This post assumes that the reader has knowledge of basic vector calculus and physics.

In the study of fluid dynamics the term ‘momentum transport’ is thrown around quite often and often in conjunction with the idea of ‘momentum accumulation’; it is a rather difficult concept to understand because it is quite impossible to *see* momentum. Hopefully this post will help to clarify the concept.

We will start with an easy to visualize concept: ‘mass transport’. Let us assume that there is a laminar and steady fluid flow of density \(\rho\) and velocity \(\mathbf{v}\). We would like to know the rate \(K_T\) at which mass is transported in this flow. The answer—

\[ K_T = \rho |\mathbf{v}| \]Makes sense until you consider the units of \(K_T\) and realize that they are \(\frac{\text{g}}{\text{cm}^2 \cdot \text{s}}\) which definitely doesn't make much sense. In fact it is difficult to determine at this point what the correct units of the rate of mass transport should even be. To solve this problem we need to consider mass transport from the other side of the equation: ‘mass accumulation’.