Wavy Water Entry

When an object like a sphere enters the water, it drags air into the water behind it, creating a cavity. Depending on the sphere’s impact speed, the cavity might close first under the water, forming a deep seal, or at the surface with a surface seal. But, as this video points out, water often isn’t still. Here, they explore how the sphere’s entry changes when there are ripples on the water surface. (Video and image credit: M. Ibrahim et al.; via GFM)

Intuition suggests that a flat rock will hit the water with greater force than a spherical one, and experiments uphold that. But a flat rock, interestingly, doesn’t produce the greatest impact force. Instead, it’s a slightly curved rock that experiences peak impact forces. Researchers found this happens because of the thin layer of air that coats the front of the impacting object. For flat faces, this layer is relatively thick and provides a cushioning effect that reduces the peak force and spreads out the impact. In contrast, a slightly curved convex surface traps a thinner air layer, and that lack of cushioning maximizes the impact force. (Image credit: J. Wixom; research credit: J. Belden et al.; via APS Physics)

https://fyfluiddynamics.com/2024/08/curved-rocks-hit-harder/

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