The implications of atmospheric gravity waves for wind farm and turbine design

Understanding how wind turbines interact with large-scale atmospheric phenomena is an increasingly important issue for wind farm developers. With the latest 15 MW turbines reaching heights of 270 m, several studies have predicted that farms of these turbines will be able to induce Atmospheric Gravity Waves (AGWs). These buoyancy-driven waves are triggered as a result of a farm vertically deflecting thermally stratified flow above the turbine array. In particular, as the temperature inversion above the North Sea is typically located at heights near the top of a 15 MW turbine, farms in this region may be especially susceptible to generating AGWs. Hence, with most European offshore wind farms based in the North Sea, understanding the interactions with, and impact of, these waves is vital for yield prediction.

Recent studies have shown that AGWs cause a redistribution of flow at the farm scale, altering wind farm power production. As a result, AGWs provide a new challenge for wind farm planning and raise questions about whether farm design can influence how the AGW is triggered, and if these AGWs also have impacts at the turbine scale. To address these questions, large eddy simulations using actuator line turbine representations were undertaken. These simulations replicated the typical atmospheric turbulence, Coriolis force and thermal parameters seen in the North Sea.

First, the middle turbine of an infinitely wide row was simulated. The turbine triggered an AGW, and the flow field was compared to a wave-free case. The AGW was found to cause upstream flow deceleration, accelerate bypass flow above the turbine wake, and cause pockets of acceleration within the wake itself at AGW troughs. Overall, this led to faster wake recovery than in a wave-free case.

Following this, simulations were conducted using two turbines aligned in the streamwise direction, each representative of the middle turbine in an infinitely wide row. Introducing a second turbine triggered stronger AGWs, magnifying their effects on the flow. Furthermore, by varying the position of the downstream turbine, it was possible to both amplify and dampen the AGW produced, along with causing a shift in the wave phase. The power of the second turbine was found to vary sinusoidally with the change in turbine position. When in line with an AGW trough, the second turbine even outperformed the first despite sitting in its wake. However, the increased power came at the cost of a higher mean blade loading and an increase in cyclic loading.

This work demonstrates that AGWs can impact intra-farm flows and turbine performance. Additionally, it confirms an interdependence between AGWs and wind farm turbine spacing. Given the variation in the AGW with spacing, it may become an important factor in design which considers both intra-farm and farm-to-farm scale flows.