Universal mechanism for air entrainment during liquid impact

Hendrix, M.H.W.; Bouwhuis, Wilco; Meer, Devaraj van der; Lohse, Detlef; Snoeijer, Jacco H.

doi:10.1017/jfm.2015.757

Cited by 62 publications

(68 citation statements)

References 33 publications

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“…The initial fast regime lasts for about the first 5 mm of penetration, corresponding to about 2 drop diameters D. Here we have scaled the time with the impact time D/U . The initial penetration velocity is expected to be about half the drop impact velocity, owing to the additional virtual mass from the pool liquid; see Hendrix et al [46]. Indeed, for the largest impact velocity U, the initial asymptotes are in reasonable agreement with this, when we include the difference in the drop vs pool densities.…”

Section: Results For 500 Cst Dropsupporting

confidence: 62%

Vortex-induced buckling of a viscous drop impacting a pool

Beilharz

Thoroddsen

2017

Phys. Rev. Fluids

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CitationLi EQ, Beilharz D, Thoroddsen ST (2017) We study the intricate buckling patterns which can form when a viscous drop impacts a much lower viscosity miscible pool. The drop enters the pool by its impact inertia, flattens, and sinks by its own weight while stretching into a hemispheric bowl. Upward motion along the outer bottom surface of this bowl produces a vortical boundary layer which separates along its top and rolls up into a vortex ring. The vorticity is therefore produced in a fundamentally different way than for a drop impacting a pool of the same liquid. The vortex ring subsequently advects into the bowl, thereby stretching the drop liquid into ever thinner sheets, reaching the micron level. The rotating motion around the vortex pulls in folds to form multiple windings of double-walled toroidal viscous sheets. The axisymmetric velocity field thereby stretches the drop liquid into progressively finer sheets, which are susceptible to both axial and azimuthal compression-induced buckling. The azimuthal buckling of the sheets tends to occur on the inner side of the vortex ring, while their folds can be stretched and straightened on the outside edge. We characterize the total stretching from high-speed video imaging and use particle image velocimetry to track the formation and evolution of the vortex ring. The total interfacial area between the drop and the pool liquid can grow over 40-fold during the first 50 ms after impact. Increasing pool viscosity shows entrapment of a large bubble on top of the drop, while lowering the drop viscosity produces intricate buckled shapes, appearing at the earliest stage and being promoted by the crater motions. We also present an image collage of the most intriguing and convoluted structures observed. Finally, a simple point-vortex model reproduces some features from the experiments and shows variable stretching along the wrapping sheets.

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Section: Results For 500 Cst Dropsupporting

confidence: 62%

Vortex-induced buckling of a viscous drop impacting a pool

Beilharz

Thoroddsen

2017

Phys. Rev. Fluids

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“…The Laplace equation, ∇ 2 φ = 0, for the flow potential φ, is solved along the liquid interface, using the axisymmetric BI method Oguz et al 1995;Pozrikidis 1997;Bergmann et al 2009;Gekle et al 2010;Bouwhuis et al 2012Bouwhuis et al , 2013Bouwhuis et al , 2015Hendrix et al 2016). The dynamic boundary condition at the interface is the unsteady Bernoulli equation, which includes both hydrostatic pressure and surface tension.…”

Section: Numerical Methods 221 Bi Simulationsmentioning

confidence: 99%

“…This implies, first, that we neglect any small bubble entrapment caused by the increased gas pressure between the pool and the lowest drop, which is a valid assumption, as the length scale of the air bubble entrapment is much smaller than the drop size (Bouwhuis et al 2012Hendrix et al 2016) and will not influence the flow dynamics on the length scale of the cavity. Second, the neglect of the airflow implies that the spherical drops within the train all fall down undecelerated and undeformed (as depicted in figure 2).…”

Section: Parameters and Assumptionsmentioning

confidence: 99%

“…In contrast, for impact on a pool on these small length scales, capillary effects are expected to be much more significant when compared to the impact of millimetresized drops, or even dominating over gravity (Aristoff & Bush 2009). Capillary effects on the small air bubbles resulting from the air-film rupture have been investigated (Bouwhuis et al 2012;Lee et al 2012;Bouwhuis et al 2015;Hendrix et al 2016), but air bubbles due to the collision of surface waves within the cavity (called 'regular bubble entrapment', as described in Pumphrey & Elmore (1990), , Wang et al (2013), Chen & Guo (2014) for millimetre-sized drops) for single impacting microdrops is still ongoing research. Impacting microjets and microdrop trains on liquid pools have not yet been studied in detail.…”

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confidence: 99%

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Impact of a high-speed train of microdrops on a liquid pool

et al. 2016

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A train of high-speed microdrops impacting on a liquid pool can create a very deep and narrow cavity, reaching depths more than 1000 times the size of the individual drops. The impact of such a droplet train is studied numerically using boundary integral simulations. In these simulations, we solve the potential flow in the pool and in the impacting drops, taking into account the influence of liquid inertia, gravity and surface tension. We show that for microdrops the cavity shape and maximum depth primarily depend on the balance of inertia and surface tension and discuss how these are influenced by the spacing between the drops in the train. Finally, we derive simple scaling laws for the cavity depth and width.

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“…Water droplets, for example, can break off ligaments and entrain air upon impact with the free-surface (see Figure 24a). This type of air entrainment has been studied extensively primarily in simplified configurations (Esmailizadeh and Mesler, 1986;Oguz and Prosperetti, 1990;Hasan and Prosperetti, 1990;Tomita et al, 2007;Ray et al, 2015;Hendrix et al, 2016). In such case, a droplet falling towards the free surface traps air between it and the water surface.…”

Section: Analysis Of Turbulent Structuresmentioning

confidence: 99%

Air Entrainment and Surface Fluctuations in a Turbulent Ship Hull Boundary Layer

Masnadi

Erinin

Washuta

et al. 2019

Journal of Ship Research

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The air entrainment due to the turbulence in a free surface boundary layer shear flow created by a horizontally moving vertical surface-piercing wall is studied through experiments and direct numerical simulations.In the experiments, a laboratory-scale device was built that utilizes a surface-piercing stainless steel belt that travels in a loop around two vertical rollers, with one length of the belt between the rollers acting as a horizontally-moving flat wall. The belt is accelerated suddenly from rest until reaching constant speed in order to create a temporally-evolving boundary layer analogous to the spatially-evolving boundary layer that would exist along a surface-piercing towed flat plate. To complement the experiments, Direct Numerical Simulations (DNS) of the two-phase boundary layer problem were carried out with the domain including a streamwise belt section simulated with periodic boundary conditions. Cinematic Laser-Induced Fluorescence (LIF) measurements of water surface profiles in two vertical planes oriented parallel to the belt surface (wall-parallel profiles) are presented and compared to previous measurements of profiles in a vertical plane oriented normal to the belt surface (wall-normal profiles). Additionally, photographic observations of air entrainment and measurements of air bubble size distributions and motions are reported herein. The bubble entrainment mechanisms are studied in detail through the results obtained by the DNS simulations. Free surface features resembling breaking waves and traveling parallel to the belt are observed in the wallparallel LIF movies. These free surface features travel up to 3 times faster than the free surface features moving away from the belt in the wall-normal LIF movies. These breaking events are thought to be one of the mechanisms by which the air is entrained into the underlying flow. The bubble size distribution is found to have a characteristic break in slope, similar to the Hinze scale previously observed in breaking waves (Deane and Stokes, 2002). The number of bubbles, their velocity, and size are re-ported versus depth from the experimental data. These results are qualitatively similar to results obtained by the simulations. Finally, several entrainment mechanisms are found in the simulations and their prevalence in the free surface boundary layer is assessed.

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Universal mechanism for air entrainment during liquid impact

Cited by 62 publications

References 33 publications

Vortex-induced buckling of a viscous drop impacting a pool

Vortex-induced buckling of a viscous drop impacting a pool

Impact of a high-speed train of microdrops on a liquid pool

Air Entrainment and Surface Fluctuations in a Turbulent Ship Hull Boundary Layer

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