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The rotor hub system is recognized as one of the primary contributors to helicopter parasite drag and is one of the inherent limiters to maximum helicopter forward-flight speed. Despite its importance for the performance of rotorcraft, complex bluff-body flows of the rotor hub have received only intermittent attention since the first studies in the 1950s. This work presents a comprehensive review of experimental and computational research over the past 60 years on rotor hub flows and describes the challenges associated with component and interference drag, effect of the hub near wake on pylon and fuselage flows, and the long-age wake of rotor hubs at high Reynolds number with the associated size and strength of flow structures that interact with the empennage. Computational technology is assessed as an emerging design tool for hub/pylon design, and, with its ability to capture downstream flow unsteadiness, empennage/tail surface aeroelasticity and controllability. The work concludes with recommendations of future experimental and computational evaluations to advance the community's understanding of rotor hub flows and mitigation of its adverse effects for future rotorcraft. Nomenclature Zvertical coordinate axis, m (positive upward) μ rotor advance ratio, U ∞ / R Rotor ν fluid kinematic viscosity, m 2 /s ρ fluid density, kg/m 3 rotor rotational speed, revolutions per minute 022007-2

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A 1:17 scale model of a notional rotor hub of a large helicopter was tested in the Pennsylvania State University Applied Research Laboratory 12 in. test-section water tunnel. Objectives of the experiment were to quantify the effects of Reynolds number, advance ratio, and hub geometry configuration on the drag and shed wake of the rotor hub. A range of flow conditions was tested, with hub-diameter-based Reynolds numbers ranging from 1.0 × 10 6 to 2.6 × 10 6 and advance ratios ranging from 0.2 to 0.6, as well as nonrotating cases. Five hub geometry configurations were tested with various combinations of components including blade stubs, spiders, scissors, the swashplate, pitch links, and beanie fairing. Measurements included the steady and unsteady hub drag and particle image velocimetry at two downstream locations. Results include time-averaged and phase-averaged analyses of the unsteady drag and wake velocity. A strong dependence of the steady and unsteady hub drag and wake on the advance ratio, Reynolds number, and configuration was observed, demonstrating the importance of adequate Reynolds number scaling for model helicopter rotor hub tests.

Computational fluid dynamics was used to study the effectiveness of micro-ramp vortex generators to control oblique shock boundary layer interactions. Simulations were based on experiments previously conducted in the 15 x 15 cm supersonic wind tunnel at NASA Glenn Research Center. Four micro-ramp geometries were tested at Mach 2.0 varying the height, chord length, and spanwise spacing between micro-ramps. The overall flow field was examined. Additionally, key parameters such as boundary-layer displacement thickness, momentum thickness and incompressible shape factor were also examined. The computational results predicted the effects of the micro-ramps well, including the trends for the impact that the devices had on the shock boundary layer interaction. However, computing the shock boundary layer interaction itself proved to be problematic since the calculations predicted more pronounced adverse effects on the boundary layer due to the shock than were seen in the experiment. = compressible boundary-layer displacement thickness θ = compressible boundary-layer momentum thickness