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 A12: Drops: General I |
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Chair: Monica Nitsche, University of New Mexico Room: Georgia World Congress Center B217 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A12.00001: Shape oscillations in an initially nonspherical raindrop Mounika Balla, Manoj Kumar Tripathi, Kirti Sahu The dynamics of a nonspherical droplet falling in air is investigated via three-dimensional numer- ical simulations of the Navier-Stokes and continuity equations. The main focus of this work is to study the effects of the relative influence of surface tension force over the inertial force, and the orientation of the droplet. The range of parameters considered in this study is similar to a falling raindrops. A theoretical analysis is also conducted by neglecting gravity and in the limit of creep- ing flow. The expressions of the frequency and decay rate of the shape oscillations of a raindrop are derived, and compared against the the direct numerical simulation results. To the best of our knowledge, the influence of these physical parameters has not been considered so far in the context of raindrops. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A12.00002: Experimental investigation of a freely falling liquid drop in air Meenu Agrawal, Rajat Kumar Katiyar, Kirti Sahu, Badarinath Karri The dynamics of a freely falling liquid drop in air was examined experimentally using a high speed camera. To obtain the freely falling drop we placed a drop on a plate and propelled the drop by using a pneumatic cylinder arrangement. This drop then reaches a terminal height before it starts to freely fall and then the drop dynamics are recorded. We investigate how the drop deforms initially and during free fall as a function of the drop velocity, size of droplet and liquid properties such as viscosity. In addition, the influence of the surface such as hydrophobic surface is an important parameter for initial drop contact angle. This work was motivated to experimentally validate the dynamics of a non-spherical liquid droplet falling in air which had been presented using simulations by Agrawal et al. [1]. [1] M. Agrawal, A. R. Premlata, M. K. Tripathi, B. Karri and K. C. Sahu, “Nonspherical liquid droplet falling in air”, PHYSICAL REVIEW E., 2017, 95,033111 |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A12.00003: An Experimental Study on Wind-Driven Runback Behavior of Water Droplets over Surfaces with Different Wettabilities Liqun Ma, Yang Liu, Hui Hu An experimental study is performed to quantify the transient runback behavior of water droplet/rivulet flows as driven by boundary layer winds. The experimental study is conducted in a low-speed wind tunnel available at Iowa State University. A suite of advanced flow diagnostic techniques, which include high-speed photographic imaging, digital image projection (DIP), particle image velocimetry (PIV), are used to quantify the transient runback behavior of wind-driven water droplets over test plates. In addition to measuring the airflow velocity field around the wind-driven water droplets/rivulets, dynamic shape changes and stumbling runback motion of the water droplets/rivulets are also measured in real time in terms of water film thickness distribution, contact line moving velocity and wet surface area over the test plates with different wettabilities. The findings derived from the present study are very helpful to gain a better understanding about the important microphysical process pertinent to aircraft icing phenomena. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A12.00004: Dynamics of water droplet on a flat plate with airflow Yewon Kim, Hyungmin Park In this study, water droplet removal on the plates with different wettability is experimentally investigated in a wind tunnel. The droplets (5-20 μl) are placed on PET (hydrophilic) and PTFE (hydrophobic) plates, over which the air flows with different flow speed (up to 20 m/s) and acceleration (3.3-8 m/s^{2}), and their dynamics are recorded with high-speed cameras. It is found that the droplets show different behaviors depending on the distance from the edge of the plate, droplet volume, flow acceleration, and plate wettability. In most cases, the oscillation of the droplet begins at the low velocity regime, and then droplet shedding occurs to the downstream direction with a specific shape at higher air velocity. In some cases, however, e.g., droplet placed very near the leading edge of plate, the droplet tends to move upstream or it is trapped at a specific position while oscillating, and satellite drops are separated and shed to downstream from the original one. This phenomenon is closely related to the boundary layer developments over the plate, and we would discuss the relation between the droplet dynamics and the flow over and around the plate and droplet. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A12.00005: Simulating two-phase flows using a thermodynamically consistent coupled Cahn-Hilliard Navier-Stokes framework Makrand A Khanwale, Alec D Lofquist, Soojung Claire Hur, Hari Sundar, Baskar Ganapathysubramanian We study the evolution of interfaces in two-phase flows with moderate density ratios. We use a thermodynamically consistent coupled Cahn-Hilliard Navier-Stokes based formulation. Mass conservation and energy stability is explicitly satisfied in the discrete formulation. We use a finite element based spatial discretisation with the variational multiscale approach to solve the momentum equations. We use a fast octree based adaptive meshing strategy which is massively parallel, thus allowing DNS of multiple droplets/bubbles in flow. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A12.00006: Dynamics of sessile drops: symmetry classes and a minimal model Elizabeth Wesson, Paul H Steen, Alex Townsend, Joshua McCraney, Joshua Bostwick We consider two approaches to the question of how liquid drops vibrate on a solid support. First, a spectral approach: Motions of a drop on a vertically oscillated support exhibit vibrational modes. These are solutions of a “droplet Schrödinger equation” which is reduced to an eigenvalue problem for mode shapes (characterized by azimuthal and polar wavenumbers) and scaled frequencies. The solutions exhibit spectral splitting and mixing when symmetry is broken. We propose a method of identifying observed motions of drops via a neural network. Triangle drop approach: It is also advantageous for application purposes to simulate large populations of droplets that are translating and deforming dynamically; this motivates a minimal model of a moving droplet. We consider a 2D triangular drop whose motions are governed by pressure and surface tension under Newton’s law. The resulting four-dimensional dynamical system includes fixed points, periodic, quasi-periodic and chaotic trajectories. We suggest a connection to real-world drops via the Steiner circumellipse. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A12.00007: Ratchet Mechanism of Drops Climbing a Vibrated Oblique Plate Hang Ding, Xi Zhu, Peng Gao, Xi-Yun Lu In this talk, we will report the ratchet mechanism of drops climbing a vibrated oblique plate based on three‐dimensional direct numerical simulations, which for the first time reproduce the existing experiment (Brunet et al., Phys. Rev. Lett., vol. 99, 2007, 144501). With the help of numerical simulations, we identify an interesting and important wetting behaviour of the climbing drop; that is, the breaking of symmetry due to the inclination of the plate with respect to the acceleration leads to a hysteresis of the wetted area in one period of harmonic vibration. In particular, the average wetted area in the downhill stage is larger than that in the uphill stage, which is found to be responsible for the uphill net motion of the drop. A new hydrodynamic model is proposed to interpret the ratchet mechanism, taking account of the effects of the acceleration and contact angle hysteresis. The predictions of the theoretical analysis are in good agreement with the numerical results. |
Sunday, November 18, 2018 9:31AM - 9:44AM |
A12.00008: Stretching and Shearing of Immiscible Droplets in Laminar Flows Aditya N Sangli, Artiom Kostiouk, David Bigio Droplet deformation in non-stagnant extensional flows is studied where a translating droplet undergoes deformation. A planar converging channel with bounding walls based on a hyperbolic equation was built and imposed a near constant extensional rate along the centerline of the channel in the streamwise direction. The deformation of a Silicone oil droplet is studied in this regime. To study shear influenced deformation, droplets were injected offset from the centerline where the strain rate tensor is strongly influenced by a shear component. Plots of drop draw ratio vs time showed that droplets with low initial Capillary number deformed and achieved a steady shape in the flow. Droplets with higher initial Capillary number experienced continuous deformation. From the instantaneous Capillary number of the droplet, and over many experiments, a heuristic inference of the critical Capillary number was made. These experiments, combined with a computational study, will aid in understanding non-stagnant extensional flows in channels as an analogue to commonly studied stagnant extensional flows in a four-roll mill. |
Sunday, November 18, 2018 9:44AM - 9:57AM |
A12.00009: Droplets with interfacial viscosity: dynamics, rheology, and breakup Vivek Narsimhan In this talk, we discuss the dynamics of droplets with a thin layer of viscous, insoluble surfactant whose mechanics are described by interfacial viscosity, i.e., a Boussinesq-Scriven constitutive law. We develop analytical theories to quantify droplet shape under flow in the limit of weak deformation, to a sufficient level of approximation where one can extract information about non-linear rheology and droplet breakup. In shear flow and extensional flows, we calculate how interfacial viscosity alters the extra stress of a dilute suspension of droplets. We also investigate how shear and dilatational viscosities influence droplet breakup and droplet migration in wall-bounded shear flow. These theories highlight the extent to which surface viscosity alters droplet dynamics, and we discuss how one can extend our theories to include effects such as surface tension gradients and viscoelastic surfaces. In the last part of the talk, we discuss a peculiar result that is related to the translational speed of a droplet with interfacial viscosity. It turns out interfacial shear viscosity plays a minimal role in modifying droplet drag when its shape is spherical. We discuss physical mechanisms and scaling theories that explain this observation. |
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