Volume viscosity (${\zeta }_B$)

Bulk viscosity is the dissipative response to a fluid's volume expansion or compression, distinct from the shear response. For a Newtonian isotropic fluid its isotropic dissipative stress includes ${\zeta }_B(\nabla \cdot \mathbf{v})I$, and nonnegative ${\zeta }_B$ gives nonnegative compression-related dissipation. A condensate's relative-density-flow bulk coefficient must be identified from its particular hydrodynamic equation; it is not automatically numerically identical to a dimensional Newtonian viscosity.