Application of Fast Pressure-Sensitive Paint to an Oscillating Wind Turbine Airfoil

In the unsteady flow environment experienced by wind turbine blades, large excursions in local angle of attack and significant three-dimensional flows arise that complicate the prediction of stall onset and unsteady loading. Accurate predictions of the dynamic loads are critical to the pursuit of lightweight, reliable structures to lower the cost of wind energy (Ref. 1). To this end, diagnostic measurement tools capable of fine spatial resolution and high frequency response are important for better understanding unsteady aerodynamic effects on blade pressure distribution. The sparseness of conventional pressure transducers has historically provided motivation for the development of pressure-sensitive paint (PSP), an optical surface pressure measurement technique with inherently fine spatial resolution. Within the last decade, the bandwidth of certain paint formulations has been significantly increased to resolve unsteady flows; a flat frequency response on the order of several kHz is now readily achievable (Ref. 2). PSP consists of luminescent molecules adhered to the test surface by a thin binder layer, typically on the order of 10 microns. The luminophore responds to the local partial pressure of oxygen, which is directly proportional to absolute air pressure. An illumination source such as a light-emitting diode or expanded laser beam excites the luminophore, and the emitted light intensity captured by a scientific-grade camera is converted to absolute pressure via calibration. Each point on the painted surface thus responds as a molecular-sized transducer, with spatial resolution limited by the camera pixel size. Recent advancements in unsteady PSP data acquisition and processing techniques have been developed for rotating blades to account for errors caused by model movement and deformation in nonuniform illumination fields. The single-shot lifetime technique (Ref. 3) and motion capturing technique (Ref. 4) are two methods which have arisen. Due to its straightforward procedure and commonality of required hardware, the single-shot technique has been used across a range of small test facilities (Ref. 5, 6) and large-scale wind tunnels (Ref. 7). PSP measurements have been historically limited to the compressible flow regime to achieve sufficient signal-to-noise ratio in the data images (Ref. 8, 9). Recent wind tunnel testing with unsteady PSP has been geared toward helicopter applications (Ref. 10, 11), although temperature error due to compressibility effects has been noted. Under isothermal conditions or with an accurate temperature correction available, laser-based excitation and highly reflective paints can enable instantaneous PSP measurements at Mach numbers near M ≈ 0.15, corresponding to dynamic pressures of approximately 1.5 kPa (Ref. 6, 12). Unsteady aerodynamic effects can result in even larger pressure differences, considering that peak suction levels on oscillating airfoils can reach factors of the free stream dynamic pressure (Ref. 13). With the present availability of suitable paints, the above considerations present an opportunity for PSP to be deployed as a measurement tool for resolving the global pressure distribution on wind turbine blades.