Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session C37: Multiphase Flows: Turbulence |
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Chair: Antonino Ferrante, Univeristy of Washington Room: 619 |
Sunday, November 24, 2019 8:00AM - 8:13AM |
C37.00001: Wavelet-spectral analysis of droplet-laden isotropic turbulence Andreas Freund, Antonino Ferrante The energy spectrum for homogeneous isotropic turbulence is computed using the Fourier transform of the velocity field. In the case of multiphase turbulent flows, the velocity field is nonsmooth at the interface between the carrier fluid and the dispersed phase, so the Fourier energy spectra exhibit spurious oscillations at high wavenumbers. An alternative definition of the spectrum uses the wavelet transform, which can handle discontinuities locally while additionally preserving spatial information about the field. We propose using the wavelet energy spectrum to study multiphase turbulent flows and a new decomposition of the wavelet energy spectrum into three contributions corresponding to the carrier phase, droplets, and interaction between the two. Lastly, we apply the new wavelet-decomposition tools in analyzing the DNS data of droplet-laden decaying isotropic turbulence. Our results show that, in comparison to the spectrum of the single-phase case, the droplets (i) do not affect the carrier-phase energy spectrum at high wavenumbers, (ii) increase the total energy spectrum at high wavenumbers by increasing the interaction energy spectrum at these wavenumbers, and (iii) decrease the total energy at low wavenumbers by increasing the dissipation rate at these wavenumbers. [Preview Abstract] |
Sunday, November 24, 2019 8:13AM - 8:26AM |
C37.00002: Direct numerical simulation of droplet-laden homogeneous shear turbulence Pablo Trefftz-Posada, Antonino Ferrante We have performed direct numerical simulations (DNS) of droplet-laden homogeneous shear turbulence (DLHST) at initial $Re_{\lambda}=75$ with 6260 droplets of diameter approximately equal to the Taylor lengthscale (i.e, 5\% droplet volume fraction). The droplet to carrier-fluid density and viscosity ratios have been set to 10. The droplet Weber number based on the r.m.s. velocity has been varied between $0.1 \leq We_{rms} \leq 5$. First, we present our numerical methods for overcoming the challenges of simulating DLHST. Then, we present the effects of varying the shear number ($Sh$) on the budget of turbulence kinetic energy (TKE). For example, in the two-fluid TKE equation the power of the surface tension is directly proportional to the rate of change of droplet surface area (with opposite sign). DNS results show the effects of shear on droplet deformation/breakup/coalescence and how this affects the power of the surface tension, and, thus, the evolution of TKE. [Preview Abstract] |
Sunday, November 24, 2019 8:26AM - 8:39AM |
C37.00003: Wavelet analysis of spectral energy transfer in two-way coupled particle-laden turbulence Miralireza Nabavi Bavil, Mario Di Renzo, Jeonglae Kim This study investigates the effects of two-way coupling between the carrier and dispersed phases on the spectral transfer of turbulent kinetic energy (TKE) using the wavelet multiresolution analysis (WMRA). Direct numerical simulations of decaying homogeneous isotropic turbulence laden with inertial particles are performed at Stokes numbers $\textrm{St}_k$ = 0, 1 and 10 with the point-particle assumption. TKE decreases monotonically as $\textrm{St}_k$ increases, while this is not the case for the dissipation rate. A multiresolution analysis based on orthonormal wavelet transform is developed to evaluate spectral energy transfer due to different physical mechanisms near particle clouds interacting in two ways with the carrier-phase turbulence. Spectral statistics conditioned on the Eulerian particle number density are examined to understand the physical trends of spectral energy transfer as a function of Stokes number and discuss their significance in modeling. [Preview Abstract] |
Sunday, November 24, 2019 8:39AM - 8:52AM |
C37.00004: Cascades of bubbles in turbulent breaking waves Wai Hong Ronald Chan, Perry Johnson, Javier Urzay, Parviz Moin Turbulent breaking waves entrain air cavities that break up and coalesce to form polydisperse clouds of bubbles. A bubble-tracking algorithm is developed to identify bubble breakup and coalescence events in interface-capturing two-phase numerical simulations, and to quantify the resulting transfers of air between bubbles of different sizes. The time evolution of the volume- and ensemble-averaged bubble size distribution resulting from imbalances of the averaged transfer fluxes is described by a population balance equation with models for breakup and coalescence kernels. The formalism resembles the phenomenology of the Richardson-Kolmogorov energy cascade in single-phase turbulence. In order to demonstrate the presence of a bubble cascade, the transfer of air mass in bubble-size space by breakup and coalescence is examined for an ensemble of simulations of turbulent breaking waves. For breakup, a quasi-local transfer is observed in which the net transfer of air across a certain bubble size primarily depends on the number and breakup frequency of bubbles of similar sizes. This quasi-locality suggests that the statistics of bubble breakup at intermediate sizes are largely independent of the smallest and largest bubbles, in support of the idea of a bubble breakup cascade. [Preview Abstract] |
Sunday, November 24, 2019 8:52AM - 9:05AM |
C37.00005: Low pressure events of finite size bubbles in homogeneous isotropic turbulence Mehedi Hasan Bappy, Alberto Vela-Martin, Pablo Carrica, Gustavo Buscaglia The study of the behavior of bubbles in turbulent flow is fundamental to the understanding of many two-phase flow applications such as cavitation inception. As gas nuclei evolve in a turbulent flow, the pressure fluctuations can dip below the vapor pressure and trigger cavitation events. Pressure statistics along trajectories of finite bubbles in isotropic homogeneous turbulence is investigated using direct numerical simulations at two $Re_{\lambda}$ (150, 240). A modified Maxey-Riley equation is solved to track bubbles of different sizes in the turbulence field. The results show that larger bubbles are more attracted by the vortex cores and spend longer times at low-pressure regions. This has significant impact on the PDF of the pressure experienced by the bubbles, as well as on the statistics of low-pressure events (i.e., average frequency, distribution of duration and inter-event delays). The effect of gravity on these statistics is also addressed. It is shown that gravity is a bubble transport mechanism that competes with flow induced pressure gradients, making the bubbles less sensitive to low-pressure vortex cores. [Preview Abstract] |
Sunday, November 24, 2019 9:05AM - 9:18AM |
C37.00006: Liquid film breakup induced by turbulent shear flow Melissa Kozul, Pedro Costa, James R. Dawson, Luca Brandt Gas turbine engines commonly employ prefilming airblast atomizers for liquid fuel injection. Supplied from holes or slits upstream, the liquid fuel forms a thin film over the prefilming surface before being driven to the atomizing edge by a turbulent flow through the turbine. The use of a second air stream on the other side of the prefilmer to prevent fuel accumulation means the breakup and eventual atomization of the liquid film occurs in the shear zone formed by the co-flowing air streams (Aigner \& Wittig, J. Eng. Gas Turbines Power (1988) vol. 110, pp. 105 - 110). We consider a simplified numerical setup using a recently developed volume of fluid method (Ii et al., J. Comput. Phys. (2012) vol. 231, pp. 2328 - 2358) to simulate this multiphase problem. A liquid film is sandwiched between sheared turbulent gas flows from a precursor simulation, which serve to deform and then rupture the liquid film. The simplified setup allows us to systematically vary parameters such as film thickness and turbulent gas flow Reynolds number to gauge the effect upon momentum transfer into the initially stationary liquid film. Understanding and controlling the route to breakup and atomization of liquid fuels in such systems is of primary practical concern in modern gas turbine engine design. [Preview Abstract] |
Sunday, November 24, 2019 9:18AM - 9:31AM |
C37.00007: Turbulence modulations induced by a swarm of surfactant-laden droplets Giovanni Soligo, Alessio Roccon, Alfredo Soldati We use direct numerical simulations of turbulence coupled with a two-order-parameter phase-field method to describe the complex dynamics of a swarm of surfactant-laden droplets in turbulence.Two separate Cahn-Hilliard (CH) equations define the dynamics of the interface and of the local surfactant concentration. An interfacial term, based on the Korteweg tensor, accounts for the effect introduced by a surfactant-laden, deformable interface on the flow field. In particular, the interfacial term accounts for both normal (capillary) and tangential (Marangoni) stresses at the interface. The former originate from the presence of the interface, while the latter arise whenever surface tension gradients along the interface are present. These surface tension gradients originate from a non-uniform surfactant distribution. The presence of a surfactant-laden interface thus affect the local flow field and turbulence and modulates exchanges of momentum between the droplet fluid and the carrier fluid. Here we will present the effects of different reference (clean interface) surface tension values and different types of surfactant on these exchanges. [Preview Abstract] |
Sunday, November 24, 2019 9:31AM - 9:44AM |
C37.00008: Turbulence statistics in a negatively buoyant particle plume Evan Variano, Ankur Bordoloi, Chris Lai, Laura Clark, Gerardo Veliz Plumes containing bubbles, particles and droplets are found in many natural phenomena as well as industrial applications. We report herein the turbulence statistics in a negatively buoyant multiphase plume containing heavy particles. We generate the plume by continuously releasing nylon particles of size 2 mm inside a salt-water tank via screw-conveyor based release mechanism. The two phases are refractive-index matched, that enables us to measure the local velocity in the salt-water via stereoscopic particle image velocimetry. Besides some structural differences, the turbulence statistics in a particle plume resemble that measured in a bubble plume. The turbulent kinetic energy (TKE) production by particles (or bubbles) roughly balances the viscous dissipation, except near the plume centerline. We observe a -3 power-law in the one-dimensional power-spectra of the velocity fluctuations that puts both the particle and bubble plume in a category different from single-phase shear-flow turbulence. [Preview Abstract] |
Sunday, November 24, 2019 9:44AM - 9:57AM |
C37.00009: Jammed emulsions via turbulent stirring Federico Toschi, Ivan Girotto, Gianluca Di Staso, Karun Datadien, Roberto Benzi, Prasad Perlekar, Andrea Scagliarini Stabilised dense emulsions are common in many foods and cosmetics products (e.g. mayonnaise). Such complex fluids, made of two immiscible fluid components and a stabilizing agent (e.g. surfactant), behave like an elastic solid below a critical yield stress and flow like a viscous fluid above it. More generally, stabilized emulsions display all the rich phenomenology and rheology typical of soft-glassy materials. Stabilized emulsions are often produced via large-scale turbulent stirring. This raises the questions of e.g. how the emulsion structure depends on the turbulent stirring protocol and what are the rheological properties of the obtained emulsion. We employ large-scale 3d direct numerical simulations based on the multi-component Lattice Boltzmann method and second-belt coupling to numerically investigate turbulent emulsification. We show that turbulence is effective in producing a jammed state. We report the protocol followed in order to achieve packing fractions above 70\% of the dispersed fluid phase and we characterize the yield stress of the obtained emulsions. In general, our model can be used to investigate catastrophic phase inversion, an event occurring either when the forcing intensity exceeds a threshold value or for excessive depletion of the matrix phase. [Preview Abstract] |
Sunday, November 24, 2019 9:57AM - 10:10AM |
C37.00010: Impact of turbulence on cloud microphysics of water droplets population Mina Golshan, Federico Fraternale, Marco Vanni, Daniela Tordella This work focuses on the turbulent shear-less mixing structure of a cloud/clear-air interface with physical parameters typical of cumulus warm clouds. We investigate the effect of turbulence on the droplet size distribution, in particular we focus on the distribution’s broaden- ing and on the collision kernel. We performed numerical experiments via Direct Numerical Simulations (DNS) of turbulent interfaces sub- ject to density stratification and vapor density fluctuation. Specifically, an initial super-saturation around 2% and a dissipation rate of turbu- lent kinetic energy of 100 $cm{^2}$/$s{^3}$ are set in the DNSs. The Taylor’s Reynolds number is between 150 and 300. The total number of par- ticles is around 5-10 millions, matching an initial liquid water content of 0.8 g/$m{^3}$ . Through these experiments, we provide a measure of the kernel of collisional integral operators to be compared with literature models [Saffman Turner, 1955] and possibly used inside drops Population Balance Equations (PBE) that include both processes of drops’ growth by condensation/evaporation and aggregation. [Preview Abstract] |
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