Measurements on a wind turbine wake: 3D effects and bluff body vortex shedding

TLDR

The wake velocity field of a two‑bladed wind turbine (180 mm diameter) was investigated using two‑component hot‑wire measurements, confirming that the turbine behaves as a bluff body shedding structures. All three velocity components were measured for a rotor aligned with and yawed (0°–20°) relative to the freestream, and the data were analyzed with two‑point cross‑correlations. The measurements revealed a wake rotation opposite to the turbine, a yaw‑induced lateral deflection, and a low‑frequency fluctuation that appears only at high tip‑speed ratios and varies with yaw, with the Strouhal number of this fluctuation being independent of freestream velocity or turbulence. © 2005 John Wiley & Sons, Ltd.

Abstract

Abstract The velocity field in the wake of a two‐bladed wind turbine model (diameter 180 mm) has been studied under different conditions using a two‐component hot wire. All three velocity components were measured both for the turbine rotor normal to the oncoming flow as well as with the turbine inclined to the freestream direction (the yaw angle was varied from 0° to 20°). The measurements showed, as expected, a wake rotation in the opposite direction to that of the turbine. A yawed turbine is found to clearly deflect the wake flow to the side, showing the potential of controlling the wake by yawing the turbine. An unexpected feature of the flow was that spectra from the time signals showed the appearance of a low‐frequency fluctuation both in the wake and in the flow outside the wake. This fluctuation was found both with and without freestream turbulence and also with a yawed turbine. The frequency expressed as a Strouhal number was shown to be independent of the freestream velocity or turbulence level, but the low frequency was only observed when the tip speed ratio (or equivalently the drag coefficient) was high. The shedding frequency changed also with the yaw angle. This is in agreement with the idea that the turbine sheds structures as a bluff body. The phenomenon, noticeable in all the velocity components, was further investigated using two‐point cross‐correlations of the velocity signals. Copyright © 2005 John Wiley & Sons, Ltd.