Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G13: Free Surface Flow IV: Thin Film Flow |
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Chair: Benoit Scheid, Université Libre de Bruxelles Room: 201 |
Monday, November 23, 2015 8:00AM - 8:13AM |
G13.00001: Critical inclination for absolute dripping in falling films subject to Rayleigh-Taylor instability Benoit Scheid, Wilko Rohlfs Liquid films flowing down the underside of inclined plates are subject to film flow instabilities causing a patterned and wavy topology as well as to the classical Rayleigh-Taylor (R-T) instability. The R-T instability results from the denser liquid film being located above a less dense liquid, which is in this case the ambient gaseous phase. Owing to the instability, large amplitude surface deformations form which can result in the formation of droplet and finally droplet detachement if no saturation mechanism arises. This study examines the critical angle for the R-T instability in a falling film between the regime of absolute and convective (A/C) instability using the weighted integral boundary layer approach. In the absence of saturation, the threshold determines whether immediate dripping occurs, e.g. in the entire domain, or if the instability is of convective type, such that waves and eventually drops form while the perturbation is moving downwards the inclined plate. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G13.00002: Rayleigh-Plateau instabilities on Drop on Demand Jetting Cristina Rodriguez-Rivero, Jose Rafael Castrejon-Pita, Ian Hutchings The fate of liquid filaments is a complex phenomenon; a filament can either collapse into a single drop or break-off into multiple droplets. The final result depends on the liquid viscosity, the shape and the inner dynamics of the filament. In addition, it has been suggested that Rayleigh-Plateau instabilities also play a role in the breakup. In this work we use high-speed imaging and the adequate instrumentation to control the breakup of liquid filaments generated from a Drop on Demand system. In these experiments, we induce a controlled perturbation, matching the optimal wave number from the Rayleigh-Plateau model, on liquid filaments produced by a droplet generator. Our setup can control the wave number, duration and time of the perturbation. Our results found that both the amplitude and frequency of the Rayleigh-Plateau instability are critical on the break-off behavior. This work was supported by the UK EPSRC (Grant EP/H018913/1) and the Impact Acceleration grant from the University of Cambridge (EP/K503757/1). [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G13.00003: Thin liquid film in polymer tubing : dynamics and dewetting in partial wetting condition Pascaline Hayoun, Alban Letailleur, J\'er\'emie Teisseire, Emilie Verneuil, Fran\c{c}ois Lequeux, Etienne Barthel Polymers such as PVC and Silicone are low cost materials widely used in industry to produce tubing for fluid transport. Most of these applications involve repeated, intermittent flow of liquids which can lead to unwanted contamination. This study aims at better understanding contamination mechanisms during intermittent flow in polymer tubing, and at elucidating the relation between flow, wetting and contamination. We experimentally and theoretically investigate, flow regimes as well as dewetting process at the triple line induced by gravity flow of a vertical liquid slug in a cylindrical geometry. Our results for Newtonian fluids evidence a succession of thick film formation, hydraulic jump creation in the thickness profile, oscillatory regime and destabilization leading to substrate contamination. In order to understand theoretically the flow, one crucial quantity to assess is the film thickness in the inside of the tube. Based on an absorption measurement method, we provide explanations for behaviors and flow regimes observed experimentally. [Preview Abstract] |
(Author Not Attending)
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G13.00004: Stability of Liquid Films on Strings Vineet Nair, Ishan Sharma, Viswanathan Shankar The dynamics and stability of liquid films on rigid substrates is a well studied problem with recent studies extending the analysis to flexible substrates. Here, we study the stability of a liquid film on a string. The string is a one-dimensional continuum and we consider it to be linear elastic, isotropic, homogeneous, and flexible. It is assumed that the slope made by the string is small and that the motion is planar and the displacements are in transverse direction. The liquid film is a two-dimensional continuum and we consider it to be a Newtonian fluid with uniform density, viscosity, and surface tension. We consider the cases where the string has an initial horizontal configuration and inclined configuration, including both the finite and infinite cases. We use the lubrication approximation to simplify the governing equations and boundary conditions. The fluid-solid coupling results in a set of two coupled nonlinear partial differential equations in film thickness and string displacement. Subsequently, a linear stability analysis will be carried out and the equations will be solved numerically. The ultimate objective of this study is to understand the behavior of liquid film flow over translating structures such as strings and beams that may or may not be of finite extent [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G13.00005: Inviscid instability of two-fluid free surface flow down an incline Sukhendu Ghosh, R Usha, Rama Govindarajan, Outi Tammisola Film flow down an incline is known to display an interesting array of instabilities. Such flows are often stratified in viscosity, and this stratification can create or suppress instabilities. We examine how much of this occurs through an inviscid mechanism, by modeling the velocity profile as piecewise linear. Besides obtaining qualitative agreement between viscous and inviscid results we present several limiting cases. It is interesting to show that a variation in viscosity acts via an inviscid mechanism to stabilize or destabilize the flow. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G13.00006: Gravity-driven liquid flow over a flexible beam Hyoungsoo Kim, Peter Howell, Marinela Popova, Howard Stone We study theoretically and experimentally the time dependence of a liquid spreading along a flexible beam. The flow is modeled using lubrication theory and the substrate is modeled as an (Euler-Bernoulli) elastic beam. We classify the model problem into two cases depending on the maximum beam deflection angle $\phi_{max}$ from the horizontal, i.e. a small deflection ($\phi_{max}$ $<$ $30^{\circ}$) and large deflection ($30^{\circ}$ $<$ $\phi_{max}$ $<$ $90^{\circ}$). For a small deflection case, we obtain asymptotic solutions for the liquid propagation speed for the early time and terminal time periods, which for the front position $\sigma$($t$) show power-law behaviors $\sigma$($t$) $\sim$ $t^{4/5}$ and $\sigma$($t$) $\sim$ $t^{4}$, respectively. The theoretical model also predicts the deflection angle of the beam at the propagating liquid front. We validate the results with experiments, which show good agreement with theory. Furthermore, for large beam deflections, we obtain experimental results demonstrating power-law behaviors, $\sigma$($t$) $\sim$ $t$ and $\phi$($t$) $\sim$ $t^{2}$ for the early time period. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G13.00007: A model for liquid film in steam turbine Amelie Simon, Meryem Marcelet, Jean-Marc Herard, Jean-Marc Dorey, Michel Lance Wetness in steam turbines induces losses and erosion. Drops are created due to the fast expansion of the steam (homogeneous nucleation) and the impurities in the steam (heterogeneous nucleation). The droplets grow and some among them settle on the blade leading to a thin liquid film. This film may then be atomized into coarse water drops which crash on the following blades. The liquid film configuration is a thin film on a curved surface, created by the drop deposit and under high steam friction. In steam turbines, the liquid film is subject to high rotational effect (rotor) and/or to negative gravity. Moreover, due to interfacial instabilities, some drops are torn off from the film. The retained approach is an integral formulation of the Navier-Stokes equation (or shallow water equation) with specific terms. The derivation of these equations requires some closure laws for the convection contributions, the Coriolis terms and for terms related to the additional mass coming from the drops deposit. Once chosen, mathematical and mechanical analyses are performed (hyperbolicity, entropy, galilean and rotational invariance). A two-dimensional code has been developed based on finite volume method to simulate numerically this liquid film model for steam turbines. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G13.00008: Healing Capillary Films Zhong Zheng, Marco Fontelos, Sangwoo Shin, Howard Stone We study the dynamics of a healing viscous thin film driven by surface tension, i.e., the inward spreading of a film to fill a “hole” in a thin film. A fourth-order nonlinear partial differential equation is obtained to characterize the time evolution of the film thickness and the novel part of study is then to seek self-similar solutions of the second kind. In this way, we are able to obtain a self-similar solution that describes the interface shape, with the scaling exponent determined by solving a nonlinear eigenvalue problem. The self-similar solution is then compared with the full numerical solution of the partial differential equation, and we observe good agreement. Laboratory experiments have also been conducted using various silicone oils, and the time evolution of the front location and the interface shape can be obtained. A comparison between the theoretical predictions and the experimental observations produces good agreement in both the front location and the interface shape. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G13.00009: Viscoelastic Liquid Curtain Luc Lebon, Laurent Limat, Antoine Gaillard, Julien Beaumont, Henri Lhuissier We have investigated experimentally the properties and stability of viscoelastic curtains, falling from a long thin slot and maintained laterally by two highly wetting wires. We have observed several original facts, compared to the seminal work of Brown and Taylor [1] on Newtonian curtains: (1) The stability with respect to breaking is considerably enhanced by the use of appropriate polymers. Even strange tree-like falling filament structures can be also stabilised, though less interesting for applications. (2) Specific instabilities can be observed, when the amount of polymers is excessive, with spatial and temporal modulations of the coating thickness. (3) Even the base state is modified, and does NOT reduce at large scale to a free fall, even slightly displaced vertically from the expected profile. We present this experimental exploration and also some attempts of analytical modeling based on Rheological theories of complex fluids. \\[4pt] [1] D. R. Brown, ``A study of the behaviour of a thin sheet of moving liquid,'' \textit{J. Fluid Mech}., \textbf{10}, 297-305 (1961) [Preview Abstract] |
Monday, November 23, 2015 9:57AM - 10:10AM |
G13.00010: Cracking the chocolate egg problem: polymeric films coated on curved substrates Pierre-Thomas Brun, Anna Lee, Joel Marthelot, Gioele Balestra, François Gallaire, Pedro Reis Inspired by the traditional chocolate egg recipe, we show that pouring a polymeric solution onto spherical molds yields a simple and robust path of fabrication of thin elastic curved shells. The drainage dynamics naturally leads to uniform coatings frozen in time as the polymer cures, which are subsequently peeled off their mold. We show how the polymer curing affects the drainage dynamics and eventually selects the shell thickness and sets its uniformity. To this end, we perform coating experiments using silicon based elastomers, Vinylpolysiloxane (VPS) and Polydimethylsiloxane (PDMS). These results are rationalized combining numerical simulations of the lubrication flow field to a theoretical model of the dynamics yielding an analytical prediction of the formed shell characteristics. In particular, the robustness of the coating technique and its flexibility, two critical features for providing a generic framework for future studies, are shown to be an inherent consequence of the flow field (memory loss). The shell structure is both independent of initial conditions and tailorable by changing a single experimental parameter. [Preview Abstract] |
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