Bulk viscosity in the Navier-Stokes equations

This article discusses the physics associated with the bulk viscosity coefficient mu(b) as it appears in the compressible Navier-Stokes equations. Thus, the active rotational and vibrational modes of a polyatomic molecule are discussed with emphasis on the importance of local thermodynamic equilibrium. This condition is necessary if mu(b) is to only depend on the thermodynamic state, as required for the Navier-Stokes equations. A new perturbation formulation is provided for the absorption and dispersion coefficients and the entropy production that holds even when mu(b) greatly exceeds the shear viscosity, as is the case for gases such as CO2. A comparison for CO2 shows excellent agreement between exact results and the low-frequency formula used for the absorption coefficient. This agreement stems from a large value for the Peclet number. (C) 1998 Elsevier Science Ltd. All rights reserved.