Greek Lower Case Letter
Phi can be written two ways
or (each Internet Browser does it differently).

Also, sometimes Upper
Case Phi, , and Upper Case Psi, , is used for
Velocity Potential and Stream Function, respectively.

It was shown previously that
two-dimensional incompressible, inviscid, and irrotational flow can be
described by the
velocity potential, φ, or
stream function, ψ, using the Laplace's equation:

In the following sections, some simple plane potential flows (e.g., uniform flow, source and sink, vortex and doublet) will be introduced. Also, since Laplace's equation is linear, various solutions can be combined to form other solutions. Therefore, some of the real flow problems (e.g., half-body) are obtained by combining these simple plane potential flows using the method of superposition.

Uniform flow is the simplest form of potential flow. For flow in
a specific direction, the velocity potential is

φ = U (x cosα + y sinα)

while the stream function is

ψ = U (y cosα - x sinα)

where α represents the angle between the flow direction and the x-axis (as shown in the figure). Recall that the
velocity potential and
stream function are related to the component velocity in the 2 dimensional
flow field as follows:

Cartesian coordinates:

and

Cylindrical coordinates:

and

Note that the lines of the constant velocity potential (equipotential
lines) are orthogonal to the lines of the stream function (streamlines).

When a fluid flows radially outward from a point source, the
velocities are

v_{r} = m / (2πr) and v_{θ} = 0

where m is the volume flow rate from the
line source per unit length. The velocity potential and stream function
can then be represented as

respectively. When m is
negative, the flow is inward, and it represents a sink. The volume
flow rate per unit depth, m, indicates the strength of the
source or sink. Note that as r approaches zero, the radial velocity goes
to infinity. Hence, the origin represents a singularity. As shown
in the figure, the equipotential lines are given by the concentric circles
while the streamlines are the radial lines.

A vortex can be obtained by reversing the velocity potential and stream
functions for a point source such that

φ = Kθ and ψ =
-K ln(r)

where K is a constant indicating the strength of the vortex. Now, the
equipotential lines are radial lines while the streamlines are given
by the concentric
circles.
The
velocities
of a vortex
are,

v_{r} = 0 and v_{θ} =
K/r

The strength of a vortex can be described using the concept of circulation
(Γ), which is defined as,

where the integral is taken around a closed arbitrary curve C. For
irrotational flows (),
the circulation becomes

However,
when the closed curve consists of a singularity point (such as the case
of a vortex), the circulation is non-zero,

Through considerable manipulation (i.e., geometric relationships and
trigonometric identities), the above equation can be rewritten as

For small values of a gives,

A doublet is obtained by letting the distance between the source and sink
approach zero (i.e., distance "a" tends to zero) which means r/(r^{2} - a^{2}) -> 1/r. The stream function for
a doublet then becomes

ψ = -Ksinθ/r

where K is a constand equal to ma/π, and is called the strength of the doublet.
The streamlines of a typical doublet are shown in the figure. The corresponding velocity potential is

φ = Kcosθ/r

For simplicity, the details of the velocity potential derivation are not given here.
Students are encouraged to go through the above derivation process themselves
for practice.

Flow around a half-body can be obtained by the superposition of a uniform
flow with a source. The combined stream function is given by

ψ = ψ_{uniform
flow} + ψ_{source} =
U r sinθ + (m/2π) θ

and the corresponding velocity potential is

φ = φ_{uniform
flow} + φ_{source} = U r cosθ +
(m/2π)
ln(r)

The velocity components are given by

Flow
Past a Oval-type Body

It is interesting to note, the streamlines of this combined function can be used to represent a oval-like shape in a flow stream. The stagnation point of the flow can be used to define the half-bodyshape. The location of the stagnation point can be determined by setting v_{r} and v_{θ} equal to zero, yielding

θ = π and r_{stagnation} = b = m/2πU

The streamline that passes through the stagnation point is then obtained
as

ψ_{stagnation} =
m/2 = πbU

By replacing this streamline with a solid boundary, one can then clearly see that flow around a half-body can indeed be represented by the superposition of a uniform flow with a source. The magnitude of the resultant velocity, V, at any point of the flow field is given by

The shape of the top or bottom half-body is found by putting ψ_{stagnation} value back into the stream function where θ = π. This gives,