Interplay between circulatory and non-circulatory effects during interaction of counter-translating lifting bodies

Aerodynamic interactions between closely spaced lifting bodies are fundamental to many natural and engineered systems, including flapping insect wings, multi-rotor aerial vehicles and vertical-axis wind turbines. During operation, these systems undergo periodic counter-translating interactions (cross-over) in which adjacent wings or blades translate past each other in close proximity, producing strong unsteady coupling and transient load excursions. This study investigates the canonical form of this problem: two counter-translating lifting bodies in a hydrodynamic towing facility to isolate the fluid mechanics of a single cross-over event. Time-resolved load and flow-field measurements reveal that the unsteady forces arise from a combination of induced effects and transient pressure fields imposed by the mutual interaction between the wings. The resulting disturbance behaves as a self-induced gust, generated collectively by the coupled system rather than externally imposed. Comparison with linear unsteady aerodynamic models demonstrates that the measured response most closely follows Miles’s travelling-gust formulation. This connection establishes a continuum between externally imposed and self-induced gust interactions. A parametric analysis further identifies the primary non-dimensional groups governing the phenomenon, Reynolds number, aerofoil thickness, bound circulation and inter-wing spacing, and quantifies their influence on the transient lift response. Together, these results reveal a new regime of multi-body unsteady aerodynamics, where the interplay between circulatory and non-circulatory effects govern the short-time dynamics of interacting lifting surfaces.