Velocity potential

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

It is used in continuum mechanics, when a continuum occupies a simply-connected region and is irrotational. In such a case, $$\mathrm{\nabla }\times 𝐮=0,$$ where u denotes the flow velocity. As a result, u can be represented as the gradient of a scalar function $\varphi$: $$𝐮=\mathrm{\nabla }\varphi =\frac{\partial \varphi }{\partial x}𝐢+\frac{\partial \varphi }{\partial y}𝐣+\frac{\partial \varphi }{\partial z}𝐤.$$

$\varphi$ is known as a velocity potential for u.

A velocity potential is not unique. If $\varphi$ is a velocity potential, then $\varphi +f(t)$ is also a velocity potential for u, where $f(t)$ is a scalar function of time and can be constant. 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.

Usage in acoustics

In theoretical acoustics, it is often desirable to work with the acoustic wave equation of the velocity potential $\varphi$ instead of pressure p and/or particle velocity u. $${\mathrm{\nabla }}^2\varphi -\frac{1}{c^2}\frac{{\partial }^2\varphi }{\partial t^2}=0$$ 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 $\varphi$ 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 $$p=-\rho \frac{\partial \varphi }{\partial t}.$$

See also

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