Rocking Turbines Stir Their Own Wakes, Boosting Floating Wind Farm Power

Floating offshore wind turbines are never still. Anchored to the seabed by mooring lines or tension legs, they sway, pitch and surge as waves and wind push against their platforms, and the rotor rides back and forth with them. For years, engineers have treated this perpetual motion mostly as a problem: a source of unsteady loads, fatigue and control headaches. But a new wind tunnel study suggests the rocking may carry an unexpected gift. Researchers at Hamburg University of Technology, the University of Oldenburg and EPFL report that the back-and-forth surge motion of a floating turbine can dramatically accelerate the recovery of its wake, the region of slowed, turbulent air trailing behind the rotor, meaning that turbines parked downwind could harvest significantly more energy than current models predict.

The study, published in the journal Wind Energy Science by Christian W. Schulz, Michael Hölling, Joachim Peinke and Thomas Messmer, goes further than previous work in two important ways. First, the team pushed their experiments into a range of motion frequencies never explored before, more than tripling the span covered by earlier studies. Second, in that uncharted territory they stumbled upon a previously unreported flow phenomenon: wake recovery improves sharply when the surging of the platform runs just slightly slower than the blades themselves sweep past the tower. The discovery hints at a hidden mechanism by which the interplay between platform motion and rotor rotation can stir the wake into mixing faster with the surrounding air.

The experiments took place in the large wind tunnel of the University of Oldenburg, in a closed test section three metres wide, three metres high and thirty metres long. The team used the TUHH model turbine, a two-bladed rotor with a diameter of 0.93 metres, mounted on a tower and hub fitted with a linear actuator capable of driving surge motions at frequencies up to 23 hertz with amplitudes up to 20 millimetres. Nineteen hot-wire sensors, aligned horizontally at hub height and mounted on a movable cart, measured the wind speed across the wake at distances from one to ten rotor diameters downstream. Most runs were performed at an inflow speed of three metres per second under nearly laminar conditions, with a turbulence intensity of just 0.3 percent, so that the effect of the motion could be isolated cleanly before being tested against turbulent inflows as well.

Two dimensionless numbers emerged as the master keys to the problem. The first is the platform Strouhal number, which compares the frequency of the surge motion to the time it takes the wind to travel one rotor diameter. The second is the relative velocity amplitude, the ratio of the surge velocity to the incoming wind speed. Previous studies had shown that these parameters govern the unsteady aerodynamic loads on a surging rotor, but it remained unclear whether they also fully characterise what happens to the wake. The new measurements confirm that they do: the shape of the wake recovery curve, with all its peaks and valleys, is dictated by the Strouhal number, while the strength of the recovery enhancement at each peak scales with the velocity amplitude. Higher motion amplitudes consistently produced stronger recovery, echoing earlier numerical and experimental findings.

In the low-frequency regime, the results align neatly with what the community already knew. For Strouhal numbers between roughly 0.2 and 0.6, the surging motion enhances wake recovery, with an optimum near a Strouhal number of 0.3. Power spectra of the velocity fluctuations in the wake’s shear layer reveal why: instead of the broadband energy distribution characteristic of natural wake meandering, the flow responds dominantly at the platform’s motion frequency, indicating the formation of a coherent, pulsating structure that transports momentum from the free stream into the wake. This is precisely the kind of motion-induced mixing that earlier numerical and experimental studies had inferred, and the new data now pin it down with unprecedented clarity across a wide parameter space.

The genuine surprise came at high frequencies. Between Strouhal numbers of roughly 2 and 4, the wake of the surging turbine recovers almost exactly like that of a fixed one, as if the motion were too fast to matter. But beyond a Strouhal number of 4, recovery suddenly climbs again, peaking at a Strouhal number of 5.39 before collapsing abruptly when the motion frequency reaches the blade passing frequency. At that peak, the power spectra show a sharp, unexpected peak at a low reduced frequency of about 0.35, which corresponds exactly to the difference between the Strouhal number of the blade passing frequency and that of the platform motion. In other words, the wake organises itself into a coherent structure at a frequency that neither the rotor nor the platform produces on its own, but that emerges from their interaction.

The authors propose a compelling physical explanation rooted in the returning wake effect, the phenomenon by which vortices shed from the blade trailing edges circle back and modulate the loads on the blades. When the surge frequency matches the blade passing frequency, vortices are emitted at identical azimuthal angles in every motion cycle, creating a persistent, frozen pattern in the wake. Introduce a slight mismatch between the two frequencies, and this pattern begins to rotate around the rotor axis by a few degrees each cycle, distributing the velocity minima and maxima along a helix that winds downstream. When the rotation frequency of this helix, given by the difference between the blade passing and motion frequencies, approaches the wake’s natural frequency of about 0.3, it resonates with a natural mode of the wake and triggers vigorous mixing. Intriguingly, this produces a near-field flow pattern resembling the helix wake mixing strategy, a control technique in which dynamic individual pitch control deliberately creates a helical wake to accelerate recovery, except that here the ocean does the work for free.

The experiments also revealed a striking sensitivity to how hard the turbine pulls on the wind. By varying the tip speed ratio between 5 and 10, the team swept the thrust coefficient from 0.58 to 0.82. At the lowest thrust, the recovery enhancement from surge motion was nearly negligible, amounting to less than 3 percent compared with a fixed turbine, while at a thrust coefficient of 0.80 the improvement exceeded 15 percent. The explanation lies in the shear between the wake and the ambient flow: a lightly loaded rotor produces a weak, stable velocity deficit that resists disturbance, whereas a heavily loaded rotor generates sharp velocity gradients and unstable shear layers that readily respond to small excitations and grow into large-scale coherent structures. This matters enormously for floating wind, because modern 15-megawatt-class turbines operate at high thrust near rated conditions, exactly where the motion effect is strongest.

Sceptics might wonder whether these delicate flow structures could survive in the churning atmosphere of a real offshore wind farm. To test this, the team repeated key measurements using an active grid that generated turbulent inflows with intensities between 2.9 and 6 percent. The dominant peaks at Strouhal numbers of 0.3 and 5.39 persisted, although their height relative to the fixed-case recovery diminished as turbulence grew, and the enhancement remained detectable up to a turbulence intensity of 4.7 percent for the higher motion amplitude. The authors caution that laboratory turbulence is not directly comparable to full-scale atmospheric turbulence, so the real-world magnitude of the effect could be markedly higher or lower. Still, the persistence of both regimes under turbulent conditions strengthens the case that the findings are not a wind tunnel artefact.

The implications for floating wind energy are potentially substantial. Wake losses, the power sacrificed because upwind turbines leave slowed air in the path of their neighbours, are among the largest efficiency drains in any wind farm, and floating arrays placed in deep waters will concentrate turbines in ever tighter configurations. If the natural surge motion of floating platforms, driven by nothing more than ocean waves, can be characterised, predicted and perhaps even tuned through platform design or mooring layout to exploit these mixing mechanisms, downwind turbines could see measurably higher inflow speeds without any additional control effort. With wave-induced motions of today’s giant rotors reaching platform Strouhal numbers as high as 10, the newly discovered high-frequency regime falls squarely within realistic operating conditions. The authors’ next challenge is to characterise the new helical mode in detail and to translate their model-scale results into reliable full-scale predictions, a step that could turn the ocean’s restless motion from an engineering liability into an asset for the floating wind farms of the coming decades.

Subject of Research: Wake recovery of floating offshore wind turbines under surge motion

Article Title: Brief communication: A novel wake mixing phenomenon and key parameters for wake recovery of floating wind turbines subjected to surge motions

Article References: Schulz, C. W., Hölling, M., Peinke, J., & Messmer, T. (2026). Brief communication: A novel wake mixing phenomenon and key parameters for wake recovery of floating wind turbines subjected to surge motions. Wind Energy Science, 11(10), 3775-3783. https://doi.org/10.5194/wes-11-3775-2026

Image Credits: AI Generated

DOI: 10.5194/wes-11-3775-2026

Keywords: floating offshore wind, wake recovery, surge motion, wind tunnel experiments, Strouhal number, thrust coefficient, coherent structures, helix wake mixing, unsteady aerodynamics, wind farm efficiency, turbulence, blade passing frequency