In this field of aeronautical science, the flow visualization has been a nebulous observation
simply because air is invisible. The use of saw dust, tufts, fluorescent dye in oil with black light, a
recently developed laser beam and smoke, are some of th methods and materials used to attempt
flow visualization.
The use of saw dust increases the difficulty of maintenance in the wind tunnel and its
associated equipment. The tuft and oil methods show flow on the surface of the model. the laser
beam has not yet been perfected as a usable tool for this purpose. Whatever the method used, the
substance introduced into the tunnel should be non-corrosive, non-toxic, and the equipment must
be safe to handle.
The use of smoke appears to combine all the best characteristics of available methods. In
the early days, rotten wood was the popular source of smoke, (Ref. 1 and 2). However, smoke
produced by the burning wood was an eye irritant and caused an accumulation of tars.
The technique of evaporating oil by various methods was introduced to eliminate some of
the problems of the smoke, (Ref. 2). The study of smoke generators has also been reported in
England and Australia,...

In general, flow visualization is an experimental means of examining the flow pattern around a body or over its surface. The flow is "visualized" by introducing dye, smoke or pigment to the flow in the area under investigation. The primary advantage of such a method is the ability to provide a description of a flow over a model without complicated data reduction and analysis.

(Detalis taken from Department of Aerospace and Ocean Engineering,Virginia Polytechnic Institute and State University, Blacksburg, VA)

A video showing the flow of dye over and through a Nascar and Truck illustrating fluid mechanics and aerodynamics. 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.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 ...

Hydrodynamic cavitation inside a freefloating water drop produced in microgravity. (Credits: ESA, Danail Obreschkow, Philippe Kobel, Nicolas Dorsaz, Aurèle de Bosset, Mohamed Farhat).

http://en.wikipedia.org/wiki/Cavitation

Cavitation is a general term used to describe the behavior of voids or bubbles in a liquid. Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation and non-inertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave. Such cavitation often occurs in pumps, propellers, impellers, and in the vascular tissues of plants. Non-inertial cavitation is the process where a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Such cavitation is often employed in ultrasonic cleaning baths and can also be observed in pumps...