The impact of optical turbulence on particle image velocimetry

Abstract

Particle image velocimetry (PIV) is a well-established tool to collect high-resolution velocity and turbulence data in the laboratory. PIV measurements are based on using a laser sheet to illuminate a flow seeded with small particles and taking quick successive images or image pairs of the illuminated particle field with a CCD or CMOS camera. The movement of the particles between images can be used to infer flow field velocities over an image area. During experiments at the Simulated Turbulence and Turbidity Environment (SiTTE) laboratory tank, we observed a marked influence of optical turbulence, i.e. strong temperature gradients leading to changes in the index of refraction, on particle imaging in PIV. The particles look blurred and have a “shooting star” appearance. PIV is routinely used in flows with very high temperature gradients, such as nuclear reactor cooling rods, but the optical path length is typically very short (on the order of cm), and no such effect is generally considered for measurements in liquids. We investigated the effect of optical turbulence on PIV imaging for various optical path lengths (0.5m to 2m) and turbulence strengths. Velocities from the PIV measurements were calculated using the algorithms provided within Dantec’s Dynamic Studio and compared to velocities from concurrent velocity point measurements with a Laser Doppler Velocimetry system. The results indicate that optical turbulence can affect PIV measurements in liquids, and that depending on the strength of the optical turbulence and path length, care needs to be taken to mediate this effect using appropriate post-processing techniques when inferring velocities from PIV data.