The viscous damping for a Newtonian-type fluid is specified by two material constants μb and μs, known as bulk viscosity and shear viscosity, respectively. The constitutive law is defined by separating the strains into two parts namely, volumetric strain and deviatoric strain (traceless) components. The volumetric strain rate multiplied by the bulk viscosity gives a pressure, and the deviatoric strain rate multiplied by a shear viscosity gives deviatoric stress. Total stress is then the sum of pressure and deviatoric stress:

Using the velocity expression from Equation 3-7, it can be shown that