Unsteady Thermal and Flow Structures of an Impinging Sweeping Jet

Sweeping jets are increasingly employed in thermal management and flow control applications due to their inherent unsteadiness and ability to cover wide surface areas. This study investigates the unsteady heat transfer and flow structures of an impinging sweeping jet, with particular emphasis on the influence of Reynolds number and nozzle-to-plate spacing. A combined diagnostic approach, utilising non-synchronous high-speed infrared thermography and planar particle image velocimetry, is adopted to independently measure wall heat flux distributions and flow field dynamics. Time-averaged Nusselt number maps reveal a progressive widening of the thermal footprint as the nozzle-to-plate distance increases, accompanied by a reduction in peak heat transfer intensity. In contrast, the fluctuating component exhibits a double-lobe structure that becomes more pronounced at higher Reynolds numbers. Velocity measurements confirm the presence of a bifurcated jet core and shear-driven unsteadiness, with fluctuating velocity components and Reynolds shear stress spatially correlated with zones of enhanced heat transfer fluctuations. A direct spatial comparison between velocity and heat transfer fluctuations demonstrates that wall-normal velocity unsteadiness dominates the thermal response at low nozzle-to-plate distances, while streamwise fluctuations become increasingly significant at larger spacings. Finally, the full-field heat transfer data are embedded into a reduced-order parametric space, demonstrating that the complex system behaviour can be effectively represented by a low-dimensional manifold governed by Reynolds number and nozzle-to-plate spacing. The findings underscore the importance of the fluctuating thermal component in sweeping jet impingement and highlight the potential of data-driven models for predicting surface heat transfer in oscillatory jet systems.