Unsteady Thermal and Flow Structures of an Impinging Sweeping Jet

Abstract

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 (Re) and nozzle-to-plate spacing (L/D). A combined diagnostic approach, utilising non-synchronous high-speed infrared thermography and planar particle image velocimetry, independently measures 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 Re. 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, where the complex system behaviour is effectively represented by a low-dimensional manifold governed by Re and L/D. This manifold is used to perform an efficient surrogate model capable of predicting both mean and fluctuating Nusselt number maps for untested combinations of Re and L/D within the parametric space, serving as a tool for the preliminary sizing and design of heat transfer enhancement devices.