Aspects of the Dynamic Response of a Small Wind Turbine Blade in Highly Turbulent Flow: Part 1 Measured Blade Response

Abstract

This paper presents some dynamic measurements acquired from the surface of a composite constructed test blade on an operating 400W three-bladed wind turbine. The structural design of the composite blade was modified from the original design to reduce its stiffness in flapwise bending. Signals from 10 strain gauges attached to the blade were acquired every 45° in the azimuthal domain and at a fixed rate of 140Hz in the time domain. The test turbine was located at a highly turbulent site with the turbulence intensity between 0.3 and 0.5 at all wind speeds. Signals from the gauges show the cyclic response of the blade was dominated by the 1P frequency with influences from 2P, 3P and 4P frequencies increasing with the radial position of the gauge on the blade. Cyclic variation in the root bending moment was nearly sinusoidal, with a small plateau in the peak possibly due to the effects of large yaw angle on the blade bending moment. The blade's response to yaw was investigated for both normal yaw events, i.e. those experienced by the turbine many times during a typical day, and for extreme yaw events, i.e. which occurred infrequently. During a normal yaw event with a yaw rate of 70°/s, the cyclic variation in blade strain near the hub connection increased from 210 μstrain¹ before the event to 1500 μstrain at the peak of the event. An extreme yaw event produced a yaw rate of 160°/s with the cyclic variation in strain increasing from 200 μstrain to 2600 μstrain.