  The current study investigates the long-age wake behind rotating helicopter
hub models composed of geometrically simple, canonical bluff body shapes. The
models consisted of a 4-arm rotor mounted on a shaft above a 2-arm (scissor)
rotor with all the rotor arms having a rectangular cross section. The relative
phase between the 2- and 4-arm rotors was either $0\deg$ (in-phase) or $45\deg$
(out-of-phase). The rotors were oriented at zero angle-of-attack and rotated at
30 Hz. Their wakes were measured with particle-image-velocimetry within a water
tunnel at a hub diameter based Reynolds number of $820,000$ and an advance
ratio of $0.2$. Mean profiles, fluctuating profiles and spectral analysis using
time-series analysis as well as dynamic mode decomposition were used to
characterize the wake and identify coherent structures associated with specific
frequency content. The canonical geometry produced coherent structures that
were consistent with previous results using more complex geometries. It was
shown that the dominant structures (2 and 4 times per hub revolution) decay
slowly and were not sensitive to the relative phase between the rotors.
Conversely, the next strongest structure (6 times per hub revolution) was
sensitive to the relative phase with almost no coherence observed for the
in-phase model. This is strong evidence that the 6 per revolution content is a
nonlinear interaction between the 2 and 4 revolution structures. This study
demonstrates that the far wake region is dominated by the main rotor arms wake,
the scissor rotor wake and interactions between these two features.
