Fluid dynamics
Fluid dynamics, also often called fluid mechanics, is the branch of physics that deals with the flow of fluids, i.e., liquids and gases. It is an adaptation of Newton's laws to a medium that is treated as if it were continuous. That is, the molecular structure of matter is, for the most part, not considered within the science of fluid dynamics. One therefore classifies fluid dynamics as being a continuum theory. Also, in most of fluid dynamics the underlying mechanical laws are taken as those of classical physics, i.e., the effects of relativity or quantum theory are usually ignored.
Since the most common liquid on Earth is water, and the most common gas is air, fluid dynamics encompasses the descriptions of water and air, sometimes called, respectively, hydrodynamics and aerodynamics. In fact, one of the major results of fluid dynamics is the understanding that over a wide range of conditions the flow of air and the flow of water can be addressed using the same set of equations from physics. There are unique areas of either subject that require special considerations. For example, in hydrodynamics we wish to address problems having to do with the motion of waves on the surface of a body of water. Similarly, in aerodynamics we wish to address problems where the compression of the gas is taken into account. This subfield of aerodynamics is often referred to as gas dynamics.
Issues of fluid dynamics are addressed by experiment, theory and analysis, and increasingly by computation. The field of computational fluid dynamics or CFD, as it is often called, has grown immensely in step with the increasing power of computers and the development of ever more efficient and ingenious algorithms for flow simulation. Similarly, the field of experimental fluid dynamics employs ever more sophisticated techniques involving lasers and high-speed imaging. Many of the images produced by experiment or computation of fluid flows have great aesthetic appeal.
While fluid dynamics may be considered a mature subject, since it has been pursued for several centuries and most of the great physicists, engineers and applied mathematicians have made contributions to it, it still holds many unresolved problems. The problem of turbulence is usually cited as one of the great unsolved mysteries of fluid dynamics and by extension of classical mechanics. While many properties of turbulent flows are understood, a deductive theory of turbulence from the basic equations of fluid dynamics has not been given. Indeed, the phenomenology of fluid flows is considerably better understood, through theory and experiments, than the basic mathematical properties of the governing equations. Physicists and engineers typically proceed with their analyses under the assumption that the basic equations have the necessary properties of smoothness and well-posedness, but some of these properties have not been proven and present major challenges.
There are numerous subfields of fluid dynamics, each with their own flavor and distinct set of problems and issues. There are also a number of technical terms unique to fluid dynamics, which are commonly used by workers in the field, but which require explanation. Some key items in this category and some major subfields of fluid dynamics are listed below: