Fluid Mechanics with teacher's notes

Section 9-3
Demonstration 7
Fluid flow around a
table-tennis ball
Purpose Introduce students to
some of the interesting effects of
fluid flow.
Materials table-tennis ball glued
to the end of a piece of light
string, water faucet
Procedure Tell the class that this
section covers the surprising
phenomena of fluids in motion
“sucking in” things around them.
Hold the end of the string so
that the ball is a few centimeters
below the faucet but away from
the stream path. When you turn
on the faucet, the ball is attracted
to the water.
Demonstration 8
Fluid flow through two
cans
Purpose Further introduce the
effects of fluid flow.
Materials two soda cans, flat
surface
Procedure Lay the soda cans on
their sides approximately 2 cm
apart on a table or other flat surface.
Ask students what would
happen if you were to blow air
between the two cans. Ask a student
volunteer to blow between
the cans. The cans will move
toward each other. Explain that
the air moves faster between the
cans, lowering the pressure
between them and drawing
them together.
332
9-3 SECTION OBJECTIVES
• Examine the motion of a
fluid using the continuity
equation.
• Apply Bernoulli’s equation to
solve fluid-flow problems.
• Recognize the effects of
Bernoulli’s principle on fluid
motion.
332
Chapter 9
9-3
Fluids in motion
FLUID FLOW
Figure 9-10
The water flowing around this cylinder exhibits laminar
flow and turbulent flow.
ideal fluid
a fluid that has no internal
friction or viscosity and is
incompressible
Have you ever gone canoeing or rafting down a river? If so, you may have
noticed that part of the river flowed smoothly, allowing you to float
calmly or to simply paddle along. At other places in the river, there may have
been rocks or dramatic bends that created foamy whitewater rapids.
When a fluid, such as river water, is in motion, the flow can be characterized
in one of two ways. The flow is said to be laminar if every particle that
passes a particular point moves along the same smooth path traveled by the
particles that passed that point earlier. This path is called a streamline. Different
streamlines cannot cross each other, and the streamline at any point coincides
with the direction of fluid velocity at that point. The smooth stretches of
a river are regions of laminar flow.
In contrast, the flow of a fluid becomes irregular, or turbulent,
above a certain velocity or under conditions that can
cause abrupt changes in velocity, such as where there are
obstacles or sharp turns in a river. Irregular motions of the
fluid, called eddy currents, are characteristic of turbulent flow.
Examples of turbulent flow are found in water in the wake of
a ship or in the air currents of a severe thunderstorm.
Figure 9-10 shows a photograph of water flowing past a
cylinder. Hydrogen bubbles were added to the water to
make the streamlines and the eddy currents visible. Notice
the dramatic difference in flow patterns between the laminar
flow and the turbulent flow. Laminar flow is much easier
to model because it is predictable. Turbulent flow is
extremely chaotic and unpredictable.
The ideal fluid model simplifies fluid-flow analysis
Many features of fluid motion can be understood by considering the behavior
of an ideal fluid. While discussing density and buoyancy, we assumed all of
the fluids used in problems were practically incompressible. A fluid is incompressible
if the density of the fluid always remains constant. Incompressibility
is one of the characteristics of an ideal fluid.
The term viscosity refers to the amount of internal friction within a fluid. Internal
friction can occur when one layer of fluid slides past another layer. A fluid with
a high viscosity flows more slowly through a pipe than does a fluid with a low
viscosity. As a viscous fluid flows, part of the kinetic energy of the fluid is
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