Air flow behind wind turbines

Wind turbine wakes have been studied by analysing a large set of atomospheric data, from a wind farm with four turbines sited on a flat coastal area. The results obtained have been generalized to allow tests against data from other full scale wind turbines as well as wind tunnel simulations. These comparisons are found to give very satisfactory results. Most results refer to single turbine wakes. The thrust coefficient, C-T, is found to be a better parameter for description of wake characteristics than wind speed, because it implicitly includes the effect of regulation. It is also found that down wind travel time is more convenient to use than down wind distance in this context. The travel time, t(0), to the end of the near-wake region, i.e. to the point where a single velocity deficit peak first appears, is found to be inversely proportional to the rotational frequency of the turbine and to the turbulence intensity of the ambient air flow and proportional to the ratio of the wake radius and the hub height. For larger travel times, i.e. for the far-wake region, it is found that the centreline relative velocity deficit decreases with the logarithm of the time travelled and is parametrically dependent on t(0) and C-T. The combined wake of two turbines aligned with the wind direction is found to have a lower relative velocity deficit than the wake of a single turbine for the same travel time. This is explained by the inverse relation between t(0) and turbulence intensity, the latter being much higher in the double wake, initially. (C) 1999 Elsevier Science Ltd. All rights reserved.