Fluid Mechanics - An-Najah National University

[Pages:69]Fluid Mechanics

Fluid Statics

[3-1]

Dr. Mohammad N. Almasri

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[3] Fall ? 2010 ? Fluid Mechanics

Dr. Mohammad N. Almasri

[3-1] Fluid Statics

Fluid Pressure

Fluid pressure is the normal force exerted by the fluid per unit area at some location within the fluid

Fluid pressure has the units:

N/m2

Pascal 1 Pa = 1 N/m2

psi pound-force per square inch = 6,894.757 Pa

Bar 1 bar = 105 Pa = 100 Kpa

Atmosphere 1 atm = 101,325 Pa = 1.01325 bars

1 kgf/cm2 = 9.807 ? 104 N/m2

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[3] Fall ? 2010 ? Fluid Mechanics

Dr. Mohammad N. Almasri

psia (pounds-force per square inch absolute) -- gauge pressure plus local atmospheric pressure

[3-1] Fluid Statics

Absolute Pressure

Absolute pressure is the actual pressure at a given position

However, the absolute pressure is measured relative to absolute zero pressure

This is why most pressure-

measuring devices (called

gages) read zero in the

atmosphere

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[3] Fall ? 2010 ? Fluid Mechanics

Dr. Mohammad N. Almasri

[3-1] Fluid Statics

Gage and Vacuum Pressures

When the pressure gages read a pressure value this value would be the difference between the absolute pressure and the local atmospheric pressure

This difference is called gage pressure (Pgage) Pgage = Pabs ? Patm

If Pgage is negative then it is called vacuum pressure Pvac = Patm - Pabs

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[3] Fall ? 2010 ? Fluid Mechanics

Dr. Mohammad N. Almasri

[3-1] Fluid Statics

Gage and Vacuum Pressures

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[3] Fall ? 2010 ? Fluid Mechanics

Dr. Mohammad N. Almasri

[3-1] Fluid Statics

Gage Pressure

The gage used to measure the air pressure in an automobile tire reads the gage pressure

Therefore, the common reading of 32 psi indicates a pressure of 32 psi above the atmospheric pressure

At a location where the atmospheric pressure is 14.3 psi, for example, the absolute pressure in the tire is 32 + 14.3 = 46.3 psi

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[3] Fall ? 2010 ? Fluid Mechanics

Dr. Mohammad N. Almasri

[3-1] Fluid Statics

Example

A vacuum gage connected to a chamber reads 5.8 psi at a location where the atmospheric pressure is 14.5 psi

Determine the absolute pressure in the chamber We know that Pabs = Patm ? Pvac = 14.5 ? 5.8 = 8.7 psi

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[3] Fall ? 2010 ? Fluid Mechanics

Dr. Mohammad N. Almasri

[3-1] Fluid Statics

Pressure at a Point

Consider a small wedgeshaped fluid element of unit length (into the page) in equilibrium

The mean pressures at the

three surfaces are Pn, Px, and Pz, and the force acting on a surface is the

product of mean pressure

and the surface area

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[3] Fall ? 2010 ? Fluid Mechanics

Dr. Mohammad N. Almasri

[3-1] Fluid Statics

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