Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session Q10: Drops: Instability, Break-up and Splashing IIDrops
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Chair: Georgios Katsikis, Stanford University Room: 503 |
Tuesday, November 21, 2017 12:50PM - 1:03PM |
Q10.00001: Isolated drops from capillary jets by means of Gaussian wave packets Francisco Javier Garcia, Heliodoro Gonzalez, Alfonso Arturo Castrejon-Pita, Jose Rafael Castrejon-Pita, Francisco Jose Gomez-Aguilar The possibility of obtaining isolated drops from a continuous liquid jet through localized velocity perturbations is explored analytically, numerically and experimentally. We show that Gaussian wave packets are appropriate for this goal. A temporal linear analysis predicts the early evolution of these wave packets and provides an estimate of the breakup length of the jet. Non-linear numerical simulations allow us both to corroborate these results and to obtain the shape of the surface of the jet prior to breakup. Finally, we show experimental evidence that stimulating with a Gaussian wave packet can lead to the formation of an isolated drop without disturbing the rest of the jet. [Preview Abstract] |
Tuesday, November 21, 2017 1:03PM - 1:16PM |
Q10.00002: Numerical simulations of an impinging liquid spray in a cross-flow Sreekar Gomatam, Vengadesan S, Chakravarthy S R The characteristics of a liquid spray in a uniform cross-flow field are numerically simulated in this study. A hollow cone liquid spray is injected perpendicular to the air stream flowing through a rectangular duct under room temperature and pressure. An Eulerian-Lagrangian framework is adopted to simulate the spray in cross-flow phenomenon. The cross-flow velocity is varied from 6-12 m/s while the liquid injection pressure is varied from 0.3-0.6 MPa. The liquid droplets from the injected spray undergo breakup and/or coalescence further in the cross-flow. Moreover, the spray injected into the cross-flow impinges on the opposite wall resulting in the formation of a liquid film. This liquid film disintegrates further into discrete droplets because of the impingement of the droplets from the spray and the shear from the cross-flow. The overall distribution of the droplets in the cross-flow for varying conditions is studied in detail. The evolution of the liquid film with space and time for varying conditions is also investigated. Suitable sub-models are used to numerically model the droplet break-up, coalescence, liquid film formation and disintegration, splashing of the droplets on the film and subsequent formation of daughter droplets. [Preview Abstract] |
Tuesday, November 21, 2017 1:16PM - 1:29PM |
Q10.00003: Modelling Oil Droplet Breakup in a Turbulent Jet Rachel Philip, Ian Hewitt, Peter Howell In a deep-sea oil spill, a broken pipe near the seabed can result in the release of a turbulent oil jet into the surrounding ocean. The jet's shearing motion will typically cause the oil to break up into smaller droplets, which are then more readily dispersed and decomposed by sea microbes. In order to understand this natural clean-up process, we develop analytical and numerical models for the drop size distribution at different locations in the jet. This involves examining and unifying disparate scales, from the macroscopic jet to the microscopic droplets. We first examine the turbulent jet and we can use its self-similarity to simplify our models. We then turn to the droplet scale, considering the rate at which drops are deformed and broken up. Droplet deformation is precipitated by the jet's turbulent mixing and shearing and thus depends on the macroscopic jet models. We combine these large and small scale models to determine the droplet size distribution, as it varies with jet location. By varying the initial conditions and parameters in these models, we obtain insights into the factors affecting this droplet breakup process and how it may be optimised. [Preview Abstract] |
Tuesday, November 21, 2017 1:29PM - 1:42PM |
Q10.00004: Computational Study of Droplet Trains Impacting a Smooth Solid Surface David Markt Jr, Ashish Pathak, Mehdi Raessi, Seong-Young Lee, Emma Zhao The study of droplet impingement is vital to understanding the fluid dynamics of fuel injection in modern internal combustion engines. One widely accepted model was proposed by Yarin and Weiss (JFM, 1995), developed from experiments of single trains of ethanol droplets impacting a substrate. The model predicts the onset of splashing and the mass ejected upon splashing. In this study, using an in-house 3D multiphase flow solver, the experiments of Yarin and Weiss were computationally simulated. The experimentally observed splashing threshold was captured by the simulations, thus validating the solver's ability to accurately simulate the splashing dynamics. Then, we performed simulations of cases with multiple droplet trains, which have high relevance to dense fuel sprays, where droplets impact within the spreading diameters of their neighboring droplets, leading to changes in splashing dynamics due to interactions of spreading films. For both single and multi-train simulations the amount of splashed mass was calculated as a function of time, allowing a quantitative comparison between the two cases. Furthermore, using a passive scalar the amount of splashed mass per impinging droplet was also calculated. [Preview Abstract] |
Tuesday, November 21, 2017 1:42PM - 1:55PM |
Q10.00005: 3D numerical simulations of oblique droplet impact onto a deep liquid pool Hanneke Gelderblom, Sten A Reijers, Marise Gielen, Pascal Sleutel, Detlef Lohse, Zhihua Xie, Christopher C. Pain, Omar K. Matar We study the fluid dynamics of three-dimensional oblique droplet impact, which results in phenomena that include splashing and cavity formation. An adaptive, unstructured mesh modelling framework is employed here, which can modify and adapt unstructured meshes to better represent the underlying physics of droplet dynamics, and reduce computational effort without sacrificing accuracy. The numerical framework consists of a mixed control-volume and finite-element formulation, a volume-of-fluid-type method for the interface-capturing based on a compressive control-volume advection method. The framework also features second-order finite-element methods, and a force-balanced algorithm for the surface tension implementation, minimising the spurious velocities often found in many simulations involving capillary-driven flows. The numerical results generated using this framework are compared with high-speed images of the interfacial shapes of the deformed droplet, and the cavity formed upon impact, yielding good agreement. [Preview Abstract] |
Tuesday, November 21, 2017 1:55PM - 2:08PM |
Q10.00006: Multi-scale simulations of droplets in generic time-dependent flows Felix Milan, Luca Biferale, Mauro Sbragaglia, Federico Toschi We study the deformation and dynamics of droplets in time-dependent flows using a diffuse interface model for two immiscible fluids. The numerical simulations are at first benchmarked against analytical results of steady droplet deformation, and further extended to the more interesting case of time-dependent flows. The results of these time-dependent numerical simulations are compared against analytical models available in the literature, which assume the droplet shape to be an ellipsoid at all times, with time-dependent major and minor axis. In particular we investigate the time-dependent deformation of a confined droplet in an oscillating Couette flow for the entire capillary range until droplet break-up. In this way these multi component simulations prove to be a useful tool to establish from "first principles" the dynamics of droplets in complex flows involving multiple scales. [Preview Abstract] |
Tuesday, November 21, 2017 2:08PM - 2:21PM |
Q10.00007: Squeezing and de-wetting of a shear thinning fluid drop between plane parallel surfaces: capillary adhesion phenomenon Thomas Ward The radial squeezing and de-wetting of a thin film of viscous shear thinning fluid filling the gap between parallel plane walls is examined both experimentally and theoretically for gap spacing much smaller than the capillary length. The interaction between motion of fluid in the gap driven by squeezing or de-wetting and surface tension is parameterized by a dimensionless variable, $F$, that is the ratio of the constant force supplied by the top plate (either positive or negative) to surface tension at the drop's circumference. Furthermore, the dimensionless form of the rate equation for the gap's motion reveals a time scale that is dependent on the drop volume when analyzed for a power law shear thinning fluid. In the de-wetting problem the analytical solution reveals the formation of a singularity, leading to capillary adhesion, as the gap spacing approaches a critical value that depends on $F$ and the contact angle. Experiments are performed to test the analytical predictions for both squeezing, and de-wetting in the vicinity of the singularity. [Preview Abstract] |
Tuesday, November 21, 2017 2:21PM - 2:34PM |
Q10.00008: Breakup of a thin drop under a stagnation point flow Alireza Hooshanginejad, Sungyon Lee, Michael Shelley Recent studies by Hooshanginejad and Lee (2017) have demonstrated complex depinning behaviors of a partially wetting droplet under wind. Motivated by this study, we examine the coupled evolution of a 2D thin drop and external wind, when it is initially held against a fast stagnation point flow. Our drop lubrication model employs the potential flow and Prandtl boundary layer theory for outer flow to compute the internal drop flow corresponding to drop deformations. Furthermore, both the analytical and numerical steady state solutions provide a partial prediction for the drop’s final shape and help identify the range of droplet sizes that undergo a breakup for the given flow condition. [Preview Abstract] |
Tuesday, November 21, 2017 2:34PM - 2:47PM |
Q10.00009: Boundary-layer effects in droplet splashing Guillaume Riboux, Jose Manuel Gordillo A drop falling onto a solid substrate will disintegrate into smaller parts when its impact velocity exceeds the so called critical velocity for splashing. Under these circumstances, the very thin liquid sheet ejected tangentially to the solid after the drop touches the substrate, lifts off as a consequence of the aerodynamic forces exerted on it and finally breaks into smaller droplets, violently ejected radially outwards, provoking the splash. Here, the tangential deceleration experienced by the fluid entering the thin liquid sheet is investigated making use of boundary layer theory. The velocity component tangent to the solid, computed using potential flow theory provides the far field boundary condition as well as the pressure gradient for the boundary layer equations. The structure of the flow permits to find a self similar solution of the boundary layer equations. This solution is then used to calculate the boundary layer thickness at the root of the lamella as well as the shear stress at the wall. The splash model presented in Riboux \& Gordillo, PRL, 2014, which is slightly modified to account for the results obtained from the boundary layer analysis, provides a very good agreement between the measurements and the predicted values of the critical velocity for the splash. [Preview Abstract] |
Tuesday, November 21, 2017 2:47PM - 3:00PM |
Q10.00010: Role of unsteadiness and local dynamics in the selection of secondary droplet sizes in drop impact fragmentation L. Bourouiba, Y. Wang Understanding the dynamics of drop fragmentation upon impact on a finite surface has a wide range of industrial and environmental applications, including predicting and controlling the transport of pathogen-bearing droplets created from contaminated surfaces and leaves. Upon impact on a small solid surface, a drop first expands into a sheet in the air, surrounded by a rim, that itself can destabilize into ligaments, that can, in turn, shed droplets. Eventually, the sheet ruptures or retracts. This process is inherently unsteady and multiscale. Yet, the quantification of droplet sizes and discussion of the underlying mechanisms linking them to the ligaments have typically not accounted for unsteadiness. In a combined experimental and theoretical study, we revisit the problem of secondary droplet generation during unsteady sheet expansion in the air from impacts on finite surfaces. We discuss how unsteadiness and local dynamics shape the sizes and speeds of the secondary droplets generated. [Preview Abstract] |
Tuesday, November 21, 2017 3:00PM - 3:13PM |
Q10.00011: Rim destabilization in unsteady sheet expansion in the air from drop impact on small surfaces Y. Wang, L. Bourouiba Understanding the dynamics of drop fragmentation upon impact on a finite surface has a wide range of industrial and environmental applications, including predicting and controlling the transport of pathogen-bearing droplets created from contaminated surfaces and leaves. Upon impact on a finite solid surface, a drop first expands into a sheet in the air, surrounded by a rim, that itself can destabilize into ligaments that can, in turn, shed droplets. This process is inherently unsteady. Yet, analysis of the destabilization of the rim surrounding the sheet has so far neglected this inherent unsteadiness and it remained unclear which instability selects for the ligament numbers and their growth. We discuss the results of a combined experimental and theoretical study where a universal constraint on the rim dynamics is discovered. This constraint on the rim dynamics throughout the unsteady sheet expansion enables us to derive and validate a unified mathematical model of sheet expansion in the air from drop impact on small surfaces. [Preview Abstract] |
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