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1st Rotor Hub Flow Prediction Workshop



A helicopter’s rotor hub system is a significant contributor to parasite drag (produced by non-lifting helicopter components) and is a prominent source of interactional aerodynamics with the empennage and tail.

As Future Vertical Lift (FVL) concepts are advanced for next-generation high-speed rotorcraft, new knowledge is required with respect to the physical understanding and predictive computational capability of these complex flows. In the past few years, fundamental research sponsored through the Vertical Lift Research Centers of Excellence (VLRCOE) and Office of Naval Research (ONR) programs at the Pennsylvania State University and the Georgia Institute of Technology produced new experimental and computational data on bluff- body flows in general and rotor hubs in particular.

The Applied Research Laboratory (ARL) at Penn State is home to the 48-inch (122-cm) diameter Garfield Thomas Water Tunnel, the second largest water tunnel in the United States. This unique facility is particularly suited for fundamental research on rotor hub flows due to its ability to test quarter- scale rotor hub models at full-scale Reynolds number in a test section that allows for non-invasive optical flow diagnostics — i.e. Laser-Doppler Velocimetry (LDV), Particle Image Velocimetry (PIV) and Stereo Particle Image Velocimetry
(SPIV) — up to the “long-age wake,” where coherent flow structures generated by the hub interact with the empennage and tail surfaces.

A major finding from the prior VLRCOE and ONR studies is that high Reynolds number testing is required to characterize the complex flows in the wakes of rotor hub systems, in particular those long-age wakes that influence aeromechanical interactions near the empennage. The capture of detailed characteristics of long-age wakes is challenging both from experimental and computational points of view.



The “1st Rotor Hub Flow Prediction Workshop” was held at Penn State on June 23 with the objective of bringing together computational fluid dynamics (CFD) experts to compare simulation data to measurements previously conducted in the Garfield Thomas Water Tunnel. Quantitative comparisons between CFD and measured data focused on drag of the various hub components and the evolution of coherent structures in the near-, mid- and long-age wakes up to seven rotor radii downstream. The Fluid Dynamics Research Consortium (FDRC) at Penn State sponsored the one-day event through cost share funding committed to a current Vertical Lift Consortium (VLC) National Rotorcraft Technology Center (NRTC) project, “Computational and Experimental Investigation of Interactional Aerodynamics Relevant to Rotor Hub and Empennage Flows;” Dr. Louis Centolanza was the technical monitor at the US Army Aviation Development Directorate (ADD) for this effort. In addition to CFD efforts directly supported by VLC/NRTC at Penn State and Georgia Tech, participation included CFD experts from US academia, industry and government.

The event began with a plenary talk on the physics and testing of rotor hub flows. CFD participants addressed challenges with respect to grid generation, boundary conditions, discretization schemes, solver convergence, and the computational cost for domain connectivity information (DCI). The day concluded with a tour of the Garfield Thomas Water Tunnel, including inspection of a new rotor hub model (tested in August 2016) as part of a current VLRCOE project at Penn State.

At the workshop, participating CFD turbulence closures ranged from URANS to hybrid URANS/LES approaches in the HELIOS (FUN3D, NSU3D) and OVERFLOW structured/ unstructured solvers as well as commercially-available CFD codes. Prof. Marilyn Smith (Georgia Tech) and Mark Potsdam (Army ADD) coordinated the HELIOS efforts among academia, industry and government; Postdam provided a common HELIOS grid that was applied by most HELIOS participants, with variations in discretization schemes, turbulence models, and boundary conditions. ARL Penn State (James Coder and Norm Foster) focused primarily on simulations with the OVERFLOW code. These included VLC/NRTC-supported work on high-order overset interpolation and new developments in transition and turbulence modeling within OVERFLOW.



HELIOS, a framework that permits complex analyses by the inclusion of structured and unstructured CFD codes with overset capability, was developed by the US Department of Defense as part of the CREATE-AV program with specific emphasis on rotary-wing systems. The unstructured CFD solvers, such as NASA’s FUN3D shown here, permit efficient mesh generation around complex geometries such as helicopter rotor hubs (left), with high-resolution visualization of wake flowfields (right) made possible by available post-processing tools. (Graphics courtesy of Mark Potsdam of Army ADD and Prof. Marilyn Smith of Georgia Tech)



OVERFLOW is a block-structured finite-difference code originally developed by NASA in the 1990s. Its overset capabilities enable simulations of complex aerodynamic shapes such as Space Shuttle launches, full aircraft and helicopter configurations, and helicopter main rotor hubs (left) by dividing complex shapes into grids of overlapping “blocks.” This allows OVERFLOW to produce flow results at high accuracy and to produce high-resolution flowfield visualizations (right). (Graphics courtesy of James Coder and Norm Foster of Penn State)



The “1st Rotor Hub Flow Prediction Workshop” is a successful example of how collaborative efforts between the VLRCOEs at Penn State (Prof. Schmitz) and Georgia Tech (Prof. Smith) has broadened its impact to include additional partners from the rotorcraft community, with focused efforts on validating and further developing the computational tools needed for future high-speed rotorcraft.

Follow-up work is anticipated, and several abstracts were submitted for AHS International’s Forum 73. As discussions and simulations of current data continue, blind comparison runs are planned in 2017 of an “Interactional Aerodynamics Experiment,” sponsored by VLC/NRTC, where an instrumented horizontal tail surface will be installed in the long-age wake of the scaled rotor hub and tested at full-scale Reynolds number.


About the Author

Sven Schmitz is an Associate Professor of Aerospace Engineering at The Pennsylvania State University and a faculty member in the Vertical Lift Research Center of Excellence at Penn State.