On the impact of a concave nosed axisymmetric body on a free surface

Abstract

We report on an experimental study of the vertical impact of a concave nosed axisymmetric body on a free surface. Previous studies have shown that bodies with a convex nose, like a sphere, produce a well defined splash with a relatively large cavity behind the model. In contrast, we find that with a concave nose, there is hardly a splash and the cavity extent is greatly reduced. This may be explained by the fact that in the concave nosed case, the initial impact is between a confined air pocket and the free surface unlike in the convex nosed case. From measurements of the unsteady pressure in the concave nose portion, we show that in this case, the maximum pressures are significantly lower than the classically expected "water hammer" pressures and also lower than those generally measured on other geometries. Thus, the presence of an air pocket in the case of a concave nosed body adds an interesting dimension to the classical problem of impact of solid bodies on to a free surface.The classical problem of rigid body-free surface interaction has many features, which make it interesting to study. Impact of solids in to water comprises a complicated series of events that occur both above and below the water surface, and depends on the configuration of the object used. There have been a large number of studies on this topic including some classical ones. 1-3 The primary motivation for these studies have been their relevance in applications like the landing of sea planes, spacecrafts and rocket parts on ocean surface, the slamming of a high speed ship with the water surface, supercavitation around rotating blades of hydraulic machinery and ship propellers operating near a free surface, and the flow field associated with air dropped underwater systems.When an object, say a sphere, impacts vertically into water from air, the hydrodynamic phenomena of interest are the splash, cavities, and jets. 3 The first two form at early times after impact; whereas, jets appear at much later times. Even though the splash forms above the water surface, its dynamics can have an important influence on the extent of cavity formed in the wake of the object that in turn influences its underwater motion. A systematic and comprehensive investigation 4 was undertaken to study the influence of various parameters on splash and cavity formation accompanying vertical impact of solid objects on a water surface. The parameters considered in this study included the density and pressure of the atmosphere above water, the velocity, size and, to a limited extent, the nose shape of the objects. More recent studies on spheres and axisymmetric slender bodies have shown that some other parameters like surface properties and spin imparted to the object can have important influence on splash and cavity formation. [5][6][7][8][9] Other than the hydrodynamic features like splash and cavity formation, there is an important structural perspective to the water impact problem. The central ques- tion is what is the pressure experienced by the object at impact; the t...

Water impact of a surface-patterned disk

“…The role of confined air in reducing the peak pressure was also described by Mathai et al. (2015), who found that a concave-nosed body exhibited a smaller peak pressure than a flat or convex-nosed body because of the presence of trapped air. In summary, our systematic experimentation has revealed that the compression and depressurization of trapped air bubbles during the initial impact phase leads to the rise and fall of the impact force and accounts for a reduction in the peak impact force.…”

“…Regarding axisymmetric slender bodies with different nose geometries pointing downward, those with ogive noses experience much smaller impact forces than those with flat noses (Bodily, Carlson & Truscott 2014). Mathai, Govardhan & Arakeri (2015) reported a reduction in peak pressure with a concave-nosed body, the result of air trapped inside the concave hole. In addition to a change in object shape, the conditions surrounding an impacting object were varied.…”

Slamming dynamics of diving and its implications for diving-related injuries

“…Naturally, these oscillations have a different origin from slamming itself. Flat (and concave) surfaces trap air bubbles in the process of piercing the water surface, and the oscillations of these bubbles have been found to give rise to the fluctuations in the force measurements ( 27 ). Assuming that the concave arch supporting human feet traps a bubble of radius w 2 (width of the feet model as shown in fig.…”

Bioinspired Universal Approaches for Cavity Regulation during Cylinder Impact Processes for Drag Reduction in Aqueous Media: Macrogeometry Vanquishing Wettability

“…By optimizing their toe’s shapes and striking posture, basilisk lizards can stabilize the air cavity long enough to supply sufficient upward support. Therefore, the geometric shape of the solid objects plays an equally important role in creating, regulating, and stabilizing the underwater air–water interface; however, it is often ignored. − In fact, the geometric shape [i.e., axisymmetric slender bodies (always cylinders) and spheres] and nose shape of objects (i.e., cone, ogive, flat, hemisphere, and concave) significantly influence the water entry, cavity formation, and cavity stabilization processes.…”