CE 230-Engineering Fluid Mechanics

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CE 230-Engineering Fluid Mechanics

• Week 2
• Pressure Variation in Static Fluids
• its Applications

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What can we say about the pressure at a point in
a static fluid?

• Consider equilibrium of the shown element

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Conclusion

• Only normal forces exist in static fluid
• Pressure at a point in a static fluid is
  independent of direction

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Pressure variation in a static fluid
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Relation between gage and absolute pressure
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Pressure variation in a static fluid
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Pressure difference between two points in a
static fluid is the weight of unit fluid column
between them

• dp/dz -? z ve up
• This equation is valid as long as we are in the
  same fluid.
• Two possibilities
• Constant ?
• Variable ?

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What is the pressure at a depth of 35 ft in the
tank?
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What is the pressure at the bottom of the tank?
S(oil)0.8
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Problem 3.16What is the maximum pressure in the
odd tank?
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Manometry

• Use of pressure change with elevation to
  determine pressure at a point
• Use of pressure change with elevation to
  determine the difference in pressure between two
  points

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Problem 3.30Find pressure at pipe B.
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Problem 3.39Find pressure at pipe A.
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Problem 3.31 If the air is under atm. P. the
water position is 1.Find P if l40 cm. Dia
ratio8, density 800 kg/m3
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Determine the weight of the man if the piston
weighs 50 lb and area in contact with water is
1.5 ft2
3 ft
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Problem 3.42determine the difference in pressure
between points A and B.
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What if ? is variable? (Compressible fluid)

• For air say
• ? pg/RT
• in troposphere TT0-a(z-z0)
• Integrate basic equation to get pressure
  variation

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Figure 3.5 (p. 39)Temperature variation with
altitude for the U.S. standard atmosphere in July
(2).