Exploring atmospheric stability effects on wind turbine wakes utilizing a small-scale wind turbine in a field environment

Abstract

A small-scale model wind turbine (diameter of 1.17m, hub height of 6.25m) was studied in a field experiment wherein turbine wake measurements were taken with the intent of studying the effects of atmospheric stability on the turbine wake. Flow profiles of the site inflow and the turbine wake were made with two meteorological towers outfitted with rakes of sonic anemometers. By using a movable model-scale turbine, it was possible to characterize the wake at 1.5, 3, 6, and 12 diameters downstream of the rotor.

The persistence of the wake (and subsequent reduction in power production by turbines) during stable conditions has thus far been attributed to a lower amount of kinetic energy, k, in the atmosphere, which results in less turbulent momentum transport. Some consideration, however, should be given to the contributions of density differences (buoyancy jumps) across the wake interface. It is demonstrated that buoyancy jumps will have an adverse effect on momentum transport during a stably-stratified atmosphere, thus causing the velocity deficit within the wake to persist for a longer downstream distance when compared to a neutrally-stratified atmosphere. It is furthermore evident that buoyancy jumps create buoyancy forces which have a greater magnitude than those contained within the freestream. It is shown analytically that the buoyancy jumps will vary according to turbine size and lateral (spanwise) position within the wake. In the case of the small-scale turbine, the swirling motion of the wake is found to be the cause of mixing within the wake, which creates the buoyancy jumps, though tip and hub vortices may also play a part. The variation of entrainment rate with stability is also investigated.

Additionally, the spectra of the horizontal and vertical components of the wind velocity within and above a wind-turbine wake, at various downstream distances, is explored. It is shown that the behavior of the spectra varies depending on ABL stability conditions; specifically, the spectral energy of fluctuations within the wake indicate that small-scale motions have more energy contained within them when the atmosphere is unstable, and the majority of the small-scale motion energy is injected into the wake by the tip vortices. The effect of stability on heat fluxes within the wake is shown, providing evidence that the high-frequency scales have large amounts of upward heat flux in the stable and neutral cases when compared to the convective condition, which sees very little heat flux at small scales.