The dynamic stall development on a pitching airfoil at Re = 10⁶ was investigated by time-resolved surface pressure and velocity field measurements. Two stages were identified in the dynamic stall development based on the shear layer evolution. In the first stage, the flow detaches from the trailing edge and the separation point moves gradually upstream. The second stage is characterized by the roll up of the shear layer into a large scale dynamic stall vortex. The two-stage dynamic stall development was independently confirmed by global velocity field and local surface pressure measurements around the leading edge. The leading edge pressure signals were combined into a single leading edge suction parameter. We developed an improved model of the leading edge suction parameter based on thin airfoil theory that links the evolution of the leading edge suction and the shear layer growth during stall development. The shear layer development leads to a change in the effective camber and the effective angle of attack. By taking into account this twofold influence, the model accurately predicts the value and timing of the maximum leading edge suction on a pitching airfoil. The evolution of the experimentally obtained leading edge suction was further analyzed for various sinusoidal motions revealing an increase in the critical value of the leading edge suction parameter with increasing pitch unsteadiness. The characteristic dynamic stall delay decreases with increasing unsteadiness, and the dynamic stall onset is best assessed by critical values of the circulation and the shear layer height which are motion independent.

As the wind energy industry is maturing and wind turbines are becoming larger, there is an increasing need for cost-effective monitoring and data analysis solutions to understand the complex aerodynamic and acoustic behaviour of the flexible blades. Published measurements on operating rotor blades in real conditions are very scarce due to the complexity of the installation and use of measurement systems. However, recent developments in electronics, wireless communication and MEMS (micro-electromechanical systems) sensors are making it possible to acquire data in a cost-effective and energy-efficient way. In this work, therefore, a cost-effective MEMS-based aerodynamic and acoustic wireless measurement system that is thin, non-intrusive, easy to install, low power and self-sustaining is designed and tested in a wind tunnel. The measurement system does not require an electrical connection to the wind turbine and can be mounted and removed without damaging the blade.The results show that the system is capable of delivering relevant results continuously, although work needs to be done on calibrating and correcting the pressure signals as well as on refining the concept for the attachment sleeve for weather protection in the field. Finally, two methods for using the measurements to provide added value to the wind energy industry are developed and demonstrated: (1) inferring the local angle of attack via stagnation point detection using differential pressure sensors near the leading edge and (2) detecting and classifying leading edge erosion using instantaneous snapshots of the measured pressure fields. Ongoing work involves field tests on a 6 kW operating wind turbine in Switzerland.

The flow around a pitching NACA0015 airfoil with an oscillating trailing-edge flap is experimentally investigated to characterize the influence of the flap kinematics on the development of dynamic ...

We present an experimental study of the aerodynamic forces on a thick and cambered airfoil at a high Reynolds number (Re) ($3.6\times {10}^6$), which is of direct relevance for wind turbine design. Unlike thin airfoils at low chord-based Reynolds numbers, no consistent description currently exists for the stall process on such airfoils. We consider two chordwise rows of instantaneous wall pressure measurements, taken simultaneously at two spanwise locations over a range of angles of attack. We show that around maximum lift conditions, a strong asymmetry is observed in the statistics of the normal force on each chord. In this range of angles of attacks, the pressure fluctuations are largest in the adverse pressure gradient region, and the fluctuation peak along the chord is systematically located directly upstream of the mean steady separation point, indicating intermittent flow separation. Moreover, the fluctuations are characterized by bistability in both space and time: For each spanwise location, large excursions of the local wall pressure between two different levels can be observed in time (jumps), and these excursions are highly anticorrelated between the two spanwise locations (spatial switches). The characteristic timescale for the switches is found to be well correlated with the amplitudes of the fluctuations. Application of proper orthogonal decomposition (POD) analysis to each row of sensors confirms that the flow separation is an inherently local, three-dimensional, and unsteady process that occurs in a continuous manner when the angle of attack increases. The correlation between the dominant POD mode amplitudes is found to be a good indicator of bistability. For all angles of attack, most of the fluctuations can be captured with the two most energetic POD modes. This suggests that force fluctuations near the maximal lift could be modelled by a low-order approach for monitoring and control purposes.