Wall effects on fluid-structure interaction of tandem flapping foils operating in energy extraction mode

This study investigates the energy harvesting performance of single and tandem fully active flapping foils in confined flow environments at a Reynolds number of 1100. Two tandem NACA0015 foils, undergoing identical sinusoidal heaving and pitching motions, are adopted using optimised kinematic parameters derived from prior research. Numerical simulations were conducted by solving the unsteady incompressible RANS equations using the finite-volume method with the SST k-ω turbulence model considered and prescribed foil kinematics implemented via user-defined functions. The influence of wall confinement is systematically examined by varying the channel width (H/c) and tandem spacing (L/c). Results reveal that wall confinement significantly enhances energy extraction efficiency when H/c ≤ 7.5, with the single foil achieving a 45.4% increase at H/c = 2.0 compared to unconfined conditions. With the tandem configuration, the performance of the fore foil is predominantly governed by wall confinement, exhibiting minimal influence from the aft foil when L/c ≥ 10. Conversely, increased confinement improves the performance of the aft foil by accelerating wake recovery behind the fore foil, thereby restoring upstream momentum, enhancing the effective inflow to the aft foil, and improving its heaving power with amplified power fluctuations at certain tandem spacings. A maximum efficiency of 57.4% is attained at L/c = 10.5 and H/c = 2.0, representing a 66.7% improvement over the single foil in unconfined flow and reflecting a high-performance level for flapping foil systems operating at selected Reynolds number. Flow structure analysis further reveals that increased wall confinement enhances wake-boundary layer interaction, transforming the fore foil's wake pattern from the classical 2S mode into a 2(P + S) mode. This transition subsequently modifies wake interactions for the aft foil. These findings underscore critical roles of wall confinement and foil arrangement in optimizing energy harvesting, offering valuable guidance for designing high-efficiency energy harvesting devices in constrained flow environments.