An experimental investigation of rotor wakes by means of time-resolved PIV measurements

Rotor wakes generated by a four-bladed isolated rotor of a scaled helicopter model in hover have been investigated by time-resolved particle image velocimetry. Two rotor speeds, i.e., Ω/2π equal to 29.9 Hz and 26.3 Hz, have been considered resulting in flow regimes defined by a tip Mach number and a chord Reynolds number of ∼0.20 and ∼1.50×105 or ∼0.17 and ∼1.32×105, respectively. The rotor disk generates aerodynamic loads for a thrust coefficient of CT=5.7×10−3, and a rotor solidity of σ=0.116. It is found an intensification of thrust fluctuations as the rotor speed increases, i.e., the amplitude of the standard deviation at 29.9 Hz intensifies approximately of 3.5 times more than that observed at 26.3 Hz. This enables the inspection of how aerodynamic loads, featured by two levels of fluctuation amplitudes, impact the spatial organization and dynamics of the wake structures. Data-driven decomposition methods, i.e., Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD), have been exploited for analyzing the most prominent coherent structures. A wake contraction inward the radial direction occurs suddenly below the rotor disk. The early wake ages feature overlapped trajectories of the tip vortices indicating a regular formation process. Conversely, as the tip vortices advect further downstream, their trajectories progressively depart from each other, and engulfment events appear. The dominance of the periodic shedding of the tip vortices was captured by both modal decompositions. The first pair of POD modes describe the spatial organization of the tip vortices and the engulfment. The corresponding time coefficients include multiple frequency peaks discerning the most prominent contribution ascribed to the tip vortex shedding for both rotor speeds. POD analysis highlights slightly higher efficiency at slower rotor speed, exhibiting a more spectral purity for the first couple of time coefficients. Despite multiple harmonics featuring the most energetic flow structures, the dominant pulsation attains a Blade Passing Frequency BPF=1 for both rotor speeds. Similarly, DMD analysis captures the tip vortices at BPF=1. However, the shear layer embeds additional flow structures pulsating at different harmonics, that include wake characteristics at BPF=0.25, secondary representation of the tip vortices scattering in the shear layer (BPF=1.5 and 2).