A Numerical Investigation of Multirotor Systems with Vortex-Generating Modes for Regenerative Wind Energy: Validation Against Experimental Data

Abstract. The current work describes and assesses multirotor systems consisting of pared multirotor and rotor-sized wings, dubbed atmospheric boundary layer (ABL)-control devices, in the rotor's near wake region. The ABL-control devices create vortical flow structures that can accelerate the vertical momentum flux from the flow above the wind farm into the wind farm flow, thus augmenting the wake-recovery process. Understanding the wake-wide impacts of this novel ABL-controlling strategy is crucial to determining the feasibility of using such a strategy in utility-scale wind farms. This work provides numerical assessments of a single multirotor system accompanied by different ABL-controlling setups. The wind flow is modeled via steady-state Reynolds-averaged Navier–Stokes computations. The multirotor and ABL-controlling devices are modeled using three-dimensional actuator surface models based on the Momentum theory. Input force coefficient data for the actuator surface models and validation data for the numerical computations were measured from a scaled model at TU Delft's Open Jet facility. The performance of the ABL-controlling devices was assessed via the net momentum entrained from the flow above the wind farm flow and the total pressure and power available in the wake. It was found that when the ABL-controlling strategy is adopted, the vertical momentum flux becomes the primary mechanism for wake recovery such that for configurations with two or four ABL-controlling wings, the total wind power in the wake recovers 95 % of the free-stream value at locations as early as x/D ≈ 6 downwind of a multirotor system, which is about one order of magnitude faster than what is seen for the baseline wake without ABL-controlling capabilities.