Dual-Volume and Four-Pulse Tomo PIV using polarized laser lights

Abstract

A demonstration experiment and first assessment of a dual-volume and four-pulse tomo PIV system operating with two pairs of perpendicularly polarized laser light pulses and respective polarizing filters for separation of the scattered particle light is presented. The polarizing filters are placed in front of eight sCMOS double-frame cameras forming two independent tomo PIV systems consisting of four cameras each. The whole system is able to provide data-sets of four subsequent particle image distributions on separate frames with uncorrelated ghost particles in the reconstructed measurement volumes. The system can be applied in a four pulse mode to low and high speed air flows measuring particle displacements with enhanced accuracy in the same volume or alternatively in a mode in which particle doubleimages in two neighbouring measurement volumes are captured simultaneously or with any time difference. The used principle to separate the two tomo PIV systems relies on the fact that for chosen seeding particles, in our experiment small spherical liquid droplets of DEHS in the order of 1 μm diameter, the mode of polarization does not change significantly during light scattering. The flow of investigation is a part of the separation and shear region of a turbulent boundary layer flow downstream of a backward facing step. After tomographic reconstruction of the volumetric particle distributions from four subsequently captured images of both systems a multi-frame pyramid-like evaluation has been applied with a 3D correlation technique. An assessment of sources of uncertainty using the proposed set-up within a wind-tunnel environment concerning vibrations and signal separation and a first comparison of the improvements of the velocity vector estimation based on multi-frame correlation with standard evaluation approaches is given.