The bulk viscosity μB of a fluid is also known as the volume viscosity or the second viscosity. It is often not used in classical CFD applications but it plays a role in acoustics as well as in high-velocity compressible flows. The bulk viscosity is associated with losses due to expansion and compression, that is, the compressional resistance. The parameter describes the difference between the mechanical and thermodynamic pressures. It is often described as representing all the effects that describe the time it takes to establish thermodynamic equilibrium for vibrational and rotational motion of the fluid molecules (see Ref. 18). In some sense, it can be said to describe low frequency relaxation processes.

In acoustics, the losses associated with the bulk viscosity only really play a role in high-frequency ultrasound propagation. The bulk viscosity term in the thermoviscous acoustics or the linearized Navier–Stokes equations is sometimes used to model the frequency dependent losses due to relaxation processes. Various models exist to include these effects; details can be found in Ref. 17.