This study numerically investigates active control of natural convection in a square-in-square enclosure using oscillating flexible fins through a fluid–structure interaction approach. The working fluid occupies the region between an outer cold square enclosure and an inner heated square. Four flexible fins are installed inside the enclosure and subjected to a horizontal sinusoidal force at their free tips to induce periodic deformation and enhance thermal mixing. The effects of Grashof number (Gr=3×10^5-7.5×10^5 ), excitation frequency (f=0.5-3┤ Hz), fin length (l/h=7/24, 9/24, 11/24), and elastic modulus (E=5×10^6-1×10^7 ┤ Pa) are examined. The results show that the oscillating flexible fins generate unsteady vortical structures, disturb the thermal boundary layers around the heated inner square, and promote fluid exchange between hot and cold regions. Increasing the fin length improves the penetration of the oscillating fins into thermally active regions. Reducing the elastic modulus also enhances the global thermal performance, and the most flexible case (E=5×10^6 ┤ Pa) gives the highest average Nusselt number. The heat transfer response is wall-dependent: among the tested frequencies, the bottom wall shows its highest response at an intermediate excitation frequency, whereas the top and lateral walls benefit more from higher-frequency oscillation. For the overall heat transfer from all hot walls, the best-performing case within the investigated parameter range is obtained at f=3 Hz, E=5×10^6 Pa, and l/h=11/24. Under this condition, the total heat transfer enhancement reaches approximately 90% at Gr=3×10^5 and remains about 62% at Gr=7.5×10^5 compared with the corresponding without-fin reference case. These findings demonstrate that periodically forced flexible fins can serve as an effective active thermal management strategy for improving buoyancy-driven convection in confined enclosures.
