Fluid Statistics

Topics: Pressure, Atmospheric pressure, Gas / Pages: 8 (1824 words) / Published: Sep 5th, 2013
Chapter 3 Fluid Statics: Definitions
Statics: ∑F = 0. In statics we have only pressure as surface force and weight as body force. Thus, when fluids are still, the pressure is balanced by the fluid weight. No relative motion between adjacent fluid layers. Shear stress is zero Only _______ can be acting on fluid surfaces Gravity force acts on the fluid (____ force) Applications: Pressure variation within a reservoir Forces on submerged surfaces Buoyant forces
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Pressure
Pressure is defined as a normal force exerted by a fluid per unit area.

p = Lim

Δ F dF = ΔA → 0 Δ A dA

where F is the normal force acting over the area A. Dimension: F/L2 Units of pressure are N/m2, which is called a pascal (Pa). Since the unit Pa is too small for pressures encountered in practice, kilopascal (1 kPa = 103 Pa) and megapascal (1 MPa = 106 Pa) are commonly used. Other units include bar, atm, kg/cm2, lbf /in2=psi.
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Absolute, gage, and vacuum pressures
Actual pressure at a give point is called the ________________. Most pressure-measuring devices are calibrated to read zero in the atmosphere, and therefore indicate ___________________. Pressure below atmospheric pressure are called ___________________.

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Absolute, gage, and vacuum pressures
Pgage = Pabs − Patm Gage pressure Absolute pressure p Standard atmospheric pressure Local atmospheric pressure Suction vacuum (gage pressure) Pvac = Patm − Pabs

1 atmosphere 101.325 kPa 14.7 psi 10.34 m H20 760 mm Hg

Local barometer reading

Absolute zero (complete vacuum)

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Absolute, gage, and vacuum pressures
Actual pressure at a give point is called the absolute pressure. Most pressure-measuring devices are calibrated to read zero in the atmosphere, and therefore indicate gage pressure, Pgage = Pabs − Patm. Pressure below atmospheric pressure are called vacuum pressure, Pvac = Patm − Pabs. Pressure has a magnitude, but not a specific direction, and thus

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