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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:
- Diffusion and dispersion of particles, tracers, and contaminants in fluid flows
- The velocity profile near the boundary of a flow (see Law of the wall)
- Transport of sediment in a channel
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% to 10% of the mean flow velocity.
For river base case, the shear velocity can be calculated by Manning's equation.
- n is the Gauckler–Manning coefficient. Units for values of n are often left off, however it is not dimensionless, having units of: (T/[L1/3]; s/[ft1/3]; s/[m1/3]).
- Rh is the hydraulic radius (L; ft, m);
- the role of a is a dimension correct factor. Thus a= 1 m1/3/s = 1.49 ft1/3/s.
Instead of finding and for your specific river of interest, you can examine the range of possible values and note that 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 can also be defined in terms of the local velocity and shear stress fields (as opposed to whole-channel values, as given above).
Friction Velocity in Turbulence
The friction velocity is often used as a scaling parameter for the fluctuating component of velocity in turbulent flows. One method of obtaining the shear velocity is through non-dimensionalization of the turbulent equations of motion. For example, in a fully developed turbulent channel flow or turbulent boundary layer, the streamwise momentum equation in the very near wall region reduces to:
By integrating in the y-direction once, then non-dimensionalizing with an unknown velocity scale u∗ and viscous length scale ν/, the equation reduces down to:
Since the right hand side is in non-dimensional variables, they must be of order 1. This results in the left hand side also being of order one, which in turn give us a velocity scale for the turbulent fluctuations (as seen above):
Here, τw refers to the local shear stress at the wall.
Planetary boundary layer
Within the lowest portion of the planetary boundary layer a semi-empirical log wind profile is commonly used to describe the vertical distribution of horizontal mean wind speeds. The simplified equation that describe it is
where is the Von Kármán constant (~0.41), is the zero plane displacement (in metres).
The zero-plane displacement () is the height in meters above the ground at which zero wind speed is achieved as a result of flow obstacles such as trees or buildings. It can be approximated as 2/3 to 3/4 of the average height of the obstacles. For example, if estimating winds over a forest canopy of height 30 m, the zero-plane displacement could be estimated as d = 20 m.
Thus, you can extract the friction velocity by knowing the wind velocity at two levels (z).
- Whipple, K. X. (2004). "III: Flow Around Bends: Meander Evolution" (PDF). MIT. 12.163 Course Notes.