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 M27: Flow Instability: Complex Fluids and Multiphase Flow 
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Chair: Alparslan Oztekin, Lehigh University Room: Georgia World Congress Center B315 
Tuesday, November 20, 2018 8:00AM  8:13AM 
M27.00001: Inertioelastic flow instability at a stagnation point Noa Burshtein, Konstantinos Zografos, Amy Q Shen, Robert J. Poole, Simon J Haward High molecular weight polymer additives can suppress inertial flow instabilities and reduce turbulent drag. Yet exactly how the polymer does this is difficult to study. For Newtonian fluids, beyond a critical Reynolds number (Re_{c}), an inertial instability in the crossslot geometry results in the formation of a single steady streamwise vortex, providing a good model system to study vortex flow. A novel configuration of crossslot allows flow velocimetry in the crosssection of the vortex as Re is increased. Using polymeric fluids of incrementally increasing elasticity (El), we are able to precisely quantify the critical conditions for instability and the subsequent intensification of streamwise vorticity. Increasing El at low levels causes dramatic reductions in both Re_{c} and the vorticity growth. For higher El, vorticity is completely suppressed and a purelyelastic instability emerges beyond a critical Weissenberg number (Wi_{c}). Dimensionless phase diagrams in ReWi and ReEl phase space delineate regions of stable flow from those of inertia and elasticitydominated instabilities. Our findings shed valuable insight into the action of polymer on vortices and bridge the gap between inertial and elastic instabilities in intersecting flows. 
Tuesday, November 20, 2018 8:13AM  8:26AM 
M27.00002: Effect of varying fluid rheology on viscoelastic fluidstructure interactions between a flexible cylinder and wormlike micelle solution Anita Dey, Yahya ModarresSadeghi, Jonathan Rothstein It is well known that when a flexible or flexiblymounted structure is placed perpendicular to a Newtonian fluid flow, it can oscillate due to the shedding of vortices at high Reynolds numbers. Unlike Newtonian fluids, viscoelastic fluid flow can become unstable even at infinitesimal Re due to a purely elastic flow instability occurring at large Weissenberg numbers. We have recently shown that elastic flow instabilities can drive the motion of different flexible structures including sheets and cylinders. The fluctuating fluid forces exerted on the structures from the elastic flow instabilities can grow large enough to result in significant motion of the flexible structure, characterized into periodic 1D and 2D oscillations. In this talk, we present an investigation into the effects of varying fluid viscosity and relaxation time on the oscillatory response of the flexible structure, in terms of the elastic Mach number, in order to distinguish the complex flow instabilities observed during the 1D and 2D oscillations. The occurrence of the oscillations of a flexibly mounted, rigid cylinder at a frequency comparable to the structural natural frequency will be studied to further investigate the possibility of a window of lockin behavior for viscoelastic fluidstructure interactions. 
Tuesday, November 20, 2018 8:26AM  8:39AM 
M27.00003: Stability of stratified flows through neoHookean softgelcoated walls Dinesh Bhagavatula, S Pushpavanam The work focuses on the effect of elastohydrodynamic coupling between the fluids and the softgelcoatings on different instabilities that arise in the flow. The fluids are assumed to be Newtonian and incompressible. The dynamics in the softgel layers is captured using a neoHookean solid model. There is a first normal stress difference which characterizes the base state in this model. The neoHookean model is relevant in the context of large displacement fields. First the effect of using a consistent neoHookean model for softgel layers on different instabilities in twophase flows is analysed. We then determine the effect of soluble surfactants on the stability of the system. The dynamics of the solute is captured using a species transport equation. The results obtained from linear stability theory of single phase flows show the same scaling laws as those found experimentally in the literature. A long wave interfacial instability arises at the liquidliquid interface. A TollmienSchlichting wave type instability is seen at higher Reynolds numbers. A new gelliquid instability arises at gelliquid interface. It is shown that these instabilities can be stabilized by the softgel layers . Insights into the physical mechanism driving the instabilities are discussed. 
Tuesday, November 20, 2018 8:39AM  8:52AM 
M27.00004: Bistability of buoyancydriven exchange flow in vertical conduits: a dynamical approach Davide Picchi, Jenny Suckale, Ilenia Battiato Buoyancydriven exchange flows are common to a variety of natural and engineering processes, ranging from persistently active volcanoes to the cementing process in wells. In these systems, gravity is the only driving force which triggers the counterflow of the flowing phases. However, even in laminar flow conditions, the effects of gravity on exchange flows are not yet completely understood. We use a coreannular flow solution for vertical conduits and reveal the existence of two steadystate solutions: one with fast flow in a thin core and another one with relatively slow flow in a thick core. By analysis of laboratory experiments, we interpret the existence of two equilibrium configurations in the framework of bistability. Specifically, we show the existence of two regimes: for viscosity ratios of order one, the solution is unique and can be identified by maximizing the flux; instead, for viscosity ratios above a critical value, the system tends to minimize the potential energy. Finally, we develop a simplified dynamic model for predicting the system evolution from a nonequilibrium state in order to elucidate the impact that initial conditions have on the two equilibrium configurations. 
Tuesday, November 20, 2018 8:52AM  9:05AM 
M27.00005: Pattern formation in oilinwater emulsions exposed to a salt gradient Ying Liu, Bhargav Rallabandi, Lailai Zhu, Ankur Gupta, Howard A Stone We describe a radial configuration where horizontal flows create regular patterns. In particular, we experimentally show that when a drop of an oilinwater emulsion with high salt concentration in its aqueous phase is injected into a HeleShaw cell filled with salt solution with low concentration, the drop spreads and a “flower” pattern of the emulsion appears. We also conduct numerical simulations and the numerical results agree well with experimental results both qualitatively and quantitatively for describing the main features of this patternforming instability. The numerical results also confirm the hypothesis that the change of the fluid density caused by salt and oil is the key. Initially, the emulsion is denser than the ambient fluid, so it spreads along the bottom. However, as it spreads, salt diffuses much faster than the oil droplets so that the emulsion drop finally becomes less dense than the ambient fluid. This transport introduces an unstable vertical density gradient. We also describe the corresponding secondary flow, which is in the form of an azimuthal instability. The criterion for pattern formation, the growth rate of the instability, and the most unstable mode of the instability are also discussed. 
Tuesday, November 20, 2018 9:05AM  9:18AM 
M27.00006: Rotational instability of viscosity stratified flow Saunak Sengupta, Sukhendu Ghosh, Sandeep Saha, Suman Chakraborty Instability of stratified multiphase flow has drawn the attention in a rotating platform because of its potential role in micromixing for medical instruments.Centrifugal actuation can play an important role in driving the flow and Coriolis force can make enhance the mixing in a short span by destabilizing the flow. Here, we focus on the impact of the Coriolis force on viscositystratified flow with a miscible layer. Modal stability analysis is used to estimate the critical parameters, namely Rotation number, Reynolds number and wave numbers, which are responsible to vary the instability mechanism for different viscosity contrast. The study explores competing influence of rotational forces against the viscous forces. Correspondingly, rotational direction shows a significant effect on the spatiotemporal instability mechanism. Usually, miscible viscosity stratified flow with streamwise disturbance becomes more unstable for thinner mixed layer. On the contrary, our numerical computation confirms complete opposite scenario for Coriolis based instability of a miscible system with spanwise disturbances. The primary mechanism for the same can be inspected through the base flow of the system. Comparison between two and three dimensional instability is done.

Tuesday, November 20, 2018 9:18AM  9:31AM 
M27.00007: The combined effects of shear and buoyancy on phase boundary stability Srikanth Toppaladoddi, John S. Wettlaufer We study the effects of externally imposed shear and buoyancy driven flows on the stability of a solidliquid interface. By reanalyzing the data of Gilpin et al. [J. Fluid Mech., 99(3), 619 (1980)] we show that the instability of the icewater interface observed in their experiments was affected by buoyancy effects, and that their velocity measurements are more accurately described by MoninObukhov theory. A linear stability analysis of shear and buoyancy driven flow of melt over its solid phase shows that buoyancy is the only destabilizing factor and that the regime of shear flow here, by inhibiting vertical motions and hence the upward heat flux, stabilizes the system. It is also shown that all perturbations to the solidliquid interface decay at a very modest strength of the shear flow. However, at much larger shear, where flow instabilities coupled with buoyancy might enhance vertical motions, a reentrant instability may arise. 
Tuesday, November 20, 2018 9:31AM  9:44AM 
M27.00008: Enhanced time dependent injection rate for multilayer stable HeleShaw flows Prabir Daripa, Craig Gin There are classical asymptotic results on the time dependent injection rate for a stable radial displacement of one viscous fluid by a less viscous one in a HeleShaw cell. However, the injection rate can be faster in a multilayer setting. In particular, we find numerically that flows with more fluid layers can be stable with faster timedependent injection rates than comparable flows with fewer fluid layers. In particular, we show that for large times, the timedependent injection rate for a stable radial HeleShaw flow increases at a rate that is proportional to the number of interfaces to the twothirds power. 
Tuesday, November 20, 2018 9:44AM  9:57AM 
M27.00009: Formation of a heavy particle curtain Daniel Freelong, Patrick Wayne, Gregory Vigil, C. Randall Truman, Peter V Vorobieff In studies of shockdriven flows of multiphase media, an initial condition that is commonly produced involves a particleseeded curtain of finite thickness, which is gravitydriven and can have planar or initially perturbed interfaces. We investigate the process of formation of such a curtain, to provide detailed data for numerical modeling, to develop and validate diagnostics, and to study the relevant physics. As our observations show, even for a relatively modest volume fraction of the particles (above 1%), the flow is heavily dominated by the particle dynamics, and the average velocity magnitude is nearly quadratic as the function of the downstream distance. At the same time, the front of the forming curtain manifests evolving perturbations which could be considered in the context of the new class of accelerationdriven multiphase flow instabilities. 
Tuesday, November 20, 2018 9:57AM  10:10AM 
M27.00010: Jet Stability During Direct Contact Condensation Maxwell Brennan, Kristofer Dressler, Arganthaël Berson, Gregory Nellis Saturated vapor injected through a straightedged orifice into crossflowing subcooled liquid presents two predominant regimes: (1) a stable conical jet and (2) an unstable behavior characterized by bubble formation and collapse. The effect of process variables and nozzle diameter on the transition between these regimes and the frequency of bubble formation and collapse in the second regime is examined. The frequencies of jet oscillation and vortex bubble collapse events are investigated using acoustic and highspeed visualization techniques to relate undesirable noise to physical phenomena. 
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