Researchers show mechanics of how drag on a partially submerged object may be several times greater than drag on a fully submerged object made of the same material.
One of the most common and practically useful experiments in all of fluid dynamics involves holding an object in air or submerging it fully underwater, exposing it to a steady flow to measure its resistance in the form of drag. Studies on drag resistance have led to technological advances in airplane and vehicle design and even advanced our understanding of environmental processes.
For instance, the researchers -- led by Brown engineers Robert Hunt and Daniel Harris -- found that drag on the spheres increased the moment they touched the water, no matter how water repellent the sphere material was. Each time, the drag increased substantially more than what was expected and continued to increase as the spheres were lowered, beginning only to drop when the spheres were fully beneath the water.
The researchers also found that the sphere's level of water repellency plays a key role in the drag forces it experiences. This is where things get a bit counterintuitive. "Superhydrophobic materials are often proposed to reduce drag, but, in our case, we found that superhydrophobic spheres when almost fully immersed have a much larger drag than the sphere made of any other water repellency," Hunt said."In trying to decrease the drag, you might actually increase it substantially.""The water wants nothing to do with this superhydrophobic sphere so it does anything that it can to, sort of, get out of the way of the sphere," Harris said.
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