Light scattering efficiency of oil smoke seeding droplets in PIV systems

Particle Image Velocimetry (PIV) technique produces the velocity vector fields of various aerodynamic flows by the correlation between a set of successive images. The flow is usually seeded by lightweight microscopic particles, which scatter the optical energy towards camera sensors. A monochromatic light sheet illuminates the seeding particles. In large-scale PIV tests, cameras are usually located at a considerable distance from the laser sheet, and sometimes at angles where the light scattering is minimal. This can create dark images, or unbalanced brightness between the different angles of observation, which negatively affect the quality of the image-correlation and velocity measurements. For this reason, we usually choose particles that have a relatively high refractive index for seeding. Fluid droplets with as small diameters as 0.3 microns have recently become popular due to their appealing aerodynamic properties. The scattering pattern can be surprisingly different from one seeding material to another, even when the particles have the same size. The oil droplets have a considerably lower refractive index than the conventional solid seeding particles, and therefore, a lower scattering efficiency. This paper discusses the influence of the refractive index of the seeding particles on the light scattering efficiency using the Mie theory (of scattering) and experimental observations.