1. Direct viasualization:
Adding a die of using smoke comes under this category. The flow patterns can de
directly tracked using this technique. Does not give too much of quantitaive informations.
Using the principle of scattering of light beams near very sharp corners.
Examples include Laser Doppler Velocimetry (LDV), Particle Image Velocimetry (PIV), Liquid Crystal Thermography (LCT).
3. Refractive index based:
Uses the property of bending of light beam with varying refractive index. Examples
include Interferometry, Schlieren, Shadowgraph.