Mitigation of surface reflection in PIV measurements

Abstract

Surface reflections of high-intensity laser light are a common concern when conducting particle image velocimetry (PIV) measurements. Consequences range from a poor signal-to-noise ratio (overexposure) in near-surface areas up to camera sensor damage. The severity depends on the interplay between three factors: surface properties, laser light intensity and relative camera position. In stereoscopic or tomographic PIV setups, material selection is often the only factor which can be adapted. We present a systematic comparative study, involving different materials and surface treatments. Their potential to mitigate surface reflections is quantified against the reference case of a flat black painted wooden surface. The largest reduction of surface reflection intensity is obtained by applying fluorescent paint on wood or by employing electropolished steel. The more widely used flat black painted wood shows poor behavior.

Fluorescent PIV using Atomized Liquid Particles

“…A frequent challenge for PIV experimentalists is the occurrence of surface laser reflection, or "flare", in flows around models [Paterna et al (2013)], flows involving free surfaces [Pedocchi et al (2008)], and studies of near-wall regions in general [Cadel et al (2016)]. Flare is detrimental to PIV correlations because it decreases the image contrast and signal-to-noise ratio (SNR) in the vicinity of surfaces.…”

“…Light that is scattered by surfaces can be far more intense than particle light scattering, potentially leading to saturation of sensor pixels and the loss of flow vectors [Chennaoui et al (2008)]. Paterna et al (2013) also note the possibility of camera sensor damage from intense flare, although with modern CMOS cameras this is less of a concern.…”

“…Attempts to reduce laser flare often involve modifications to surfaces themselves, such as a coat of black paint [Wernet (2000)] or clear surfaces that match the fluid's index of refraction [Uzol et al (2002)]. The comparative study of materials and surface treatments by Paterna et al (2013) concluded that a coating of fluorescent paint was the most effective method of reducing flare in air-based experiments, given that no material can match the refractive index of air. The fluorescent paint technique relies on laser-induced fluorescence (LIF), the emission of light that is red-shifted relative to any absorbed laser light.…”

POD-based background removal for particle image velocimetry

“…When PIV experiments are performed over opaque objects, proper material selection or adequate surface treatment can limit the prominence of reflections by reducing omnidirectional light scattering [7]. Alternatively, light reflections can be minimized by suitable camera observation angle [8].…”

Automated mask generation for PIV image analysis based on pixel intensity statistics

“…hindering the measurement quality (Westerweel et al 2005). If the required particle image displacement measurement is in the near vicinity of a surface, special care must be taken in adequate surface preparation during the experimental phase (Paterna et al 2013), images must be properly pre-processed by means of, e.g., background subtraction (Mendez et al 2017), or advanced image interrogation processes are to be adopted (Ronneberger et al 1998;Gui et al 2003;Usera et al 2004). Such advanced PIV analysis routines typically involve the exclusion of object regions within cross-correlation windows as to minimise distortions in the cross-correlation map, consequently limiting associated displacement bias errors .…”