Physics of tacking a sailboat
Credit:
Jonathan King/NYU
Possibly the most challenging basic move for beginner sailors is learning how to tack to sail upwind. Done correctly, the sail will flip around into a mirror image of its previous shape. And in competitive sailboat racing, a bad tack can lose the race. So physicists at the University of Michigan decided to investigate the complex fluid dynamics at play to shed more light on the tricky maneuver, according to a paper published in the journal Physical Review Fluids.
After modeling the maneuver and conducting numerical simulations, the physicists concluded that there are three primary factors that determine a successful tack: the stiffness of the sail, its tension before the wind hits, and the final sail angle in relation to the direction of the wind. Ideally, one wants a less flexible, less curved sail with high tension prior to hitting the wind and to end up with a 20-degree final sail angle. Other findings: It’s harder to flip a slack sail when tacking, and how fast one manages to flip the sail depends on the sail’s mass and the speed and acceleration of the turn.
DOI: Physical Review Fluids, 2025. 10.1103/37xg-vcff (About DOIs).
Elephant ears inspire building design
Maintaining a comfortable indoor temperature constitutes the largest fraction of energy usage for most buildings, with the surfaces of walls, windows, and ceilings contributing to roughly 63 percent of energy loss. Engineers at Drexel University have figured out how to make surfaces that help rather than hamper efforts to maintain indoor temperatures: using so-called phase-change materials that can absorb and release thermal energy as needed as they shift between liquid and solid states. They described the breakthrough in a paper published in the Journal of Building Engineering.
The Drexel group previously developed a self-warming concrete using a paraffin-based material, similar to the stuff used to make candles. The trick this time around, they found, was to create the equivalent of a vascular network within cement-based building materials. They used a printed polymer matrix to create a grid of channels in the surface of concrete and filled those channels with the same paraffin-based material. When temperatures drop, the material turns into a solid and releases heat energy; as temperatures rise, it shifts its phase to a liquid and absorbs heat energy.
