Velocity potential

A velocity potential is a scalar potential used in potential flow theory. It was introduced by Joseph-Louis Lagrange in 1788.[1]

It is used in continuum mechanics, when a continuum occupies a simply-connected region and is irrotational. In such a case, ∇×𝐮=0, where u denotes the flow velocity. As a result, u can be represented as the gradient of a scalar function Φ: 𝐮=∇Φ =∂Φ∂x𝐢+∂Φ∂y𝐣+∂Φ∂z𝐤.

Φ is known as a velocity potential for u.

A velocity potential is not unique. If Φ is a velocity potential, then Φ + a(t) is also a velocity potential for u, where a(t) is a scalar function of time and can be constant. In other words, velocity potentials are unique up to a constant, or a function solely of the temporal variable.

The Laplacian of a velocity potential is equal to the divergence of the corresponding flow. Hence if a velocity potential satisfies Laplace equation, the flow is incompressible.

Unlike a stream function, a velocity potential can exist in three-dimensional flow.

## Contents

* 1 Usage in acoustics
* 2 See also
* 3 Notes
* 4 External links

## Usage in acoustics

In theoretical acoustics,[2] it is often desirable to work with the acoustic wave equation of the velocity potential Φ instead of pressure p and/or particle velocity u. ∇2Φ−1c2∂2Φ∂t2=0 Solving the wave equation for either p field or u field does not necessarily provide a simple answer for the other field. On the other hand, when Φ is solved for, not only is u found as given above, but p is also easily found—from the (linearised) Bernoulli equation for irrotational and unsteady flow—as p=−ρ∂Φ∂t.

## See also

* Vorticity
* Hamiltonian fluid mechanics
* Potential flow
* Potential flow around a circular cylinder

## Notes

1. ↑ Anderson, John (1998). A History of Aerodynamics. Cambridge University Press. ISBN 978-0521669559. [page needed]
2. ↑ Pierce, A. D. (1994). Acoustics: An Introduction to Its Physical Principles and Applications. Acoustical Society of America. ISBN 978-0883186121. [page needed]

## External links

* Joukowski Transform Interactive WebApp

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