Air flow behind wind turbines

Wind turbine wakes have been studied by analysing a large set of atomospher ic 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 c omparisons are found to give very satisfactory results. Most results refer to single turbine wakes. The thrust coefficient, C-T, is found to be a bett er parameter for description of wake characteristics than wind speed, becau se it implicitly includes the effect of regulation. It is also found that d own wind travel time is more convenient to use than down wind distance in t his 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 an d to the turbulence intensity of the ambient air flow and proportional to t he 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 veloci ty deficit decreases with the logarithm of the time travelled and is parame trically dependent on t(0) and C-T. The combined wake of two turbines align ed with the wind direction is found to have a lower relative velocity defic it than the wake of a single turbine for the same travel time. This is expl ained by the inverse relation between t(0) and turbulence intensity, the la tter being much higher in the double wake, initially. (C) 1999 Elsevier Sci ence Ltd. All rights reserved.