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10:45   Multiphase Flows 4 10:45 15 mins #369 Numerical study of gravity effects on the symmetry and development of particle-laden flows Xinchen Zhang, Graham Nathan, Zhao Tian, Cheng Chin Abstract: In the majority of particle-laden flow experiments, due of the limitation of laboratory space, using a horizontal pipe or a tilted pipe with a certain angle to the horizontal plane is more achievable and common than a vertical pipe. That might result in an asymmetry in both fluid characteristics and particle behaviour due to the gravity, like the difference of particle distribution between vertical and horizontal pipes shown in Figure 1. In this research, the influences of the gravity bias effect on both the characteristics and turbulence development of particle-laden flows are studied using Large Eddy Simulations (LES). This research aims at investigating the critical regimes of various particle-laden flow parameters, such as the Stokes number, volume fraction and tilted angle of the pipe, that when gravity bias effect becomes negligible, i.e. the study of the symmetry of flows. This research is then extended to identify the correlation between these properties mentioned above and the pipe length to achieve a fully developed turbulent stage, i.e. the study of the convergence of flows. A periodic and long straight pipe models are employed in this research for the studies of the symmetry and convergence respectively. The simulations are conducted based on the experimental parameters and also validated against the results of Lau & Nathan [1, 2]. The following flow parameters, Stokes number Sk0, volume fraction Φv and tilted angle θ ranging from 0.15 ≤ Sk0 ≤ 44.8, 10-6 ≤ Φv ≤10-3 and 0° ≤ θ ≤ 90° respectively, are considered in the particle-laden flows with a bulk Reynolds number of Reb = 20000. Additionally, research of the convergence of the mean velocity and turbulence intensity profiles along the straight pipe is conducted, to identify the required pipe lengths of particle-laden flows to achieve a fully developed stage. The results suggest that the volume fraction and tilted angle play an important role in the variation on the symmetry and convergence of particle-laden flows, while the Stokes number shows a weaker correlation with the gravity bias effect. These results contribute to the advanced understanding of particle-laden flow and guiding future experiments in this field. 11:00 15 mins #319 Modulation of very large scale motions by inertial particles David Richter, Guiquan Wang Abstract: Very large-scale motions (VLSMs) extending to over 20h (where h is the boundary layer thickness) are found in very high Reynolds number, wall-bounded turbulent flows and are distinct from the well-understood large-scale motions (LSMs) which form canonical streaks and hairpin vortices. These long, meandering features are observed to be energetic, carrying 40-65% of the kinetic energy and 30-50% Reynolds shear stress, and in environmental flows, these anisotropic structures also have significant influence on the dispersion of pollutants, sand, and other constituents. At the same time, the general understanding of turbulence modulation by inertial particles is itself a formidable challenge, and nearly all numerical studies of two-way coupling in particle-laden wall turbulence have been restricted to low Reynolds numbers. It is therefore the aim of this investigation to study the effects of particles on VLSMs, in particular focusing on the question of whether particles act directly or indirectly (via LSMs) on these very large motions. To achieve this goal, direct numerical simulations two-way coupled with inertial particles are employed in open channel flow at friction Reynolds numbers of 550 and 950 and over a wide range of particle Stokes numbers. In the wall-normal direction, particle distributions (mean/preferential concentration) exhibit two distinct behaviors in the inner flow and outer flow, corresponding to the two highly anisotropic LSM and VLSM characteristic structures. This ultimately results in particle inertia's non-monotonic effects on the VLSMs: low inertia (compared to the timescales of the inner layer) and high inertia (compared to the timescales of the outer layer) both strengthen the VLSMs whereas moderate and very high inertia have little influence. We do not observe any attenuation of VLSM energy. This is seen clearly in the streamwise energy spectra, where energy contained at VLSM wavelengths is enhanced throughout the entire vertical extent of the channel. Through conditional tests, low and high inertia particles enhance VLSMs following two distinct routes. Low inertia particles promote VLSMs indirectly through the enhancement of the regeneration cycle (the self-sustaining mechanism of LSMs) in the inner region whereas high inertia particles enhance the VLSM directly through contribution to the Reynolds shear stress at similar temporal scales in the outer region. To better understand low-inertia particles' enhancement of VLSMs, additional simulations performed at transitional Reynolds numbers in planar Couette flow are utilized to map out the mechanisms by which low-inertia particles enhance and accelerate the traditional LSM regeneration cycle (thus triggering the laminar-to-turbulence instability at lower critical Reynolds numbers) while high-inertia particles inhibit the regeneration cycle and suppress transition. By altering the regeneration of these LSMs, low-inertia particles are found in the high Reynolds number flow to enhance VLSMs via a nonlinear, upscale energy transfer between LSMs and VLSMs. This finding is consistent with current understanding of inner/outer interactions in wall-bounded turbulent flows. In contrast to the low-inertia particles indirectly modifying VLSMs by interacting with LSMs in the inner layer, high-inertia particles operate directly on VLSMs, enhancing their energy content via modifications to the spectral Reynolds stress budget in the outer layer of the flow. Thus while these two distinct Stokes number particles have similar enhancement signatures in the VLSM energy content, the mechanism by which this is achieved is quite different. 11:15 15 mins #325 Direct numerical simulation of particle clustering in the wake of flow past a circular cylinder Zhaoyu Shi, Håkon Strandenes, Fengjian Jiang, Lihao Zhao, Helge Ingolf Andersson Abstract: Particle-laden flows are encountered in many natural and engineering applications, such as sediment and scouring of suspended particles with the flows around offshore wind turbine foundations. Of particular interest is how inertial particles move and cluster in the wake vortices. Particle-laden flow around a circular cylinder with diameter D is simulated at three different Reynolds numbers Re. Only Stokes drag force acts on the particles whose inertia is quantified by means of the particle relaxation time τ. The Stokes number St=τU/D is introduced as a dimensionless measure of particle inertia. The wake and the vortex shedding are known to become increasingly more complex with increasing Reynolds number. The wake at Re = 100 remains two-dimensional with a unique shedding frequency. Nevertheless, we observe distinct Stokes number effects to the left panels in Figure 1 with rather complex clustering patterns of the most inertial particles. In the lower part of the transition-in-wake regime at Re = 200, ’Mode A’ instabilities with wavelength ≈ 4D emerge and make the wake three-dimensional [1]. The streamwise vortices associated with ’Mode A’ instability change sign twice every shedding period and evidently introduce a new time scale into the wake flow. We address the yet unanswered question whether inertial particles concentrate within or outside of these secondary vortices. This will affect the wake’s mixing ability. We also perform a direct numerical simulation at Re = 3900, at which the three-dimensional flow field is almost fully turbulent [1], as shown in Figure 1(d). At this Reynolds number, the wake turbulence extends over a fairly wide range of time- (and length-) scales, in addition to the time scale associated with the primary vortex shedding. Here, we explore how the particles concentrate preferentially by means of Voronoï analysis [3] and correlations between the turbulent vorticity and the local Voronoï volume. We show that inertial particles cluster differently in the turbulent wake than in wall turbulence. 11:30 15 mins #481 Cluster of inertial particles and fluid acceleration in turbulence Sunao Oka, Susumu Goto Abstract: Inertial particles form clusters in developed turbulence and the spatial distribution depends on the Stokes number (i.e. particle relaxation time normalized by the Kolmogorov time). In order to predict the location of the clusters with an arbitrary Stokes number, we propose surfaces defined in terms of the coarse-grained fluid acceleration. We numerically simulate the turbulence with the Reynolds number based on the Taylor length being 900 and the motion of small heavy particles. By choosing the proper coarse-graining scale, the surfaces spatially correlate to the clusters with a wide range of the Stokes numbers. The relation between the Stokes number and the coarse-graining scale obeys the power law consistent with Kolmogorov’s similarity. 11:45 15 mins #450 TURBULENCE MODIFICATION OF A PARTICLE-LADEN FLOW IN A ROCKET ENGINE MODEL Sabrina Kalenko, Alex Liberzon Abstract: Particle-laden turbulent flow is the key for the optimal performance of a solid propellant rocket engine that uses dispersed alumina particles to increase combustion efficiency. We study turbulence - particles interaction in a complex geometry of a rocket engine chamber with gas flow accelerating through the nozzle. Accurate estimate of turbulent kinetic energy (TKE) at various regions of the chamber is important for both the design of abrasive protection, and combustion performance of an engine. TKE was shown to increase [2, 5, 6] or decrease [1, 4, 3] in presence of the dispersed particle phase, and the flow case with acceleration is less explored. We use Particle Image Velocimetry (PIV) measurements of the turbulent air flow (the carrier phase) and Particle Tracking Velocimetry (PTV) measurements of inertial particles (the solid phase) in a simplified two dimensional model of a rocket engine chamber cross-section. We study air flow at Re up to 23×10^4 based on the velocity at the nozzle. Tracers (1μm propylene glycol aerosol) and alumina particles (∼350μm) are injected upstream the chamber entrance, above the measurement volume. A relatively high average mass loading (φ >1) of dispersed inertial alumina particles (St =10^4) leads to the spatially varying two-way coupling flow regime. The combination of the two measurement methods allows to quantify relative velocity that characterises fluid-particle interactions. The interaction of high Reynolds number turbulent flow with the high mass loading of inertial alumina particles results in the overall increase of TKE across the chamber, as compared to the unladen case. Furthermore, we observe decrease in the streamwise component of the root-mean-square velocity, u′x as the flow approaches the nozzle (x/Lt→0) in Fig. 1a. This seem to be directly related to the slowly increasing local mass loading of the inertial particles which are moving towards the entrance of the nozzle, shown in Fig. 1b. We infer that it is a combined effect of the local mass loading and increase in particles relative velocity in respect to the flow acceleration 12:00 15 mins #555 Effects of Stokes number on particle mechanics in a free-shear jet Bianca Viggiano, Jeremy Vessaire, Romain Volk, Mickaël Bourgoin, Laurent Chevillard, Raúl Bayoán Cal Abstract: Particle mechanics are chaotic, especially in the turbulent flow regime as it leads to enhanced aggregate growth rate through increased collisions and in turn, the onset of particle breakup \cite{babler2010,derksen2012}. Within the turbulent flow regime, the Stokes number becomes the relevant parameter with respect to inertial particle dynamics. Statistical studies of the dependence of particle mechanics on Stokes number have been performed on experimental data in isotropic turbulence \cite{salazar2008,saw2008}. These studies investigate flow dynamics in an idealized field, but particle aggregation can occur in bounded and unbounded shear flows, where the flow becomes highly anisotropic. It is therefore important to identify the mechanism that lead to aggregation within a spatially dependent regime. This study experimentally investigates the effects of localized atmospheric conditions on the aggregation and breakup of particles at a turbulent/non-turbulent interface of a free-shear jet. Characterization of the physical mechanism leads to increased controllability and predictability of particle-laden flow processes and is relevant to many geophysical flows. The experiments were conducted in the Lagrangian Exploration Module (LEM) at ENS-Lyon shown in figure~\ref{LEM}. A vertical pipe apparatus with a 4 mm exit diameter was used to feed water into the LEM, creating a free-shear jet within center of the module. The flow was seeded with particles of varying density and size to provide sufficient variation in the Stokes drag. The data was obtained via three high speed cameras at a frequency of 3600 Hz with a total record length of over 60 seconds. 3D particle tracking velocimetry (PTV) was used to quantify particle mechanics within and outside of the jet. PTV allows for in-depth spatial measurements which are equipped to create highly converged and unbiased probabilistic descriptions of the flow. 12:15 15 mins #277 EXACT CALCULATION OF ENERGY FLUX RATE IN TURBULENT FERROFLUIDS Sukhdev Mouraya, Supratik Banerjee Abstract: We investigate the total energy conservation in ferrofluids taking into the interaction between the external magnetic field and the fluid. Using two point statistics we can formulate exact relations in the inertial zone of the turbulence. This exact relation shows that (u × ω), (M × H), M · ∇H and (ω × M) play the major role in energy cascading. 12:30 15 mins #280 ELECTRO-VORTEX FLOW IN LIQUID METAL BATTERIES Caroline Nore, Pedro Ziebell Ramos, Wietze Herreman, Loïc Cappanera, Jean-Luc Guermond, Norbert Weber Abstract: We study the generation of rotational flows in electrically conducting fluids due to the Electro-Vortex-Flow (EVF) phenomenon, i.e. the interaction of a non-uniform current with the magnetic field it generates. We have been developing a so-called code SFEMaNS since 2001 [2] capable of simulating the nonlinear magnetohydrodynamic (MHD) equations in heterogeneous domains (with electrical conductivity or magnetic permeability jumps) in axisymmetric geometries and with several fluids [1]. Liquid Metal Batteries are composed of three layers of fluids (liquid metal electrode–electrolyte– liquid metal electrode) of different densities lying over each other and stabilized by gravity. These batteries are prone to magnetohydrodynamical instabilities (e.g. the Tayler instability [3], the Metal Pad Roll instability [4], etc.) which may deform the electrode–electrolyte interfaces until the ultimate situation of short circuit when the two metals touch each other. In this talk we first discuss the typical intensity and structure of the axisymmetric flow in a liquid metal column covered by many previous studies. After that, we focus on EVF in liquid metal batteries (see figure 1). We discuss the deformation of the electrolyte-liquid metal interfaces caused by EVF and we characterize how EVF helps in mixing the bottom alloy layer. [1] L. Cappanera, J.-L. Guermond, W. Herreman, and C. Nore. Momentum-based approximation of incompressible multiphase fluid flows. International Journal for Numerical Methods in Fluids, 86(8):541–563, 2018. [2] J.-L. Guermond, J. Léorat, F. Luddens, C. Nore, and A. Ribeiro. Effects of discontinuous magnetic permeability on magnetodynamic problems. Journal of Computational Physics, 230:6299–6319, 2011. [3] W. Herreman, C. Nore, L. Cappanera, and J.-L. Guermond. Tayler instability in liquid metal columns and liquid metal batteries. Journal of Fluid Mechanics, 771:79–114, 2015. [4] N. Weber, P. Beckstein, W. Herreman, G. M. Horstmann, C. Nore, F. Stefani, and T. Weier. Sloshing instability and electrolyte layer rupture in liquid metal batteries. Physics of Fluids, 29(5):054101, 2017.