Shear velocity

Shear velocity, also called friction velocity, is a form by which a shear stress

Shear stress

A shear stress, denoted , is defined as a stress which is applied parallel or tangential to a face of a material, as opposed to a normal stress which is applied perpendicularly.-General shear stress:...

 may be re-written in units of velocity

Velocity

In physics, velocity is the rate of change of position. It is a vector physical quantity; both speed and direction are required to define it. In the SI system, it is measured in meters per second: or ms^-1. The scalar absolute value of velocity is speed...

. It is useful as a method in fluid mechanics

Fluid mechanics

Fluid mechanics is the study of how fluids move and the forces on them. Fluid mechanics can be divided into fluid statics, the study of fluids at rest, and fluid dynamics, the study of fluids in motion. It is a branch of continuum mechanics, a subject which models matter without using the...

 to compare true velocities, such as the velocity

Velocity

In physics, velocity is the rate of change of position. It is a vector physical quantity; both speed and direction are required to define it. In the SI system, it is measured in meters per second: or ms^-1. The scalar absolute value of velocity is speed...

 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
  Diffusion
  Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material...
  
   and dispersion
  Dispersion
  Dispersion may refer to:In physics:*Dispersion *Dispersion *Dielectric dispersion*Dispersive mass transfer, in fluid dynamics, the spreading of mass from areas of high to low concentration**Atmospheric pollution dispersion...
  
   of particles, tracers, and contaminants in fluid flows
• The velocity profile near the boundary of a flow (see logarithmic 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, also called friction velocity, is a form by which a shear stress

Shear stress

A shear stress, denoted , is defined as a stress which is applied parallel or tangential to a face of a material, as opposed to a normal stress which is applied perpendicularly.-General shear stress:...

 may be re-written in units of velocity

Velocity

In physics, velocity is the rate of change of position. It is a vector physical quantity; both speed and direction are required to define it. In the SI system, it is measured in meters per second: or ms^-1. The scalar absolute value of velocity is speed...

. It is useful as a method in fluid mechanics

Fluid mechanics

Fluid mechanics is the study of how fluids move and the forces on them. Fluid mechanics can be divided into fluid statics, the study of fluids at rest, and fluid dynamics, the study of fluids in motion. It is a branch of continuum mechanics, a subject which models matter without using the...

 to compare true velocities, such as the velocity

Velocity

In physics, velocity is the rate of change of position. It is a vector physical quantity; both speed and direction are required to define it. In the SI system, it is measured in meters per second: or ms^-1. The scalar absolute value of velocity is speed...

 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
  Diffusion
  Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material...
  
   and dispersion
  Dispersion
  Dispersion may refer to:In physics:*Dispersion *Dispersion *Dielectric dispersion*Dispersive mass transfer, in fluid dynamics, the spreading of mass from areas of high to low concentration**Atmospheric pollution dispersion...
  
   of particles, tracers, and contaminants in fluid flows
• The velocity profile near the boundary of a flow (see logarithmic 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 about 1/10 of the mean flow velocity.
Where is the shear stress in an arbitrary layer of fluid and is the density

Density

The density of a material is defined as its mass per unit volume. The symbol of density is ρ .- Formula :Mathematically:where: is the density, is the mass, is the volume....

of the fluid.

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