LED illumination for planar PIV in large-scale hydraulic laboratories

Abstract

This paper presents the application, and implications of line arrays of light emitting diodes (LEDs) as an illumination source for planar particle image velocimetry (PIV) measurements in large-scale hydraulic laboratories. The use of class 4 lasers, commonly applied as illumination source in these PIV measurements, requires strict safety precautions (i.e. to prevent safety hazards for people working in their vicinity), specifically when applied at hydraulic experimental setups that are not located in a laser lab. To examine the applicability of an alternative light source, differences between LED- and laser-based illumination for PIV are analyzed. A theoretical analysis, in which a so-called motion blur parameter ((Formula presented) ) is introduced, shows that for moderate flow velocities in large-scale setups, image blur can be avoided, even for relatively long (millisecond) pulse widths. The light sheet thickness, width and intensity of a pulsed laser and a line array of both continuous and pulsed LEDs are measured and compared. Based on these properties a safety assessment is made, from which it is concluded that the application of arrays of LEDs for PIV measurements applied in liquid flows requires significantly less safety precautions than in case a class 4 PIV laser is used. Planar (2D) PIV measurements have been performed with both a pulsed and a continuous LED as light source for two testcases in large hydraulic scale-models. Time-averaged velocity field results from the LED-based PIV measurements show good resemblance to both PIV measurements obtained with a class 4 laser as well as to pitot tube measurements. It is shown that the time-averaged PIV vector fields are influenced by motion blur, resulting in a distinct bias towards smaller velocities when increasing motion blur. The two testcases show that linear LED arrays can serve as a suitable alternative illumination source for planar PIV measurements in large-scale hydraulic laboratories in case motion blur remains limited. Specifically, LED line arrays are considered useful for the time-average quantification of predominantly 2D, low to moderate flows in a relatively large domain.