Shear velocity

Shear velocity, also called friction velocity, is a form by which a shear stress may be re-written in units of velocity. It is useful as a method in fluid mechanics to compare true velocities, such as the velocity of a flow in a stream, to a velocity that relates shear between layers of flow.

Shear velocity is used to describe shear-related motion in moving fluids. It is used to describe:

Shear velocity also helps in thinking about the rate of shear and dispersion in a flow. Shear velocity scales well to rates of dispersion and bedload sediment transport. A general rule is that the shear velocity is between 5% and 10% of the mean flow velocity.

For river base case, the shear velocity can be calculated by Manning's equation.

Instead of finding ${\displaystyle n}$ and ${\displaystyle R_{h}}$ for the specific river of interest, the range of possible values can be examined; for most rivers, ${\displaystyle u^{*}}$ is between 5% and 10% of ${\displaystyle \langle u\rangle }$:

For general case

where τ is the shear stress in an arbitrary layer of fluid and ρ is the density of the fluid.

Typically, for sediment transport applications, the shear velocity is evaluated at the lower boundary of an open channel: