Pressure based active load control of a blade in dynamic stall conditions

Abstract

A proportionate controller is investigated experimentally for unsteady load alleviation purposes on a 2D wing model with a trailing edge flap. This study is supposed to be a simplified version of a Smart Rotor, a mechanism which deals with the reduction of unsteady loads acting on a wind turbine and aims to understand the behavior of this type of control under dynamic stall conditions. The controller acts on the velocity of the flaps, and pressure sensors are used to detect the unsteady loads, which are generated by actuating the wing model in a sinusoidal motion. Two different regimes are considered: attached flow (oscillations around zero angle of attack, where little flow separation is expected) and dynamic stall (oscillations close to the static stall angle). The influence of actuation frequency and controller time lag is also studied, as these parameters were found to be crucial for such a concept during the literature review. A reduction of 87.5% in the standard deviation of the lift is obtained for a frequency of 0.2Hz and time lag in the control system of 12ms for attached flow conditions. The reduction of the standard deviation of the lift deteriorates for increased frequency and time lag. The proposed controller is also able to reduce the loads during dynamic stall, although the reduction is smaller, close to 40% in the reduction of the lift coefficient. A maximum reduction of 59% in the peak-to-peak loads experienced during dynamic stall is observed, for 0.2Hz and a time lag of 12ms. Time lag is again crucial for this reduction, and larger and faster actuation systems are needed to fully compensate for the larger drop in loads. However, the active load control of the loads during dynamic stall can negatively affect the aerodynamic damping of the model, since the flap actuation is dependent on the detachment of the leading-edge vortex from the surface of the wing. The flap actuation is also shown to delay the onset of dynamic stall, by increasing the static stall angle with respect to the case without flap deflection.