Field experiment and analysis of the wake behind a horizontal-axis wind turbine

Abstract

Wind turbines work in a complex natural environment, where their flow field shows the characteristics of high turbulence levels. Due to the complexity and instability of the atmospheric boundary layer flow, as well as the influence of the scale effect, wind tunnel test cannot reflect the real working condition of wind turbine. So field experiments are very necessary to study wind turbine flow characteristics in actual wind field. In this investigation, field experiments of the wake behind a wind turbine were carried out to get velocity distribution in it. The field experiment system is composed of a 33 kW wind turbine, a 20 m high anemometer tower, a wake velocity measurement platform and the controlling device. The wind turbine is horizontal-axial, two-bladed, upwind-type, with variable pitch angles, 14.8 m in diameter. Wake velocity measurement platform is composed of an 18 m high hydraulic lift, anemometer installation platform and three sets of the US CSAT3 three-dimensional ultrasonic anemometer. Inflow parameters were gauged, such as wind speed, wind direction, atmospheric pressure, temperature and humidity. Besides, the operating conditions of the wind turbine were recorded, including pitch angle, yaw angle, rotor speed. It is discussed that the time domain and power spectrum characteristics of wake velocity at a measuring point located at one rotor diameter downwind from the rotor plane. The results show that there are larger velocity deficits in the wake. The axial velocity deficit rate at the measuring point is between 35.1% and 54.17%. The change of the velocity in vertical direction is small. The velocity in lateral direction is slightly larger than the velocity component of inflow, which reflects the expansion characteristics of the wake. Besides, the turbulent kinetic energy at the measuring point shows a periodic variation, and the vortex sheet passage frequency is similar to the rotation frequency of the wind turbine. Meanwhile, all of the power spectra of turbulent velocity in three directions at the measuring point show a characteristic of slope for –1 at the section of low frequency, which mean the turbulent flow is in the classical production subrange.