A new study shows that partially submerged objects can experience more drag than fully submerged ones. This unexpected result comes from the excess fluid that piles up ahead of the object, as seen in the image above, where flow is moving from left to right. The experiments used centimeter-sized spheres and showed that the maximum drag on a nearly-submerged sphere could be 300-400% greater than the drag on a fully submerged sphere.
Even more surprisingly, they found that water-repellent hydrophobic coatings — which are often suggested for drag reduction — actually increased the drag even further on partially submerged spheres. That’s because the water-repelling coating caused an even larger build-up of fluid ahead of the sphere, increasing the pressure on the front side of the sphere and creating even more drag. Spheres with a hydrophilic coating had less water build-up and thus lower drag.
The study suggests that — at the centimeter-scale — drag physics at the air-water interface may be more complicated than we assume. (Image and research credit: R. Hunt et al.; via Physics World; submitted by Kam-Yung Soh)