A flipper is a typically flat forelimb evolved for movement through water. Various creatures have evolved flippers, for example penguins (also called "wings"), cetaceans (e.g. dolphins and whales), pinnipeds, and reptiles, such as some varieties of turtle and the now-extinct plesiosaurs, mosasaurs, ichthyosaurs, and metriorhynchids.
Usage of the terms "fin" and "flipper" are often inconsistent, even in the scientific literature. The hydrodynamic control surfaces of a fish are always referred to a "fins" and never as "flippers", and limbs which have evolved into fin-like structures are usually (but not always) called "flippers" rather than fins, but the dorsal structure on cetaceans is called the "dorsal fin" and the large cetacean tails are referred to primarily as flukes but occasionally as "caudal fins".
Some flippers are very efficient hydrofoils, analogous to wings (airfoils), used to propel and maneuver through the water with great speed and maneuverability (see Foil (fluid mechanics)). Some flippers are less foiled, with the appendages still apparent, as in the webbed forefeet seen in amphibious turtles.
In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including aerodynamics (the study of air and other gases in motion) and hydrodynamics (the study of liquids in motion). Fluid dynamics has a wide range of applications, including calculating forces and moments on aircraft, determining the mass flow rate of petroleum through pipelines, predicting weather patterns, understanding nebulae in interstellar space and modelling fission weapon detonation,
Fluid dynamics offers a systematic structure—which underlies these practical disciplines—that embraces empirical and semi-empirical laws derived from flow measurement and used to solve practical problems. The solution to a fluid dynamics problem typically involves the calculation of various properties of the fluid, such as flow velocity, pressure, density, and temperature, as functions of space and time.
Before the twentieth century, hydrodynamics was synonymous with fluid dynamics. This is still reflected in names of some fluid dynamics topics, like magnetohydrodynamics and hydrodynamic stability, both of which can also be applied to gases.Foil (fluid mechanics)
A foil is a solid object with a shape such that when placed in a moving fluid at a suitable angle of attack the lift (force generated perpendicular to the fluid flow) is substantially larger than the drag (force generated parallel to the fluid flow). If the fluid is a gas, the foil is called an airfoil or aerofoil, and if the fluid is water the foil is called a hydrofoil.