Capped Helical J-Shaped Blades Give Vertical-Axis Wind Turbines a Powerful Boost

Vertical-axis wind turbines have long promised a kind of wind energy that fits where people actually live: on rooftops, beside highways, and in the gusty, chaotic air of cities. Unlike their towering horizontal-axis cousins, these machines spin no matter which way the wind blows, and they tolerate turbulence that would batter conventional propeller-style turbines. Yet they have always carried an awkward pair of flaws. Darrieus-type lift-based rotors, the most efficient of the family, are notoriously bad at starting themselves from rest, while the helical blades that smooth out their jerky torque tend to sacrifice raw power. A new computational study now suggests that a cleverly reshaped blade, capped at its tips with thin plates, can attack both problems at once, delivering dramatic gains in torque and a substantially stronger kick to get the rotor turning in the first place.

The research, published in the journal Results in Engineering, was carried out by Maziyar Moghtadaeifar, Ramin Farzadi, and Majid Bazargan, who built their design around a radical idea that has been gathering momentum in wind engineering: the J-shaped airfoil. A conventional blade cross-section, such as the standard NACA0021 profile used here as a baseline, is a closed teardrop shape. A J-shaped airfoil takes that profile and simply removes part of the pressure side, the face of the blade that faces away from the turbine’s rotation axis, leaving a hollow, scoop-like section bounded by a wall just one millimeter thick. The opening begins at the point of maximum thickness and runs all the way to the trailing edge, transforming the blade into something between a wing and a curved trough.

That hollow matters. When wind sweeps across a J-shaped blade, air circulates inside the concave cavity, and the resulting changes in pressure distribution alter how lift and drag act on the rotor. Earlier experimental and numerical work had shown that such profiles can improve self-starting, because the open cavity behaves somewhat like a drag device at low speeds, but often at a cost to peak efficiency. The innovation in the new study is to combine this profile with a helical configuration: three blades, each twisted sixty degrees around the rotation axis, mounted on a rotor with a radius of 0.99 meters and a height of 1.15 meters. Helical blades smooth the torque output because some section of some blade is always optimally positioned relative to the wind, reducing the violent fluctuations that plague straight-bladed designs and making the rotor less dependent on the angle at which it happens to have stopped.

To test the concept, the team ran three-dimensional unsteady Reynolds-averaged Navier-Stokes simulations using the shear-stress transport k-omega turbulence model, a combination that previous validation studies have identified as among the most accurate for capturing the complex, separated flows around vertical-axis rotors, including the transient phenomenon of dynamic stall. The computational domain stretched thirty-two rotor radii downstream and twelve radii in the other directions, with a blockage ratio of just 1.61 percent, and a sliding-mesh cylinder carried the spinning blades. The researchers verified their setup against published experimental data for two different variable-pitch turbines, then ran careful mesh and time-step independence studies, refining the grid until the cycle-averaged torque coefficient changed by less than one percent and settling on a time step of half a degree of rotation per step.

The first headline result concerns how the J-shaped profile itself performs. Across low and medium tip speed ratios, the ratio of blade speed to wind speed that governs rotor behavior, the J-shaped helical turbine outperformed a geometrically identical machine with conventional NACA0021 blades. At a tip speed ratio of 1.11, for example, the J-shaped rotor produced a torque coefficient 26.59 percent higher than the full-profile machine, with the advantage concentrated in the upwind passage between azimuth angles of thirty and one hundred twenty degrees, where dynamic stall dominates. The cavity appears to reshape the internal flow structure in a way that enlarges the pressure difference between the two sides of the blade, and the first torque peak shifts to a later azimuth angle and grows taller. Averaged across all the operating conditions examined, the profile swap alone was worth roughly a ten percent improvement in torque.

But the J-shape carried a hidden liability that emerged at high speed. At a tip speed ratio of 1.77, the open-tipped J-shaped turbine’s performance collapsed, falling more than fifty percent below the conventional rotor. The culprit was the blade tip. Because the J-profile is hollow, vortices do not merely shed from the tip the way they do on a solid wing; they can leak out through the open cavity itself, spilling high-energy flow and destroying the pressure difference the blade depends on. Tip vortices, the swirling structures that form wherever lift-generating surfaces end abruptly, are a familiar efficiency tax on all rotors, but the open J-section makes the leakage far more damaging precisely when the turbine is spinning fast enough to generate its best power.

The team’s fix was disarmingly simple: cap the tips. They added thin NACA0021-shaped plates, one millimeter thick, to both ends of each blade, sealing the hollow section like lids on a tube. The effect was enormous. At the problematic tip speed ratio of 1.77, the caps boosted the torque coefficient by roughly 122 percent compared with the uncapped J-shaped turbine, lifting it about eight percent above even the conventional full-profile machine. Averaged over all five tip speed ratios studied, from 0.44 to 1.77, the caps raised the J-shaped turbine’s torque coefficient by about 46 percent, and the capped J-shaped design beat the conventional helical turbine by about 48 percent overall. In power-coefficient terms, obtained by multiplying torque by tip speed ratio, the capped machine dominated across the operating range.

By slicing each blade into twenty sections along its height and tracking the torque of each, the researchers traced exactly where the gains came from. At a mid-range tip speed ratio of 1.33, the upwind half of the rotor improved by just over twenty percent with caps installed, while the downwind half gained a more modest eight percent, with the largest benefits near the tips themselves. Flow visualizations revealed the mechanism: without caps, the vortex forming inside the blade’s concave cavity escapes outward over the open edge; with caps in place, the vortex is redirected toward the trailing edge, and pressure builds inside the cavity, strengthening the force that drives rotation. The caps essentially convert a leaking bucket into a sealed one, letting the J-profile’s pressure advantage accumulate instead of draining away through the blade ends.

Perhaps most important for the urban applications that motivate this whole line of research, the capped design starts better. The team held the rotor stationary in a seven-meter-per-second wind and computed the starting torque at azimuth angles from zero to 105 degrees in fifteen-degree increments. The capped J-shaped turbine beat the conventional helical rotor at every single angle, with improvements ranging from about four percent to a striking 46.6 percent at ninety degrees, and an average gain of roughly 21.5 percent across all positions. For a technology whose central weakness has always been the dead zone where lift-based rotors cannot overcome their own drag, a design that simultaneously raises efficiency, smooths torque, and strengthens self-starting is a meaningful step forward. The authors caution that they fixed the cut ratio and cap thickness rather than optimizing them, so further gains may remain on the table. But the message of the study is clear: sometimes the path to better wind energy is not a bigger machine or a fancier material, but a hollow blade with its ends sealed shut, turning the city’s messy gusts into cleaner, steadier power.

Subject of Research: Aerodynamic performance and self-starting behavior of a helical J-shaped vertical-axis wind turbine with capped blade tips

Article Title: Aerodynamic performance and self-starting characteristics of a novel helical J-shaped vertical-axis wind turbine with blade-tip caps

Article References: Moghtadaeifar, M., Farzadi, R., & Bazargan, M. (2026). Aerodynamic performance and self-starting characteristics of a novel helical J-shaped vertical-axis wind turbine with blade-tip caps. Results in Engineering, 32, Article 113308. https://doi.org/10.1016/j.rineng.2026.113308

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113308

Keywords: vertical-axis wind turbine, J-shaped airfoil, helical blades, blade-tip caps, tip vortices, self-starting torque, CFD simulation, URANS, SST k-omega, dynamic stall, urban wind energy, power coefficient