Effect of blade aspect ratio on the performance tradeoff between figure of merit and bending-torsion dynamics of wind turbines with synthetic jets

Abstract

In this experimental study, two three-bladed rotors with a blade aspect ratio of 7.79 and 9.74 representing a 25% increase were tested. Each blade contains an S809 airfoil with zero pre-built twist and 20 high-performance synthetic jet actuators distributed along the span. The rotor was tested at four rotor speeds, Ω of 250, 500, 750, and 1,000 revolutions per minute (RPM) and three collective blade pitch angles, θc of 2, 5, and 8 degrees. Rotor thrust was measured using a high-capacity load cell, and blade bending and torsion was measured at four equally spaced radial locations on the blade defined by r/R= 0.24, 0.48, 0.72, and 0.96 using four onboard inertial measurement units. The spanwise flow over the suction surface of the blade was measured using laser Doppler velocimetry (LDV) techniques. The local Reynolds number produced with the test rotor speeds at the measurement locations ranges from 8.31 × 104≤Re∞≤ 9.49 × 105. It was found that rotor aerodynamic efficiency as quantified by the figure of merit is directly linked to blade aspect ratio and the magnitude of spanwise flow over the blade. The rotor with a higher blade aspect ratio produces less overall spanwise flow and is more aerodynamically efficient, however the blade also generates larger mean and fluctuating bending and torsion displacements, which are undesirable for aeroelastic stability. The use of synthetic jets on the blade mitigates the root mean square of the bending and pitch displacements above Ω= 250 RPM and θc= 2°. It was found that at Ω= 1,000 RPM, the blade structures undergo a second mode bending response indicating a condition of structural instability due to higher aerodynamic loading. At this rotor speed, flow control on the outboard of the blade reduces unsteady bending as quantified by arms up to a maximum of 36.5%, and unsteady torsion, θrms up to 22.9% compared to the baseline for the rotor with an aspect ratio of 9.74.