Hydraulics | Research & Encyclopedia Articles

Hydraulics

Hydraulics is the practical application of fluid mechanics. It deals with engineering and scientific devices that control the flow, or movement, of liquids, such as valves, pipes, pneumatic controls, and nozzles; the manmade and natural environments that store, release, or channel liquids, such as dams, pumps, tanks, turbines, rivers, and canals; and the instruments that measure the flow of liquid through particular containers, channels, or systems, such as flow meters and pressure gauges. In large part, the theory of hydraulics is based on fluid mechanics, a dynamic field of science that has enjoyed explosive growth in the mid-1800s, and under certain conditions, hydraulics can also be applied to the flow of gases through a system. This application of concepts of hydraulics to gases generally occurs when the densities of the gases are kept within parameters at which the principles of fluid mechanics apply. Hydraulics encompasses the study of the flow of any liquid, but it is often applied to oil or water.

Investigations in hydraulics are dependent primarily on the mathematical concepts of density (the mass of an object over a particular volume), force, pressure (the force exerted by an object over a particular area), and mass. In turn, these concepts are governed by the known principles of classical physics, such Newton's laws, the laws of thermodynamics, conservation of mass, conservation of energy, and laws of motion. The principles of hydraulics on which modern hydraulic-power systems, including hydraulic lifts and brakes, are based is derived largely from the works of French scientist Blaise Pascal (1623-1662) and Swiss physicist Daniel Bernoulli (1700-1782).

Pascal's Law

While pursuing medical studies and in particular studying the flow of blood through the body, Pascal declared in 1652 that pressure applied to any confined, incompressible liquid will be applied equally to the liquid and the container holding the liquid in all directions, with no loss. This statement is known as Pascal's law (or Pascal's principle) and can be succinctly written that pressure_in = pressure_out. It implies that even if pressure is applied to just a portion of the liquid, the pressure will be distributed over the entire volume of the liquid and the surface containing that liquid. For example, if an incompressible liquid is contained in a cylindrical container and then pressure is applied to just one of the circular faces of the cylinder, the change in pressure will distribute equally within the liquid and the cylinder's surface. An equilibrium will eventually result due to the distribution of pressure. Of course, if the pressure exceeds the strength of the material containing the liquid, the cylinder will burst, distributing the pressure over a larger volume.

Hydraulic Lever

The hydraulic lever is based on the concept of Pascal's law. In a simple hydraulic lever, a finite amount of incompressible liquid is placed between two pistons of unequal surface area. Pressure applied to the smaller piston results in a force being applied to the smaller piston, and as a result, a proportional force is applied to the larger piston. The relationship in applied force is described by the equation force_out = force_in * (area_out / area_in). The amount of work, which is equal to force times distance, exerted on both systems is equal, providing the useful result that a small force exerted over a large distance can be transformed into a larger force exerted over a small distance. A car jack operates on this principle, as one has to pump the handle many times (that is, move the handle a significant distance) to lift the car a short distance off the ground.

Bernoulli's Principle

Simply stated, Bernoulli's principle (also known as Bernoulli's equation) asserts that the speed of moving fluid in inversely proportional to the pressure of the fluid (if traveling horizontally). If the moving liquid must transverse an elevation change, Bernoulli's equation must account for gravitational force, becoming p + 1/2(v² + (gv = k, where p is pressure, ( is density, g is the gravitational constant, v is the velocity of the liquid, and k is a constant. As a result, fluid moving from a wider pipe to a smaller pipe increases in speed, and fluid moving from a smaller pipe to a larger pipe decreases in speed.

The principles of hydraulics have widespread applicability to science, engineering, and every day life, especially when applied to systems that require the transfer of power. Aircraft controls, for example, are based on hydraulic concepts, as are many braking systems. Modern industrial processes often use hydraulics to perform activities requiring more force than a human can exert.