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# Potential flow

Potential flow, also know as irrotational flow in fluid dynamics is steady flow defined by the equations

(zero rotation = no viscosity)

(zero divergence = volume conservation)

Equivalently,

where:

• v is the vector fluid velocity
• Φ is the fluid flow potential, scalar
• " ×" is curl
• " ·" is divergence.

The equations above imply , or Laplace's equation, holds.

Together with the Navier-Stokes equations and the Euler equations, these equations can be used to calculate solutions to many practical flow situations. In two dimensions, potential flow reduces to a very simple system that is analysed using complex numbers (see potential flow in 2d))

Unfortunately, potential flow bears very little resemblance to many flows that are encountered in the real world. For example, potential flow excludes turbulence, which is commonly encountered in nature. Richard Feynman considered potential flow to be so unphysical that the only fluid to obey the assumptions was "dry water".

Potential flow also makes a number of predictions, such as d'Alembert's paradox, which states that the drag on any object moving through an infinite fluid otherwise at rest is zero.

More precisely, potential flow cannot account for the behaviour of flows that include a boundary layer. Someone once said that flows are split into those that are observed but never calculated, and those that are calculated but never observed.

Nevertheless, understanding potential flow is important in many branches of fluid mechanics. In particular, simple potential flows such as the free vortex and the point source possess ready analytical solutions. These flows correspond closely to real-life flows over the whole of fluid mechanics; in addition, many valuable insights arise when considering the deviation (often slight) between an observed flow and the corresponding potential flow.

## Analysis

Potential flow in two dimensions is simple to analyse using complex numbers, viewed for convenience on the Argand diagram.

The basic idea is to define a holomorphic function . If we write

then the Cauchy-Riemann equations show that
(it is conventional to regard all symbols as real numbers; and to write
```and ).
```
The velocity field , specified by
then satisfies the requirements for potential flow:
and

Lines of constant are known as streamlines and lines of constant are known as equipotential lines (see equipotential surface).

The two sets of curves intersect at right angles, for

## Examples: general considerations

Note that multi-valued functions such as the natural logarithm may be used, but attention must be confined to a single Riemann surface.

## Examples: Power laws

If

then, writing , we have

and

### Power law with

If , that is, a power law with , the streamlines (ie lines of constant ) are a system of straight lines parallel to the x-axis. This is easiest to see by writing in terms of real and imaginary components:

thus giving and .

### Power law with

If , then and the streamline corresponding to a particular value of are those points satisfying

which is a system of rectangular hyperbolae. This may be seen by again rewriting in terms of real and imaginary components. Noting that and rewriting it is seen (on simplifying) that the streamlines are given by

The velocity field is given by , or

In fluid dynamics, this corresponds to a stagnation point. Note that the fluid velocity is zero at the origin (this follows on differentiation of at ).

The streamline is particularly interesting: it has two (or four) branches, following the coordinate axes, ie and .

As no fluid flows across the x-axis, it (the x-axis) may be treated as a solid boundary (remember that the physical system to which this analysis corresponds is an inviscid (ie zero viscosity) fluid; there are thus no boundary layers to worry about). It is thus possible to ignore the flow in the lower half-plane where and to focus on the flow in the upper half-plane.

With this interpretation, the flow is that of a vertically directed jet impinging on a horizontal flat plate.

The flow may also be interpreted as flow into a 90 degree corner if the regions specified by (say) and are ignored.

### Power law with

If the resulting flow is a sort of hexagonal version of the case considered above. Streamlines are given by .

### Power law with

if , the streamlines are given by

This is more easily interpreted in terms of real and imaginary components:

Thus the streamlines are
circles that are tangent to the x-axis at the origin. The velocity field is given by

The circles in the upper half-plane thus flow anticlockwise, those in the lower half-plane flow clockwise.  Note that speeds go as ; and the speed at the origin is infinite.