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 H03: Focus Session: Direct Numerical Simulations of Fluid Interfaces, Deformation and Break-Up in Turbulence |
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Chair: Olivier Desjardins Room: 201 |
Monday, November 25, 2019 8:00AM - 8:13AM |
H03.00001: DNS of turbulent flows laden with deformable bubbles or droplets: Overview of methods Invited Speaker: Said ELGHOBASHI Turbulent flows laden with liquid droplets or gas/vapor bubbles are ubiquitous in nature and engineering applications. In nature, examples include rain, air bubbles in the upper ocean, and vapor bubbles in geysers. Engineering applications include liquid fuel sprays in all types of combustion engines, paint sprays, spray drying in the pharmaceutical industry as well as food processing, and water vapor bubbles in nuclear reactor cooling systems or those created by cavitation in the wakes of ship propellers, just to list a few. This lecture focuses on direct numerical simulations (DNS) of turbulent flows laden with droplets or bubbles. DNS of these flows are more challenging than those of flows laden with solid particles due to the surface deformation in the former. The numerical methods to be discussed are classified by whether the initial diameter, $d$, of the bubble/droplet is smaller or larger than the Kolmogorov length scale, $\eta$. The lecture discusses DNS of deformable small spherical bubbles/droplets ($d<\eta$) via a phenomenological model.\footnote[1]{Elghobashi, S. `` Direct Numerical Simulation of Turbulent Flows Laden with Droplets or Bubbles", Annu. Rev. Fluid Mech. 2019, 51:217-244.} [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H03.00002: DNS of turbulent flows laden with deformable bubbles or droplets: Recent advances Said Elghobashi Turbulent flows laden with liquid droplets or gas/vapor bubbles are ubiquitous in nature and engineering applications. In nature, examples include rain, air bubbles in the upper ocean, and vapor bubbles in geysers. Engineering applications include liquid fuel sprays in all types of combustion engines, paint sprays, spray drying in the pharmaceutical industry as well as food processing, and water vapor bubbles in nuclear reactor cooling systems or those created by cavitation in the wakes of ship propellers, just to list a few. We discuss recent advances in the numerical methods\footnote[1]{Elghobashi, S. `` Direct Numerical Simulation of Turbulent Flows Laden with Droplets or Bubbles", Annu. Rev. Fluid Mech. 2019, 51:217-244.} of deformable large spherical bubbles/droplets whose initial sizes are larger than the Kolmogorov length scale, $\eta$. The methods include the Conservative Level Set (CLS), Volume of Fluid (VOF), Front Tracking (FT), Phase Field Model (PFM), and Lattice Boltzmann (LB). [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H03.00003: Effects of droplet deformation and breakup/coalescence on turbulence kinetic energy Antonino Ferrante, Pablo Trefftz-Posada We have performed direct numerical simulations (DNS) of droplet-laden homogeneous isotropic and shear turbulence (DLHIT \& DLHST) using our volume-of-fluid/pressure-correction method for two-fluid incompressible flows, FastP$^*$ (Dodd \& Ferrante, {\em J. Comp. Phys.}, 2014). For the DLHIT case, we validate the DNS results of the droplet deformation by comparison with available experiments for droplet diameter of order of the Taylor length scale of turbulence. Then, we have derived the governing equations of turbulence kinetic energy (TKE) for DLHIT \& DLHST for the carrier-phase, droplet-phase and two-fluid flow. In the two-fluid TKE equation, there is the power of surface tension term, $\Psi_\sigma(t)$, which represents the rate of change of the surface energy for the interface between the two fluids. $\Psi_\sigma(t)$ is proportional, with opposite sign, to the rate of change of total droplet surface area. Thus, as the total droplet surface area changes in time through droplet deformation and breakup/coalescence, $\Psi_\sigma(t)$ acts as source (or sink) of turbulence kinetic energy. Accordingly, through the power of the surface tension, droplet coalescence acts as a source of TKE and breakup acts as a sink of TKE. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H03.00004: Direct Numerical Simulations of Bubble Break-up in Turbulence Luc Deike, Alienor Riviere, Wouter Mostert We present direct numerical simulations of bubble break-up in a three-dimensional homogeneous and isotropic turbulent flow. We consider the effect of the turbulent Reynolds number and the bubble Weber number on the break-up dynamics, the number of child bubble created together with their size and the break-up frequency. An ensemble of simulation is done in order to study these quantities statistically. For Weber number slightly above the critical value number, we retrieve binary break-up with two child bubble of similar size, while for large Weber number, we observe more complex break-up patterns with successive break-up events and the formation of a large number of much smaller bubbles. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H03.00005: Direct numerical simulation of bubble-induced turbulence at high Reynolds number Alice Jaccod, Alessio Innocenti, Stephane Popinet, Sergio Chibbaro \noindent Among the various kind of multiphase flows, bubbly flows represent a challenging and key field of investigation for their particular dynamics and their applications in several fields. Important experiments have been carried out in the last decades but a precise understanding of bubble-induced turbulence is still lacking. In the present work, a study of this phenomenon is presented, by performing fully resolved two and three dimensional numerical simulations of bubbles, rising up under the effect of buoyancy. Bubbles, initially placed at rest at the bottom of a channel, experience a large transfer rate with the liquid, resulting in an agitated turbulent motion, called pseudo-turbulence. Varying the physical parameters of the problem as the bubble volume fraction and increasing the Reynolds number, it's possible to outline a large phenomenology of the dispersed phase flows.\\ \noindent An investigation of energy spectra and velocity fluctuations probability density function has been done, for both two and three dimensional cases. Moreover, we performed a scale-by-scale analysis of energy transfer, to highlight the spatial range within which a direct or inverse energy cascade is present. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H03.00006: Breakage, coalescence and droplet size distribution of surfactant-laden droplets in wall-bounded turbulence Alfredo Soldati, Giovanni Soligo, Alessio Roccon The dynamics of surfactant-laden droplets in a turbulent channel flow is investigated using direct numerical simulations of turbulence coupled with a two-order-parameter phase-field method; the first-order parameter describes the dispersed phase morphology, while the second one the surfactant concentration. The problem is characterized by the complex interplay among flow field, interface, and surfactant distribution: these three factors are deeply intertwined and determine the overall dynamics of the dispersed phase. Shear stresses deform the interface, changing the local curvature and thus surface tension forces, but also advect surfactant over the interface. In turn, local increases of surfactant concentration reduce surface tension, changing the interface deformability and producing tangential (Marangoni) stresses. Finally, the interface feeds back to the local shear stresses via the capillary stresses, and changes the local surfactant distribution as it deforms, breaks and merges. These complex interactions determine the steady-state droplet size distribution, which is found to be in good agreement with previous experimental observations and numerical simulations. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H03.00007: Breakup at the resolution limit Marcus Herrmann The process of atomization is characterized by a vast range of time and length scales present in the flow. In fact, when the topological change of the phase interface occurs, i.e. a liquid structure breaks into smaller structures, the length scale goes to zero. Thus simulations of atomization are not able to resolve all length and time scales at all times. Typically, simulations are under-resolved during the final stages of breakup, relying on the properties of the numerical methods used, i.e., the properties of the methods' inherent numerical errors, to capture the topology change events correctly. This reliance on numerical errors to reproduce physical processes is questionable, but unavoidable without dedicated, breakup models that do not rely on the local mesh resolution to initiate breakup. In this talk, we will discuss how different interface capturing techniques perform during the final stages of breakup, using the breakup of a ligament in a test bed as an example. The results can give insights into the drop size distributions obtained in detailed simulations of atomizers, especially concerning smaller drop sizes near the mesh resolution limit. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H03.00008: Lattice Boltzmann Simulations of Interface Deformation and Breakup in Turbulent Flow Over Superhydrophobic and Liquid-Infused Surfaces Amirreza Rastegari, Rayhaneh Akhavan Interface deformation and breakdown in turbulent flow over Super-Hydrophobic (SH) and Liquid-Infused (LI) surfaces is investigated by Direct Numerical Simulation (DNS) using a two-phase, single relaxation time, free-energy lattice Boltzmann method. In this method, the dynamics of a diffuse interface is incorporated into the governing equations using a Peng-Robinson free-energy functional. This obviates the need for explicit tracking of the interface or pinning of the contact line. DNS studies were performed in turbulent channel flows with longitudinal microgrooves of width $15 \le g^{+0} \le 64$ in base flow wall units, at solid fractions of $\phi_s =1/16$ or $1/2$ on both walls. Simulations were performed at a base flow friction Reynolds number of $Re_{\tau_0} \approx 222$, with viscosity ratios of $\mu_{ext}/\mu_{int} = 10$, $20$ and $55$, and Weber numbers of $10^{-3} \le We_{\tau_0} = \rho u_{\tau_0} \nu/\sigma \le 10^{-2}$. Analysis of the results shows that interface deformation and contact line motion can significantly reduce the magnitude of drag reduction compared to DNS results obtained in turbulent flow with `idealized', flat SH or LI interfaces. In addition, the simulations identify the conditions for contact line depinning and interface breakdown. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H03.00009: Analysis of the statistics of droplet sizes in atomization. Stephane Zaleski, Yue Ling, Daniel Fuster, Gretar Tryggvason From CPU-intensive simulations of quasi planar coflowing liquid and gas jets at high velocity, we obtain dynamics remarkably similar to experiments. Simulations are performed using the VOF method and the Paris simulator code. Four grids of increasing resolution are used. The distribution of droplet sizes is observed and shown to correspond to a log-normal distribution. The effect of statistical error and finite grid size is analyzed. It is shown that there are good indication of convergence of the probability distribution function upon grid refinement. The manned of convergence is analyzed. It is shown that PDFs obtained by VOF methods and by level set methods converge differently. The numerical mechanism for this difference is hypothesized. The {\em physical} mechanisms for the generation of this probability distribution function are also discussed. The dependence of these distributions on the grid resolution is a key point of future analyses. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H03.00010: Asymmetric primary breakup of a round liquid jet with non-zero injection angle Bo Zhang, Yue Ling In previous simulations of liquid jet atomization, the injection velocity is generally considered to be aligned with the nozzle axis. As a result, the near-field instability development is axisymmetric. Nevertheless, in many fuel injection applications, the liquid flow direction at the nozzle inlet is not perpendicular to the inlet plane. The effect of the angle between the inlet velocity and nozzle axis on the primary breakup of a round liquid jet is unclear and will be investigated in the present study through high-fidelity simulation. The non-zero inlet angle induces an azimuthal variation of velocity in the liquid jet, which in turn influences the macro-scale jet dynamics, such as jet deflection and asymmetric deformation of the head of liquid jet. The simulations results of the global features, including jet deflection angle and temporal evolution of penetration length, are compared with experimental data and good agreement is achieved. The non-zero inlet angle is shown to impact small-scale interfacial wave formation and breakup. Wavelet analysis is performed to characterize the azimuthal variation of the interfacial waves in the near field. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H03.00011: Characterizing the dynamics of complex multifluid flows undergoing topology changes Gretar Tryggvason, Jiacai Lu Although disperse multiphase flows, where one fluid appears as discrete drops or bubbles dispersed in another contiguous phase, have been widely studied, such flows are generally only seen when the volume fractions of the fluids are very different. For comparable void fractions, particularly if the flow is turbulent, we expect to see a complex dynamic interface whose topology changes repeatedly as fluid masses coalesce and break apart. Describing and modeling such churn-turbulence flows is challenging. Approaches drawn from studies of heterogeneous solids, rheology, and premixed combustion provide some guidance, but do not cover the full complexities of a dynamic interface and fluid turbulence. The challenges involve both finding the appropriate statistical descriptions as well as the generation of coarser models. We discuss these challenges; review the various ways the flow can be described and modeled and show examples from our recent work on complex turbulent buoyant two fluid flows in vertical channels and the breakup of periodic liquid jets. [Preview Abstract] |
Monday, November 25, 2019 10:23AM - 10:36AM |
H03.00012: Simulating high void fraction flows undergoing massive topology changes in vertical channels Jiacai Lu, Gretar Tryggvason Turbulent multifluid flows in vertical channels, where the topology of the interface between the different fluids repeatedly changes due to the breakup and coalescence of fluid masses, are examined by numerical simulations, using a finite volume/front-tracking method where the interface is tracked by connected marker particles and the flow equations solved on a regular structured grid. When a film of one fluid, separating blobs of a different fluid, becomes sufficiently thin, it is ruptured. At low volume fraction of one phase, one phase usually consists of disperse drops or bubbles, but as its volume fraction increases the interface structures changes from to more complex irregular shapes, where each fluid is often highly interconnected. Here the focus is on flows where the volume fractions are comparable. The evolution of various integral quantities, such as the average flow rate, wall-shear, and interface area and structure are monitored and compared for different governing parameters such as void fraction and Weber number. Various averages of the flow field and the phase distribution, over planes parallel to the walls, are examined and compared, and the microstructure is examined using two-point correlation functions and other measures. [Preview Abstract] |
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