Fluid–structure interaction analysis of an asymmetric thin downstream bluff body for a tandem bladeless wind turbine

The present study investigates a fixed thin downstream bluff body (TDB) as a passive flow-control strategy for enhancing the post-onset oscillatory response and energy-harvesting performance of a bladeless wind turbine operating through flow-induced vibration (FIV). The proposed asymmetric TDB reorganizes the wake interaction between the oscillating primary cylinder and the fixed downstream body, thereby strengthening aerodynamic excitation and fluid–structure energy transfer over the investigated operating range. The experiments examined fifteen TDB geometries grouped according to cross-sectional shape, together with four aspect-ratio values and multiple spacing ratios. The effects of these parameters were evaluated through vibration response, power generation, flow visualization, particle image velocimetry (PIV), proper orthogonal decomposition (POD), pressure recovery, Reynolds shear stress, velocity deficit, turbulent kinetic energy, and enstrophy analyses. The results demonstrate that the introduction of a properly designed TDB fundamentally reorganizes the wake dynamics. An optimum spacing range of 0.3 ≤ Z / D ≤ 0.4 was identified, while AR = 0.17 was the best-performing aspect ratio within the investigated range, resulting in voltage and power enhancements of approximately 56% and 81%, respectively, compared with the baseline configuration without a TDB. Among all investigated geometries, the asymmetric TDB exhibited the best overall performance within the tested configurations, producing approximately 7.5% higher voltage and 15.5% higher power than the primary rectangular TDB, together with a stronger sustained post-onset oscillatory response. Flow-field analyses revealed enhanced wake confinement, increased circulation and vorticity, stronger coherent vortex interaction, delayed pressure recovery, and greater concentration of energy within low-order POD modes. Enstrophy analysis further showed reduced mean enstrophy and transport accompanied by increased fluctuating enstrophy, indicating a transition toward a more coherent and energetically efficient wake. These findings demonstrate that asymmetric downstream bluff-body design provides an effective approach for enhancing fluid–structure energy transfer and improving the viability of bladeless wind energy harvesting technologies.