These particulates are generally small enough to be carried by the fluid but large enough to be picked up using a flow visualization technique, such as particle image velocimetry (PIV). In reference to aerodynamic testing, such as wind tunnel testing, water tunnel testing, or any other test investigating the flow of a fluid which may be invisible to the naked eye, seeding a flow is often the only way to take visual measurements. Simple examples of a seeded flow include the introduction of smoke into a low speed wind tunnel to see the general path of the air, or injecting colored dye into a water tunnel to see secondary flow structures such as hairpin vortices.^{[citation needed]}

As stated in The Handbook of Fluid Dynamics, an ideal seeding particle should have uniform properties such that its density is the same as the fluid that it's added to.^[1]

• Flow visualization
• Fluid dynamics
• Fluid mechanics
• Streamlines, streaklines, and pathlines