Data-Driven Analysis of Cycle-to-Cycle Variations, Scatter, and Furcation in the UH-60A Wind Tunnel Rotor Airloads Measurements

A data-driven clustering algorithm based on proper orthogonal decomposition was applied to assess the scatter found in the UH-60A wind tunnel airloads measurements. Upon verifying the capability of the algorithm, pushrod loads, blade surface pressures, sectional loads, and torsional moments were analyzed. Spatial eigenmodes resulting from the decomposition provided the optimal basis; projection of the individual cycles onto the high singular value modes allowed visualizing the statistical distribution of data over the entire azimuth. While not all cases showed furcation in the data, the bimodal distribution was found in the high-thrust cases, where statistically normal distribution is generally assumed. Consequent clustering of the measured cycles produced an excellent correlation among clusters found in the pushrod loads, blade surface pressure, and torsional moment that suggests a common source for furcation in the data. The cycles assigned to one group repeatedly showed distinguishable variations from the other group in terms of the presence/absence of a dynamic stall vortex, azimuthal occurrence of the stall, chordwise location of separation, reattachment, and so on. When one of the clusters is smaller in size compared to the other, the conventional phase average obscured all the intricate features even when the loads are substantially higher than the larger cluster. In general, clustering the dataset when warranted showed not only higher peak loads but also lower variance for both the clusters across the entire azimuth compared to the conventional simple phase-average results. Computational simulations were conducted using CREATETM-AV Helios towards understanding the underlying flow field. Misinterpreted earlier as under-/overpredictive when compared with the simple phase-average data, Helios results consistently showed significantly improved correlation with one of the two clusters. Combining the clustered results and the flow visualization provided by Helios, aperiodicity in the spatial location and the strength of both the trim tab vortices and tip vortices have been hypothesized as potential sources of furcation.