What makes fluid flow

Fluid dynamics is the study of the flow of liquids and gases, usually in and around solid surfaces. For example, fluid dynamics can be used to analyze the flow of air over an airplane wing or over the surface of an automobile. It also can be used in the design of ships to increase the speed with which they travel through water.

Scientists use both experiments and mathematical models and calculations to understand fluid dynamics. A wind tunnel is an enclosed space in which air can be made to flow over a surface, such as the model of an airplane. Smoke is added to the air stream so that the flow of air can be observed and photographed. The data collected from wind tunnel studies and other experiments are often very complex. Scientists today use models of fluid behavior and powerful computers to analyze and interpret those data.

The field of fluid dynamics is often subdivided into aerodynamics and hydrodynamics. Aerodynamics is the study of the way air flows around airplanes and automobiles with the aim of increasing the efficiency of motion. Hydrodynamics deals with the flow of water in various situations such as in pipes, around ships, and underground. Apart from the more familiar cases, the principles of fluid dynamics can be used to understand an almost unimaginable variety of phenomena such as the flow of blood in blood vessels, the flight of geese in V-formation, and the behavior of underwater plants and animals.

Flow patterns in a fluid gas or liquid depend on three factors: the characteristics of the fluid, the speed of flow, and the shape of the solid surface. Three characteristics of the fluid are of special importance: viscosity, density, and compressibility. Viscosity is the amount of internal friction or resistance to flow. Water, for instance, is less viscous than honey, which explains why water flows more easily than does honey. All gases are compressible, whereas liquids are practically incompressible; that is, they cannot be squeezed into smaller volumes.

Flow patterns in compressible fluids are more complicated and difficult to study than those in incompressible ones. Fortunately for automobile designers, at speeds less than about miles kilometers per hour, air can be treated as incompressible for all practical purposes. Also, for incompressible fluids, the effects of temperature changes can be neglected. Boundary layer: The layer of fluid that sticks to a solid surface and through which the speed of the fluid decreases.

Laminar: A mode of flow in which the fluid moves in layers along continuous, well-defined lines known as streamlines. Flow patterns can be characterized as laminar or turbulent. The term laminar refers to streamlined flow in which a fluid glides along in layers that do not mix. The flow takes place in smooth continuous lines called streamlines.

You can observe this effect when you open a water faucet just a little so that the flow is clear and regular. If you continue turning the faucet, the flow gradually becomes cloudy and irregular.

This condition is known as turbulent flow. The Mach number is a measurement used in fluid dynamics that compares the velocity of an object traveling through a fluid to the speed of sound in that fluid.

Imagine an airplane flying just above the ocean at a speed of miles per hour meters per second. The Mach number is named after Austrian physicist and philosopher Ernst Mach — , who pioneered the study of supersonic faster than sound travel. The Mach number is especially important in the field of fluid dynamics because fluids flow around an object in quite different ways.

For example, when an airplane flies at a speed greater than the speed of sound, sound waves are not able to "get out of the way" of the airplane. Shock waves are produced, resulting in the sonic booms heard when an airplane exceeds the speed of sound. Aircraft designers have to take differences in fluid behavior at different Mach numbers into account when designing planes that take off and climb to altitude at speeds in the subsonic less than the speed of sound region, then pass through the transonic about equal to the speed of sound region, and cruise at speeds in the supersonic region.

Bernoulli's principle. Swiss mathematician Daniel Bernoulli — was the first person to study fluid flow mathematically. For his research, Bernoulli imagined a completely nonviscous and incompressible or "ideal" fluid.

In this way, he did not have to worry about all the many complications that are present in real examples of fluid flow. The mathematical equations Bernoulli worked out represent only ideal situations, then, but they are useful in many real-life situations. A simple way to understand Bernoulli's result is to picture water flowing through a horizontal pipe with a diameter of 4 inches 10 centimeters.

Then imagine a constricted section in the middle of the pipe with a diameter of only 2 inches 5 centimeters. Bernoulli's principle says that water flowing through the pipe has to speed up in the constricted portion of the pipe. Hydrodynamics deals primarily with the flow of water in pipes or open channels.

Geology professor John Southard's lecture notes from an online course, " Introduction to Fluid Motions " Massachusetts Institute of Technology, , outline the main difference between pipe flow and open-channel flow: "flows in closed conduits or channels, like pipes or air ducts, are entirely in contact with rigid boundaries," while "open-channel flows, on the other hand, are those whose boundaries are not entirely a solid and rigid material.

Due to the differences in those boundaries, different forces affect the two types of flows. For instance, most city water systems use water towers to maintain constant pressure in the system. This difference in elevation is called the hydrodynamic head. Liquid in a pipe can also be made to flow faster or with greater pressure using mechanical pumps.

The flow of gas has many similarities to the flow of liquid, but it also has some important differences. First, gas is compressible, whereas liquids are generally considered to be incompressible. Balachandran describes compressible fluid, stating, "If the density of the fluid changes appreciably throughout the flow field, the flow may be treated as a compressible flow.

Second, gas flow is hardly affected by gravity. The gas most commonly encountered in everyday life is air; therefore, scientists have paid much attention to its flow conditions. Wind causes air to move around buildings and other structures, and it can also be made to move by pumps and fans. One area of particular interest is the movement of objects through the atmosphere. This branch of fluid dynamics is called aerodynamics, which is "the dynamics of bodies moving relative to gases, especially the interaction of moving objects with the atmosphere," according to the American Heritage Dictionary.

Problems in this field involve reducing drag on automobile bodies, designing more efficient aircraft and wind turbines, and studying how birds and insects fly. Generally, fluid moving at a higher speed has lower pressure than fluid moving at a lower speed. This phenomenon was first described by Daniel Bernoulli in in his book " Hydrodynamica ," and is commonly known as Bernoulli's principle.

It can be applied to measure the speed of a liquid or gas moving in a pipe or channel or over a surface. This principle is also responsible for lift in an aircraft wing, which is why airplanes can fly. Because the wing is flat on the bottom and curved on the top, the air has to travel a greater distance along the top surface than along the bottom.

To do this, it must go faster over the top, causing its pressure to decrease. This makes the higher-pressure air on the bottom lift up on the wing. Scientists often try to visualize flow using figures called streamlines, streaklines and pathlines. McDonough defines a streamline as "a continuous line within a fluid such that the tangent at each point is the direction of the velocity vector at that point. A streakline, according to McDonough, is "the locus [location] of all fluid elements that have previously passed through a given point.

However, in the case of turbulent or unsteady flow, these lines can be quite different. Most problems in fluid dynamics are too complex to be solved by direct calculation. In these cases, problems must be solved by numeric methods using computer simulations.

This area of study is called numerical or computational fluid dynamics CFD , which Southard defines as "a branch of computer-based science that provides numerical predictions of fluid flows. Small changes at the beginning can result in large differences in the results.

The accuracy of simulations can be improved by dividing the volume into smaller regions and using smaller time steps, but this increases computing time.