Impacts of Free-falling Spheres on a Deep Liquid Pool with Altered Fluid and Impactor Surface Conditions
Summary
This protocol demonstrates the basic experimental configuration for water entry experiments with free-falling spheres. Methods for the alteration of liquid surface with penetrable fabrics, the preparation of chemically non-wetting spheres, and steps for splash visualization and data extraction are discussed.
Abstract
Vertical impacts of spheres on clean water have been the subject of numerous water entry investigations characterizing cavity formation, splash crown ascension and Worthington jet stability. Here, we establish experimental protocols for examining splash dynamics when smooth free-falling spheres of varying wettability, mass, and diameter impact the free surface of a deep liquid pool modified by thin penetrable fabrics and liquid surfactants. Water entry investigations provide accessible, easily assembled and executed experiments for studying complex fluid mechanics. We present herein a tunable protocol for characterizing splash height, flow separation metrics, and impactor kinematics, and representative results which might be acquired if reproducing our approach. The methods are applicable when characteristic splash dimensions remain below approximately 0.5 m. However, this protocol may be adapted for greater impactor release heights and impact velocities, which augurs well for translating results to naval and industry applications.
Introduction
The characterization of splash dynamics arising from vertical impacts of solid objects on a deep liquid pool1 is applicable to military, naval and industrial applications such as ballistic missile water entry and sea surface landing2,3,4,5. The first studies of water entry were conducted well more than a century ago6,7. Here, we establish clear in-depth protocols and best practices for achieving consistent results for water entry investigations. To aid valid experimental design, a method is presented for the maintenance of sanitary conditions, alteration of interfacial conditions, control of dimensionless parameters, chemical modification of impactor surface, and visualization of splash kinematics.
Vertical impacts of free-falling hydrophilic spheres on the quiescent fluid show no sign of air-entrapment at low velocities8. We find that the placement of thin penetrable fabrics atop the fluid surface causes cavity formation due to forced flow separation1. A meager amount of fabric on the surface amplifies splashing across a range of moderate Weber numbers while sufficient layering attenuates splashing as spheres overcome drag at fluid entry1. In this article, we explain protocols suitable for establishing the effects of material strength on the water entry of hydrophilic spheres.
Cavity forming splashes from hydrophobic impactors show the ascension of a well-developed splash crown, followed by the protrusion of the primary jet high above the surface when compared to their water-liking counterparts8. Here, we present an approach for achieving water repellency through chemically modifying the surface of hydrophilic spheres.
With the advent of high-speed cameras, splash visualization and characterization have become more attainable. Even so, established standards in the field call for the use of a single camera orthogonal to the primary axis of travel. We show that the use of an additional high-speed camera for overhead views is necessary to adjudge spheres strike the intended location.
Protocol
Representative Results
Discussion
This protocol describes the experimental design and best practices for investigations of free-falling spheres onto a deep liquid pool. We begin by highlighting steps necessary for configuring the experiment for vertical impacts. It is important to create an ideal splash environment with the use of a sufficiently large splash zone such that wall effects are negligible9, and a suitable visual scale for extracting kinematics12,13,<…
Disclosures
The authors have nothing to disclose.
Acknowledgements
The authors would like to acknowledge the College of Engineering and Computer Sciences (CECS) at the University of Central Florida for funding this project, Joshua Bom and Chris Souchik for splash imagery and Nicholas Smith for valuable feedback.
Materials
| 3D Printer | FlashForge | Creator Pro | Dual Extrusion |
| Alcohol | Swan | M314 | 99% Isopropyl |
| BNC Cables | Thorlabs | 2249-C-24 | |
| Caliper | Anytime Tools | 203185 | Dial |
| Camera | Photron | Mini AX-100 | 16GB Ram |
| Computer | Dell | Windows 7 Pro | |
| Fabric | Georgia Pacific | 19378 | Toilet Paper |
| Fabric | Kleenex | 10036000478478 | Tissue |
| Laser Cutter | Glowforge | Basic | |
| Lights | GS Vitec | LT-V9-15 | Multi-LED |
| Microscope | Keyence | VHX-900F | Digital |
| Retort Stand | VWR | VWRF08530.083 | |
| Router | ASUS | RT-N12 | Off Network |
| Ruler | Westcott | 10432 | Meter Ruler |
| Software | Open-Source | Tracker | Video Analysis |
| Software | Photron | Fastcam Viewer | Video Recording |
| Sphere | Amazon | 8DELSET | Delrin |
| Spray | Rust-Oleum | 274232 | Water Repelling |
| Surfactant | Dawn | 37000973782 | Liquid Soap |
| Surfactant | USP Kosher | 5 Gallons | Glycerin |
| Tensile Tester | MTS | Model 42 | |
| Trigger Switch | Custom Made | ||
| Water Tank | Mr. Aqua | MA-730 | Non-Tempered Glass |
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