78th Annual Meeting of the APS Division of Fluid Dynamics (Nov 23 — 25, 2025)

V049: Between the twists and the turns lies the shadow: Free surface thermal signatures of a submerged plume

Flows at the water surface, when thermally scanned, appear striking - rhythmic, convoluted motions folding and fading as they evolve. This exhibit presents long-wave infrared imagery, capturing the rise of a plume as it impacts the free surface at height of 0.22m. The sequence of images shows free-surface features transitioning from a plume-like to jet-like flow, sweeping across the source-based Reynolds-Richardson number parameter space (600 < Re < 8600, 0.001 < Ri < 0.3). Because these are the true surface temperature fields, they have been processed to capture the evolution of dominant thermal length scales with changing sub-surface conditions. Furthermore, these temperature fields can also be processed like tracer patterns-for example, with particle image velocimetry algorithms-to extract relevant length scales corresponding to the velocity fields. Beyond their visual appeal, these images demonstrate how free-surface thermal signatures can serve as a remote diagnostic of submerged flows, offering a safer and more time and cost effective alternative to intrusive in-situ measurements.