On the Lack of Enhanced Vertical Mixing near the Ground under the Wake of a Wind Turbine during the 2016 VERTEX Field Campaign

The goal of this paper is to study if and how wind turbines affect local microclimate near the ground, especially air temperature. The literature is highly divided about the ultimate effect on temperature near the ground, with studies suggesting warming, cooling, both, or neither. Despite the controversy, the widely accepted mechanism by which wind turbine wakes may affect the surface has been enhanced vertical mixing. Vertical mixing can be described via turbulent kinetic energy (TKE) and the vertical components of the momentum fluxes. If vertical mixing is enhanced near the ground due to the presence of wind turbine wakes, then all these properties are expected to increase. However, no study to date has conclusively verified such enhancement.

To fill this knowledge gap, the VERTEX (VERTical Enhanced miXing) measurement campaign was conducted in 2016 from late August to early October near a 2-megawatt wind turbine (with a hub height H=80 m and diameter D=90 m) in Lewes, Delaware. Fifteen flux towers were deployed at distances 5D-15D downwind of the wind turbine to measure near-surface properties including wind speed, temperature, and humidity. A 50-m meteorological tower was also instrumented with sonic anemometers and temperature and humidity sensors at 5 different heights (10 m, 25 m, 33 m, 42 m, 49 m). Two scanning lidars were also deployed to locate and characterize the wind turbine wake by sequential scans with either fixed azimuth angle (RHI) or fixed elevation angle (PPI) with a scanning period of 30 minutes.

A wake detection algorithm was applied to the PPI scans to detect the location of the wake and thus determine which stations are under the wake for each scanning period. The atmospheric stability during each wake case was determined based on the Obukhov length L at a far-field flux station that is unaffected by the wake. Fifteen wake cases were identified, two of which had missing data issues. The remaining 13 were classified as follows: nine neutral, one unstable, and three stable. To assess vertical mixing, the differences in TKE, vertical momentum flux, and heat flux between the station under the wake and a nearby station unaffected by the wake were calculated and compared to the same differences during a reference period when neither stations were under the wake and the stability was the same.

The results indicate a general lack of enhancement of vertical mixing near the ground at the sites under the wake, with many cases suggesting a decrease of it. All nine neutral cases showed a very consistent reduction of both TKE and vertical momentum fluxes. Because of the high number of neutral cases, the confidence in these findings is high. For unstable conditions, the only case shows again a decrease of both TKE and vertical momentum fluxes. For stable conditions, all three cases show a reduction of all vertical momentum fluxes, but for TKE the cases are not consistent, with one showing a decrease, another showing no change, and the third having an increase. However, this third case shows an increase in TKE while at the same time a decrease in the vertical flux of vertical momentum (w’w’), which is the only TKE term that is linked to vertical mixing (the other two TKE terms are u’u’ and v’v’ and relate to horizontal turbulent mixing). We therefore propose that, even in stable conditions, vertical mixing is reduced or unchanged and that any TKE increase is only caused by enhanced horizontal, not vertical, mixing.

The key to explaining the reduction in vertical mixing near the ground due to wind turbine wakes lays in the difference between the wind shear profiles upstream and downstream of a wind turbine. The wind turbine blades extract kinetic energy from the flow, which causes a wind speed deficit over the rotor area that propagates downwind with the wake. The wind speed deficit acts as a momentum sink and ‘sucks in’ the higher momentum from below the wake, reducing the wind shear in the region between the ground and the bottom-tips of the rotor compared to the same region upstream before the turbine. The reduction of wind shear will cause a reduction of TKE production and weaker vertical momentum fluxes. Thus, vertical mixing is reduced.