Wind Tunnel Simulation of Wind Turbine Wakes in Neutral, Stable and Unstable Offshore Atmospheric Boundary Layers

Relatively little is known about wind turbine wake development in stable and convective wind flow. From the UpWind [1] study wind conditions in areas of the North Sea are significantly non-neutral for about 70 percent of the time. The stable atmospheric boundary layer is particularly complex, and if the stability is sufficiently strong the boundary layer depth is comparable with the tip height of large wind turbines. The 'imposed' condition, the potential temperature gradient above the boundary layer, is therefore important. In a first phase of work, measurements have been made in the wake of a model wind turbine in neutral, stable and unstable offshore atmospheric boundary layer simulations, the neutral case being the reference case. The stable case is for weak surface-layer stability, but typical strong imposed stability above. The wake deficit decreases more slowly, consistent with the lower level of boundary layer turbulence, but after about 3 rotor diameters it ceases growing in height as a direct consequence of the imposed stability. Temperature and heat transfer are increased in the wake, the latter though only from about 3 diameters. The unstable cases show more rapid reduction of the velocity deficit, and higher levels of turbulence. The greater depth of the unstable layer means that conditions above are much less important.