  The current study uses large eddy simulations to investigate the transient
response of a utility-scale wind turbine wake to dynamic changes in atmospheric
and operational conditions, as observed in previous field-scale measurements.
Most wind turbine wake investigations assume quasi-steady conditions, but real
wind turbines operate in a highly stochastic atmosphere, and their operation
(e.g., blade pitch, yaw angle) changes constantly in response. Furthermore,
dynamic control strategies have been recently proposed to optimize wind farm
power generation and longevity. Therefore, improved understanding of dynamic
wake behaviors is essential. First, changes in blade pitch are investigated and
the wake expansion response is found to display hysteresis as a result of flow
inertia. The timescales of the wake response to different pitch rates are
quantified. Next, changes in wind direction with different timescales are
explored. Under short timescales, the wake deflection is in the opposite
direction of that observed under quasi-steady conditions. Finally, yaw changes
are implemented at different rates, and the maximum inverse wake deflection and
timescale are quantified, showing a clear dependence on yaw rate. To gain
further physical understanding of the mechanism behind the inverse wake
deflection, the streamwise vorticity in different parts of the wake is
quantified. The results of this study provide guidance for the design of
advanced wake flow control algorithms. The lag in wake response observed for
both blade pitch and yaw changes shows that proposed dynamic control strategies
must implement turbine operational changes with a timescale on the order of the
rotor timescale or slower.
