Momentum flux

momentum flux

The vertical flux of horizontal momentum, equal to the force per unit area, or stress.

The Reynolds stress (τ_R) can be determined from the covariance of the fluctuations of the horizontal (u′, v′) and vertical (w′) wind components, by

where ρ is air density. A direct measurement is possible with eddy-correlation techniques, or an indirect determination can be made using Monin–Obukhov similarity flux-profile relationships (
also called universal functions). The relation between the velocity scale (friction velocity u_*) and the momentum flux is

Momentum flux can be associated with either mean velocity components, internal gravity waves, or with turbulent velocity fluctuations. For turbulence, the momentum flux is also called the Reynolds stress. For waves, it is related to mountain wave drag.

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