Hydraulics, << hy DRAW lihks, >> is the branch of physics that studies the behavior of liquids at rest and in motion. Some laws of hydraulics apply to both gases and liquids. But they apply only under certain conditions, such as when gases flow at low velocity and are not compressed. Scientists sometimes divide the study of hydraulics into two categories—hydrostatics and hydrodynamics. Hydrostatics describes the behavior of liquids at rest. Hydrodynamics describes their behavior in motion.
Hydraulics is studied and applied in two of the specialty fields of engineering: civil engineering and mechanical engineering. Civil engineers use the principles of hydraulics mainly to study the flow of water in open or partially filled pipes. They design embankments and levees that provide flood control along rivers, as well as canals, irrigation systems, and sewage systems. They also plan water supply systems for cities and towns.
Mechanical engineers are usually more concerned with the flow of fluids in filled pipes under pressure. They use the principles of hydraulics to design hydraulic machines. These machines include hydraulic turbines, hydraulic brakes, power steering for cars, controls for airplanes and spacecraft, and construction equipment.
Pressure from water or some other fluid supplies the power that runs hydraulic machines. Some hydraulic machines, including turbines, operate by the force of a flowing fluid. Turbines are large wheels that change the potential energy of a fluid into work that can be used to power an electric generator or perform some other type of work. Steam or gas powers other kinds of turbines.
Other hydraulic machines, called hydraulic presses, increase and transfer force through a fluid from one piston to a larger piston. Industry uses hydraulic presses for such jobs as lifting heavy loads and pressing metal parts. One form of hydraulic press is called a hydraulic jack.
Liquids at rest
Use of hydrostatic principles
is fundamental to the design of many machines and instruments. The design of hydraulic presses and the operation of such instruments as manometers and barometers depend on laws of hydrostatics.
A simple hydraulic press consists of a cylinder that contains two pistons, one smaller than the other. The cylinder is filled with a fluid. A force applied to the smaller piston is transferred through the fluid to the larger piston. The force increases in direct proportion to the ratio of the area of the larger piston to the area of the smaller piston. Automobile brakes operate on the principle of the hydraulic press. Pressure applied to the brake pedal is transmitted through a liquid to brake shoes, which press against the car’s wheels.
A manometer measures the pressure exerted by a liquid or a gas. The simplest kind of manometer is a U-shaped tube with open ends. The tube contains a liquid, usually mercury or water, which fills the bottom of the U and rises a short distance in each arm. The liquid in the manometer must have a different weight per unit of volume than the substance whose pressure is to be measured. One arm is connected to the place where the pressure is to be measured. The other arm remains open to the atmosphere. The difference in liquid level in the arms indicates the pressure compared to atmospheric pressure. See Manometer .
A barometer measures atmospheric pressure. It is used primarily to forecast weather and to measure heights of mountains. One type of barometer consists basically of a tube containing mercury. At sea level, a column of mercury about 30 inches (76 centimeters) high equals in weight the force of atmospheric pressure on that column. The level of mercury in the tube rises or falls as the atmospheric pressure changes. See Barometer .
Laws of hydrostatics
describe the behavior of liquids at rest.
One principle of hydrostatics states: The pressure caused by the weight of a column of fluid is determined by the height of the column. The shape of the column does not affect the pressure that is exerted by the fluid.
Suppose that a pipe extends in a zigzag pattern from the ceiling to the floor. Another pipe of equal diameter extends straight down from the ceiling to the floor. The second pipe is, of course, shorter than the first. If both pipes are filled with water, the longer one will hold more water. But the water pressure at the bottom of both pipes will be equal because the height of both columns of water is equal.
An illustration of the above principle is the buoyant force exerted by a fluid on an object submerged in it. The bottom of an object submerged in water is deeper than the top of the object. Therefore, the column of water pressing on the bottom of the object is taller than the column pressing on the top of the object. Thus, the water exerts an upward force on the object, called a buoyant force. Buoyant forces act to keep boats afloat.
Archimedes’ principle
comes from the principle discussed above. It states: An object placed in a fluid seems to lose an amount of weight equal to the weight of the fluid it displaces. A buoyant force exerted on the object by the fluid causes the apparent loss of weight. Archimedes, a Greek mathematician, developed this principle during the 200’s B.C.
To illustrate this principle, imagine that a 1-liter metal can is placed in water. If the can weighs 3 kilograms, it will displace 1 liter of water, which weighs 1 kilogram. Thus, the buoyant force equals 1 kilogram. The can will then seem to weigh only 2 kilograms—that is, 1 kilogram less than its original weight.
Pascal’s law
states: A fluid in a container transmits pressure equally in all directions. Blaise Pascal, a French scientist and mathematician, developed this law during the A.D. 1600’s.
To illustrate Pascal’s law, take a bottle with a neck opening of 1 square inch and fill it with water. Put a plug in the neck so that the plug seals the top of the bottle and moves freely in the neck. Place a 1-pound weight on the plug. The pressure of the water will increase by 1 pound over each square inch (1 pound per square inch, or 1 psi) of the inside surface of the bottle.
Liquids in motion
Use of hydrodynamic principles
is basic to hydraulic engineering and to the design of certain machines. Engineers use hydrodynamics when planning water supply systems, canals, and irrigation systems. They also apply hydrodynamic principles to the design of airplanes and to the construction of certain hydraulic machines, such as water turbines.
Engineers consider many factors to determine the proper pressure for a water supply system. For instance, the height of the reservoir or tank affects the pressure in water flow from the reservoir, and this pressure determines the flow rates that can be obtained. Most reservoirs have a long series of pipes that connects the source of water with its destination. Such factors as pipe size and the friction between the water and the pipes affect the flow of water.
Laws of hydrodynamics play a fundamental role in the design of water turbines. Some water turbines are submerged in rivers, and the normal flow of water provides power. Other water turbines are located at the bases of dams. Water pressure created by the dam is used to speed up the water when it enters the turbine. The turbines then change the kinetic energy of the moving water into rotational energy, which turns generators that produce electric power.
Laws of hydrodynamics
describe the behavior of flowing fluids. Fluid flow may be steady or unsteady. An unsteady flow results from changes in the velocity, temperature, or pressure of a fluid. The movement of a fluid around obstructions may also cause an unsteady flow.
There are three basic laws of hydrodynamics. All of them apply only to steadily flowing liquids.
The principle of continuity in fluid flow
states: The velocity of a fluid flowing through a pipe increases as the area of the pipe decreases, and decreases as the pipe’s area increases.
The nozzle of a garden hose uses the principle of continuity. The nozzle decreases the size of the opening at the end of the hose. As a result, water flows faster through the nozzle than through the hose.
Bernoulli’s principle,
also called Bernoulli’s law, states: The pressure of a fluid increases as its velocity decreases, and decreases as the fluid’s velocity increases. Daniel Bernoulli, a Swiss mathematician, developed this principle during the 1700’s.
Bernoulli’s principle is used in the design of airplane wings. Engineers curve the upper surface of the wing so air will flow faster over the top than it does over the bottom of the wing. The faster-flowing air exerts less pressure on the top of the wing. As a result, the greater pressure under the wing lifts the airplane.
Torricelli’s law
states: The velocity with which a liquid flows through an opening in a container equals the velocity of a body falling from the surface of the liquid to the opening. Evangelista Torricelli, an Italian physicist, developed this law during the 1600’s.
According to Torricelli’s law, a stream of water flowing through a hole 10 feet (3 meters) below the water surface in a dam has the same velocity as a stone falling that 10-foot distance. Torricelli’s law does not apply to gases because a gas has no surface. The velocity at which a gas flows from a container depends on the pressure under which the gas is confined in the container.
