In fluid dynamics it is critically important to see the patterns produced by flowing fluids, in order to understand them. We can appreciate this 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. Fluid dynamics is the subdiscipline of fluid mechanics that studies fluids (liquids and gases) in motion. ... A pattern is a form, template, or model (or, more abstractly, a set of rules) which can be used to make or to generate things or parts of a thing, especially if the things that are generated have enough in common for the underlying pattern to be inferred or discerned... A subset of the phases of matter, fluids include liquids and gases, plasmas and, to some extent, plastic solids. ... See: transparency (optics) alpha compositing GIF#Transparency transparency (overhead projector) market transparency transparency (telecommunication) transparency (computing) For X11 pseudo-transparency, see pseudo-transparency. ...

On another level, we know the governing equations of fluid motion (the Navier-Stokes equations), 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. The Navier-Stokes equations, named after Claude-Louis Navier and George Gabriel Stokes, are a set of equations that describe the motion of fluid substances like liquids and gases. ... In mathematics, and in particular analysis, a partial differential equation (PDE) is an equation involving partial derivatives of an unknown function. ...

On still another level the Navier-Stokes equations are pattern 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 and the tornado, the smoke ring and the mushroom cloud, the singing of wires in the wind and the collapse of a historic bridge due forced oscillations from vortex shedding. The Navier-Stokes equations, named after Claude-Louis Navier and George Gabriel Stokes, are a set of equations that describe the motion of fluid substances like liquids and gases. ... A pattern is a form, template, or model (or, more abstractly, a set of rules) which can be used to make or to generate things or parts of a thing, especially if the things that are generated have enough in common for the underlying pattern to be inferred or discerned... Vortex created by the passage of an aircraft wing, revealed by coloured smoke A vortex (pl. ... A tornado in central Oklahoma. ...

Flow visualization is the art of making these patterns visible. In experimental fluid dynamics, flows are visualized by three methods: surface flow visualization, particle tracer methods, and optical methods. Surface flow visualization reveals the flow streamlines in the limit as a solid surface is approached. Colored oil applied to the surface of a wind tunnel model provides one example (the oil responds to the surface shear stress and forms a pattern). Particles, such as smoke, can be added to a flow to trace the fluid motion. We can illuminate the particles with a sheet of laser 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 a method known as particle image velocimetry. Finally, some flows reveal their patterns by way of changes in their optical refractive index. These are visualized by optical methods known as the shadowgraph, schlieren photography, and interferometry. In fluid dynamics, a streamline is the path that an imaginary massless particle would make if it followed the flow of a fluid in which it was embedded. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... // Experiment using a (likely argon) laser. ... Particle Image velocimetry (PIV) is an optical method used to measure velocities and related properties in fluids. ... The refractive index (or index of refraction) of a material is the factor by which the phase velocity of electromagnetic radiation is slowed in that material, relative to its velocity in a vacuum. ... a) prehistoric shadowgraphy, b) sunlight shadowgram of a martini glass, c) focused shadowgram of a common firecracker explosion, d) Edgerton shadowgram of the firing of a legendary AK-47 submachine gun (images and artwork by G.S. Settles, Penn State Gas Dynamics Lab) Shadowgraph is an optical method that reveals... Schlieren photography is a visual process that is used to photograph the flow of air (or other compressible fluids) around objects. ... This article or section is in need of attention from an expert on the subject. ...

In computational fluid dynamics 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. Computational fluid dynamics (CFD) is the use of computers to analyze problems in fluid dynamics. ...

References

Merzkirch, W., Flow visualization, New York:Academic Press, 1987.

Van Dyke, M., An album of fluid motion, Stanford, CA:Parabolic Press, 1982.

Samimy, M., Breuer, K. S., Leal, L. G., and Steen, P. H., A gallery of fluid motion, Cambridge University Press, 2004.

Settles, G. S., Schlieren and shadowgraph techniques: Visualizing phenomena in transparent media, Berlin:Springer-Verlag, 2001.