1. Flow over immersed bodies. Boundary layer. Analysis of inviscid flow.
Lecture 7
Flow over immersed bodies.
Boundary layer. Analysis of inviscid flow.

2. Flow over immersed bodies
flow classification: 2D, axisymmetric, 3D
bodies: streamlined and blunt
Shuttle landing: examples of various body types

3. Lift and Drag
shear stress and pressure integrated over the surface of a body create force
drag: force component in the direction of upstream velocity
lift: force normal to upstream velocity (might have 2 components in general case)

4. Flow past an object Dimensionless numbers involved
for external flow: Re>100 dominated by inertia, Re<1 – by viscosity

5. Flow past an object
Character of the steady, viscous flow past a circular cylinder: (a) low Reynolds number flow, (b) moderate Reynolds number flow, (c) large Reynolds number flow.

6. Boundary layer characteristics
for large enough Reynolds number flow can be divided into boundary region where viscous effect are important and outside region where liquid can be treated as inviscid

7. Laminar/Turbulent transition
Near the leading edge of a flat plate, the boundary layer flow is laminar.
f the plate is long enough, the flow becomes turbulent, with random, irregular mixing. A similar phenomenon occurs at the interface of two fluids moving with different speeds.

8. Boundary layer characteristics
Boundary layer thickness
Boundary layer displacement thickness:
Boundary layer momentum thickness (defined in terms of momentum flux):

9. Prandtl/Blasius boundary layer solution
Let’s consider flow over large thin plate:
approximations:
than:
boundary conditions:

10. Prandtl/Blasius boundary layer solution
as dimensionless velocity profile should be similar regardless of location:
dimensionless similarity variable
stream function

11. Drag on a flat plate
Drag on a flat plate is related to the momentum deficit within the boundary layer
Drag and shear stress can be calculated just by assuming some velocity profile in the boundary layer

12. Transition from Laminar to Turbulent flow
The boundary layer will become turbulent if a plate is long enough
turbulent profiles are flatter and produce larger boundary layer

13. Inviscid flow no shearing stress in inviscid flow, so
equation of motion is reduced to Euler equations

14. Bernoulli equation
let’s write Euler equation for a steady flow along a streamline
now we multiply it by ds along the streamline

15. Irrotational Flow
Analysis of inviscide flow can be further simplified if we assume if the flow is irrotational:
Example: uniform flow in x-direction:

16. Bernoulli equation for irrotational flow
always =0, not only along a stream line
Thus, Bernoulli equation can be applied between any two points in the flow field

17. Velocity potential
equations for irrotational flow will be satisfied automatically if we introduce a scalar function called velocity potential such that:
As for incompressible flow conservation of mass leads to:
Laplace equation

18. Some basic potential flows
As Laplace equation is a linear one, the solutions can be added to each other producing another solution;
stream lines (y=const) and equipotential lines (f=const) are mutually perpendicular
Both f and y satisfy Laplace’s equation

19. Uniform flow
constant velocity, all stream lines are straight and parallel

20. Source and Sink
Let’s consider fluid flowing radially outward from a line through the origin perpendicular to x-y plane from mass conservation:

21. Vortex
now we consider situation when ther stream lines are concentric circles i.e. we interchange potential and stream functions:
circulation
in case of vortex the circulation is zero along any contour except ones enclosing origin

22. Shape of a free vortex
at the free surface p=0:

23. Doublet let’s consider the equal strength, source-sink pair:
if the source and sink are close to each other:
K – strength of a doublet

24. Summary

25. Superposition of basic flows
basic potential flows can be combined to form new potentials and stream functions. This technique is called the method of superpositions
superposition of source and uniform flow

26. Superposition of basic flows
Streamlines created by injecting dye in steadily flowing water show a uniform flow. Source flow is created by injecting water through a small hole. It is observed that for this combination the streamline passing through the stagnation point could be replaced by a solid boundary which resembles a streamlined body in a uniform flow. The body is open at the downstream end and is thus called a halfbody.

27. Rankine Ovals
a closed body can be modeled as a combination of a uniform flow and source and a sink of equal strength

28. Flow around circular cylinder
when the distance between source and sink approaches 0, shape of Rankine oval approaches a circular shape

29. Potential flows
Flow fields for which an incompressible fluid is assumed to be frictionless and the motion to be irrotational are commonly referred to as potential flows.
Paradoxically, potential flows can be simulated by a slowly moving, viscous flow between closely spaced parallel plates. For such a system, dye injected upstream reveals an approximate potential flow pattern around a streamlined airfoil shape. Similarly, the potential flow pattern around a bluff body is shown. Even at the rear of the bluff body the streamlines closely follow the body shape. Generally, however, the flow would separate at the rear of the body, an important phenomenon not accounted for with potential theory.

30. Effect of pressure gradient
d’Alemberts paradox: drug on an object in inviscid liquid is zero, but not zero in any viscous liquid even with vanishingly small viscosity
inviscid flow
viscous flow

31. Effect of pressure gradient
At high Reynolds numbers, non-streamlined (blunt) objects have wide, low speed wake regions behind them.
As shown in a computational fluid dynamics simulation, the streamlines for flow past a rectangular block cannot follow the contour of the block. The flow separates at the corners and forms a wide wake. A similar phenomenon occurs for flow past other blunt objects, including bushes. The low velocity wind in the wake region behind the bushes allows the snow to settle out of the air. The result is a large snowdrift behind the object. This is the principle upon which snow fences are designed

32. Drag on a flat plate
Drag coefficient diagram

33. Drag dependence
Low Reynolds numbers Re<1:

34. Drag dependence
Moderate Reynolds numbers. Drag coefficient on flat plate ~Re-½; on blunt bodies relatively constant (and decreases as turbulent layer can travel further along the surface resulting in a thinner wake

35. Examples

36. Examples
The drag coefficient for an object can be strongly dependent on the shape of the object. A slight change in shape may produce a considerable change in drag.

37. Problems
6.60. An ideal fluid flows past an infinitely long semicircular hump. Far from hump flow is uniform and pressure is p0. Find maximum and minimum pressure along the hump. If the solid surface is y=0 streamline, find the equation for the streamline passing through Q=p/2, r=2a.
9.2. The average pressure and shear stress acting on the surface of 1 m-square plate are as indicated. Determine the lift and drag generated.
9.12. Water flows past a flat plate with an upstream velocity of U=0.02 m/s. Determine the water velocity 10mm from a plate at distances 1.5mm and 15m from the leading edge.