Experimental aerodynamic research conducted in a specialized wind tunnel demonstrates that pairing counter-rotating wind turbines in a side-by-side dual-rotor system enables unprecedented control over downstream wake trajectories and accelerates the recovery of kinetic energy in the airflow. By systematically manipulating the rotational sense of two closely spaced model turbines alongside symmetrical inward and outward yaw angles, researchers observed that specific aerodynamic alignments can either induce an intentional collision between the two turbine wakes or force them to diverge cleanly. These steering mechanisms overcome the persistent velocity deficits typical of isolated wakes, offering practical aerodynamic strategies to replenish the wind resource for downstream turbines in dense multi-row arrays.

The investigation addresses a growing trend in renewable energy engineering toward multi-rotor architectures designed to extract significantly more electricity from a given parcel of land or marine space. Over recent decades, the wind industry has pursued ever-increasing rotor diameters and multi-megawatt platforms exceeding fifteen megawatts for offshore deployment, yet structural, logistical, and economic limitations have spurred interest in multi-rotor configurations. Previous computational simulations and field deployments—including scaled prototypes tested by European industrial consortia and multi-megawatt floating dual-rotor systems launched in China—suggest that multi-rotor systems can decrease structural manufacturing and maintenance costs while providing favorable wake recovery characteristics. However, numerical models such as rotating actuator disk simulations have frequently underestimated the rotational momentum imparted to the wake, creating an urgent need for rigorous empirical measurements to validate simulated aerodynamic interactions and elucidate the physical mechanisms governing wake development across three spatial dimensions.

To establish high-fidelity empirical benchmarks, the research team conducted systematic aerodynamic trials inside a Göttingen-type wind tunnel featuring an extensive enclosed test section measuring three meters wide, three meters high, and thirty meters long. Ambient atmospheric conditions, including temperature, relative humidity, and barometric pressure, were continuously tracked and factored into comprehensive instrument calibrations executed at the start and conclusion of every experimental session. Wake generation was achieved using two identical MoWiTO 0.6 model wind turbines, which feature rotor diameters of 0.58 meters and incorporate low-Reynolds airfoils capable of reaching thrust coefficients near 0.8, mirroring the operational aerodynamics of full-scale utility turbines. Each model turbine was operated using an automated generator torque scheme and a proportional-integral-derivative pitch controller, ensuring stable and repeatable rotor performance in the partial load regime under an undisturbed laminar inflow velocity of five meters per second monitored by an upstream Prandtl tube.

The experimental architecture placed the two model turbines at a lateral nacelle-to-nacelle spacing of 1.07 rotor diameters, maintaining a physical clearance of approximately four centimeters between blade tips to prevent collisions while maximizing aerodynamic coupling. Measurements were systematically recorded across two distinct operational configurations: a counter-right arrangement, wherein the counterclockwise-rotating turbine stood to the right of the clockwise-rotating unit when viewed looking downwind, and a counter-left arrangement, where the counterclockwise turbine was positioned on the left. The downstream wake field was mapped across horizontal planes spanning from 0.75 to 10.0 rotor diameters behind the rotor plane using an automated traversing array carrying nineteen horizontally spaced hot-wire anemometers positioned at hub height. Operating at a sampling frequency of six kilohertz with low-pass filtering applied at three kilohertz, the sensor array acquired 240 seconds of high-resolution velocity data at each spatial coordinate, providing an extensive statistical dataset across nine symmetrical yaw angles ranging from negative twenty to positive twenty degrees.

Before evaluating dual-rotor interactions, the investigators analyzed single-rotor dynamics to establish how blade rotation and yaw misalignment shape an isolated wake. When a single turbine was operated without yaw, the strongest velocity deficit emerged between four and five rotor diameters downstream, recovering slowly due to the smooth laminar inflow and persistent tip vortices that inhibited lateral momentum exchange. However, applying positive or negative yaw displacement not only shifted the peak deficit closer to the rotor but also revealed a pronounced directional asymmetry governed by blade rotation. The wake of a clockwise-rotating rotor deflected far more effectively toward its right, whereas a counterclockwise rotor exhibited superior deflection toward its left. This phenomenon stems from angular momentum conservation: the wake rotates in the opposite direction of the rotor blades, sweeping the persistent turbulent wake shed by the turbine support tower upward into the horizontal measurement plane on one side while drawing high-velocity freestream air downward on the other.

Transferring these rotational dynamics into the unyawed dual-rotor baseline revealed substantial structural differences between the two rotational pairings even prior to yaw adjustment. When operated at zero degrees yaw, the counter-right configuration exhibited almost no lateral expansion by ten diameters downstream, maintaining a concentrated and narrow wake structure along the wind tunnel centerline. In sharp contrast, the unyawed counter-left setup underwent substantial lateral expansion, spreading its wake boundaries outward to approximately 1.25 rotor diameters on either side of the center. Analysis of turbulent kinetic energy demonstrated that the counter-right arrangement concentrated flow fluctuations along the central shear layer where the counter-rotating blade passages met, whereas the counter-left layout developed its highest turbulent energy along its outer lateral perimeters. These contrasting spatial patterns demonstrated that the direction of blade rotation fundamentally dictates how individual single wakes merge, transport turbulence, and exchange momentum with the surrounding undisturbed flow.

The application of symmetrical inward yaw—directing the thrust vectors of both nacelles toward the wind tunnel centerline—produced dramatic differences in wake behavior between the two rotational arrangements. In the counter-right setup yawed twenty degrees inward, the two single wakes merged rapidly into an exceptionally compact wake exhibiting a total lateral width of less than one rotor diameter at six diameters downstream. The mutual inward deflection triggered an immediate collision and destabilization of the blade tip vortices, generating intense turbulent kinetic energy along the central axis between one and six diameters downstream. This rapid vortex breakdown and subsequent mixing with the entrained tower wake facilitated vigorous momentum transfer across the outer wake boundaries. Conversely, when the counter-left setup was yawed inward by twenty degrees, the two wakes converged around four diameters downstream to create a broad, uniform wake roughly two diameters wide that persisted through ten diameters, accompanied by localized secondary flow deceleration between four and seven diameters downwind.

When both turbines were yawed symmetrically outward by twenty degrees, the flow fields diverged along entirely different trajectories, yielding exceptionally favorable conditions for downstream power extraction. In the counter-right configuration, the individual wake centroids traveled downstream along parallel paths before beginning to interact and merge near ten diameters behind the turbines. In the counter-left outward layout, however, the two single wakes drifted persistently away from the central axis, remaining completely segregated throughout the entire ten-diameter measurement domain. Because the internal blade motion in the counter-left setup naturally sweeps downward toward the center, the outward yaw alignment generated a central low-pressure trough that vigorously entrained undisturbed high-speed air from above the rotor plane directly into the space between the turbines. This outward divergence enabled the individual single wakes to decay rapidly, achieving a normalized velocity deficit below 0.5 approximately two rotor diameters sooner than the corresponding counter-right outward configuration.

To evaluate aerodynamic performance systematically across all eighteen yaw and rotational alignments, the authors derived several quantitative parameters, including the normalized velocity deficit, the streamwise wake recovery rate, and the spanwise-integrated relative wind speed. The recovery rate showed that flow acceleration initiates earliest on the specific side of each turbine where the blades move upward, confirming that downward wake rotation pulls high-momentum air into the measurement plane. In terms of integrated relative wind speed across the dual-rotor span, unyawed baselines suffered extended velocity deficits before recovering, whereas yawed configurations initiated recovery significantly closer to the rotor plane. The counter-left arrangement yawed twenty degrees outward began recovering just one diameter downstream, attaining an integrated relative wind speed above 0.75 at ten diameters. The counter-right setup yawed twenty degrees inward achieved the highest recovery overall, exceeding 0.80 relative wind speed at ten diameters despite a 12.6 percent average power penalty incurred by the upstream dual rotor due to yaw misalignment.

While these wind tunnel findings reveal compelling aerodynamic mechanisms for wind farm optimization, the authors emphasize several experimental constraints and operational considerations that must inform full-scale interpretation. The miniature MoWiTO 0.6 turbines feature support towers that are proportionally larger relative to the rotor swept area than towers on commercial multi-megawatt turbines, amplifying the aerodynamic influence of the tower wake within the measurement plane. Furthermore, the experiments were conducted under smooth, laminar inflow conditions, whereas real atmospheric boundary layers feature pervasive background turbulence and vertical wind shear that naturally accelerate tip-vortex breakdown and lateral mixing. Nonetheless, the experimental results demonstrate that strategic coordination of rotational direction and symmetrical yaw steering provides dual rotors with a powerful mechanism to steer wake trajectories away from downstream machines or trigger deliberate wake-wake collisions that accelerate velocity recovery, establishing foundational aerodynamic principles for optimizing the layout and control of future multi-rotor wind farms.

Subject of Research: Aerodynamic wake interactions and steering in counter-rotating dual-rotor wind turbine systems

Article Title: Exploring wake interactions in a counter-rotating dual-rotor system for different yaw configurations: an experimental study

Article References: Maus, J., Peinke, J., & Hölling, M. (2026). Exploring wake interactions in a counter-rotating dual-rotor system for different yaw configurations: an experimental study. Wind Energy Science, 11(11), 4111-4123. https://doi.org/10.5194/wes-11-4111-2026

Image Credits: AI Generated

DOI: 10.5194/wes-11-4111-2026

Keywords: wind energy, dual-rotor turbines, wake steering, fluid dynamics, wind tunnel experiments, renewable energy, aerodynamics, wake recovery, Exploring, wake, interactions, counter-rotating