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 M26: General Fluid Dynamics: Rotating Flows and Multiphysics Phenomena 
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Chair: Amir Gat, Technion, Israel Institute of Technology Room: Georgia World Congress Center B314 
Tuesday, November 20, 2018 8:00AM  8:13AM 
M26.00001: Rotating Polygonal Depression Soliton Clusters on the Surface of a Liquid Ring. Hamid Ait Abderrahmane, Pooya Soltanian, Hoi Dick Ng, Georgios Vatistas We report the first experimental observation of rotating depression soliton sets on the inner surface of a viscous liquid ring, carrying background waves. These occur within a rotating shallow layer of oil inside a stationary cylindrical container. The solitons are organized either in single, two, or regular polygonal (triangle and hexagon) clusters; they travel in unison at a higher speed than the background traveling waves without changing their shapes. The spectral power density reveals a possible energy exchange between the soliton clusters and the background mixed radialazimuthal modulations through wave radiation. 
Tuesday, November 20, 2018 8:13AM  8:26AM 
M26.00002: Flow induced by a rotating cone: velocity field and stability properties Antonio Segalini, Simone Camarri The flow induced by a cone which rotates around its symmetry axis is a canonical case of interest both from a fundamental and an applied viewpoints. Despite its simplicity, it has received little attention if compared to the case of a rotating disk. The boundarylayer equations in the case of a rotating cone allow for a selfsimilar solution (see Wu, Appl. sci. Res. 8, 1959) analogous to the von Kármán one for a rotating disk. This solution implies that the wallnormal velocity does not vanish far from the wall, at difference with what expected for a cone of finite size. Moreover, the cone apex induces a largescale motion in comparison to the boundary layer thickness, which is absent for the rotating disk. In this work we extend the analysis by Wu (1959) to account for viscous corrections and largescale motion due to the apex, arriving to an original correction which is also selfsimilar. We furthermore explore the influence of the proposed correction on the estimated stability properties of the flow using a weaklydivergent approach based on the assumption of a slow evolution in the streamwise direction. In addition to a standard approach, which is firstorder and in which elliptic terms are excluded, we also propose and apply here a secondorder extension. 
Tuesday, November 20, 2018 8:26AM  8:39AM 
M26.00003: Thin Film Flow with Partial Slip in NonIsothermal Rimming Flow Jonathon M P Nicholson, Antonino La Rocca, Donald Giddings, Edward Kay, Outi Tammisola Within typical aeroengine bearing chambers, complex fluid flows wholly coat the chamber surface as thin films. Injection of externally cooled oil droplets are used to dissipate heat from the chamber. For increasing demands on the operation conditions, detailed thermal modelling is critical in predicting and maintaining film flows to avoid oil degradation. We simulate the thermal model of a rimming flow on a simple chamber geometry for the exploration of key characteristics on flow dynamics and thermodynamics. Utilisation of a depth averaged mathematical formulation allows for a boundary slip condition at the fluidsolid interface, which is appropriate since surface effects become apparent for thinner films. The thermal characteristics are explored for a range of flows, including those of modest inertial effects retained due to depthaveraging. The attributes of smooth and pooled flows are detailed for the thermal cases on temperature distribution, with emphasis on slip affecting the ability of heat transfer. We present the impact of slip on the thermal model where enhanced velocities at the interfaces may influence heat extraction and provide an overview of the thermal parameters on the system. 
Tuesday, November 20, 2018 8:39AM  8:52AM 
M26.00004: Machine learning predictions for magnetic field time evolution in a ThreeMeter liquid sodium spherical Couette experiment Artur Perevalov, Ruben E Rojas Garcia, Itamar Shani, Brian R Hunt, Daniel Perry Lathrop The source of the Earth's magnetic field is the turbulent flow of liquid metal in the outer core. Our experiment's goal is to create an Earthlike dynamo to explore the mechanisms of generating the magnetic field and to understand the dynamics of the magnetic and velocity fields. We observe subdynamo states that show gain in the applied magnetic field. Prediction of the magnetic field in MHD turbulence field is a challenging problem. We present results of mimicking the experiment by a reservoir computer deep learning algorithm and compare the results with predictions that are done by other techniques. The experiment is a threemeter diameter outer sphere and a onemeter diameter inner core model with the gap filled with liquid sodium. The spheres can rotate independently up to 4 and 14 Hz respectively, giving a Reynolds number up to 1.5*10^{8}. Two external electromagnets apply magnetic fields, while 33 Hall sensors measure the resulting fields. We use this data to train a reservoir computer to predict the time evolution and mimic waves in the experiment. Surprisingly accurate predictions can be made for several magnetic dipole time scales. This shows that such a complicated MHD system’s behavior can be predicted. 
Tuesday, November 20, 2018 8:52AM  9:05AM 
M26.00005: Magnetic Levitation Stabilized by Streaming Fluid Flows David Fairhurst, Kyle Baldwin, JeanBaptiste de Fouchier, Patrick Atkinson, Richard Hill, Michael Swift Stable levitation is a tantalizing concept, with prospects for frictionless transport, containerless storage, or contactfree manipulation. We find that the ubiquitous magnetic stirrer provides a simple method, seen but overlooked since the device’s invention in 1942. The magnetic stir bar or "flea" will levitate indefinitely, stabilized by an interplay between magnetic forces and fluid inertia, at drive speeds usually avoided due to "spinout" and loss of mixing efficiency. We study the onset of levitation and quantify the flea's motion (a combination of vertical oscillation, spinning and "waggling"), finding excellent agreement with an analytical model. The waggling motion drives recirculating flow, which is directionally tunable via the fluid parameters. Below streaming Reynolds number Re_{s} ≈ 170 the swimstroke switches from pulling to pushing the fluid, and stabilizes the levitation. Our findings have implications for the locomotion of artificial swimmers, the development of bidirectional microfluidic pumps and as an alternative to sophisticated levitators.

Tuesday, November 20, 2018 9:05AM  9:18AM 
M26.00006: Effects of Geometric Change on Supercritical Fluid Flow Dynamics and Mixing in GasCentered LiquidSwirl Coaxial Injectors Liwei Zhang, Vigor Yang The present work investigates supercritical fluid flow dynamics and mixing in gascentered liquidswirl coaxial injectors, which are used in in the main chambers of oxidizerrich stagedcombustion engines. Gaseous oxygen (GOX) is axially directed through a center post at a temperature of 687.7 K. Kerosene is tangentially introduced into the outer coaxial swirler at a temperature of 492.2 K. The mean chamber pressure of 253.0 bar substantially exceeds the thermodynamic critical pressures of oxygen and kerosene. The end of the GOX post is recessed from the entrance of the taper region, which is connected downstream to an open domain. Changes in geometric parameters in recess lengths and in the taper regions are found to have noticeable influences on the flow and mixing fields. The present abstract focuses on the latter case. First, adjusting the injector geometry accelerates the development of the GOXkerosene mixing layer. Second, stretching and amalgamation of vortices disturb the energy cascade toward highwavenumber eddies. Third, flow recirculation and reduction in the bulk axial velocity increase the residence time of the reactive mixture, facilitating flame stabilization. Results and discussions will be presented in detail at the meeting. 
Tuesday, November 20, 2018 9:18AM  9:31AM 
M26.00007: On the collision of a rigid pendulum with a deformable mambrane in a viscous fluid Roberto Verzicco, Giorgio Querzoli In Nature and technology, the interaction between two bodies is always mediated by a fluid that, depending on its properties, can heavily influence the dynamics of the whole system. While for gasses, the fluid density is much smaller than those of the bodies and the presence of the former can be neglected, if the fluid is a liquid, its inertia and viscous stresses produce significant loads and its effect has to be accounted for. In this study we have considered a rigid spherical pendulum impacting on thin rubber membrane in water either experimentally (by highspeed contour tracking and particle image velocimetry) and by numerical simulations of the Navier–Stokes equations coupled with a fluid/structure interaction algorithm. We have found that general model that is flexible enough to cope with arbitrary deformable objects is missing from the literature and in this study we aim at progressing in that direction. The properties of both, pendulum and membrane have been changed independently and different impact regimes have been reproduced. The most common impact models are tested and benchmarked against the experimental results and a physics based general collison model is presented and validated. 
Tuesday, November 20, 2018 9:31AM  9:44AM 
M26.00008: Inflation and deflation dynamics of waterfilled balloons Dotan Ilssar, Amir Daniel Gat This work presents simplified modelling of the fully coupled behavior of a spherical, hyperelastic balloon exhibiting bistability, under a timedependent externallydictated input pressure. The reduced order model, derived under the assumption of laminar flow at high Reynolds numbers, distinguish between the flow regime during deflation, where the potential flow theory can be applied, and inflation where boundary layer separation occurs, giving rise to an internal jet. Combining the bistable elasticity of the balloon and the effects of the entrapped fluid, yields a nonlinear oscillator equation describing the extensional motion of the balloon, assuming the latter does not deviate significantly from sphericity. A twostep model verification, based on finite element computations is presented. First, the different forces applied by the entrapped fluid on the balloon are examined separately utilizing several degenerated simulations, where the elasticity of the balloon is ignored. Next, a fully coupled finite element simulation is executed and compared to the reduced order model, showing a good correlation. A possible application of the suggested system could be serving as a building block for soft robots, exploiting multiple bistable elements. 
Tuesday, November 20, 2018 9:44AM  9:57AM 
M26.00009: Breathing from Underground: The Effect of Atmospheric Stability on Mass and Heat Transport in Termite Mounds Saurabh Saxena, Neda Yaghoobian Termite mounds are massive, complex structures, built collectively by millions of minuscule insects – termites. The ventilation and gasexchange function of these oddshaped structures have been related to several phenomena including metabolismdriven natural convection within the mound, the effect of turbulent atmospheric wind flow on the mound superstructure, and the diurnally variable surface temperature and environmental conditions. In this study, we aim to use computational modeling to investigate the role of geometry, atmospheric stability, and turbulent intensity in the mound ventilation mechanism. We specifically focus on the termite mounds of the subfamily Macrotermitinae that are known for their massive coneshaped, overground structures, believed to be built for the ventilation of the termites’ subterranean nest. Numerical simulations are performed over a simplified geometry, representing the key features of these mounds, exposed to the turbulent atmospheric wind flow. The internal and external flow features under different stability conditions are examined to reveal the diurnal variations of the heat and respiratory gas transport from the mound underground nest. 
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