theory, where turbulence is considered to be responsible for the outward
transport of angular momentum in the accretion disk. In turbulent flows,
vortices transport momentum over their length scales providing the mechanism of
viscosity that is controlled by mass entrainment. We have earlier proposed an
entrainment model for the particular case of the relativistic jets in the radio
galaxy 3C31. In this paper, we further constrain the model parameters. The
model (in the non-relativistic part) is successfully tested versus experimental
and simulation data on the Reynolds stresses of free mixing layers and predicts
the Smagorinsky constant $C_\mathrm{S} \approx 0.11$, which is consistent with
the experimental range for shear flows $C_\mathrm{S} \approx 0.1-0.12$. For
accretion disks, the entrainment model allows us to derive the same accretion
mass rate as in the Shakura \& Sunyaev's $\alpha$-model without appealing to
the turbulent kinematic viscosity $\nu_\mathrm{t}$, and the viscosity parameter
$\alpha$ derived in the form $\displaystyle \alpha = -\frac{8}{3} \beta
s_\mathrm{T} \frac{\mathrm{v_t}^2}{c_\mathrm{s}^2}$ depends on the power
$s_\mathrm{T}$ of the temperature slope along the disk radius, $T\propto
r^{s_\mathrm{T}}$, and quadratically on the turbulent velocity $\mathrm{v_t}$.
