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The 'Rotor Hub Flow Prediction Workshops' have been productive collaborations between experimental and computational efforts in the important area of high-Reynolds number model testing of rotor hubs and associated complex interactional aerodynamics in the long-age wake as relevant to current and future rotorcraft. As such the hub flow workshops have joined the ranks of past successful collaborations such as the UH-60 Airloads and HART-II workshops. This paper begins by describing the basic physics of rotor hub flows and gives a brief summary of recent water-tunnel test campaigns. Following, the evolution of the hub flow workshops is summarized, with emphasis on the productive interactions between experimentalists and computational participants. A compilation of computational blind comparison results against measured data for all three workshops thus far is presented. Challenges associated with uncertainties in both experiments and computations and their effect on quantitative comparisons are discussed. In particular, emphasis is given to the 'Lessons Learned' on both sides and an outlook into remaining challenges and next research steps in the area of rotor hub flows is provided.

Rotor hub parasite drag remains a primary obstacle to improving the forward-flight capabilities of helicopters. As part of a rotor hub flow physics project at the Vertical Lift Research Center of Excellence (VLRCOE) at Penn State, this investigation was designed to improve the understanding of the interactional aerodynamics and wake flow physics of counter-rotating coaxial rotor hubs and explore designs for reducing the rotor hub drag factor, Kfe. These experiments measured the time-averaged and time-varying drag on four rotor hub designs, each with unique blade stubs. The four shapes tested were the DBLN 526 airfoil, 3.25:1 Rectangle, 4:1 Ellipse, and the novel profile named the Optimized Cambered Shape (OCS). Load data was collected at four Reynolds numbers ranging from 3.77×105 to 1.51×106 and advance ratios ranging from .25 to .6. Additionally, stereoscopic particle-image velocimetry (SPIV) measured the three velocity components at two downstream locations in the wake of the DBLN 526 rotor hub at Re=1.13×106 and advance ratios of .25 and .6, providing insight into and visualizing the development of the wake. Presented here is the compiled load data and calculated Kfe from these experiments, as well as the flow fields at the near- and midwake locations, with discussion of new knowledge gained of the coaxial rotor hub wakes.