Innumerable times in a day, boats enters the water from dry land, seabirds dive-catch prey from the ocean, and seaplanes touch down smoothly between the waves.
Objects entering water is a common phenomenon, but a complete understanding of the physics of water entry continues to be mysterious; particularly when it relates to cases where a solid object enters a water body containing other solid objects - for example, a seagull diving into the sea’s rocky patch.
This fairly untouched area of research is being investigated by researchers at the NYU Tandon School of Engineering who have reported a series of unexpected findings that may result in strategies for reducing the strain of water entry on seaplanes, marine vessels, and space-crew capsules engineered for water landing.
“Many studies of water entry overlook the presence of solid, stationary objects like ice or rocks in the water, and it is clear that these items can affect objects entering the water and change the physics of impact,” stated Maurizio Porfiri, professor of mechanical and aerospace engineering at NYU Tandon and lead author of the study titled “Solid Obstacles Can Reduce Hydrodynamic Loading During Water Entry,” which has been published in the journal Physical Review Fluids.
Collaborators of Porfiri include Ghania Benbelkacem, NYU Tandon Adjunct Faculty Member in the Department of Mechanical and Aerospace Engineering, and Mohammad Jalalisendi, a recent doctoral degree graduate in Porfiri’s team.
Together with his collaborators in the Dynamical Systems Laboratory, Porfiri developed an experiment by utilizing a solid wedge plunging into a water tank that contains a neutrally buoyant cylinder. Depth, pressure, and acceleration were measured by sensors in the setup. A particle image velocimetry was used by the team to view flow and determine the speed of water jets generated by the wedge as it lands in the water. Analyses showed that the presence of the cylinder in the water considerably altered the physics of impact on the wedge in surprising ways.
Surprisingly, the researchers observed a reduction in pressure in the pile-up - the fluid area where a high-speed jet is generated as the wedge hits - on the side of the wedge nearest to the cylinder. According to Porfiri and his team, this decrease is caused by the cylinder that restricts the fluid on that side, and as a result less amount of water was displaced upon impact.
Conversely, in a conflicting finding, the researchers observed an increase in pressure toward the keel of the wedge, suggesting a complex, rich impact from the cylinder. While more research is required to resolve these contradictory findings, the team notes the significance of continuing to study the interplay between objects entering the water and stationary items in water.
It is clear that there are sympathetic interactions between these objects, and as we gain a better understanding of them, it may lead to designs and materials that mitigate some of the strain on marine vessels that travel in occluded waters, especially those exploring and navigating polar regions.
Maurizio Porfiri, Professor of Mechanical and Aerospace Engineering
This study was supported by a grant from the Solid Mechanics Program of the Office of Naval Research, with Dr. Y. D. S. Rajapakse as the program manager.