EF 152 – Physics for Engineers
Summer, 2013
Recitation 1.6 Hydrodynamics, Bernoulli
Objectives
• See Bernoulli’s principle in action through several demonstrations
• Learn about laminar and turbulent flow
• Apply impulse-momentum to fluids
Task 1. Demonstration of Bernoulli’s principle
A. Ping pong ball in a funnel experiment
Attach a straw to the funnel using clear tape as shown. Place the ping pong ball in
the funnel and blow. What would you expect to happen?
What actually happens?
Explain this in terms of Bernoulli’s Principle.
Try turning the funnel as though to dump out the ping-pong ball while blowing on it. What happens?
When does the ball fall out?
B. Tie a 12 inch piece of string to each of two ping pong balls. Hold the ping pong balls so
they hang approximately 1 cm apart. Blow a steady stream of air between the two. What
happens to the ping pong balls?
C. Bernoulli’s Water Gun. Place one straw in the water, and blow across the top of it with the
other straw. Predict what will happen.
D. Rising paper strip experiment
Cut a piece of paper 2 in. by 6 in. Hold the narrow end, with the other end hanging down,
in front of your mouth and blow across the top. See what happens to the paper.
If you put paper clips on the lower end of the strip, how many does it take to keep the
Bernoulli effect from raising the paper? Put your result here: ______________
Based on the weight of the paper and the paper clips, how fast were you blowing? _______________
E. Air Jet and Ping Pong Ball
Attach a 3-4” long thread to the end of a straw and to the ping pong ball.
Blow vigorously through the straw while holding the ping pong ball in line
with the tube so that it is in the path of the rapidly moving air. Then let go
of the ping pong ball. What happens?
F. Look at the demonstration of Bernoulli’s principle at http://mitchellscience.com/bernoulli_principle_animation.
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EF 152 – Physics for Engineers
Task 2. Why do golf balls have dimples? or Poiseuille’s equation and Stoke’s Law revisited.
Poiseuille’s equation and Stoke’s Law are based on laminar flow.
Summer, 2013
Laminar and turbulent flow are seen in
water flow over the hull of a submarine.
: fluid flows in parallel layers.
: fluid flows in a chaotic fashion.
Drag forces can actually be less with turbulent flow than with laminar flow.
This is shown in the pictures of a bowling ball flowing through water at 17
mph. The bowling ball (8.5 in. diameter) on the right has a 4 in. diameter
patch of sand grains cemented to the nose.
Whether the flow is laminar or turbulent is characterized by the
. The Reynolds number (Re) is a dimensionless
number that gives a measure of the ratio of inertial forces to viscous forces.
For a sphere, the Reynolds number is calculated as:
Re =
vDρ
η
Re = Reynolds number
v = velocity
D = diameter
ρ = density of fluid
η = viscosity
The drag coefficient for a sphere as a function of Reynolds
number is shown in the following graph.
The great Professor Schleter hits a golf ball with a speed of 70 m/s (157 mph). Determine the Reynolds number
-5
3
and drag coefficient for the golf ball. Dgolf ball = 41.7 mm; ηair = 1.8x10 Pa·s; ρair = 1.20kg/m .
Task 3. Most difficult concept
Look over Fall 2012 Exam 1. Vote on the most difficult problem. Work through that problem as a group.
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