Numerical and experimental investigations of HAWT near wake predictions using Particle Image Velocimetry and Actuator Disk Method

Abstract

The wake flow characteristics downstream from the rotor blade of small Horizontal Axis Wind Turbine (HAWT) have been investigated using Particle Image Velocimetry and an Actuator Disk Method developed in OpenFOAM at low Reynolds number of Re ≈ 47 000 for four TSR values of 6, 5.5, 5 and 4.5. The wind tunnel measured mean velocity distributions along the rotor radius, for two TSRs of 5.5 and 4.5 corresponding respectively to the optimum operating point of the HAWT and the rotational augmentation effect appearance, have been compared quantitatively at five downstream planes of 0.225D, 0.45D, 0.9D, 1.8D and 2.4D in order to investigate the near wake evolution and the nacelle effect on the flow field characteristics. At a distance of 0.225D downstream from the rotor mostly affected by the rotor blade predicted performances, the 3D corrections for stall delay phenomenon consideration have provided noticeable improvements for the predicted velocity field near the root of the rotor blade especially the axial and tangential velocity components. However, by getting further from the rotor i.e. at a distance of 2.4D, the flow field is mostly affected by the nacelle and the velocity field has loosed its structured distribution which has been explained as the wake expansion occurrence. The near wake upper limit has been located at a distance of 1.8<ZD≤2.4 downstream from the rotor. Also, the turbulent kinetic energy and the specific turbulence dissipation fields have been investigated qualitatively where it has been demonstrated that the rotor blade tip is the principal element causing the turbulence production in contrary to the nacelle which causes the turbulence dissipation.