Applications of Fluid Principles
by Ron Kurtus (revised 17 April 2013)
There are several major applications of the special properties of fluids.
The pressure of fluids can be amplified through the use of hydraulic mechanisms. Changes in pressure with the velocity of the fluid allow airplanes to fly. Fluids are also used to reduce friction.
Questions you may have include:
- How can pressure by amplified through hydraulic mechanisms?
- How do airplanes fly?
- How is friction reduced?
This lesson will answer those questions. Useful tool: Units Conversion
Hydraulics is the scientific field that concerns fluids--usually liquids--that are in partially enclosed containers, such that you can apply pressure in one area. An example is a cylinder with a piston.
Pressure from single piston
If you have a cylinder filled with a liquid and apply a force to a piston on one end of the cylinder, the pressure on the walls of the cylinder equal the force divided by the area of the piston in the cylinder.
P = F/A
- P is the pressure on the walls of the cylinder in newtons per square meters (N/m2)
- F is the force on the piston in newtons N
- A is the area of the top of piston in m2
Pressure on second piston
Now, if the first cylinder was connected to a second cylinder of larger diameter, the pressure inside that cylinder would be the same P, but the force F2 applied to the larger piston would now be:
P = F2/A2
Pressure is the same
The pressure for both is the same. Thus,
F2/A2 = F/A
F2 = (FA2)/A
For example, if F = 100 pounds and A = 5 square inches, then P = 20 pounds/square inch. P is the same on both pistons.
Force greater on large piston
If the larger piston had an area of A2 = 25 square inches, and the pressure remained at P = 20 pounds/square inch, then the resulting force on that piston would be:
F2 = (FA2)/A = (100 x 25) / 5 = 500 pounds.
This is a mechanical advantage, similar to that seen with levers.
Used in brakes
Hydraulic mechanisms are used in the brakes in your car. The force applied on the brake pedal is multiplied on the brake drums. Another use is to jack up a heavy item, like a truck.
Velocity reduces pressure
The scientist Bernoulli discovered that the air pressure in a tube goes down when the velocity of the air in the tube increases. This discovery became known as Bernoulli's Principle.
Used by airplanes
The greatest application of this principle is used in airplanes. The wing of an airplane is usually curved on top and flat on the bottom. This shape is called the airfoil. When the air moves over the curved top portion of the wing or airfoil, it speeds up because of the shape. This lowers the pressure with respect to the bottom part of the wing. Lower pressure on the top results in the lift required to keep the airplane aloft.
The principle is so simple, but not very obvious.
But if the airfoil gives lift, how can an airplane fly upside down?
If the airplane is going fast enough, other factors influence the lift. When the plane is upside-down, it is really flying at a slight angle, so it is going slightly upward to compensate for the loss of lift.
Some airplanes--such as an airliner--can have great difficulty flying upside down. Usually only smaller stunt planes and military craft can do this maneuver.
Solids can have rough surfaces. Even microscopic roughness can result in a substantial resistive force of friction when two solids are rubbed together, as well as wear on the parts.
Fluids offer little resistance
On the other hand, a fluid does not have a rough surface and rubbing a solid along a fluid results in little resistive force. Instead of friction, the resistance is due to the thickness or viscosity of the liquid, which affects its ability to move and change its shape.
Used as lubricants
The reduction of friction of two solids can then be achieved by separating them by a layer of a fluid, so the solid surfaces are not in direct contact. This is called lubrication. Water can be used as a lubricant, but it also evaporates quickly. Oils are typically used to lubricate parts and prevent friction, as well as excessive wear from the friction. In some small, high-speed parts, such as the hard-drive of your computer, air is used as a lubricant.
Hydraulics use fluid pressure to create the same mechanical advantage as a lever. The Bernoulli Principle allows airplanes to fly from the lift created by reduced air pressure on the top of their wings. Fluids also can be used to reduce friction.
Apply what you know
Resources and references
Fluid Mechanics by Ira M. Cohen and Pijush K. Kundu; Academic Press (2004) $74.95
Vectors, Tensors and the Basic Equations of Fluid Mechanics by Rutherford Aris; Dover Publications (1990) $14.95
Fundamentals of Fluid Mechanics by Bruce R. Munson, Donald F. Young, Theodore H. Okiishi; Wiley (2001) $37.95
Questions and comments
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Applications of Fluid Principles