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Effect of ambient gas on cavity formation for sphere impacts on liquids
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Williams, Hollis (2022) Effect of ambient gas on cavity formation for sphere impacts on liquids. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3922089
Abstract
Formation of a splash crown and a cavity following the impact of a sphere on a body of liquid is a classical problem. In the related problem of droplet splashing on a flat surface, it has been established that the properties of the surrounding gas can influence the threshold speed at which splashing occurs. At lower impact speeds, this is due mainly to the influence of gas kinetic effects, since the height of the gas lubrication film which is displaced during dynamic wetting is often comparable to the mean free path of the gas. At higher Weber and Reynolds numbers, on the other hand, inertial effects dominate and the density of the gas becomes important in determining whether a splash occurs. Since the influence of the ambient gas on the droplet impact has been known since the beginning of the twentieth century, it is of theoretical importance to investigate whether a similar phenomenon occurs during entry of projectiles into liquids and test the applicability of the wetting theory which is used for droplet impacts. In this work, we present experimental and theoretical investigations of the influence of the ambient gas on cavity formation during a sphere impact on a free liquid surface. In the experiments, a vacuum chamber was used to provide ambient conditions from atmospheric pressure down to 1/100-th of an atmosphere, whist high-speed photography is used to provide detailed images both of the cavity beneath the surface and the splash crown and jets which form above the surface. To elucidate the primary forces at work, we vary the radius of the sphere, the properties of the liquid, the ambient gas pressure, and the type of gas. It is found that the threshold entry speed for cavity formation is influenced by the density of the surrounding gas, whereas changing the mean free path of the gas has no effect. We attribute this phenomenon to the gas slowing the sealing of the thin crown sheet behind the sphere. This assertion is supported with detailed optical measurements of the liquid sheet thickness. In the range of parameters considered, the splash crown influences the movement of the contact line. Finally, we extend our investigations by performing simulations of the impact of a hydrophobic sphere on the surface of a body of water in the presence of ambient air above the liquid. For this purpose, we employ a three-dimensional Eulerian volume of fluid (VOF) multiphase model which uses overlapping meshes. We find that decreasing ambient gas density is not able to suppress cavity formation in the case of hydrophobic spheres.
Item Type: | Thesis (PhD) | ||||
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Subjects: | Q Science > QD Chemistry T Technology > TA Engineering (General). Civil engineering (General) T Technology > TP Chemical technology |
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Library of Congress Subject Headings (LCSH): | Splashes, Impact, Boundary layer, Sphere, Gas dynamics, Fluid dynamics, Curves on surfaces | ||||
Official Date: | September 2022 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | School of Engineering | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Denissenko, Petr | ||||
Format of File: | |||||
Extent: | 128 pages :illustrations (some colour) | ||||
Language: | eng |
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