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 F21: Flow Instability: Multiphase FlowInstabilities

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Chair: Katarzyna Kowal, Northwestern University Room: 706 
(Author Not Attending)

F21.00001: LInear stability analysis of granular TaylorCouette flow Meheboob Alam The linear stability analysis of the circular Couette flow of granular materials is carried out in the rapid shear regime. The kinetictheory based continuum models, with a separate balance equation for granular/fluctuation energy, is employed, and the underlying rheological model is likely to be valid for a range of density from the dilute to dense regime. The steady base state equations for the case of rotating inner cylinder and stationary outer cylinder are solved numerically; it is found that the inelastic dissipation can make the flow radially inhomogeneous even in the narrowgap limit. The linear stability equations are solved using spectral collocation method. The onset of Taylorlike vortices and the effects of inelastic dissipation and compressibility on them are analysed. [Preview Abstract] 
Monday, November 20, 2017 8:13AM  8:26AM 
F21.00002: Modulation of flow transition in microbubble TaylorCouette flow Tomoaki Watamura, Yuji Tasaka, Yuichi Murai To examine the interaction mechanism of bubbles and flows that result in drag reduction, the effect of the presence of microbubbles on a flow state is experimentally investigated in a TaylorCouette flow with azimuthal waves. The average diameter of the bubbles is 60 $\mu$m, which is smaller than the maximum length scale of vortices in the flow, and the maximum void fraction is $1.2 \times 10^{4}$ at the maximum case. The modifications of the fluid properties, bulk density and effective viscosity are expected to have a small effect on modifying flow states. The power of the basic wave propagating in the azimuthal direction is enhanced; its modulation, however, is decreased by adding microbubbles in the flow regime corresponding to modulated Taylor vortex flow. Moreover, the gradient of the azimuthal velocity near the walls, source of the wall shear stress, decreases by 10\%. The modified velocity distribution by adding microbubbles is comparable to that obtained with a 20\% lower Reynolds number. Microbubbles in the wavy Taylor vortices are visualized and exhibit a preferential distribution and motion at the crests and troughs of the waviness. We discussed the effect of the extra buoyant force due to the inhomogeneously distributed microbubbles in wavy structures on the flow. [Preview Abstract] 
Monday, November 20, 2017 8:26AM  8:39AM 
F21.00003: Shock tube Multiphase Experiments John Middlebrooks, Roy Allen, Manoj Paudel, Calvin Young, Ben Musick, Jacob McFarland Shock driven multiphase instabilities (SDMI) are unique physical phenomena that have farreaching practical applications in engineering and science. The instability is present in high energy explosions, scramjet combustors, and supernovae events. The SDMI arises when a multiphase interface is impulsively accelerated by the passage of a shockwave. It is similar in development to the RichtmyerMeshkov (RM) instability however, particletogas coupling is the driving mechanism of the SDMI. As particle effects such as lag and phase change become more prominent, the SDMI's development begins to significantly deviate from the RM instability. We have developed an experiment for studying the SDMI in our shock tube facility. In our experiments, a multiphase interface is created using a laminar jet and flowed into the shock tube where it is accelerated by the passage of a planar shockwave. The interface development is captured using CCD cameras synchronized with planar laser illumination. This talk will give an overview of new experiments conducted to examine the development of a shocked cylindrical multiphase interface. The effects of Atwood number, particle size, and a second acceleration (reshock) of the interface will be discussed. [Preview Abstract] 
Monday, November 20, 2017 8:39AM  8:52AM 
F21.00004: Effect of the boundary layer thickness on the hydrodynamic instabilities of coaxial atomization under harmonic flow rate and swirl ratio fluctuations Corentin Jorajuria, Nathanael Machicoane, Rodrigo Osuna, Alberto Aliseda Breakup of a liquid jet by a high speed coaxial gas jet is a frequentlyused configuration to generate a high quality spray. Despite its extended use in engineering and natural processes, the instabilities that control the liquid droplet size and their spatiotemporal distribution in the spray are not completely understood. We present an experimental measurements of the near field in a canonical coaxial gasliquid atomizer. The liquid Reynolds number is constant at 10$^{\mathrm{3}}$, while the gas jet Reynolds number is varied from 10$^{\mathrm{4}}$10$^{\mathrm{6}}$. The liquid injection rate and the swirl ratio are harmonically modulated to understand the effect of unsteadiness on the interfacial instability that triggers primary breakup. The gas velocity is measured using a combination of hotwire anemometry and 3D PIV, resolving the gas boundary layer and the threedimensionality of the flow, particularly in the cases with swirl. The development of the hydrodynamic instabilities on the liquidgas interface is quantified using high speed visualizations at the exit of the nozzle and related to the frequency and growth rates predicted by stability analysis of this boundary layer flow. The resulting droplet size distribution is measured at the end of the breakup process via Particle Phase Doppler Anemometry and compared to stability analysis predictions statistics. [Preview Abstract] 
Monday, November 20, 2017 8:52AM  9:05AM 
F21.00005: Ultrahigh speed visualization of the flashing instability under vacuum conditions Jose Federico Hernández Sánchez, Tariq AlGhamdi, Sigurdur T. Thoroddsen We investigated experimentally the flashing instability of a jet of perfluoronhexane (PFnH) released into a lowpressure environment. Using a ultrahigh speed camera we observed the jet fragmentation occurring close to the nozzle. Using a fixed total driving pressure, we decreased systematically the vacuum pressure, investigating the transition from a laminar jet to a fully flashing jet. Our high temporal resolution allowed to visualize the detailed dynamics of external flashboiling for the first time. We identified different mechanisms of jet breakup. At chamber pressures lower than the vapor pressure the laminar jet evolves to a meandering stream. In this stage, bubbles start to nucleate and violently expand upstream the nozzle. At lower vacuum pressures the initially cylindrical jet elongates, forming a liquid sheet that breaks in branches and later in drops. At very low pressures both mechanisms are responsible for the jet breaking. We calculated the size distribution of the ejected droplets, their individual trajectories, velocities as well as the spray angle as a function of the dimensionless vacuum pressure. [Preview Abstract] 
Monday, November 20, 2017 9:05AM  9:18AM 
F21.00006: Direct numerical simulation of threedimensional liquid jet breakup Ricardo Constante, Lyes Kahouadji, Andre Nicolle, Jalel Chergui, Damir Juric, Seungwon Shin, Omar K. Matar We carry out direct numerical simulations of liquid jet dynamics and breakup using a highperformance code, {\it Blue}, which uses a hybrid technique based on the fronttracking and the levelset method; it defines the interface position through a marker function and a local triangular Lagrangian mesh. Liquid jet breakup is an example of interfacial complexity associated with multiphase flows because of the formation of ligaments and their pinch off to give rise to droplet formation. We consider the atomisation of a liquid jet released into a stagnant gas phase where the velocity is stimulated sinusoidally to promote the growth of KelvinHelmholtz instabilities, thus forming a flow system characterized by complex interfaces. The spread of cylindrical liquid jet into a coflowing external stream is also considered (essentially, a replication of the Marmottant and Villermaux experimental work). [Preview Abstract] 
Monday, November 20, 2017 9:18AM  9:31AM 
F21.00007: Direct numerical simulation of annular flows Assen Batchvarov, Lyes Kahouadji, Jalel Chergui, Damir Juric, Seungwon Shin, Richard V. Craster, Omar K. Matar Vertical countercurrent twophase flows are investigated using direct numerical simulations. The computations are carried out using Blue, a fronttrackingbased CFD solver. Preliminary results show good qualitative agreement with experimental observations in terms of interfacial phenomena; these include threedimensional, largeamplitude wave formation, the development of long ligaments, and droplet entrainment. The flooding phenomena in these counter current systems are closely investigated. The onset of flooding in our simulations is compared to existing empirical correlations such as Kutateladzetype and Wallistype. The effect of varying tube diameter and fluid properties on the flooding phenomena is also investigated in this work. [Preview Abstract] 
Monday, November 20, 2017 9:31AM  9:44AM 
F21.00008: Direct numerical simulation of annular flows with surfactants Andrius Patapas, Assen Batchvarov, Lyes Kahouadji, Jalel Chergui, Damir Juric, Seungwon Shin, Richard V. Craster, Omar K. Matar Vertical countercurrent airwater surfactantladen flows are investigated using direct numerical simulations. The computations are carried out using Blue, a fronttrackingbased CFD solver. The presence of surfactants in the flow results in Marangoni stresses. At low gas and liquid flow rates, the Marangoni effect is found to be dominant over shear stresses, resulting in the suppression of wave perturbations and wave amplitudes. Marangoni stresses are found to dominate at low gas and liquid flow rates, resulting in lack of wave development and subsequent droplet detachment thus hindering flooding. It is also found that shear stresses dominate the flow at higher gas/liquid flow rates, ``trapping” surfactant in wave crests thus favouring droplet detachment and increasing the extent of flooding. In addition to the above, we also report on the results of a parametric study to observe the effect of varying surfactant surface concentration, maximum packing concentration, bulk and interface diffusivity, and adsorption and desorption rate on the interfacial dynamics. [Preview Abstract] 
Monday, November 20, 2017 9:44AM  9:57AM 
F21.00009: Flow characterization and stability analysis in rectangular liddriven cavities with particle suspensions John Shelton, Abhishek Sunkavalli Like the canonical, twodimensional, square liddriven cavity problem that serves as its cornerstone, the twodimensional, rectangular liddriven cavity is a wellstudied extension that also generates dynamically stable, welldefined, flow structures within the laminar flow regimes. Mathematical timedependent perturbations to these flow structures have been shown to generate a region of metastability as the system transitions towards a turbulent flow regime. By replacing the mathematicallygenerated, timedependent perturbations of previous investigations into this phenomena with the particlefluid and particleparticle interactions present within a multiphase flow, a unique perspective on the stability of these flow structures within the laminar flow regimes of the twodimensional liddriven cavity can be obtained. Therefore, the objective of this study is to investigate the effect varying area fractions and relative densities of suspended granular particles have on traditionally laminar and stable flows found at Reynolds numbers of 100, 400, and 1000 of a rectangular liddriven cavity with an aspect ratio of 1.5. These studies and analyses will aid in the determination how granular materials can be used to enhance desirable flow characteristics of fluid behaviors. [Preview Abstract] 
Monday, November 20, 2017 9:57AM  10:10AM 
F21.00010: Morphological instabilities of rapidly solidified binary alloys under weak flow Katarzyna Kowal, Stephen Davis Additive manufacturing, or threedimensional printing, offers promising advantages over existing manufacturing techniques. However, it is still subject to a range of undesirable effects. One of these involves the onset of flow resulting from sharp thermal gradients within the laser melt pool, affecting the morphological stability of the solidified alloys. We examine the linear stability of the interface of a rapidly solidifying binary alloy under weak boundarylayer flow by performing an asymptotic analysis for a singular perturbation problem that arises as a result of departures from the equilibrium phase diagram. Under no flow, the problem involves cellular and pulsatile instabilities, stabilised by surface tension and attachment kinetics. We find that travelling waves appear as a result of flow and we map out the effect of flow on two absolute stability boundaries as well as on the cells and solute bands that have been observed in experiments under no flow. [Preview Abstract] 
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