Investigation on heat transfer and coherent structures of rotational sweeping impingement jets with different frequencies and angles

• Rotational impinging jets with different sweeping frequencies/angles are proposed. • High-fidelity large eddy simulation is performed to reveal the coherent structures. • Unsteady heat transfer is driven by spring-like vortical structures in sweeping jets. • Optimal sweeping modes enhance heat transfer and reduces temporal instability. While sweeping jets generated by fluidic oscillators have been regarded as an effective means of enhancing heat transfer, their unsteady flow and thermal behaviors remain insufficiently understood. The present study introduces an actively rotational sweeping jet and examines both time-averaged and unsteady behaviors under different frequencies ( Str ) and inclined angles ( α ). Through high-fidelity large eddy simulations, we elucidate the vortex formation, coherent structure evolution, and their contributions to heat transfer enhancement and uniformity. Results show that sweeping jets enhance surface-averaged Nusselt number ( Nu ) by up to 3.25% and reduce spatial non-uniformity by 52.41%, compared to the classical steady jet. Additionally, pressure loss is reduced by 14.51%, leading to a 7.74% improvement in overall thermal performance. Unsteady analysis reveals a spring-like spiral vortex structure forming in early jet development, whose profile could be influenced by sweeping frequency and angle. These vortices periodically transport the migration of high- Nu regions along the radial direction, which weakens the stagnation effect and promotes wall jet uniformity. Probability distribution analysis indicates that optimal sweeping parameters (e.g., α = 30°, Str = 0.33) can improve time-averaged efficiency while reducing temporal instability in heat transfer. Power spectral density analysis further confirms that jet sweeping disrupts the classical -5/3 energy cascade by both effects of vortex shedding and periodic forcing, leading to anisotropy and altered inertial sub-range development.