Fluids Lecture 12 Notes

1. Stream Function

2. Velocity Potential

Reading: Anderson 2.14, 2.15

Stream Function

Definition

Consider defining the components of the 2-D mass flux vector ~V as the partial derivativesof a scalar stream function, denoted by (x, y):

u =

y, v =

x

For low speed flows, is just a known constant, and it is more convenient to work with ascaled stream function

(x, y) =

which then gives the components of the velocity vector ~V .

u =

y, v =

x

Example

Suppose we specify the constant-density streamfunction to be

(x, y) = lnx2 + y2 =

1

2ln(x2 + y2)

which has a circular funnel shape as shown in the figure. The implied velocity componentsare then

u =

y=

y

x2 + y2, v =

x=

x

x2 + y2

which corresponds to a vortex flow around the origin.

x

y

vortex flow example

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Streamline interpretation

The stream function can be interpreted in a number of ways. First we determine the differ-ential of as follows.

d =

xdx +

ydy

d = u dy v dx

Now consider a line along which is some constant 1.

(x, y) = 1

Along this line, we can state that d = d1 = d(constant) = 0, or

u dy v dx = 0 dy

dx=

v

u

which is recognized as the equation for a streamline. Hence, lines of constant (x, y) arestreamlines of the flow. Similarly, for the constant-density case, lines of constant (x, y) arestreamlines of the flow. In the example above, the streamline defined by

lnx2 + y2 = 1

can be seen to be a circle of radius exp(1).

x

y Vd

dx

dy

= 1

u

v = 0

Mass flow interpretation

Consider two streamlines along which has constant values of 1 and 2. The constantmass flow between these streamlines can be computed by integrating the mass flux alongany curve AB spanning them. First we note the geometric relation along the curve,

n dA = dy dx

and the mass flow integration then proceeds as follows.

m = B

A~V n dA =

BA

(u dy v dx) = B

Ad = 2 1

Hence, the mass flow between any two streamlines is given simply by the difference of theirstream function values.

2

x

yV + d

dx

dy

A

B

V

n^

n^

= 2

dA

= 1

Continuity identity

Consider the mass flux field ~V (x, y) specified by some (x, y). Computing the divergenceof this field we have

(~V)

=(u)

x+

(v)

y=

x

(

y

)

y

(

x

)=

2

y x

2

x y= 0

so that any mass flux field specified via (x, y) will automatically satisfy the steady masscontinuity equation. In low speed flow, a similar computation shows that any velocity fieldspecified via (x, y) will automatically satisfy

~V = 0

which is the constant-density mass continuity equation. Because of these properties, usingthe stream function to define the velocity field can give mathematical simplification in manyfluid flow problems, since the continuity equation then no longer needs to be addressed.

Velocity Potential

Definition

Consider defining the components of the velocity vector ~V as the gradient of a scalar velocitypotential function, denoted by (x, y, z).

~V = =

x+

y+ k

z

If we set the corresponding x, y, z components equal, we have the equivalent definitions

u =

x, v =

y, w =

z

Example

For example, suppose we specify the potential function to be

(x, y) = arctan

(x

y

)

which has a corkscrew shape as shown in the figure. The implied velocity components arethen

u =

x=

y

x2 + y2, v =

y=

x

x2 + y2

3

xy

vortex flow example

which corresponds to a vortex flow around the origin. Note that this is exactly the samevelocity field as in the previous example using the stream function.

Irrotationality

If we attempt to compute the vorticity of the potential-derived velocity field by taking itscurl, we find that the vorticity vector is identically zero. For example, for the vorticityx-component we find

x w

y

v

z=

y

z

z

y=

2

yz

2

zy= 0

and similarly we can also show that y = 0 and z = 0. This is of course just a manifestationof the general vector identity curl(grad) = 0 . Hence, any velocity field defined in termsof a velocity potential is automatically an irrotational flow . Often the synonymous termpotential flow is also used.

Directional Derivative

In many situations, only one particular component of the velocity is required. For example,for computing the mass flow across a surface, we only require the normal velocity component.This is typically computed via the dot product ~V n. In terms of the velocity potential, wehave

~V n = n =

n

where the final partial derivative /n is called the directional derivative of the potentialalong the normal coordinate n. The figure illustrates the relations.

n =

= =

1 = 2

3

4

V n =. ^ 66n

V =

In general, the component of the velocity along any direction can be obtained simply bytaking the directional derivative of the potential along that same direction.

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