Water entry without surface seal: Extended cavity formation

Mansoor, Mohammad M.; Marston, Jeremy; Vakarelski, Ivan Uriev; Thoroddsen, Sigurdur T

Water entry without surface seal: Extended cavity formation

Type

Article

Authors

Mansoor, Mohammad M.

Marston, Jeremy
Vakarelski, Ivan Uriev

Thoroddsen, Sigurdur T

KAUST Department

Clean Combustion Research Center
High-Speed Fluids Imaging Laboratory
Mechanical Engineering Program
Physical Science and Engineering (PSE) Division

Online Publication Date

2014-03-04

Print Publication Date

2014-03

Date

2014-03-04

Abstract

We report results from an experimental study of cavity formation during the impact of superhydrophobic spheres onto water. Using a simple splash-guard mechanism, we block the spray emerging during initial contact from closing thus eliminating the phenomenon known as 'surface seal', which typically occurs at Froude numbers Fr= V0 2/(gR0) = O(100). As such, we are able to observe the evolution of a smooth cavity in a more extended parameter space than has been achieved in previous studies. Furthermore, by systematically varying the tank size and sphere diameter, we examine the influence of increasing wall effects on these guarded impact cavities and note the formation of surface undulations with wavelength λ =O(10)cm and acoustic waves λa=O(D0) along the cavity interface, which produce multiple pinch-off points. Acoustic waves are initiated by pressure perturbations, which themselves are generated by the primary cavity pinch-off. Using high-speed particle image velocimetry (PIV) techniques we study the bulk fluid flow for the most constrained geometry and show the larger undulations ( λ =O (10cm)) have a fixed nature with respect to the lab frame. We show that previously deduced scalings for the normalized (primary) pinch-off location (ratio of pinch-off depth to sphere depth at pinch-off time), Hp/H = 1/2, and pinch-off time, τ α (R0/g) 1/2, do not hold for these extended cavities in the presence of strong wall effects (sphere-to-tank diameter ratio), ε = D 0/Dtank 1/16. Instead, we find multiple distinct regimes for values of Hp/H as the observed undulations are induced above the first pinch-off point as the impact speed increases. We also report observations of 'kinked' pinch-off points and the suppression of downward facing jets in the presence of wall effects. Surprisingly, upward facing jets emanating from first cavity pinch-off points evolve into a 'flat' structure at high impact speeds, both in the presence and absence of wall effects. © 2014 Cambridge University Press.

Citation

Mansoor, M. M., Marston, J. O., Vakarelski, I. U., & Thoroddsen, S. T. (2014). Water entry without surface seal: extended cavity formation. Journal of Fluid Mechanics, 743, 295–326. doi:10.1017/jfm.2014.35