Spatially-resolved free-surface signatures of submerged counter-rotating vortex pairs.

Free surface signatures offer a valuable means to predict the underlying sub-surface dynamics in the ocean. Momentum sources such as submarines generate counter-rotating vortex pairs that propagate downstream with self-induced velocity, creating a detectable surface footprint when interacting with the surface. Several qualitative experimental studies and numerical works have been conducted on studying the signature of a single vortex pair but lack spatially resolved, quantitative information at the free surface. In this study, we investigated the free surface signatures of a counter-rotating vortex pair, generated by two wing sections placed side by side with some tip separation distance in an open-channel recirculating flume. We combine the results from free surface elevation maps, sub-surface velocity fields, and the free surface thermal field, obtained using free surface synthetic schlieren, digital particle image velocimetry, and an infrared camera, to quantitatively study the signatures and relate them with the subsurface dynamics. We present results for varying wing tip separation, submergence, and vertical direction of vortex pair propagation, to explore both primary and secondary vortex dynamics. The results of this study help in stealth navigation and sub-surface object detection in the ocean.