LES Modelling of Highly Transient Wind Speed Ramps in Wind Farms

Large eddy simulations of wind farms are often performed with canonical atmospheric conditions, where the background flow is based on precursor simulations with idealized model setups yielding statistically stationary turbulent flows. However, precursor simulations can only handle gradually changing flow conditions, and are not capable of modelling highly transient and statistically non-stationary flows, e.g. frontal passages or large gusts. Such flows frequently occur in nature, and can influence the operation—and potentially design—of wind turbines. It is generally not possible to impose non-stationary features through inlet boundary conditions, if the imposed flow violates the most fundamental assumption of micro-scale flow simulations, namely conservation of mass. This work presents a method for modelling highly transient wind speed ramps by extending and adapting the method of applying body forces to achieve specific flow scenarios, where the wind speed ramps are embedded as constrained turbulent boxes. Several scenarios with significant increases in the streamwise wind speed are simulated. Analyses of the transient wake dynamics, as the wind speed ramps propagate through large wind farms are performed to show how well the momentum is maintained throughout the numerical domain and the influence and operation of turbines during the ramp passages.