1. Lecture 15 Final Version Contents Solutions to Laplace Equation
Line Source/Sink
Line Vortex
The Circulation
Combinations of Solutions: Solid Bodies in a Potential Flow (Rankine Oval etc.)

2. What Did We Do In Last Lecture?
Flow Around a Corner of Arbitrary Angle
One can interpret real part, R(x,y), of complex function, F(z), as velocity potential and imaginary part, I(x,y), as stream function of a two-dimensional flow.
Derivative, F’=u-iv, corresponds to complex conjugate of velocity, u+iv.
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2) Streeter Wylie & Bedford ?????

3. SOLUTIONS TO LAPLACE EQUATION
We learnt that in inviscid, irrotational and incompressible flow Laplace equation determines flow field. For instance, in 2D all that is required is to solve:
(1)
subject to boundary conditions (usually no flow through a solid boundary) obtained when a body (e.g. an aerofoil) is placed in a flow which is uniform infinitely far upstream..
To satisfy boundary conditions, one often constructs a composite solution. Note that if are solutions to Eq. (1) then function is also a solution since:
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(2)

4. Continued …
In general then, if where are each solutions to Laplace equation, so must be
This process of constructing a solution is known as
LINEAR SUPERPOSITION.
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2) Streeter Wylie & Bedford ?????
Functions , are usually well-known solutions to Laplace equation and are ‘building bricks’ of a composite solution. We construct a solution according to Eq. (2), finding necessary constants Ai such that boundary conditions are satisfied.

5. Some useful solutions to the Laplace Equation as examples
We now identify some useful solutions to Laplace equation. Remember that for a potential function to be a solution to Laplace equation velocity must satisfy conditions of zero vorticity and imcompressibility (the mass-continuity equation).
If fluid is incompressible and because we are dealing with two-dimensional flows, then there must also exist a stream function. Below we will identify both the velocity potential, , and the stream function, , for particular flows.
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2) Streeter Wylie & Bedford ?????

6. Example: Uniform Flow (Revision, see earlier examples!)
Paralell to the x-axis….
Need to project free stream velocity onto radial and circumferential unit vector!
Minus sign mathematically negative sense, i.e. clockwise
In cartesians ...
In polar coordinates ...
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2) Streeter Wylie & Bedford ?????
Have already seen in previous example that this flow has zero vorticity and that it satisfies the mass-continuity equation.
We found that velocity potential and stream function are...
Velocity Potential
Stream Function
In cartesians ...
In polar coordinates ...

7. Example: Line Source/Sink
Suppose z-axis were a sort of thin-pipe manifold through which issued fluid at a total rate Q uniformly along its length b. Looking at xy plane, we would see a cylindrical outflow or line source as sketched below. If flow is perfectly uniform then flow speed is entirely in radial direction, i.e. there is no circumferential velocity.
Questions and Solutions:
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(Q1) Determine radial flow velocity ur as function of the radial coordinate r.
(A1) Apply mass-continuity to a cylindrical control volume of radius r which has source as its centre line. Surface area of control volume through which fluid has to pass is...
Since, ...
Area x Flow Velocity = Volume Flow Rate Through Area
one gets...
(1)
Will need this
repeatedly. Keep
this result in mind!
: Source Strength; Volumetric flow rate (Q) per unit length and divided by

8. Continued...
(Q2) Is this an irrotational flow? Think about deformation of fluid elements as they move away from the origin ...
(A2) Need to verify that vorticity is zero. In polars, vorticity is ...
(2)
Substituting in values for radial and azimuthal velocity gives, ...
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2) Streeter Wylie & Bedford ?????

9. (Q3) Is mass-continuity satisfied?
Continued...
(Q3) Is mass-continuity satisfied?
(A3) Evidently it will be satisfied from our derivation of radial and circumferential velocity. However, to show this formally, substitute these into mass-continuity equation. In polars mass-continuity is ...
(3)
Note symmetry / similarity of Eq. (2) and Eq. (3) in co- ordinates!
Substituting in values for radial and azimuthal velocity gives, ...
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CONCLUDE: Because irrotational and mass continuity satisfied Laplace equation must be satisfied ...
… everywhere in plane EXCEPT at origin, location of source, where radial flow speed, is infinite- Eq. (1) previous slide.
Note: A sink is opposite of source - fluid is sucked into ‘hole’. Same formulation as for source is used except that source strength becomes -m

10. (6a) (6b) Velocity Potential :
Continued...
(Q4) Evaluate velocity potential and stream function!
(A4) From their definitions, using polars ...
Stream Function :
(4a,b)
Velocity Potential :
(5a,b)
…now integrate
From (4a)
…now integrate
From (4b)
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(6a)
Compatibility required, as usual, and neglecting constant ...
…now integrate
From (5a)
From (5b)
…now integrate
Compatibility required, as usual, and neglecting constant ...
(6b)

11. Velocity Potential Stream Function = ur
Continued...
Velocity Potential
Stream Function
= ur
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2) Streeter Wylie & Bedford ?????
Again, note:
Stream function constant along streamlines (here in radial direction).
Lines of constant potential are perpendicular to streamlines
For what should be obvious reasons, polar coordinates have been used to determine the velocity potential and the stream function.
Can convert both into cartesian forms ...

12. Continued...
Conversion of stream function and velocity potential into cartesian forms.
Suppose ‘red dot’ moves on circle around origin of xy coordinate system...
One has ...
and
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Added velocity components are...
For our example circumferential velocity does not exis, hence...
with...
one gets ...
(7a,b)
And so ...

13. Velocity Potential, Eq. (6b) Stream Function Eq. (6a)
Continued...
Velocity potential and stream function can either be found using integration of velocity components in Eq. (7a,b) from previous slide or by using solutions obtained earlier (Eq. (6a,b)) and performing a direct change of variables in polar-coordinate forms (remember that these are scalar quantities).
Velocity Potential, Eq. (6b)
Stream Function Eq. (6a)
Change of Variables
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2) Streeter Wylie & Bedford ?????
Can use these forms to find velocity potential and stream function due to a source/sink which is not located at origin. If source/sink was located at (x0 , y0 ) then same analysis can be carried out using translated axes and following results apply:

14. Example : Line Vortex (1) Laplace Equation for velocity potential
For irrotational, incompressible flow from previous example we can show that stream function also satisfies the Laplace equation, i.e. that...
(1)
Recall...
Hence, Eq. (1) equivalent to...
Which is the statement that vorticity is zero!
This means that stream function for source/sink flow from previous example satisfies Laplace Eq.!!! (Recall, we started our previous example from velocity potential satisfying Laplace Eq.)
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Hence, this stream function could represent velocity potential.
Therefore define new flow where roles of velocity potential and stream function have been swapped around, i.e ...
Velocity Potential
Stream Function
where K is a constant (vortex strength). Velocity components are now easily found...
Radial Velocity Component
Circumferential Velocity Component

15. Continued...
This is a flow exclusively in tangential direction, with flow speed inversely proportional to (radial) distance from vortex centre. The larger the value of K the higher velocity at any particular radius r - this is why K is called vortex strength. Flow is akin to that seen around a plug hole… What would be a better representation of a flow near/down a plug hole?
Note that flow is not a rigid body motion with constant angular velocity and speed directly proportional to radial distance from axis of rotation!
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2) Streeter Wylie & Bedford ????? A B
Along line AB one has 1/r velocity ‘decay’. Speed downwards on interval B0 and upwards on 0A. Velocity infinite at origin; of course this cannot happen in reality. In reality viscosity removes the singularity at origin.

16. Important here…. … to realize what irrotational means!
Continued...
AGAIN… Note from a physical standpoint that flow is IRROTATIONAL everywhere … except at origin where circumferential velocity is infinite and so is vorticity.
Important here…. … to realize
what irrotational means!
… Put a piece of paper on our irrotational, potential vortex and watch it... What you observe as it goes around is...
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The EXCLAMATION MARK … … does NOT change its orientation in space as it moves around centre of the vortex! … This is because the flow is irrotational; its vorticiy is zero...!

17. THE CIRCULATION CIRCULATION IS DEFINED as the
For irrotational line vortex ‘exclamation mark did not rotate’ - in the sense of changing its orientation in space. Had it changed its orientation then flow would have been rotational. It is THIS rotation that vorticity deals with.
Nevertheless, exclamation mark moved around centre of vortex. So there was some sort of rotary motion. This aspect of flow is connected with the CIRCULTATION.
For a region of flow...
CIRCULATION IS DEFINED as the
clockwise integral around the
closed curve (that encloses the region) of the
flow velocity component along the curve.
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2) Streeter Wylie & Bedford ?????

18. Hmhhh,… wouldn’t I expect this integral to be zero?
No it is not zero! Convince yourself by looking at the example ...
(1)
So, … integral (i.e. circulation) gives a finite value! Reason is that plus/minus signs always appear in such combinations that one always adds up (positive) terms!
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Note, the larger u and v (i.e. the faster I move around curve) the larger circulation.
Similar sums as above in Eq.(1) could be written down for line vortex...
Circulation for line vortex will be constant no matter around which one of the circular streamlines I integrate. Velocity decrease as 1/r but circumference of circle increases proportional to r.

19. ! (A) Two slides ago where we defined circulation we had… So, then...
Continued...
Two slides ago where we defined circulation we had…
So, then...
(A)
For velocity potential one has:
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Hence, ...
Total Differential, 1st year Maths
So, then, ...
Circulation only depends on values of velocity potential at start and end points of the path along which we integrate!

20. OK,… Then let’s calculate the circulation for our line vortex and for our source!
(i) For LINE VORTEX the velocity potential was found to be ...
As we move along curve C surrounding the line vortex the angle changes as...
Hence,...
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(ii) For LINE SOURCE the velocity potential was found to be ...
When we move along curve C surrounding line source then radius...
Hence,...

21. Simplified version of ‘Wing Example’
Continued...
NOTES:
Value of circulation does not depend on the shape of closed curve, C. To find results above, we could chose a circular or non-circular path round the vortex (i) and source (ii).
In general, value of circulation denotes net algebraic strength of all vortex filaments contained within the closed curve. This can be seen by re-writing the line integral into a surface integral using Stokes theorem...
Circulation is an important concept in fluid mechanics: it is circulation which (together with onset of flow speed) develops lift (Magnus Force) on a spinning cylinder, spinning cricket ball and an aerofoil section.
In potential flow, line vortex is used to model circulation that occurs in real life.
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Use of word ‘Circulation’ to label integral used for its definition is slightly misleading.
It does not necessarily mean that fluid elements are moving around in circles within this flow field!
Rather, when circulation exist it simply means that line integral is finite.
For example, if airfoil below is generating lift the circulation taken around a closed curve enclosing the airfoil will be finite, although the fluid elements are by no means executing circles around the airfoil (as clearly seen from streamlines sketched in figure).
Simplified version of ‘Wing Example’
on shown on left...

22. COMBINATIONS OF SOLUTIONS: SOLID BODIES IN A POTENTIAL FLOW
Recall: Can use PRINCIPLE OF SUPERPOSITION for velocity potential.
In addition, have shown that for incompressible, irrotational flow, stream function also satisfies Laplace Eq. So can similarly construct flow solutions by combining S.F. associated with uniform flow, source/sink flow and line-vortex flow.
In fact, we will almost exclusively use stream function here because we are interested in pattern of streamlines; once we find stream function, we can use fact that it is constant along streamlines to plot out streamlines.
Uniform Flow and Source: THE RANKINE BODY
What happens if we combine... ?
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(1)
Cartesian Coordinates:
(2)
Polar Coordinates:
(1)/(2) represent complete descriptions of flow field. But what does it look like?...
To graph lines of constant
, first look for STAGNATION POINTS.
There ...
Thus, both velocity components must be zero… Differentiate to get expressions for velocity components ...

23. Cartesian Coordinates:
Continued...
Cartesian Coordinates:
(3)
(4)
For v=0 Eq. (4) requires ...
...with this Eq. (3) gives...
STAGNATION POINT at
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Polar Coordinates:
and
Since m, r positive choose...
to get a solution for
STAGNATION POINT at

24. Cartesian Coordinates
Continued...
In both cases same location for Stagnation Point ...
Cartesian Coordinates
Polar Coordinates
We repeated ourselves to demonstrate that either coordinate system can be used. In general choose the one that makes the analysis easiest.
Now use for distance between origin and stagnation point:
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Find S.L. that arrives at stagnation point and divides there. Using ...
along this S.L., use a known point - the stagnation point - to evaluate constant. With Eq (2) from above ...
where suffix s denotes ‘along particular streamline through stagnation point’.
Streamline found by equating Eq (2) to the constant and rearranging...
(5)

25. 2) Streeter Wylie & Bedford ?????
Continued...
Plotting stagnation streamline:
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2) Streeter Wylie & Bedford ?????
Stagnation streamline defines shape of (imaginary) solid half-body which may be fitted inside streamline boundary; remember flow does not cross streamline … or solid boundary. Call this special S.L. SURFACE STREAMLINE.
Body shape named after Scottish engineer W.J.M. Rankine ( ).
Now only have stagnation/surface. To get other S.L.’s, choose point, determine constant for S.L. through point and then sketch particular S.L. through this point by compiling a table as above.