Flow visualization is sub field of scientific visualization. Since most fluids are transparent (air and water), one must derive a method to visualize the fluid flow to get a better understanding of the motion both qualitatively and quantitatively. Allowing us to look at flows pass bodies (pathlines), velocity vectors, wake structures, etc. But application is much broader and spans to such application as geological, meteorological, aeronautical, and astronautical applications.
Some methods are as follow:
Surface Oil Flow Visualization, where colored oil is applied to the surface of an object and as the fluid flows pass the object the flow will create a pattern on the surface. Typically, the oil is standard 40W with fluorescent dye. Surface oil flows indicate the boundary of a flow separation since the oil cannot penetrate the separation boundary.
Particle Tracer Methods, such as Particle image velocimetry (PIV), where particles such as smoke, small solids, or oil, is added to a flow to trace the fluid motion. The particles are then illuminated by a sheet of laser (a laser with a light sheet optic) so that a camera can pick up with trace particle’s motion. Here we assume that the particle follows the stream line of the native fluid, which is generally true for smaller particles. This measurement of how well the particle follows the fluid is represented by the Stokes number (Stk) , for Stk >> 1, particles will detach from a flow, for Stk <<, particles follow fluid streamlines closely. If Stk << 0.1 tracing accuracy errors are below 1%. Using a camera to capture the motion of the tracer it can later be post-process to obtain the fluid velocity using the particle image velocimetry methods. A stereoscopic PIV can be done using 2 cameras to get a 3D velocity profile. More information can be found at: UMD.edu, openPIV, and pivLab (or pivLab Blog ).
Optical methods is a method that looks at the change in optical refractive index due to the change in the density of the fluid, such method are know as shadowgraph, schlieren photography, and interferometry.