Leading-edge Erosion Study (LEES) Project
Airfoil Performance Degradation due to Roughness and Leading-edge Erosion
Wind farms often underperform predicted power output by 10 to 30 percent relative to manufacturer predictions. A potential aerodynamic explanation is that blade roughness caused by insect impingement and leading-edge erosion decreases lift and drag as opposed to “clean” blades. These effects are difficult to test in the field because aerodynamic performance cannot be measured directly and can be affected by many factors that cannot be controlled in field experiments. This project provides aerodynamic performance data using wind tunnel measurements of representative inboard and outboard blade sections contaminated with various types and levels of roughness and leading-edge erosion. Results include aerodynamic load coefficients and measurements of laminar-to-turbulent transition location as functions of Reynolds number and angle of attack for various roughness configurations.
Measurements of section lift, drag, and pitch moment, as well as laminar-to-turbulent boundary layer transition locations, are made on two airfoils at various angles of attack, Reynolds numbers, and roughness contamination conditions. The airfoils include an 18-percent-thick NACA 63(3)-418 and 24-percent-thick NREL S814, which respectively represent the blade tip and inboard regions. Chord-based Reynolds numbers from 1.6 to 4.0e6 are tested. Randomly distributed additive roughness, characteristic of insect carcasses, are added to each airfoil. Surface area coverage between 3 and 15 percent and several roughness heights are tested. Measurements at the same conditions using trip tape also are made to assess the extent to which trip tape captures distributed roughness effects. In addition to additive roughness, the NACA 63(3)-418 includes a modular leading edge to enable testing of an eroded leading edge manufactured to match a field observation. The data in this repository include drag polars and boundary layer transition limits at various tested conditions. The intent for these data is to enable prediction of “typical” performance loss from roughness accumulation and leading-edge erosion. In addition, the laminar-turbulent transition locations are intended to aid validation of numerical simulations that include transition location as a function of operating condition and blade roughness state.
Measurements are made in the Oran W. Nicks Low-Speed Wind Tunnel (LSWT) located at Texas A&M University in College Station, Texas. The LSWT is a 2.1 m x 3.0 m wind tunnel with a maximum speed of 90 m/s. The airfoil models are mounted vertically. Both have an 813 mm chord length and span the tunnel height (3050 mm) with an approximately 12 mm gap at each wall. Various angles of attack are achieved by rotating the model using the LSWT floor turntable.
Lift and pitch moment measurements are made using surface pressure taps. Drag measurements employ a wake rake that measures the dynamic pressure profile approximately 730 mm downstream of the airfoil trailing edge. Load and angle-of-attack data are corrected for wind tunnel wall and blockage effects. Boundary layer transition location is measured using infrared (IR) thermography. Heat transfer from the air to the airfoil models is higher in turbulent regions, resulting in somewhat warmer surface temperatures where the flow is turbulent. IR images reveal different surface temperatures, which are analyzed to give the transition location.
- Geometrical angle of attack [degrees] (angle between airfoil chord line and nominal wind tunnel flow direction).
- Corrected angle of attack [degrees] (angle of attack accounting for wind tunnel wall effects).
- Sectional lift, drag, and pitch moment coefficients, cL = L/q0c, cD = D/q0c, and cM = M/q0c^2, respectively, where q0 is the reference dynamic pressure and c is the airfoil chord. cM is measured about c/4. Coefficients are corrected for wind tunnel wall and blockage effects.
- Pressure coefficients Cp = (p-p,ref)/q, relative to airfoil surface location (x/c,y/c). Pressure coefficients are not corrected for wind tunnel wall and blockage effects.
- Wake dynamic pressure profiles, q/q0.
- Boundary layer transition location on the upper (suction) and lower (pressure) airfoil surfaces as x/c.
- Infrared airfoil images as .jpg-format image files.