- Flow visualization
In

fluid dynamics it is critically important to see thepatterns produced by flowing fluids, in order to understand them. We can appreciate this on several levels: Mostfluids (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 ), but they are nonlinearpartial 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 arepattern 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 bathtubvortex and thetornado , 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 vortex shedding.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 oflaser 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 asparticle image velocimetry . Finally, some flows reveal their patterns by way of changes in their opticalrefractive index . These are visualized by optical methods known as theshadowgraph ,schlieren photography , andinterferometry .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.**See also***

Streamlines, streaklines and pathlines **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.

**External links*** [

*http://www.cg.tuwien.ac.at/~helwig/diss/node10.htm Flow visualization techniques*] .

* [https://visualization.hpc.mil/wiki/index.php/Visualization_Algorithms Flow visualization algorithms] .

* [*http://www.interactiveflows.com/links/ Educational Particle Image Velocimetry (e-PIV) - resources and demonstrations*]

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