Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session Q05: Free-surface Flows: Hydraulic Jumps and Instability |
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Chair: Julie Crockett, Brigham Young University Room: Georgia World Congress Center B207 |
Tuesday, November 20, 2018 12:50PM - 1:03PM |
Q05.00001: Axisymmetric Water Jet Impingement on Superheated Superhydrophobic Surfaces D. Jacob Butterfield, Julie Crockett Superhydrophobic (SH) surfaces, characterized by the presence of air cavities between microstructures, possess self-cleaning properties and present an attractive solution to fouling during jet impingement but also impede heat transfer. Previous research shows that film boiling occurs on superheated SH surfaces, rather than more effective nucleate boiling, as a thin insulating vapor film forms between the surface and the liquid due to these cavities. Differences between heat transfer on SH and smooth surfaces during liquid jet impingement were explored experimentally. A smooth silicon wafer or a microstructured, Teflon-coated SH surface was clamped to an aluminum block heated to temperatures up to 330C. An axisymmetric water jet impinged and cooled the surface while a high-speed camera recorded boiling behavior. Heat flux and surface temperature for various radial locations were recorded via embedded thermocouples in the block. Boiling curves specifying Leidenfrost temperature and overall local heat transfer coefficients were determined. An equation to model heat transfer as a function of surface superheat and wettability, jet Reynolds number, radial distance and impingement angle is developed. |
Tuesday, November 20, 2018 1:03PM - 1:16PM |
Q05.00002: Jet impingement on wettability-patterned surfaces Souvick Chatterjee, Uddalok Sen, Julie C Crockett, Ranjan Ganguly, Constantine Megaridis The orthogonal impingement of liquid jets on flat, impermeable substrates is useful for a number of applications. Impingement on a wettable surface possessing a sufficient downstream liquid depth leads to the formation of a classical hydraulic jump, where the jet forms a thin film on the substrate up to a distance from the point of impact and then the liquid film height suddenly increases. On the other hand, a downstream depth cannot be sustained on a superhydrophobic surface. Therefore, upon impact on such surfaces, the liquid jet spreads radially as a thin film, which subsequently breaks up into droplets that are randomly ejected. A wettability-patterning technique is demonstrated in the present work that can delay or even eliminate droplet breakup for jet impingement on horizontal superhydrophobic substrates. An analysis for the prediction of the hydraulic jump and droplet breakup locations is presented, thus facilitating the design of the wettability pattern required to cause a desired outcome. The model is further extended to determine the radial variation of the competing forces. |
Tuesday, November 20, 2018 1:16PM - 1:29PM |
Q05.00003: Investigation of Bed Pressure Fluctuations and Turbulence Distributions in a Gated Box Culvert Hamid Bazgirkhoob, David Admiraal Turbulence distributions and pressure fluctuations downstream of a sluice gate in a culvert flow with a fully submerged hydraulic jump are investigated for different Reynolds numbers based on gate height. Particle Image Velocimetry (PIV) is used to capture turbulence distributions and pressure transducers are used to monitor pressures and pressure fluctuations on the bed. Incursions of low velocity flow from the recirculation zone above the culvert inlet cause large spikes in negative and positive pressure. A finite-volume method is applied to numerically solve the governing equations for flows with the same conditions as previously run experiments. The volume of fluid (VOF) method is used to calculate the free surface in the forebay. To model turbulence stresses, Reynolds-averaged Navier-Stokes (RANS) turbulence models are employed. Reynolds stress turbulence closure Model (RSM) and Realizable k-ε turbulence model simulate the submerged hydraulic recirculation zone and the tail water free surface well, as validated with PIV experimental results. |
Tuesday, November 20, 2018 1:29PM - 1:42PM |
Q05.00004: Satellite-droplet formation regimes in the natural breakup of clean and surfactant-laden liquid threads Alejandro Martínez-Calvo, Javier Rivero-Rodríguez, Benoit Scheid, Alejandro Sevilla We report a numerical study of the natural breakup of Newtonian liquid threads whose cylindrical interface is coated with insoluble surfactants. A parametric study is performed to quantify the influence of the Laplace number, La, and the elasticity parameter, β, on the volume of the satellite droplet formed just prior to pinch-off, V_{sat}, on the mass of surfactant trapped at its surface, Σ_{sat}, on the break-up time, and on its shape. For a clean interface, we provide a new scaling law for the satellite volume normalized with the total volume, namely V_{sat} = 0.00421 La^{1.64,} valid for 0.05<La<1. When La<0.05, V_{sat} becomes negligible, and for La>10 it reaches a plateau of about 3%. In the surfactant-laden case, for La>7.5, V_{sat} displays a local minimum in β, but has small relative variations around the 3%, while the mass of surfactant increases monotonously. For 1.69<La<7.5, there is a discontinuous transition at β = β_{c}(La), in V_{sat} and Σ_{sat}. |
Tuesday, November 20, 2018 1:42PM - 1:55PM |
Q05.00005: Stability and collapse of holes in liquid layers on bounded substrates Michael Eigenbrod, Cunjing Lu, Steffen Hardt We study experimentally and theoretically the stability regime and collapse dynamics of holes in liquid layers on bounded substrates having different wettabilities. It is shown that a critical diameter d_{c} exists which is a measure indicating the onset of hole closure. d_{c} depends, among others, on the thickness of the layer, the wettability of the substrate as well as the size of the domain. The wettability of the substrate only has a minor influence on d_{c}, whereas the influence of the size of the substrate is pronounced. The experimental results are in good agreement with the theoretical predictions of the stability threshold for a broad range of wettabilities and substrate sizes. We further show that the collapse dynamics and the evolution of the minimum thickness of the liquid film after collapse obey different power laws. Surface tension triggers the collapse of the hole, whereas the dynamics of the collapse is solely determined by inertia. By contrast, the evolution of the minimum thickness of the liquid film after collapse is the result of a balance between capillary and inertial forces. Corresponding scaling coefficients are determined. |
Tuesday, November 20, 2018 1:55PM - 2:08PM |
Q05.00006: Optimal interface distortion of a round liquid jet Hanul Hwang, M. J. Philipp Hack A thorough understanding of the mechanisms driving the atomization process of liquid jets is of fundamental interest in applications ranging from medicinal spray generation to fuel injection. Atomization often begins with the deformation and distortion of the fluid interface by initially linear growth mechanisms. Depending on the parameters, the inflectional profile of a liquid jet injected into a stationary gas environment can support the growth of primary exponential instabilities of Kelvin-Helmholtz type. However, even when the eigenvalue spectrum reveals the flow as exponentially stable, interface perturbations may amplify through non-modal or algebraic mechanisms. The present work generalizes the analysis of primary distortions of the jet interface by casting it as an optimization problem for the surface-tension energy. The base flow of the linear analysis is computed in two-fluid simulations using a volume-of-fluid approach. Consideration of the budget of the perturbation kinetic energy provides insight into the principal physical mechanisms in various settings. |
Tuesday, November 20, 2018 2:08PM - 2:21PM |
Q05.00007: Sphere entry through an oil lens floating on water Linda B Smolka, Clare McLaughlin The low speed entry of a sphere onto a two-phase fluid consisting of an oil lens floating on a water surface is examined in experiments with spheres of different radii, densities and materials. Oil coats both the leading edge of the sphere as it penetrates the free surface of the two-phase fluid and the wall of the air-entraining cavity that forms behind the descending sphere. Spheres with lower inertia form smooth cavities whereas spheres with higher inertia develop a three-dimensional crumpled morphology along the cavity wall due to a shear-induced instability between the oil layer and surrounding water near the sphere front. Despite these different dynamics the sphere depth at collapse, either by deep seal or rupture, scales logarithmically with sphere mass for all of the spheres examined. We also observe a new phenomena: as air evacuates the necked region of the cavity, the oil coating the cavity forms an oil filament that tethers the two disjoint air cavities together before eventually breaking up into satellite drops. We find that the oil lens at the free surface is critical to forming an air-entraining cavity; with no oil lens only a small air pocket forms as the water completely wets the spheres either sealing the free surface or forming a quasi-static cavity. |
Tuesday, November 20, 2018 2:21PM - 2:34PM |
Q05.00008: Experimental Study of Thin Liquid-Films Flowing Under Planar Substrates Christos N Markides, Fabian Denner, Berend G M van Wachem, Serafim Kalliadasis, Benoit Scheid, Alexanros Charogiannis We undertake an experimental study of liquid films flowing under inverted planar substrates, and report on space- and time-resolved thickness measurements by laser-induced fluorescence at 330 mm from the inlet. The experiments span inclination angles β = -15 °, -30 ° and -45 °, Kapitza numbers Ka = 13.1 and 330, and Reynolds numbers Re = 0.6 - 193. When Ka = 13.1, the topology of the free surface transitions from smooth rivulets to sequences of solitary pulses with increasing Re. When Ka = 330, rivulets typically emerge above Re = 30 and the mean rivulet-amplitude increases non-monotonically. The peak amplitude shifts to higher Re at larger negative inclinations. Based on our results we conjecture that the development of rivulets can be attributed at smaller negative inclinations to a secondary Rayleigh-Taylor (RT) instability that destabilizes the suspended 2D wavefronts, and at larger negative inclinations to the primary RT instability of a flat film coating the underside of the plate. |
Tuesday, November 20, 2018 2:34PM - 2:47PM |
Q05.00009: A Locally Adaptive Mesh Densification Scheme for Resolving Singularities in Multi-Scale Free Surface Flows Christopher Anthony, Osman A Basaran Simulating free surface flows near singularities, as in the thinning of a thread/neck in drop breakup or the growth of a bridge/neck in drop coalescence, presents unique challenges in computational fluid dynamics. In order to accurately capture the dynamics and deduce scaling laws of pinch-off (coalescence) in such problems, it is necessary to resolve many different length scales simultaneously at the incipience of the singularity. A successful strategy over the years has involved use of elliptic mesh generation coupled with Galerkin finite elements as the basis of a multi-dimensional, arbitrary Eulerian Lagrangian algorithm that can accurately track the deforming free surface as the neck radius varies by up to three orders in magnitude. Attaining smaller length scales while maintaining adequate mesh density near the singularity and also discretizing the domain far away had heretofore proved prohibitively expensive. Here we present a scheme to adaptively densify regions of the mesh near the singularity without perturbing the mesh far from it allowing simulations to span length scales that differ by six to seven orders of magnitude which had heretofore proved possible only with 1D algorithms and boundary integral simulations restricted to creeping or potential flows. |
Tuesday, November 20, 2018 2:47PM - 3:00PM |
Q05.00010: Formation of spiral liquid curtains falling from a downward-facing free surface Harunori Nakagawa Yoshikawa, Christian Mathis, Shu Satoh, Yuji Tasaka The spontaneous formation of spiral patterns has been investigated intensively in excitable media, where the nonlinear coupling of excitation and recovery variables plays key roles in wave generation. Patterns rotate, in general, with spiral arms trailing the direction of rotation. We will report spiral patterns observed in a non- excitable fluid system. It consists of a downward-facing free surface of a horizontal liquid film maintained by continuous liquid feeding. The surface is unstable to the Rayleigh-Taylor (RT) instability. The resulting liquid discharge from the film occurs in different modes: in drops, in columns, and in curtains, depending on the liquid feeding rate, as reported by Pirat et al. (Phys. Rev. Lett., 92, 104501, 2004). Liquid curtains can exhibit spiral patterns with their arms leading the direction of rotation. Characterizing the formation of spiral curtains by an experiment and comparing the results with a phenomenologically developed theoretical model, we show that the patterns result from the synchronized development of the RT instability with the motion of curtains. |
Tuesday, November 20, 2018 3:00PM - 3:13PM |
Q05.00011: Sloshing experiments: Large scale circulations and instabilities Kerstin Avila, Bastian Bäuerlein, Patrycja Kotarski, Martin Dullweber The sloshing motion of fluids occurs in partly filled containers subjected to vibrations or oscillations. For example, sloshing can be catastrophic in spacecraft tanks, but is useful to damp energy e.g. in skyscrapers following an earthquake. In most applications, the shift of mass and impact on the container walls are the most relevant phenomena. Many studies have focused on these effects, studying the details of their occurrence for complex or shallow tank geometries and developing nonlinear models for simulations. In this talk, we take a different approach and revisit experimentally the fundamentals of deep water sloshing in a thin rectangular container under harmonic oscillation. Taking advantage of the quasi-two dimensional nature of the flow, we correlate the surface motion to the flow field in the bulk. We unravel nonlinear effects like hysteresis and shift of the resonance frequency that are known from other complex systems. In the vicinity of the resonance frequency, we observe large-scale circulations, which fill the entire container and enhance mixing. |
Tuesday, November 20, 2018 3:13PM - 3:26PM |
Q05.00012: Translational instability in actin lamellar fragments: a weakly nonlinear stability analysis Amarender Nagilla, Ranganathan Prabhakar, Sameer Jadhav Lamellar fragments of cells with actin filaments exhibit autonomous symmetry-breaking shape transitions and motility. We model such lamellar fragments as a fluid bound by an elastic boundary with bending resistance that is driven continuously by actin polymerization. Weakly-nonlinear stability analysis is applied to investigate the fingering instability at such an interface. We show that the dominant unstable modes can cause the translational mode to become unstable in the nonlinear regime, leading to asymmetric shapes and net motility of the fragment. We further show that a local geometry-based line model of the interface can reproduce the qualitatively similar behaviour. |
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