Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session L27: Flow Instability: Interfacial and Thin Film |
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Chair: Philipp Schlatter, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) Room: 251 E |
Monday, November 25, 2024 8:00AM - 8:13AM |
L27.00001: Interfacial phase-change and geometry modify nanoscale pattern formation in irradiated thin films Tyler P Evans, Scott A Norris When a Group IV semiconductor wafer is irradiated, it may amorphize. Beyond a certain angle of irradiation, the so-called critical angle, spontaneous pattern formation on the amorphized surface is often observed, but differences in the formed patterns, their angle-dependence (especially critical angle for pattern formation), and other behavior across various ion, target and energy combinations currently lacks a comprehensive explanation. To address this, we consider the hydrodynamic stability of ion-irradiated thin films, modeled as a modified Stokes flow, where the typically-used no-penetration boundary condition has been relaxed to a phase-change or mass conservation boundary condition. Then, we determine simple closed-form expressions for the geometry of the amorphous–crystalline interface entirely in terms of the free interface and the statistics of the implantation-induced collision cascade, an improvement on frequently-used ad hoc approximations. We find that phase-change at the amorphous–crystalline boundary, and the geometry of that boundary, impart a surprisingly strong ion-, target-, and energy- dependence. For validation of our theoretical work, we consider argon-irradiated silicon, where the presence of phase-change at the amorphous–crystalline interface appears to correctly predict the experimentally observed, strong suppression of pattern formation near 1.5 keV for that system. |
Monday, November 25, 2024 8:13AM - 8:26AM |
L27.00002: Three-dimensional wave dynamics of falling film flows on structured surfaces Andrea Düll, Lyes Kahouadji, Marion Boernhorst, Thomas Haeber, Omar K Matar, Olaf Deutschmann In applications such as falling film absorbers and reactors, unsteady film flows play a crucial role in enhancing heat and mass transfer processes. This study investigates the use of structured surfaces to induce suchtransient film instabilities. To optimize the employed structure dimensions, an in-depth understanding ofthestructure-induced wave evolution is essential, whichis developed through a combined experimental and computational approach. Experimentally, high-speed camera imaging and alight absorption technique are used to reconstruct the spatiotemporal evolution of the interface. The transient film instabilities are observed to evolve from an initially steady base flow, and an optimal structure distance-to-height ratio is identified, at which particularly strong interfacial oscillations are induced in the falling film. Detailed insights into the local flow phenomena under such resonance-like conditions are gained using three-dimensional direct numerical simulations with a hybrid front-tracking/level-set interface capturing algorithm. The simulations show that strong interfacial oscillations are generally associated with the occurrence of unsteady internal recirculation zones. In mass transfer applications, these mixing zones effectivelybroaden the concentration boundary layer, leading to a significant increase in the liquid-side mass transfer coefficient. |
Monday, November 25, 2024 8:26AM - 8:39AM |
L27.00003: Stabilising unstable steady states of falling liquid films using optimal feedback control Oscar Holroyd, Radu Cimpeanu, Susana N Gomes The dynamics of thin liquid films falling under gravity is an excellent example of a highly complex, nonlinear, interfacial flow. Such problems are typically too complex for standard control theoretical results to be applicable. We propose a method to stabilise unstable steady-state solutions to such systems using a finite number of sites at the lower boundary wall where fluid can either be injected or removed. Based on approaches proved to control the Kuramoto-Sivashinsky equation, we link the two problems we chain together a hierarchy of increasingly idealised approximations: an asymptotic expansion followed by linearising and then discretising. For the simplest approximation we can design optimal feedback controls using a linear quadratic regulator. We show that this can be successfully applied to direct numerical simulations of the full Navier-Stokes system over a wide range of parameters, even when observations are restricted to a finite number of points. |
Monday, November 25, 2024 8:39AM - 8:52AM |
L27.00004: Direct numerical simulations of turbulence in a film flowing over a rapidly spinning disc Omar K Matar, Jason Stafford, Lyes Kahouadji We consider the dynamics of a thin liquid film flowing over a rapidly spinning disc. This flow is accompanied by the formation of interfacial waves that originate near the flow inlet and travel towards the disc periphery. Several flow regimes have been identified as a result of previous experimental, modelling, and computational work; these include two-dimensional, transitional three-dimensional, and fully-three-dimensional waves. Reduced-order, weighted residual-type approaches,have been used to model theinterfacial dynamics with some success for moderate flow rates and disc rotational speeds for which the flow remains laminar throughoutthe film. Large eddy simulations have also been utilized to simulate turbulent film flows. In the present work, we use a direct numerical simulations (DNS) approach to examine the flow field within the film in the turbulent regime and its effect on the shapes of the interfacial waves. We start with a divergence-free, synthetic turbulent inflow and demonstrate the emergenceof sustained turbulence downstream of the inletand highlight how it imprints on the interface. An attempt is also made to abstract from the DNS results velocity closures with which to build reliable approximate models. |
Monday, November 25, 2024 8:52AM - 9:05AM |
L27.00005: Stability and dynamics of thin film flow on a rotating cylinder Souradip Chattopadhyay, AMAR K GAONKAR, Hangjie Ji The study of thin liquid films flowing along vertical cylinders has gained significant attention due to their rich dynamics and various industrial applications. In this work, we investigate the influence of rotation on the dynamics of a falling liquid film along the outside or inside of a rotating vertical cylinder. We explore various setups, including slippery cylinder surfaces and thermal effects. The interplay between gravity-driven film flow and cylinder rotation, in the presence of wall slippage or thermal effects, introduces complex fluid dynamics. We perform linear stability analysis and extend it beyond the linear regime. Additionally, we propose a combination of these physical effects to explain the traveling wave solution and further investigate the stability of this solution. Numerical simulations and theoretical analysis are employed to understand the intricate dynamics and heat transfer characteristics of this system. The findings offer insights into optimizing industrial applications involving heat and mass transfer or slippery cylinder surfaces. Stability analysis and numerical simulations agree well with theoretical observations. |
Monday, November 25, 2024 9:05AM - 9:18AM |
L27.00006: Role of odd viscosity in shear-imposed heated falling film down a slippery slope Souradip Chattopadhyay, Ashutosh Bijalwan, AMAR K GAONKAR We investigate the flow of a thin liquid film down a uniformly heated, slippery slope under the influence of gravity. The liquid-air interface is subjected to a constant imposed shear stress along the flow direction. Our model assumes that the time-reversal symmetry of the liquid is broken, introducing an additional viscosity coefficient known as odd viscosity. This study aims to explore the impact of odd viscosity on surface wave and shear wave dynamics in the presence of wall slippage, imposed shear stress, and thermal effects. To analyze the linear stability of this system, we have constructed an Orr-Sommerfeld type boundary value problem (OS BVP). The OS BVP is numerically solved using the Chebyshev spectral collocation method for a range of Reynolds numbers from low to high. We have found that while both the imposed shear and Marangoni number destabilize surface and shear modes, the instabilities can be mitigated by the odd viscosity coefficient. |
Monday, November 25, 2024 9:18AM - 9:31AM |
L27.00007: Self-similar dynamics of axisymmetric point rupture of highly viscous liquid sheets Ajay Harishankar Kumar, Hansol Wee, Vishrut Garg, Sumeet S Thete, Osman A Basaran High-viscosity liquid sheets arise in coating flows and polymer processing. If sufficiently thin, sheets rupture due to van der Waals (vdW) forces and exhibit self-similar dynamics. Witelski et al. (2001) investigated the axisymmetric point rupture of liquid sheets under the influence of inertia and viscosity. They uncovered the self-similar nature of the dynamics, highlighting a balance between inertial, viscous, and vdW forces as the film thins. Moreover, they showed that the similarity is of the first kind with rational power-law scaling exponents that relate the free surface height, lateral length, and lateral velocity with time remaining to rupture. They highlighted sheet rupture in three practically important geometries: axisymmetric point, line, or ring. Subsequently, Thete et al. (2016) built on their pioneering work to investigate line rupture in the low and high viscosity (Stokes) limits. In the latter, Thete et al. showed that the dynamics exhibits self-similarity of the second kind with a dominant balance between viscous and vdW forces in line rupture. Here, we investigate the dynamics of axisymmetric point and ring rupture under Stokes flow conditions, unveiling unexpected findings during point rupture indicating a heretofore unknown dependence on initial conditions. |
Monday, November 25, 2024 9:31AM - 9:44AM |
L27.00008: ABSTRACT WITHDRAWN
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Monday, November 25, 2024 9:44AM - 9:57AM |
L27.00009: Solutal Marangoni stresses delay Rayleigh-Taylor instability Minwoo Choi, Hyejoon Jun, Hyoungsoo Kim Understanding the Rayleigh-Taylor instability of volatile liquid films is crucial in widespread applications like coating, wetting and painting to prevent liquid droplets from falling. To date, the thermal effects have been mainly considered for the instability of volatile liquid films, including binary mixtures, while the solutal Marangoni effects were ignored because the major assumption was the very thin film thickness and small Peclet number. In this study, we explored how the instability of the suspended binary mixture thin film under a ceiling was influenced by the solutal Marangoni effect through both numerical and experimental investigations. Here, we systematically controlled a dimensionless Marangoni number to investigate its stability. We derived simplified long-wave evolution equations for the gas-liquid interface, and performed a linear stability analysis. Furthermore, we performed a deflectometry experiments that showed that the binary mixture thin film was more unstable when the surface tension of the more volatile component was higher than that of the other component. Conversely, when the surface tension of the more volatile component was lower, the instability was either stabilized or exhibited an oscillatory mode, which delayed the exponential increase in amplitude. As a result, we propose that binary mixtures where the volatile component has lower surface tension are more effective at preventing liquid droplet falling by delaying the exponential growth of film thickness. |
Monday, November 25, 2024 9:57AM - 10:10AM |
L27.00010: THE ROLE OF MARANGONI EFFECT ON THE NON-ISOTHERMAL FALLING FLUID FILM INSTABILITY Ayhan Yiğit Y Özel, Luca Biancofiore, Christian Ruyer-Quil The Marangoni effect, caused by temperature-induced surface tension variations along an inclined/vertical plane, creates dynamic fluid film flows with roll waves. When an inclined plane is non-isothermally heated from below, gravity and thermocapillarity drive fluid flow, leading to strong nonlinear wave evolution along the film. Forcing at the inlet can amplify thermocapillarity, increasing flow instability and enhancing wave formation. |
Monday, November 25, 2024 10:10AM - 10:23AM |
L27.00011: Abstract Withdrawn |
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