Many books on transport processes, in order to emphasize the analogy between heat transfer, mass transfer, and momentum transfer, typically regard pressure and stress mechanistically as being equivalent to momentum transfer. (There is certainly valid molecular basis for treating pressure and stress in this way).
For flow past an object, the relationship between the drag force F and the dynamic pressure ##\frac{1}{2}\rho v^2## is expressed as $$\frac{F}{A}=C_D\left(\frac{1}{2}\rho v^2\right)$$where A is the projected area of the object. Since dynamic pressure has units of momentum flux (and, mechanistically, can be regarded as a momentum flux), and since F/A also has units of momentum flux, ##C_D## is thereby sometimes regarded as a dimensionless momentum flux. I personally don't like this interpretation, and it does nothing for me.
In the case of fluid flow in a tube, the relationship between the shear stress at the wall ##\tau## and the dynamic pressure ##\frac{1}{2}\rho v^2## is expressed as $$\tau=f\left(\frac{1}{2}\rho v^2\right)$$where f is the Fanning friction factor. Since dynamic pressure has units of momentum flux (and, mechanistically, can be regarded as a momentum flux), and since the wall shear stress ##\tau## also is interpreted as momentum flux, f is thereby sometimes regarded as a dimensionless momentum flux. I personally don't like this interpretation, and it too does nothing for me.
