# Flow velocity

In continuum mechanics the macroscopic velocity,[1][2] also flow velocity in fluid dynamics or drift velocity in electromagnetism, is a vector field which is used to mathematically describe the motion of a continuum. The length of the flow velocity vector is the flow speed and is a scalar.

## Definition

The flow velocity u of a fluid is a vector field

$\mathbf{u}=\mathbf{u}(\mathbf{x},t),$

which gives the velocity of an element of fluid at a position $\mathbf{x}\,$ and time $t.\,$

The flow speed q is the length of the flow velocity vector[3]

$q = || \mathbf{u} ||$

and is a scalar field.

## Uses

The flow velocity of a fluid effectively describes everything about the motion of a fluid. Many physical properties of a fluid can be expressed mathematically in terms of the flow velocity. Some common examples follow:

The flow of a fluid is said to be steady if $\mathbf{u}$ does not vary with time. That is if

$\frac{\partial \mathbf{u}}{\partial t}=0.$

### Incompressible flow

If a fluid is incompressible the divergence of $\mathbf{u}$ is zero:

$\nabla\cdot\mathbf{u}=0.$

That is, if $\mathbf{u}$ is a solenoidal vector field.

### Irrotational flow

A flow is irrotational if the curl of $\mathbf{u}$ is zero:

$\nabla\times\mathbf{u}=0.$

That is, if $\mathbf{u}$ is an irrotational vector field.

A flow in a simply-connected domain which is irrotational can be described as a potential flow, through the use of a velocity potential $\Phi,$ with $\mathbf{u}=\nabla\Phi.$ If the flow is both irrotational and incompressible, the Laplacian of the velocity potential must be zero: $\Delta\Phi=0.$

### Vorticity

The vorticity, $\omega$, of a flow can be defined in terms of its flow velocity by

$\omega=\nabla\times\mathbf{u}.$

Thus in irrotational flow the vorticity is zero.

## The velocity potential

If an irrotational flow occupies a simply-connected fluid region then there exists a scalar field $\phi$ such that

$\mathbf{u}=\nabla\mathbf{\phi}.$

The scalar field $\phi$ is called the velocity potential for the flow. (See Irrotational vector field.)

## References

1. Duderstadt, James J., Martin, William R. (1979). "Chapter 4:The derivation of continuum description from transport equations". In Wiley-Interscience Publications. Transport theory. New York. p. 218. ISBN 978-0471044925. <templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
2. Freidberg, Jeffrey P. (2008). "Chapter 10:A self-consistent two-fluid model". In Cambridge University Press. Plasma Physics and Fusion Energy (1 ed.). Cambridge. p. 225. ISBN 978-0521733175.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
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