Coriolis effects on wind turbine wakes across atmospheric boundary layer regimes

TL;DR

This study uses large eddy simulations to analyze how Coriolis effects influence wind turbine wake deflections across different atmospheric boundary layer regimes, revealing a transition from clockwise to anti-clockwise deflections as Rossby number decreases.

Contribution

It provides the first detailed analysis of Coriolis effects on wind turbine wakes across a wide range of Rossby numbers using large eddy simulations.

Findings

Coriolis effects cause significant wake deflections.

Wake deflections switch from clockwise to anti-clockwise with decreasing Rossby number.

Coriolis-induced deflections are comparable to yaw-misalignment effects.

Abstract

Wind turbines operate in the atmospheric boundary layer (ABL), where Coriolis effects are present. As wind turbines with larger rotor diameters are deployed, the wake structures that they create in the ABL also increase in length. Contemporary utility-scale wind turbines operate at rotor diameter-based Rossby numbers, the nondimensional ratio between inertial and Coriolis forces, of O(100) where Coriolis effects become increasingly relevant. Coriolis forces provide a direct forcing on the wake, but also affect the ABL base flow, which indirectly influences wake evolution. These effects may constructively or destructively interfere because both the magnitude and sign of the direct and indirect Coriolis effects depend on the Rossby number, turbulence, and buoyancy effects in the ABL. Using large eddy simulations, we investigate wake evolution over a wide range of Rossby numbers relevant to offshore wind turbines. Through an analysis of the streamwise and lateral momentum budgets, we show that Coriolis effects have a small impact on the wake recovery rate, but Coriolis effects induce significant wake deflections which can be parsed into two regimes. For high Rossby numbers (weak Coriolis forcing), wakes deflect clockwise in the northern hemisphere. By contrast, for low Rossby numbers (strong Coriolis forcing), wakes deflect anti-clockwise. Decreasing the Rossby number results in increasingly anti-clockwise wake deflections. The transition point between clockwise and anti-clockwise deflection depends on the direct Coriolis forcing, pressure gradients, and turbulent fluxes in the wake. At a Rossby number of 125, Coriolis deflections are comparable to wake deflections induced by 17{\deg} of yaw-misalignment.