Question

Diffusion terms are not included in ____ of the Navier-Stokes equations.

(a) continuity equation

(b) y-momentum equation

(c) z-momentum equation

(d) energy equation

The question was asked during a job interview.

My query is from Navier Stokes Equation topic in section Governing Equations of Fluid Dynamics of Computational Fluid Dynamics

Answer 1

Right answer is (c) z-momentum equation

To explain I would say: Mass diffusion of the continuity equations are in general not included in the Navier-Stokes equations. This is because most of the fluid flow and thermodynamic processes do not include any change in concentration which is mass diffusion.