Multi-scale imaging of upward liquid spray in the far-field region

The fluid dynamics in the far-field of liquid sprays involves a wide range of spatio-temporal scales; challenging experimental observation and numerical modeling alike. Here we show how a combination of multiple imaging approaches, targeted to different scales, can provide useful insight into this class of flows. We performed multi-scale imaging measurements in the far-field of an upward-facing liquid spray issued into a large quiescent air chamber by a hollow-cone atomizer. Time-resolved planar laser imaging was carried out simultaneously over two nested windows: one aimed at the large-scale dynamics of the spray cone, and the other focused on the intermediate-scale motions spanning the cone half-width. Spray velocity fields were obtained from both windows via Particle Image Velocimetry (PIV) using robust phase correlation and multi-frame pyramid correlation. Additionally, Lagrangian droplet trajectories were obtained in the intermediate-scale window via Particle Tracking Velocimetry (PTV). Finally, droplet sizes and velocities were obtained via shadowgraphy using a double-pulsed LED source over a small-scale (cm-sized) window. We discuss the agreement between the different techniques, as well as the physical insight offered by the combined measurement approaches on various aspects of the flow, including: velocity self-similarity, correlated and uncorrelated motion, frequency content, and droplet size-velocity correlation. (C) 2020 The Authors. Published by Elsevier Ltd.