Wind tunnel meets supercomputer: new turbine model nails the invisible swirls behind clean power

Behind every spinning wind turbine lies an invisible architecture of swirling air that decides how much energy a whole wind farm can harvest. Now, a team of Belgian researchers has put that hidden world under an unusually rigorous double test, building a miniature turbine from scratch in a wind tunnel and then challenging a high-fidelity computer simulation to reproduce its wake, breath for breath. The study, published in Wind Energy Science, shows that a refined simulation technique can capture the fine structure of the turbulent air left behind rotating blades with remarkable accuracy, while honestly exposing where the models still fall short.

The research was led by Emmanuel Gillyns of the von Karman Institute for Fluid Dynamics and ENGIE Laborelec, together with Sophia Buckingham, Jeroen van Beeck and Grégoire Winckelmans of the von Karman Institute and UCLouvain. Their starting point was a problem that has long frustrated wind energy engineers: the air that flows off a turbine blade, known as the wake, governs how much wind reaches the turbines downstream, how heavily their structures are loaded, and ultimately how much electricity a farm produces. Getting the wake wrong in a simulation means getting the economics of wind power wrong.

To attack the problem, the team first designed and built a brand-new model-scale wind turbine, christened TWIST, short for Turbine for Wind-tunnel Investigation and Scaled Testing. The machine has a rotor diameter of 0.80 metres and a hub height of 0.61 metres, small enough to fit in a laboratory yet large enough to avoid the misleading physics that plagues tinier desktop turbines. Its blades were 3D-printed at UCLouvain from a ductile PLA+ plastic, using the SD7037-092-88 airfoil profile, a shape specifically chosen because it performs well at the low Reynolds numbers that small-scale models inevitably experience.

Scaling down a wind turbine is not simply a matter of shrinking it. The researchers had to preserve the key dimensionless numbers that control wake behaviour, including the tip speed ratio, which sets how fast the blade tips sweep through the air, and the thrust and power coefficients, which determine the velocity deficit and the energy extracted from the flow. The TWIST turbine operates at a rotor Reynolds number above 35 times ten to the fourth, a threshold above which wake statistics become largely independent of scale, meaning the miniature wake genuinely mimics what a full-size machine would produce. Blockage effects from the tunnel walls were carefully minimised, with the rotor occupying only about 8.4 percent of the test section cross-section.

The experiments took place in the L1-B wind tunnel at the von Karman Institute, a closed-loop facility with a test section two metres high, three metres wide and roughly twenty metres long. Crucially, the team did not test their turbine in smooth, artificial air. Instead, they generated a turbulent boundary layer on the tunnel floor using roughness elements, a grid and a fence, replicating the atmospheric conditions that real turbines face in the field. Hot wire anemometers then measured wind speeds along vertical lines at three locations, sampling at 20 kilohertz for five minutes per point, with uncertainties meticulously quantified and propagated through every calibration step.

On the computational side, the team ran large eddy simulations using Nek5000, an open-source, high-order spectral element code developed at Argonne National Laboratory. Large eddy simulation resolves the dominant turbulent structures directly, but modelling the full geometry of rotating blades remains computationally prohibitive, so researchers rely on the actuator line method, which represents the blades as lines of force rather than solid surfaces. The catch is that the method’s accuracy depends heavily on its implementation, and conventional versions smear out the physics near the blade tip and root, where forces change most abruptly.

Here lies the study’s central innovation: a continuous actuator line method, or cALM, that represents blade forces as polynomials on the simulation mesh nodes rather than as discrete point sources. The approach avoids spanwise averaging along the blade, applies its Gaussian force spreading only in the plane perpendicular to the blade, and samples the wind velocity independently at every grid point. This not only improves the physical fidelity of the tip vortex, the spiral of fast-spinning air shed from each blade end, but also slashes computational overhead by keeping every node independent and perfectly parallelisable. The same actuator principle was extended to the nacelle and tower, with the nacelle treated as a bluff body with an empirically tuned drag coefficient.

The team also invested enormous effort in making the simulated inflow match the wind tunnel conditions, using a modified recycling and rescaling technique in a co-simulation of two computational domains, plus a cleverly designed forcing term built from thirty closely spaced sine waves to boost turbulence where the experiment demanded it. The result was striking: the base flow error between simulation and experiment fell to a root mean square error of just 0.11 metres per second, with a mean absolute percentage error of 1.55 percent. That level of agreement means any remaining differences when the turbine is switched on can be attributed to the turbine model itself rather than to mismatches in the incoming wind.

When the results were compared, the cALM captured the essential features of the near-wake at 1.41 rotor diameters downstream, including the velocity deficit and the wake boundary, and remained consistent with measurements further out at 4.35 diameters. Most impressively, the simulation reproduced the sharp velocity gradient between the free stream and the wake, a zone where tip vortices dominate. With tip spreading disabled and a finely refined mesh, the simulated vertical shear in the transition zone came far closer to the experimental value than conventional smoothing would allow. Deviations persisted near the blade root, where the geometry transitions from airfoil to ellipse to circle and the flow becomes genuinely three-dimensional, stretching the assumptions of any actuator line model beyond its limits.

The study also probed how sensitive the wake is to blade pitch angle, running simulations at plus one and minus three degrees around the experimental operating condition, and examined how the flexible printed blades actually deform under load. Camera measurements revealed mean tip deflections of 7.5 millimetres at the top of the rotation and 6.2 millimetres at the bottom, more than the 2.9 millimetres predicted by a structural model that assumed a solid blade, yet still only about two percent of the blade span, too small to drive the observed wake discrepancies. The bigger picture is one of growing confidence: by pairing a purpose-built experimental turbine with a sharper, faster simulation method, the researchers have delivered both a reference dataset for the community and a demonstration that wind farm performance can be predicted with greater physical grounding, a quiet but meaningful step toward squeezing more clean energy from the same patch of sky.

Subject of Research: Experimental and numerical validation of wind turbine wake modeling using a model-scale turbine and large eddy simulation with a continuous actuator line method

Article Title: Investigating wake reproduction of a model-scale wind turbine: experimental measurements versus large eddy simulation with actuator line

Article References: Gillyns, E., Buckingham, S., van Beeck, J., & Winckelmans, G. (2026). Investigating wake reproduction of a model-scale wind turbine: experimental measurements versus large eddy simulation with actuator line. Wind Energy Science, 11(9), 3531-3553. https://doi.org/10.5194/wes-11-3531-2026

Image Credits: AI Generated

DOI: 10.5194/wes-11-3531-2026

Keywords: wind energy, wind turbine wake, large eddy simulation, actuator line method, wind tunnel experiments, Nek5000, atmospheric boundary layer, tip vortex, blade deformation, turbulence modeling, renewable energy, TWIST turbine