Dynamic viscosity

Dynamic viscosity $\mu$ relates Newtonian shear stress to rate of strain.

Kinematic viscosity is dynamic viscosity divided by mass density: $\nu =\mu /\rho$.

If angular velocity is independent of height, integrating the viscous stress $\rho \nu r{\Omega }^{\prime }$ gives $\Sigma \bar{\nu }r{\Omega }^{\prime }$. Thus the surface density weighted average, rather than an unweighted height average, enters vertically averaged viscous disk equations.

The viscous stress tensor records momentum flux caused by velocity gradients. For a Newtonian compressible fluid it separates into shear and bulk contributions.

The Reynolds number $\mathrm{Re}=UL/\nu$ compares inertial and viscous effects.