Physics of How Things WorkFluid in Motion
Chapter 6
Vortex Cannon
Vortex Cannon
Which way does it travel?
Why?
Vortex Cannon
Which way does it travel?
Why?
Which way does it spin?
Why?
Vortex Cannon
Which way does it travel?
Why?
Which way does it spin?
Why? flow rate ∝ 1viscosity
Vortex Cannon
Vortex Cannon
When does it stop and why?
Mr. Bernoulli
As speed increases, pressure decreases
Mr. Bernoulli
As speed increases, pressure decreases
P1 + ρgh1 +12ρv1
2 = P2 + ρgh2 +12ρv2
2
Mr. Poiseuille
ΔVΔt
= π ⋅ΔP ⋅D4
128 ⋅L ⋅η
Flow Rate = π ⋅Pressure difference ⋅Diameter4
128 ⋅Length ⋅viscocity
Example #1
How fast will honey flow through a straw that is 5” (0.127m) long (it has a diameter of 10 mm) if you have perfect lung power.
Example #2
A 68 cm tall Gatorade tank is full up to the lid which also allows air inside. How fast does the gatorade flow out of the tap when the tap is opened?
Turbulent Flow
Causes of Turbulent Flow
Viscosity of fluid
Speed of fluid
Obstacle size
Fluid density
Reynolds number
Reynolds number = fluid density ⋅obstacle width ⋅ fluid speed
viscosity
Turbulence happens when the Reynolds number exceeds 2300
viscous dominated vs. inertia dominated
Example
How fast can a small (~ 2 cm wide) fish swim before it looses control?
What about ball sports?
Curve BAll
1.Thrower gives the ball a forward speed and puts a spin on the ball
Curve BAll
2.The spin causes one side of the ball to have a higher relative air speed. Which side has the higher speed?
V
v
Curve BAll
3.Because of the higher air speed on the right side, the Reynold’s number exceeds 2300 on that side alone!
V
v
reynolds number = fluid density ⋅obstacle width ⋅ fluid speed
viscosity
Curve BAll4.Because the Reynold’s number was so high on the right
side, turbulence occurs on the right side of the ball (but not the left)
Curve BAll5.Because turbulence occurs on the right side of the ball,
the air does not stick to the ball on that side
Curve BAll
6.Because of the low relative air-speed on the left side of the ball, the air sticks to the ball (through friction).
v
Curve BAll
7.As the ball continues to rotate, it pulls the air on the left side to the right.
Curve BAll
8.Ball pulls air to the right. Therefore,air pulls the ball to the left
This is called the “Magnus Force”
Curve BAll9.The constant push to the left coupled with the
forward motion causes the ball to curve left.
Curve
Spin, turbulence, & strikes
motionSpin
Curve
Why do golf balls have dimples?
Quick Quiz
A vortex from a vortex cannon rotates
A.Clockwise (as viewed from the shooter)
B.Counterclockwise (as viewed from the shooter
C.doesn’t rotateD.From the middle forward
and then backwards on the outer edge
Quick Quiz
Why do umbrella’s flip up in a gust of wind?
A.Because of Bernoulli’s Principle
B.Because of Pascal’s Principle
C.Because of Poiseuille’s Principle
D.Because of Reynolds’ Principle