Combined Upwind/Downwind Plasma-Based Flow Control on a Vertical-Axis Wind Turbine

Abstract

Dynamic stall poses significant problems on vertical axis wind turbines (VAWTs) since the blades experience both large positive and large negative angles of attack. The resulting unsteady loads imposed on the drive-train and generator are primarily responsible for failures in the field. To control dynamic stall, a feed-forward control mechanism was designed and integrated with high-voltage, pulsed, dielectric barrier discharge (DBD) plasma actuators on a double-bladed VAWT. The actuators are pulsed to exploit flow instabilities in an on/off feed-forward configuration. A mechanical switch controlled a relay which switched between the earthed electrodes of the DBD plasma actuators which were placed on opposite sides of both VAWT blades. This enabled the high-voltage electrodes to generate plasma on alternating sides of the blades depending on which side was predicted to stall. As a result, dynamic stall was partially controlled by alternating actuators during continuous operation of the VAWT. Three types of tests were performed: actuation tests to find the output possible under different actuation conditions; tests to find the effective azimuth range of actuator operation; and transient tests to characterize the system's response. The results suggest that DBD plasma actuators have the potential to reduce unsteady loads and increase VAWT efficiency. NomenclatureA = projected turbine area C P = power coefficient ΔC P = change in power coefficient P C  = change in power coefficient with floating electrode disabled P C  = change in power coefficient with floating electrode not disabled c = chord lengthD = turbine diameter DC p = plasma duty cycle DC T = turbine duty cycle F + = reduced frequency, f mod · c / U rel f p = plasma ionization frequency f mod = modulation frequency H = turbine height N = number of plasma bursts P = turbine power output R = turbine radius Re = Reynolds number, U rel · c / ν RPM = turbine rotation speed in rotations per minute ΔRPM = change in turbine rotation speed, in rotations per minute T = torque applied by turbine T brake = torque applied by brake T mod = period of modulation frequency T p = period of plasma pulsation 1 Graduate Student, Faculty of Mechanical Engineering. 2 Associate Professor, Faculty of Mechanical Engineering, Senior Member. Downloaded by KUNGLIGA TEKNISKA HOGSKOLEN KTH on July 30, 2015 | http://arc.aiaa.org | 2 t = time t on = length of time plasma is on during a half-cycle U i = induced wind speed I U = induced wind velocity U rel = wind speed relative to turbine blade rel U = wind velocity relative to turbine blade U ∞ = freestream wind speed V p2p = peak-to-peak voltage input to actuators α = angle of attack Γ = error fraction θ = blade azimuth angle θ on = blade azimuth angles during which DBD actuators operate θ i = blade azimuth angle during which DBD actuators initiate operation θ t = blade azimuth angle during which DBD actuators terminate operation λ = ratio of blade speed to freestream wind speed (tip speed ratio) τ = time constant ω = turbine rotational s...