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 and for the specific river of interest, the range of possible values can be examined; for most rivers, is between 5% and 10% of :
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:
where τb is the shear stress given at the boundary.
Shear velocity is linked to the Darcy friction factor by equating wall shear stress, giving:
where fD is the friction factor.[1]
Shear velocity can also be defined in terms of the local velocity and shear stress fields (as opposed to whole-channel values, as given above).
