Flow visualization
Encyclopedia
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
is used to make the flow patterns visible, in order to get a qualitative or quantitative information on them.
Overview
Flow visualization is the art of making flow patterns visible. Flow visualization can be appreciated on several levels. Most fluids (air, water, etc.) are transparent, thus their flow patterns are invisible to us without some special methods to make them visible.On another level, we know the governing equations of fluid motion (the Navier-Stokes equations
Navier-Stokes equations
In physics, the Navier–Stokes equations, named after Claude-Louis Navier and George Gabriel Stokes, describe the motion of fluid substances. These equations arise from applying Newton's second law to fluid motion, together with the assumption that the fluid stress is the sum of a diffusing viscous...
), but they are nonlinear partial differential equations with very few general solutions of practical utility. We can solve them numerically with modern computer methods, but these solutions may not correspond to nature unless verified by experimental results.
On still another level the Navier-Stokes equations
Navier-Stokes equations
In physics, the Navier–Stokes equations, named after Claude-Louis Navier and George Gabriel Stokes, describe the motion of fluid substances. These equations arise from applying Newton's second law to fluid motion, together with the assumption that the fluid stress is the sum of a diffusing viscous...
are pattern
Pattern
A pattern, from the French patron, is a type of theme of recurring events or objects, sometimes referred to as elements of a set of objects.These elements repeat in a predictable manner...
generators, and natural fluid flows display corresponding patterns that can recur on scales differing by many orders of magnitude. Such fluid patterns are familiar to almost everyone: the bathtub vortex
Vortex
A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...
and the tornado
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
, the smoke ring and the mushroom cloud, the swinging of wires in the wind and the collapse of a historic bridge due to forced oscillations from aeroelastic flutter.
Methods of visualization
In experimental fluid dynamics, flows are visualized by three methods:- Surface flow visualization: This reveals the flow streamlinesStreamlines, streaklines and pathlinesFluid flow is characterized by a velocity vector field in three-dimensional space, within the framework of continuum mechanics. Streamlines, streaklines and pathlines are field lines resulting from this vector field description of the flow...
in the limit as a solid surface is approached. Colored oil applied to the surface of a wind tunnelWind tunnelA wind tunnel is a research tool used in aerodynamic research to study the effects of air moving past solid objects.-Theory of operation:Wind tunnels were first proposed as a means of studying vehicles in free flight...
model provides one example (the oil responds to the surface shear stress and forms a pattern). - Particle tracer methods: Particles, such as smoke, can be added to a flow to trace the fluid motion. We can illuminate the particles with a sheet of laserLaserA laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
light in order to visualize a slice of a complicated fluid flow pattern. Assuming that the particles faithfully follow the streamlines of the flow, we can not only visualize the flow but also measure its velocity using the particle image velocimetryParticle image velocimetryParticle image velocimetry is an optical method of flow visualization used in education and research. It is used to obtain instantaneous velocity measurements and related properties in fluids...
or particle tracking velocimetryParticle tracking velocimetryParticle tracking velocimetry is a velocimetry method, i.e a technique to measure velocity of particles. The name suggests that the particles are tracked, and not only recorded as an image as it is suggested in another form, particle image velocimetry...
methods. - Optical methods: Some flows reveal their patterns by way of changes in their optical refractive indexRefractive indexIn optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
. These are visualized by optical methods known as the shadowgraphShadowgraphShadowgraph is an optical method that reveals non-uniformities in transparent media like air, water, or glass. It is related to, but simpler than, the schlieren and schlieren photography methods that perform a similar function...
, schlieren photographySchlieren photographySchlieren photography is a visual process that is used to photograph the flow of fluids of varying density. Invented by the German physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects...
, and interferometryInterferometryInterferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...
. More directly, dyes can be added to (usually liquid) flows to measure concentrations; typically employing the light attenuation or laser-induced fluorescenceLaser-induced fluorescenceLaser-induced fluorescence is a spectroscopic method used for studying structure of molecules, detection of selective species and flow visualization and measurements....
techniques.
Application
In computational fluid dynamicsComputational fluid dynamics
Computational fluid dynamics, usually abbreviated as CFD, is a branch of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows. Computers are used to perform the calculations required to simulate the interaction of liquids and gases with...
the numerical solution of the governing equations can yield all the fluid properties in space and time. This overwhelming amount of information must be displayed in a meaningful form. Thus flow visualization is equally important in computational as in experimental fluid dynamics.