Characterization of a canonical helicopter hub wake

Abstract

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° (in-phase) or 45° (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

Wind Speed Statistics from a Small UAS and Its Sensitivity to Sensor Location

“…The projected area of the models varied with configuration and tilt angle producing ratios of solid blockage area to total area ranging from 3.4% (dual parallel at 0°tilt angle) to 9.6% (quad-plus at 30°tilt angle). This range of blockage is comparable to traditional helicopter hub water tunnel experiments with values above 8% [25,26], which those studies reported negligible impact from blockage on the results. Correcting the freestream speed for the solid blockage increases the Reynolds number and advance ratio for the 0.5 m/s condition by less than 7%.…”