Aerodynamic Interactions of Side-by-Side Rotors in Ground Proximity
H.N.J. Dekker (TU Delft - Wind Energy)
D. Ragni (TU Delft - Wind Energy)
W.J. Baars (TU Delft - Aerodynamics)
Fulvio Scarano (TU Delft - Aerodynamics)
Marthijn Tuinstra (Royal Netherlands Aerospace Centre NLR)
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Abstract
An experimental investigation is conducted to study the aerodynamic behavior of a two-rotor system in ground proximity. The counter-rotating rotors are placed side-by-side in the hovering condition. The time-averaged and unsteady flow behavior is studied when the rotor-to-rotor lateral distance and the distance between the rotors and the ground are varied. The experiments are performed using three-dimensional large-scale volumetric velocimetry with helium-filled soap bubbles as tracers, tracked by the particle motion analysis technique “Shake-The-Box.” The mean velocity field reveals the wake deflection due to the ground plane and the formation of toroidal-shape regions of separated flow below each rotor. The interaction of the wall jets formed by slipstream deflection results in a separation line with the flow emerging from the wall in a fountain-like pattern. Regimes of flow re-ingestion occur when the rotors are sufficiently far apart. The flowfield exhibits the tendency toward asymmetric states, during which the fountain flow column and the domain of re-ingestion shift closer to one of the rotors. A generic classification of flow regimes is proposed in relation to the behavior of two rotors in ground effect.