Volume viscosity (also called second viscosity or bulk viscosity or dilatational viscosity) becomes important only for such effects where fluid compressibility is essential. Dilatational viscosity is a measure of the viscous forces and viscous forces depend on the rate of compression or dilatation. Dilatational viscosity is zero for Monatomic gases at low density. For an incompressible liquid the dilatational viscosity is neglected. The dilatational viscosity is important in describing sound absorption in polyatomic gases and in describing the fluid dynamics of liquids containing gas bubbles.
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Derivation and use
The negative-one-third of the trace of the Cauchy stress tensor at the equilibrium is often identified with the thermodynamic pressure,
which only depends upon the equilibrium state potentials like temperature and density (equation of state). In general, the trace of the stress tensor is the sum of thermodynamic pressure contribution and another contribution which is proportional to the divergence of the velocity field. This constant of proportionality is called the volume viscosity.
Volume viscosity appears in the Navier-Stokes equation if it is written for compressible fluid, as described in the most books on general hydrodynamics , and acoustics , .
where
The Navier-Stokes equation above includes the dynamic viscosity μ, also known as the shear viscosity.
Measurement
The volume viscosity of many fluids is inaccurately known, despite its fundamental role for fluid dynamics at high frequencies. The only values for the volume viscosity of simple Newtonian liquids known to us come from the old Litovitz and Davis review, see References. They report the volume viscosity of water at 15 °C is 3.09 centipoise.
Modern acoustic rheometers are able to measure this parameter.
More recent studies have determined the bulk viscosity for a variety of fluids). In the latter study, a number of common fluids were found to have bulk viscosities which were hundreds to thousands of times larger than their shear viscosities. The details of the data used and estimation techniques are provided in Cramer (2012). As discussed by Cramer (2012), fluids having large bulk viscosities include those used as working fluids in power systems having non-fossil fuel heat sources, wind tunnel testing, and pharmaceutical processing.