Numerical Investigation of Water Entry of Hydrophobic Spheres

TL;DR

This study uses numerical simulations to analyze how hydrophobic spheres enter water, revealing that hydrophobic surfaces create air cavities that reduce drag compared to hydrophilic spheres.

Contribution

The paper introduces a detailed numerical approach to compare hydrophobic and hydrophilic sphere water entry, highlighting the impact of surface properties on drag reduction.

Findings

Hydrophobic spheres form air cavities during water entry.

Hydrophobic spheres have lower drag coefficients than hydrophilic ones.

Air cavity formation reduces water contact area, decreasing drag.

Abstract

We perform numerical simulations to study the dynamics of the entry of hydrophobic spheres in a pool of water using ANSYS. To track the air-water interface during the translation of the sphere in the pool of water, we use the volume of fluid (VOF) model. The continuum surface force (CSF) method computes the surface tension force. To simulate the hydrophobic surface properties, we also include wall adhesion. We perform simulations with different diameters and impact speeds of the sphere. Our simulations capture the formation of different types of air cavities, pinch-offs of these cavities, and other finer details similar to the experiments performed at the same parameters. Finally, we compare the coefficient of drag among the different hydrophobic cases. We further perform simulations of hydrophilic spheres impacting the pool of water and compare the drag coefficient with the analogous hydrophobic cases. We conclude that the spheres with hydrophobic surfaces have a lower drag coefficient than their hydrophilic counterparts. This lower drag of the hydrophobic spheres is attributed to the formation of the air cavity by the hydrophobic surfaces while translating through the pool of water, which reduces the area of the sphere in contact with water. In contrast, no such air cavity forms in the case of hydrophilic spheres.