The aerodynamic characteristics of high–lift airfoil designs is of interest for improved performance and reduced blade count in Low–Pressure Turbine (LPT) design. The present paper presents both experimental measurements as well as numerical simulation results from a single-stage LPT. The airfoils were designed for an embedded stage with a total pressure expansion ratio of 1.75 and a rotor Zweifel coefficient of 1.35. The measurement program was highly unique in that detailed measurements were obtained using a variety of different probe types, including time–resolved total pressure and hot–wires. Agreement between various measurement types was generally good, but differences beyond typically stated uncertainty bounds were noted. The computations were done using RANS and a mixing model via commercially available software. The numerical results were evaluated to determine the efficacy of this type of model for prediction and design of high–lift airfoils. The computations agreed very well with the experimental results in the midspan region, but losses were over–predicted in the lower 40% span near the hub. A basic description and understanding of the flow physics in the LPT stage are presented based on the relative agreement between the experiments and computations.

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.

Detailed measurements of total temperature and velocity in high-speed, rotating turbomachinery flows are difficult due to the high sensor frequency response required. Constant temperature hot-wires have the needed frequency response, but are sensitive to both Tt and effective wire ρU which vary significantly in high speed flows. The multiple overheat method can be used to decouple the phase locked averages of Tt and wire effective ρU from the phase locked average hot-wire voltages. The present study will demonstrate the efficacy of this technique as well as a detailed uncertainty analysis of the multiple overheat method. The overall uncertainty is primarily determined by: the quality of the hot-wire calibration, the frequency response of the constant temperature hot-wire system, and the ability of the turbomachinery facility to maintain a constant operating point while the hot-wire overheat values are changed. As a proof of concept, a single constant temperature hot-wire was operated at four different overheats at the exit of a transonic axial compressor rotor. The phase locked average Tt and wire effective ρU fields show flow features relative to the rotor including blade wakes and tip clearance flows. These phase locked average measurements were validated against each other.

In order to extend the boundaries of helicopter performance and increase forward-flight speed, it is necessary to reduce the drag on the rotor hub, which can account for as much as 30% of the total parasite drag on the helicopter. Currently, there is limited experimental data available to predict the drag force on new hub configurations. The purpose of this testing is to create a database of lift and drag at various angles of attack to aid in hub design and hub drag prediction. Testing was conducted in the 12 inch-diameter water tunnel at ARL Penn State on four shapes - DBLN 526, 4:1 Ellipse, 3.25:1 Rectangle, and a new Optimized Cambered Shape (OCS) designed at UT Knoxville. Load cell data for lift and drag were obtained for angles of attack from approximately -5 degrees to 5 degrees. Drag data were also calculated using PIV velocity fields. Results are plotted and tabulated for use in future hub drag prediction toolsets.