Transcript Slide 1

FLUID FLOW
MOTION OF OBJECTS IN FLUIDS
How can a plane fly?
Why does a cricket ball swing or a baseball curve?
Why does a golf ball have dimples?
web notes: lect6a.ppt
flow4.pdf
flight.pdf
FORCES ACTING ON OBJECT MOVING THROUGH FLUID
Resultant FR
Lift FL
drag FD
Motion of object through fluid
Fluid moving around stationary object
Uniform motion of an object through an ideal fluid ( = 0)
The pattern is symmetrical
 FR = 0
C
B
A
D
Drag force
frictional drag (viscosity)
pressure drag (eddies – lower pressure)
Drag force due
to pressure difference
motion of air
motion of object
low pressure region
rotational KE of eddies 
heating effect  increase
in internal energy 
temperature increases
NO
CURVE
high pressure region
Drag force is
opposite to the
direction of motion
motion of air
motion of object
Drag force due
to pressure difference
flow speed (high) vair + v
 reduced pressure
v
vair (vball)
MAGNUS EFFECT
flow speed (low) vair - v
 increased pressure
v
high pressure region
low pressure region
Boundary layer – air
sticks to ball
(viscosity) – air
dragged around with
ball
Velocity profile around ball
Ball moving to left and rotating clockwise
Ball moving to left and rotating clockwise
4.5
4
3.5
3
2.5
2
1.5
1
0.5
Professional golf drive
Initial speed v0 ~ 70 m.s-1
Angle ~ 6°
Spin  ~ 3500 rpm
Range ~ 100 m (no Magnus effect)
Range ~ 300 m (Magnus effect)
The trajectory of a
golf ball is not
parabolic
Golf ball with backspin (rotating CW) with air stream going from
left to right. Note that the air stream is deflected downward with a
downward force. The reaction force on the ball is upward. This
gives the longer hang time and hence distance carried.
How can a plane fly?
lift
Direction plane is moving w.r.t. the air
Direction air is moving w.r.t. plane
low
pressure

lift
low pressure drag
attack angle
momentum transfer
high
pressure
downwash
huge vortices
FLift = CL ½  vs2 A