Development of a method for obtaining local inflow angle from pressure gradient at leading edge on operating wind turbine blades

Full-scale on-site wind turbine measurements are necessary to fully understand the complex aerodynamics of the rotor blades in atmospheric flow conditions (J.G. Schepers and S.J. Schreck 2019). In particular, measuring local blade aerodynamic pressure distributions, vibrations and noise emissions would make it possible to better optimise the design of large wind turbine blades as well as to prevent damages and failure by active monitoring. Published measurements on operating rotor blades in real conditions are very scarce, due to the complexity of installation and use of the measurement system (Wu et al. 2019; Medina et al. 2011; Madsen et al. 2016). However, recent developments in electronics, wireless communication, and MEMS sensors are making it possible to acquire data in a cost-effective and energy-efficient way (Mayer et al. 2019; Niklaus et al. 2017). Therefore, a cost-effective MEMS-based smart measurement system that is thin, non-intrusive, easy to install, low power, self-sustaining and wirelessly transmitting is being developed in the Aerosense project.
For full-scale measurements, one of the most important and complicated tasks is the assessment of the incoming flow to the blade. In particular, the complex aerodynamic behaviour of rotating blades makes it challenging to define the angle of attack and the
relative wind speed. In previous work, these parameters have been measured using probes positioned at the leading-edge of the blades and a reference pressure measured in the hub or far upstream, using long tubes (Medina et al. 2011; Troldborg et al. 2013; Wu et al. 2019). As the Aerosense system will be self-sustaining and non-intrusive, the angle of attack and incoming flow velocity cannot be obtained in this manner.
In this work, a method for deducing the local angle of attack using the measured pressure gradient near the leading-edge is investigated. The pressure gradient varies depending on the incoming flow velocity and the angle of attack (or the position of the stagnation point). As the gradient of pressure is only necessary and not the true pressure values (Callegari et al. 2006), four differential pressure sensors are positioned in the first 25% of the chord on one side of the aerofoil with the same pressure reference taken at quarter chord. Differential pressure sensors make it possible to acquire more sensitive values than absolute pressure sensors, and to be able to detect small variations of angle of attack. The measured variations of pressure are fed into an algorithm based on a potential flow model passing a parabola (Saini and Gopalarathnam 2018) to estimate the angle of attack and the incoming flow velocity without the need of external measurements and reference pressure.
As a proof of concept, first tests are carried out in the wind tunnel of ETH Zurich on a NACA0018 aerofoil in static conditions for a range of Reynold's numbers between 1.5x10⁵ and 9x10⁵. Differential pressure sensors are attached to the blade with custom-made
electronics and housings. For comparison purposes, 40 conventional pressure taps linked to a pressure scanner measure the pressure distribution along the chord. Initial results show that the pressure gradient shifted by the reference pressure follows the pressure distribution from reference pressure taps and therefore have high potential to be used for the above method. This will be discussed in more detail in the presentation.