What is Momentum Flux?
Momentum flux is the transport of momentum that acts in a direction perpendicular to the direction of fluid flow. It is considered as the rate of change of horizontal momentum which is moving across a unit area, equal to force per unit area. It is also equivalent to a stress in that it is a force per unit area where the force is acting in a direction within the plane of the area.
In laminar flow, momentum flux is caused by molecular diffusion, giving rise to viscous effects. But additionally in turbulent flow there will be momentum flux due to the action of eddies (Reynolds stresses) caused by inertial forces in the fluid which usually is a much greater effect. It is possible to measure this using eddy-correlation methods. The dimensionless Reynolds number is a measure of the relative importance of these two transport mechanisms, and consequently is an indicator of whether the flow is laminar or turbulent. In free-surface flows momentum flux can also be generated by gravity waves.