flow of a fluid: example water. chapter 5yodh/phys18/fluids_lec2.pdfflow of a fluid: example water....
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Flow of a fluid: Example Water.Consider the motion of a small volume of water at a point P. It ismoving with velocity: vector v in the figure. The lines are drawn alongthe velocity vector at each point : called streamlines. Water does notflow across them.
Chapter 5
Pascal's Principle: Deals with transmission of pressuredisturbance in a fluid and its container : Statement is
pages 153154
“A Change in pressure of an enclosed incompressible fluidis conveyed undiminished to every part of the fluid and to the surfaces of the container.”
Now we consider water flow situations– needs fluid containers and pipes to guide the water . Quantities involved in understanding flow are:1. kinetic energy of water flow, 2. pressure energy of water and3. if the flow is between different heights then gravitational potential energy.
Conservation of water flow through a tube:
Flow per second across crossection at point 1: A1 v1
Flow per second across crossection at point 2: A2 v2
Water coming in at 1 must come out at 2, hence A1 v1=A2 v2
This is the equation of continuity, Note density is constant in water
How does one move water through a pipe? By pressure differences.
Fig 5.2.1 page 152
Pressure is energy per unit volume. Flow of water must conserve energy. Energy in nonturbulent fluid flow comes in three forms:(pages 154156)Pressure energyEnergy of motion or kinetic energyPotential energy due to the relative heights of the water
We will take about energy densities; i.e. energy per unit volume.
Mass of water per unit volume is water= whose units are kg /m3
Kinetic energy density KE per unit mass = 12v2
Potential energy density PE=g h
The total energy per unit mass at any point in the flow is:
Total Energy density =P12v2g h=Constant
provided there are no sources or sinks of water in the flow
This is Bernoulli's equation: Equation 5.2.4
For an incompressible fluid in steady flow, the sum of its pressure potential energy, its kinetic energy and its gravitational potential energy is constant along a streamline.
Incompressible means its density is a constant.
Why does the water come out with high speed ?
Continuity Equation: A1 v1=A2 v2
Hence v2=A1
A2
v1v1
Uxe Bernoulli's equation 12v1
2p1=12v2
2p2
Which is larger ? p1 or p2 ?
We write
p2−p1=12v1
2−A1
A2
2
v12
As A1 is greater than A2
p2p1
Faster the flow lower the pressure !
Some air flows upwards through the nozzles shown in the figure. Depending on the pressure of air in the tube the balls rise to different heights.
Chapter 6.3 pages 187190
Which way will the baseball curve ? Section 6.2
Why is accumulating cholesterol bad for you ?
Plaque formation and blood vessel constriction.
Viscocity: friction in fluid flow
Pouiseuille's Law:
V t
=
128 pL
D4
where L is the length of the tube ; is the viscocity ;
D is the diameter of the tube and p is the head of pressure
Measuring Pressure: I
partial vacuum
Air pressure supports the column ofliquid.
With mercury as fluid: 76 cm heightWith Water as fluid: 10 meters height
76 cm is 30 inches.
When weatherman says pressure isfalling and is 29.5 inches we havea low pressure.
Atomizer : pressure drops at the constriction and sucks up fluid and blowsit out in a spray.