1. The mechanics of nonviscous fluids
Chapter 13
The mechanics of nonviscous fluids
equation of continuity
Bernoulli’s Equation
static consequences of Bernoulli's equation
Blood Pressure measurements using the
sphygmomanometer
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2. Steady/Unsteady flow Turbulent Flow
In steady flow the velocity of particles is constant with time
Unsteady flow occurs when the velocity at a point changes with time. Turbulence is extreme unsteady flow where the velocity vector at a point changes quickly with time e.g. water rapids or waterfall.
When the flow is steady, streamlines are used to represent the direction of flow.
Steady flow is sometimes called streamline flow
Streamlines never cross. A set of streamlines can define a tube of flow, the borders of which the fluid does not cross
Turbulent Flow
Turbulent flow is an extreme kind of unsteady flow and occurs when there are sharp obstacles or bends in the path of a fast-moving fluid.
In turbulent flow, the velocity at a point changes erratically from moment to moment, both in magnitude and direction.
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3. Viscous/Non viscous flow
A viscous fluid such as honey does not flow readily, it has a large viscosity.
Water has a low viscosity and flows easily.
A viscous flow requires energy dissipation.
Zero viscosity – requires no energy. with no dissipation of energy. Some liquids can be taken to have zero viscosity e.g. water.
An incompressible, non viscous fluid is said to be an ideal fluid
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4. 13. 2
Stream flow
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5. The volume flow rate, Q is defined as the volume of fluid that flows past an imaginary (or real) interface.
A v represents the volume of fluid per second that passes through the tube and is called the volume flow rate Q
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6. If the area is reduced the fluid must speed up!
If the fluid is incompressible, the density remains constant throughout
Av represents the volume of fluid per second that passes through the tube and is called the volume flow rate Q
This just means that the amount of fluid moving in any “section of pipe” must remain constant.
If the area is reduced the fluid must speed up!
Speed is high where the pipe is narrow and speed is low where the pipe has a large diameter
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7. The product of the area and the fluid speed at all points along a pipe is constant for an incompressible fluid.
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8. Q: Have you ever used your thumb to control the water flowing from the end of a hose?
A: When the end of a hose is partially closed off, thus reducing its cross-sectional area, the fluid velocity increases.
This kind of fluid behavior is described by the equation of continuity.
Example:
Oil is flowing at a speed of 1.22 m/s through a pipeline with a radius of m. How much oil flows in 1 day?
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9. example
Each second 5525m3 of water flows over the 670 m wide cliff of the Horseshoe falls portion of Niagara falls .the water is a approximately 2m deep as it reach the cliff . What is its speed at that instant
example
A river flows in a channel that is 40. m wide and 2.2 m deep with a speed of 4.5 m/s.
The river enters a gorge that is 3.7 m wide with a speed of 6.0 m/s.
How deep is the water in the gorge?
The area is width times depth A1 = w1d1
Use the continuity equation v1A1 = v2A2
v1w1d1 = v2w2d2
Solve for the unknown d2.
d2 = v1w1d1 / v2w2 =(4.5 m/s)(40. m)(2.2 m) / (3.7m)(6.0 m/s) = 18 m
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10. example
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11. example
If pipe 1 diameter = 50mm, mean velocity 2m/s, pipe 2 diameter 40mm takes 30% of total discharge and
pipe 3 diameter 60mm. What are the values of discharge and mean velocity in each pipe?
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12. example
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13. example
example
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14. Example:
The volume rate of flow in an artery supplying the brain is 3.6x10-6 m3/s. If the radius of the artery is 5.2 mm,
A) determine the average blood speed.
B) Find the average blood speed if a constriction reduces the radius of the artery by a factor of 3 (without reducing the flow rate).
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15. Example:
Example,decrease in area of stream of water coming from tap.
So v2 > v1
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16. Example
if the cross-section area, A, is 1.2 x 10-3m2 and the discharge, Q is 2.4 l / s , then the mean velocity,
Example
if the area A 1 = 10 x10-3 m2 and A 2 = 3 x10-3 m 2 and the upstream mean velocity, = 2.1 m / s , then the downstream mean velocity
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17. Example
A sink has an area of about 0.25 m2. The drain has a diameter of 5 cm. If the sink drains at 0.03 m/s, how fast is water flowing down the drain?
Ad = pr2 = (p)(0.025 m)2 = 1.96 X 10-3 m3
Advd = Asvs
vd = Asvs/Ad=[(0.25 m2)(0.03 m/s)]/(1.96 X 10-3 m3)
vd = 3.82 m/s
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18. Example Aava = NAcvc (N is the number of capillaries)
The radius of the aorta is about 1.0 cm and blood passes through it at a speed of 30 cm/s. A typical capillary has a radius of about 4 X 10-4 cm and blood flows through it at a rate of 5 X 10-4 m/s. Estimate how many capillaries there are in the human body.
Aava = NAcvc (N is the number of capillaries)
Aa = pr2 = (3.14)(0.01 m)2 = 3.14 X 10-4 m2
Ac = pr2 = (3.14)(4 X 10-6 cm)2 = 5.0 X x m2
N = Aava/ Acvc (N is the number of capillaries)
N = (3.14 X 10-4 m2)(0.30 m/s) = ~ 4 billion
(5.0 X x10-4 m2)(5 X 10-4 m/s)
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19. Example: same Example:
The figure shows 3 straight pipes through which water flows. The figure gives the speed of the water in each pipe.
Rank them according to the volume of water that passes through the cross-sectional area per minute, greatest first. 6
same
Example:
Water flows smoothly through the pipe shown in the figure, descending in the process.
Rank the four numbered sections of pipe according to
(a) the volume flow rate Q through them
(b) the flow speed v through them
(c) the water pressure p within them, greatest first.
a)Same b) 1,2,3,4 c ) 4,3,2,1
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20. 1) 2) 3)
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21. (II) A (inside) diameter garden hose is used to fill a round swimming pool 6.1 m in diameter. How long will it take to fill the pool to a depth of 1.2 m if water issues from the hose at a speed of
The volume flow rate of water from the hose, multiplied times the time of filling, must equal the volume of the pool.
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22. Pressure
Pressure is the ratio of a force F to the area A over which it is applied:
A = 2 cm2
1.5 kg
P = 73,500 N/m2
The greater the force, the greater the pressure is. The greater the area, the smaller the force is.
example
a) Calculate the total force of the atmosphere acting on the top of a table that measures
(b) What is the total force acting upward on the underside of the table?
(a) The total force of the atmosphere on the table will be the air pressure times the area of the table.
(b) Since the atmospheric pressure is the same on the underside of the table (the height difference is minimal), the upward force of air pressure is the same as the downward force of air on the top of the table,
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23. example
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24. Example
The mattress of a water bed is 2.00m long by 2.00m wide and 30.0cm deep. a) Find the weight of the water in the mattress.
The volume density of water at the normal condition (0oC and 1 atm) is 1000kg/m3. So the total mass of the water in the mattress is
Therefore the weight of the water in the mattress is
b) Find the pressure exerted by the water on the floor when the bed rests in its normal position, assuming the entire lower surface of the mattress makes contact with the floor.
Since the surface area of the mattress is 4.00 m2, the pressure exerted on the floor is
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25. 13.3 Bernoulli’s Equation
Relates pressure to fluid speed and elevation
Bernoulli’s equation is a consequence of Work Energy Relation applied to an ideal fluid
Assumes the fluid is incompressible and nonviscous, and flows in a nonturbulent, steady-state manner
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26. This work is negative because the force on the segment of fluid is to the left and the displacement is to the right. Thus, the net work done on the segment by these forces in the time interval
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28. States that the sum of the pressure, kinetic energy per unit volume, and the potential energy per unit volume has the same value at all points along a streamline
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29. 13.4 static consequences of Bernoulli's equation
Fluid At Rest In a Container :
Pressure in a continuously distributed uniform static fluid varies only with
vertical distance and is independent of the shape of the container.
• The pressure is the same at all points on a given horizontal plane in a fluid.
• For liquids, which are incompressible, we have:
pb + ρgh1 = patm + ρgh2
pb = patm + ρg (h2 - h1) = patm + ρgd
Y a = y b = y c = yd
y A = y B = y C = y D h1 h2 d
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30. Absolute Pressure and Gauge Pressure
The pressure P is called the absolute pressure
Remember, P = Patm + ρ g h
P – Patm = ρ g h ( so ρ g h is the gauge pressure)
, because it is the actual value of the system’s pressure.
Gauge pressure: Pressure expressed as the difference between the pressure of the fluid and that of the surrounding atmosphere
Usual pressure gauges record gauge pressure. To calculate absolute pressure:
P = P atm + P gauge
As a fluid moves along if it changes speed or elevation then the pressure changes and vice versa.
Bernoulli’s equation is really just conservation of energy for the fluid
Bernoulli’s equation.
P + ½ ρv2 + ρg h = constant
Bernoulli’s equation shows that as the velocity of a fluid speeds up it’s pressure goes down…..this idea used in airplane wings and frisbees (difference in pressure leads to upward force we call Lift)
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31. Atmospheric Pressure and Gauge Pressure
The pressure p1 on the surface of the water is 1 atm, or x 105 Pa. If we go down to a depth h below the surface, the pressure becomes greater by the product of the density of the water , the acceleration due to gravity g, and the depth h. Thus the pressure p2 at this depth is
In this case, p2 is called the absolute pressure -- the total static pressure at a certain depth in a fluid, including the pressure at the surface of the fluid
The difference in pressure between the surface and the depth h is gauge pressure
Note that the pressure at any depth does not depend of the shape of the container, only the pressure at some reference level (like the surface) and the vertical distance below that level.
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32. example
(II) What gauge pressure in the water mains is necessary if a fire hose is to spray water to a height of 15 m?
Apply Bernoulli’s equation with point 1 being the water main, and point 2 being the top of the spray. The velocity of the water will be zero at both points. The pressure at point 2 will be atmospheric pressure. Measure heights from the level of point 1.
example
A tire gauge reads 220 kPa, what is the absolute pressure?
P - Patm = PG
P = Patm + PG
P = kPa kPa = 321 ka
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33. PA>PB PA=PC PA – Patm = ρ g h
PA = PB = PC = PD
P atm is atmospheric pressure =1.013 x 105 Pa
The pressure does not depend upon the shape of the container
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34. Variation of Pressure with Depth
• If a fluid is at rest in a container, all
portions of the fluid must be in static equilibrium
• All points at the same depth must be at the same pressure
– Otherwise, the fluid would not be in
equilibrium
Pressure changes with elevation
The pressure gradient in the vertical direction is negative
The pressure decreases as we move upward in a fluid at rest
Pressure in a liquid does not change due to the shape of the container
The fluid would flow from the higher pressure region to the lower pressure region
the pressure at a point in a fluid depends only on density, gravity and depth
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35. example
example
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36. example
Water circulates throughout a house in a hot-water heating system. If the water is pumped at a speed of 0.5m/s through a 4.0cm diameter pipe in the basement under a pressure of 3.0atm, what will be the flow speed and pressure in a 2.6cm diameter pipe on the second 5.0m above? Assume the pipes do not divide into branches.
Using the equation of continuity, flow speed on the second floor is
Using Bernoulli’s equation, the pressure in the pipe on the second floor is
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37. Pressure Measurements:
The manometer
Manometers are devices in which one or more columns of a liquid are used to determine the pressure difference between two points.
U-tube manometer
One end of the U-shaped tube is open to the atmosphere
The other end is connected to the pressure to be measured
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38. Pressure at B is P=Patm+ρgh
Pressure at A > Patm
Pressure in a continuous static fluid is the same at any horizontal level so,
Pressure at A = Pressure at B
P A = P B = Patm+ ρgh
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39. Pressure at c =pressure at D
Blood Pressure measurements using the sphygmomanometer
Pressure at c =pressure at D
PC = PD
PC = PA + ρsgh
PB = Patm + ρ gh
PA + ρsgh = Patm + ρ gh
PA = Patm + ρ g h – ρsgh
Pblood =PA = Patm + ρ g h – ρsgh
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40. Blood Pressure Sphygmometer
Blood pressure is measured with a special type of manometer called a sphygmomano-meter
Pressure is measured in mm of mercury
26.8 K P a
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41. Pressure with depth
Estimate the amount by which blood pressure changes in an actuary in the foot P2 and in the aorta P1 when the person is lying down and standing up
Take density of blood = 1060kg/m3
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42. example
A fluid of constant density ρ = 960 kg / m3 is flowing steadily through the above tube. The diameters at the sections are d 1 =100 mm and d 2 = 80 mm . The gauge pressure at 1 is p1 = 200 k N/ m2 ,and the velocity here is u 1 = 5 m /s . We want to know the gauge pressure at section 2.
The tube is horizontal, with y1 = y2 so Bernoulli gives us the following equation for pressure at section 2:
P2 = 2 x { 52 – ( 7.8 )2} /2
= pa
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43. What is the gauge pressure if: h1 = 0.4m and h2 = 0.9m?
An example of the U-Tube manometer Using a u-tube manometer to measure gauge pressure of fluid density ρ = 700 kg/m3, and the manometric fluid is mercury, with a relative density of 13.6.
What is the gauge pressure if:
h1 = 0.4m and h2 = 0.9m?
b) h1 = 0.4 and h2 = -0.1m?
pB = pC
pB = pA + ρgh1
pB = pAtmospheric + ρman gh2
Subtract patmospheric to give gauge pressure
pA = ρman gh2 - ρgh1
a) pA = 13.6 x 103 x 9.8 x x 9.8 x 0.4 = N,
b) pA = 13.6 x 103 x 9.8 x (-0.1) x 9.8 x 0.4 = N,
The negative sign indicates that the pressure is below atmospheric
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44. Example:
Fluid is flowing from left to right through the pipe. Points A and B are at the same height, but the cross-sectional areas of the pipe differ. Points B and C are at different heights, but the cross-sectional areas are the same.
Rank the pressures at the three points, from highest to lowest.
A) A and B (a tie), C
B) C, A and B (a tie)
C) B, C, A
D) C, B, A
E) A, B, C
E) PA > PB > PC
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45. Pressure Measurements:
A long closed tube is filled with mercury and inverted in a dish of mercury
Measures atmospheric pressure as ρ g h
Since the closed end is at vacuum, it does not exert any force.
p=0
Vacuum!
Vacuum pressure = 0
patm h
For mercury, h = 760 mm.
How high will water rise?
No more than h = patm/g = 10.3 m
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46. example
We can set (assume the hole is on the ground or is where we measure height from). We also have atm. So we have v2 h
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47. example
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49. example P2 =p1 + ρ (v12 –v22 ) / 2 = 180X103 + 103X(22 – 182)
= 20X 103 pa
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50. example
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51. If a fluid is at rest in a container, all portions of the fluid must be in static equilibrium
If the height doesn’t change much, Bernoulli becomes: y1 = y2
Where speed is higher, pressure is lower.
Speed is higher on the long surface of the wing – creating a net force of lift. FL
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52. example
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53. example
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54. example
1.5m
Water flows through the pipe as shown at a rate of .015 m3/s. If water enters the lower end of the pipe at 3.0 m/s, what is the pressure difference between the two ends?
A2 = 20 cm2
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55. Example
Estimate the force exerted on your eardrum due to the water above when you are swimming at the bottom of the pool with a depth 5.0 m.
We first need to find out the pressure difference that is being exerted on the eardrum. Then estimate the area of the eardrum to find out the force exerted on the eardrum.
Since the outward pressure in the middle of the eardrum is the same as normal air pressure
Estimating the surface area of the eardrum at 1.0cm2=1.0x10-4 m2, we obtain
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56. Example. A diver is located 20 m below the surface of a lake (r = 1000 kg/m3). What is the pressure due to the water?
The difference in pressure from the top of the lake to the diver is: h
r = 1000 kg/m3
DP = rgh
h = 20 m; g = 9.8 m/s2
DP = 196 kPa
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57. Example
(a) What are the total force and the absolute pressure on the bottom of a swimming pool 22.0 m by 8.5 m whose uniform depth is 2.0 m?
(a)The absolute pressure is given by Eq. 10-3c, and the total force is the absolute pressure times the area of the bottom of the pool.
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58. Example:
a) At what water depth is the pressure twice the atmospheric pressure?
b) What’s the pressure at the bottom of the 11-km-deep Marianas Trench, the deepest point in the ocean?
Take 1 atm = 100 kPa & water = 1000 kg/m3 .
(a) 
(b) Pressure increases by 1 atm per 10 m depth increment. 
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59. Applications of Bernoulli's Equation
Air speeds up in the constricted space between the car & truck creating a low-pressure area. Higher pressure on the other outside pushes them together.
Hold two sheets of paper together, as shown here, and blow between them. No matter how hard you blow, you cannot push them more than a little bit apart!
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60. Summary Fluid at Rest: PA - PB = rgh Bernoulli’s Theorem:
Streamline Fluid Flow in Pipe:
PA - PB = rgh
Horizontal Pipe (h1 = h2)
Fluid at Rest:
Bernoulli’s Theorem:
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