Resolvent Analysis of Single Rotor Wake Stability in Ground Effect

In-ground effect (IGE) plays an important role in shaping the aerodynamic performance and wake stability of micro-rotors, with direct implications for the operational safety of micro aerial vehicles (MAV s). Although the steady-state thrust enhancement associated with IGE has been widely reported, the mechanisms governing unsteady wake instability remain insufficiently understood. In this work, resolvent analysis based on an axisymmetric base flow is applied to investigate the bi-global stability of the wake generated by a single micro-rotor at Re ≈ 3.5 × 104. V alidation against experimental particle image velocimetry (PIV) data shows that proximity to the ground markedly alters the flow amplification characteristics. Compared with the shear-layer-driven instabilities observed under out-of-ground effect (OGE) conditions, the IGE wake exhibits pronounced low-frequency energy amplification associated with a convective instability in the radial wall jet. The optimal forcing modes are concentrated near the stagnation region, where they induce large-scale vortex pairing and wake meandering. These results provide insight into the physical mechanisms responsible for wake destabilization near rigid boundaries and highlight regions that are particularly sensitive to targeted flow control.