Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session T15: Flow Instability: Multiphase Flow (8:00am  8:45am CST)Interactive On Demand

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T15.00001: From Waste to Power: Pulsing Biosludge Atomization for Efficient Energy Conversion Daniel Wilson, Wayne Strasser Efficient conversion of human waste to usable energy is sought by introducing a highly concentrated nonNewtonian biosludge to a steam boiler via direct spray injection. Compared to other methods of energy conversion that require dilution and/or drying, using a more concentrated biosludge increases energy conversion efficiency, reduces water usage, and reduces fossil fuel emissions by decreasing the transportation load. The use of steam as the assisting gas for a twinfluid atomizer reduces the viscosity of the biosludge, enabling more effective atomization; however, the steam reduces boiler efficiency. Therefore, two objectives must be balanced in an atomizer design: minimizing steam usage and effective atomization of the viscous biosludge. CFD simulations provide preliminary assessments of inverted twinfluid atomizer designs when the steam flow is reduced to a desirable rate. Typical designs involve the steam flowing outside of the slurry stream, while the inverted design has the steam entering inside of the slurry stream. It is shown that the inverted design produces superior atomization and is robust in that droplet size does not change significantly for a range of acceptable steam flows. Additional difficulties arise because the viscosity of boisludge varies widely; if viscosity levels are too high, an undesirable pressure drop restricts the flow, and atomization quality suffers. To improve the robustness of this system, two PID controllers are added. The first automates the flow of biosludge based on pressure drop, and the second compensates for phase momentum ratio and controls the flow of steam based on droplet size distribution. [Preview Abstract] 

T15.00002: On a GasParticle Analogue to the RichtmyerMeshkov Instability. Part 1: Evolution of a Corrugated Particle Curtain Frederick Ouellet, Bertrand Rollin, Bradford Durant, Rahul Babu Koneru, S. Balachandar An emerging area of research in the multiphase flow community is the study of ShockDriven Multiphase Instability (SDMI) which is a gasparticle analog of the traditional twofluid RichtmyerMeshkov instability (RMI). We study the interaction of a planar air shock with a corrugated glass particle curtain through the use of numerical simulations with an EulerianLagrangian approach. The simulations track the computational particle trajectories as well as the evolution of the curtain of gas which is initially trapped inside of the particle curtain. This work focuses on the evolution of the particle curtain after interacting with the shock. Two shock Mach numbers, 1.21 and 1.5, are studied along with perturbation wavelengths of 3.6 and 7.2 mm to analyze the roles of these parameters as the evolving curtain undergoes SDMI both before and after reshock. The effect of particle loading in the curtain evolution is also investigated. A dilute curtain (roughly 0.1{\%} volume fraction) replicates the Atwood number of an airSF$_{\mathrm{6}}$ RMI experiment and a denser curtain at 26{\%} volume fraction is used to introduce additional multiphase coupling effects. This study also looks at the validity of comparing the fluidonly and multiphase Atwood numbers in the twoway coupled flow regime. [Preview Abstract] 

T15.00003: Fluid Capture Patterns in Hairy Surfaces Christopher Ushay, Etienne JambonPuillet, PierreThomas Brun Interfacial flows past dense arrays of flexible hairs are highly coupled: capillary forces of the moving interface can be strong enough to deform fibres, which in turn modifies local flow geometry. As a result, the ability to drain a preexisting layer of fluid over the deformable surface can significantly differ from the undeformed reference case. Here we fabricate “hairy” elastoporous media by casting a curing elastomer and mounting the arrays in a 1D HeleShaw cell. Upon displacing a defending phase of oil with water, elastocapillary bundling causes preferential invasion into certain pores at a regular interval. We characterize these patterns and then study pore drainage dynamics and bundle size as a function of capillary number $\mathbf{Ca}$. Models for depthaveraged fluid flow and the deflection of elastic beams are adapted to our problem to describe the deformation of the host medium due to interfacial and viscous forces. These experiments are then upscaled to higher dimensions, where oil is displaced from a HeleShaw cell textured with a 2D carpet of fibres arranged in a lattice; we present results for oil drainage and pattern formation in both rectangular and radial geometries. [Preview Abstract] 

T15.00004: On a GasParticle Analogue to the RichtmyerMeshkov Instability. Part 2: The Initially Entrapped Gas Bertrand Rollin, Frederick Ouellet, Bradford Durant, Rahul Koneru, S. Balachandar We study a planar numerical shock tube containing a corrugated particle curtain. The curtain is about 4mm thick and has a peak volume fraction of about 26{\%}. It is composed of spherical particles of 115$\mu $m in diameter with a density of 2500kg.m$$3, thus mimicking glass particles commonly used in multiphase shock tube experiments or multiphase explosive experiments. Under these conditions the gasparticle flow that follows the shock interaction with the curtain is twoway coupled. Using a EulerianLagrangian approach, we track trajectories of computational particles in the threedimensional planar shock tube as well as the air initially trapped inside the particle curtain. This work focuses on the latter. We characterize the evolution of the gas cloud inside the particle curtain for two Mach numbers, M$=$1.21 and M$=$1.50, and two dominant wavelengths, l$=$3.6mm and l$=$7.2mm, as it is advected downstream and undergoes RichtmyerMeshvov instability with features corresponding to the initial perturbation imposed on the particle curtain. We also analyze the behavior of the gas after reshock. [Preview Abstract] 

T15.00005: Linear stability analysis of a plane Poiseuille flow in a multilayer porous channel Supriya Karmakar, R Usha, Priyanka Shukla We examine a pressuredriven, incompressible, fully developed flow through a multilayer channel containing an anisotropic porous layer placed parallel to the channel walls. The channel is confined by impermeable walls and governed by the DarcyBrinkmanForchheimer equation in a porous layer along with the NavierStokes equation in liquid layers. The continuity of stress and velocity are used at the interface and noslip condition at the impermeable walls. The effect of anisotropic permeability and orientation angle on the flow and on the skin friction are discussed. Furthermore, the linear stability analysis of aforementioned configuration by considering the nonlinear inertial term in the DarcyBrinkmanForchheimer equation is performed numerically. The results are validated with the linear stability results of a flow where the channel consists of a fluid layer sandwiched between two isotropic porous layers. The present results are consistent when we neglect the inertial effect of the porous medium. It is found that the major system parameters affect significantly the stability characteristics of the flow, and therefore the inertial effect provides a useful means to control the flow instability of a multilayer porous system having anisotropic permeability. [Preview Abstract] 

T15.00006: A new pathway for the amplification of distortions to the surface of liquid jets Hanul Hwang, Parviz Moin, M. J. Philipp Hack The atomization process of liquid jets often begins with the linear amplification of small perturbations to the material interface. We identify and characterize a novel mechanism for the amplification of interface distortions of liquid jets. The mechanism is independent of the exponential instability of the flow and, depending on the parameters, can intensify small perturbations to the material interface by several orders of magnitude and at a faster pace than the exponential KelvinHelmholtz instability. Analysis of the budget of the perturbation kinetic energy sheds light on the underlying physics. It is shown that energy is extracted from the mean shear by the production term in the axial velocity component and redistributed to the radial velocity component where it is transferred into the surfacetension energy of the material interface. Parameter studies show that the gain in surface tension energy scales linearly with the Reynolds number. Furthermore, a critical Weber number is identified as a lower bound beyond which the mechanism becomes active. Asymptotic analysis establishes a powerlaw relationship of a critical Weber number to the Reynolds number which is confirmed by computational results. [Preview Abstract] 

T15.00007: Experimental Investigation of Surface Temperature Changes due to Flow Instability in a Micro/Mini Channel During FlowBoiling Heat Transfer with NonUniform Circumferential Heat Fluxes at Different Inclinations Marius Vermaak, Jaco Driker, Khellil Sefiane, Josua Meyer Flow instability was analysed during flowboiling of Perfluorohexane (FC72) in a rectangular micro/mini channel at different inclinations with onesided heating. The flow passage had a hydraulic diameter of 909~$\mu $m and an aspect ratio of 10 (5mm~x 0.5mm). Inclination angles ranged from 20\textordmasculine (downward flow) to 0\textordmasculine (horizontal flow), and up to $+$90\textordmasculine (vertical upward flow) at mass fluxes of 10, 20~and 40kg/m$^{\mathrm{2}}$s. Pressure measurements and highspeed images were used to determine the effect of instability events on the heated surface's temperature measured using infrared thermography. Inlet pressure measurements identified reverse flow events while outlet pressure measurements identified twophase mixing events. The frequency of instability events was determined using pressure transducer data and an instability threshold pressure. Flow instability was shown to decrease surface temperatures by up to 14.1\textordmasculine C. Negatively inclined channels and $+$90\textordmasculine experienced negligible instability frequencies. Horizontal and positively inclined channels had instability frequencies of up to 1.8Hz and 2.1Hz at the inlet and outlet. [Preview Abstract] 

T15.00008: Energy budget analysis of plane PoiseuilleCouette flow over a permeable surface Saman Hooshyar, Harunori N. Yoshikawa, Parisa Mirbod The stability of Poiseuille flow over a permeable surface has been studied extensively over the past years due to its vast range of engineering applications. We previously investigated how imposing a Couette flow could impact the stability of such a system by performing a linear stability analysis. It was observed that the Couette flow exerts destabilizing or stabilizing effects depending on the permeable surface properties. This study, using the energy budget analysis, aims to provide a physical interpretation of the behavior of the system. The results show that for unstable wavenumbers, the production term from the Reynolds shear stresses produces the required energy which allows the disturbances to grow. For stable wavenumbers, the disturbances are dampened since the rate of energy loss due to viscous dissipation and Darcy drag becomes greater than the production term. In general, increasing both the Couette component and the porous permeability, as well as decreasing the fluid layer thickness results in a higher production term, which consequently makes the flow less stable. We were also able to distinguish different modes by computing the kinetic energies of perturbation flows in the fluid and porous layers. [Preview Abstract] 

T15.00009: Primary instability of an airwater mixing layer: convergence between simulations, experiments and linear theory Cyril Bozonnet, Guillaume Balarac, Olivier Desjardins, JeanPhilippe Matas The KelvinHelmholtz instability occurring at the interface between a slow liquid and a fast gas is the first in a cascade of instabilities leading to spray formation. Recent progress made in the understanding of the nature of this instability and its driving mechanisms (confinement by the gas stream, surface tension, viscosity) allows now to reconcile linear theory and experiments. While numerical simulations can be used to deepen the analysis of such complex phenomena, the codes have to be carefully validated first. Multiphase code validation is made challenging by several factors: high density and momentum flux ratio, high Reynolds number, and complex topology changes. In this work, we present a systematic validation of our multiphase flow solver against experiments and linear theory for the canonical configuration of a twodimensional airwater mixing layer. Particular attention is given to the characteristics of the instability in both its linear and nonlinear regime. We discuss the accuracy of our results and the convergence of the statistics. Finally, we explore the effects of the injector geometry on the stability of the flow. [Preview Abstract] 
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