Inflow-shear-induced wake asymmetry links axial force oscillations to vertical wake meandering in active wake control

This study investigates the impacts of inflow shear on wind-turbine wakes in active wake control through a resolvent-based input–output model and large-eddy simulation (LES). The input–output model, i.e. the proposed regular perturbation resolvent (RP-Resolvent) model, provides an efficient framework for analysing the underlying mechanism, while LES provides a high-fidelity comparison. The RP-Resolvent model adopts a two-parameter regular perturbation expansion of the incompressible Navier–Stokes equations around an axisymmetric baseline wake flow, to describe the wake response under the combined action of inflow shear and time-harmonic axial rotor forcing. Analysis using the RP-Resolvent model indicates a key mechanism: inflow-shear-induced asymmetry affects wake evolution via the coupling of azimuthal modes, which generates extra volumetric forcing in the entire wake region with azimuthal structures different from the rotor forcing and alters the spatial pattern of the wake’s coherent motion. Such a mechanism is tested using dynamic induction control under sheared inflow. A counterintuitive mode transition is observed, i.e. axisymmetric rotor forcing due to axial induction control triggers vertical meandering, contrasting with the axisymmetric pulsating motion observed under uniform inflow, with meandering intensity increasing with shear rate. Large-eddy simulation confirms that the model correctly predicts the spatial pattern of coherent structures and the sensitive frequencies, and reveals significant wake-recovery enhancement. This work offers new physical insights into wind-farm flow control, suggesting that atmospheric shear can be exploited to amplify control effectiveness and increase vertical mixing.