Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session M22: Turbulence: Multiphase Flows |
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Chair: Stephane Zaleski, University Paris VI Room: 210 |
Tuesday, November 24, 2015 8:00AM - 8:13AM |
M22.00001: Realistic simulations of coaxial atomisation Stephane Zaleski, Daniel Fuster, Tomas Arrufat Jackson, Yue Ling, Matteo Cenni, Ruben Scardovelli, Gretar Tryggvason We discuss advances in the methodology for Direct Numerical Simulations of coaxial atomization in typical experimental conditions. Such conditions are extremely demanding for the numerical methods. The key difficulty seems to be the combination of high density ratios, surface tension, and large Reynolds numbers. We explore how using a momentum-conserving Volume-Of-Fluid scheme allows to improve the stability and accuracy of the simulations. We show computational evidence that the use of momentum conserving methods allows to reduce the required number of grid points by an order of magnitude in the simple case of a falling rain drop. We then apply these ideas to coaxial atomization. We show that in moderate-size simulations in air-water conditions close to real experiments, instabilities are still present and then discuss ways to fix them. Among those, removing small VOF debris and improving the time-stepping scheme are two important directions.The accuracy of the simulations is then discussed in comparison with experimental results and in particular the angle of ejection of the structures. The code used for this research is free and distributed at http://parissimulator.sf.net. [Preview Abstract] |
Tuesday, November 24, 2015 8:13AM - 8:26AM |
M22.00002: DNS of coflowing planar jet atomization: can one reach convergence? Yue Ling, Stephane Zaleski, Gretar Tryggvason, Daniel Fuster, Ruben Scardovelli, Matteo Cenni, Tomas Arrufat Atomization of a liquid jet assisted by a coflowing fast gas jet is commonly seen in fuel injection systems. Three-dimensional direct numerical simulations are performed to investigate the turbulent multiphase flow characteristics in coflowing planar jet atomization, with the interface tracked by the Volume-of-fluid method. Although many numerical simulations of atomization were reported in the recent years, whether the atomization characteristics such as droplet formation and size distribution are fully resolved is often unclear. In this work, a series of very large-scale simulations of different grid resolution (up to four billion grid points) are conducted and particular attention is focused on examining whether we can achieve converged results on the statistical atomization characteristics. The statistical characteristics of the turbulence (such as turbulence kinetic energy) and of the spray (such as droplet size distribution, liquid volume fraction, and gas-liquid interfacial area) are calculated by averaging the DNS data spatially and temporally. The complex multiscale droplet formation mechanisms due to the interaction between the interface and the turbulence are also revealed by the simulation results. [Preview Abstract] |
Tuesday, November 24, 2015 8:26AM - 8:39AM |
M22.00003: Droplet dynamics in homogeneous isotropic turbulence Daniel Albernaz, Minh Do-Quang, Gustav Amberg This study investigates the droplet dynamics in homogeneous isotropic turbulence using a lattice Boltzmann model for multiphase flows. The thermodynamics is taken into account with a non-ideal equation of state allowing phase change and by solving a scalar transport energy equation. The system is considered close to the critical point, where a saturated hydrocarbon droplet is surrounded by vapor. The droplet deformation and frequency spectra are analyzed in detail, where the surface tension and local temperature gradients play a major role. The effects of the turbulence intensity and droplet size are also discussed. The droplet behavior under turbulent flows is essential to gain in-depth insight into the different physical phenomena taking place inside sprays and liquid jets. [Preview Abstract] |
Tuesday, November 24, 2015 8:39AM - 8:52AM |
M22.00004: Rain formation via turbulent mixing of droplet distributions Mihkel Kree, Jaan Kalda It is well known that the growth of water droplets in a cloud due to vapor diffusion alone is insufficiently slow to explain the rapid onset of rain formation. In recent years, there have been several proposals of turbulent mechanisms leading to enhanced collision rates. It has been understood that a broadening of droplet size spectra can provide a sufficient boost to the collision rate. However, the broadening of the droplet size spectra also needs to be explained. Here, we propose a novel approach based on the idea that turbulent mixing brings together droplets of very different histories and hence, of very different sizes, similarly to how passive scalar fronts are formed. We provide relevant analytical estimates, and simulations based on 1D model of turbulence (stochastic triplet map similar to the Baker's map). This mapping model captures the essential stretching and folding nature of turbulent flows. The triplet mapping is accompanied by averaging of neighboring distributions, corresponding to local diffusive mixing of droplets. In particular, we study the widths (variances) of local drop size distributions, which appear to follow a power law. Accordingly, we witness occasional instances of extremely broad drop size distributions, which can trigger the rain formation. [Preview Abstract] |
Tuesday, November 24, 2015 8:52AM - 9:05AM |
M22.00005: Scalewise investigation of two-phase flow turbulence in upward turbulent bubbly pipe flows Jun Ho Lee, Hyunseok Kim, Hyungmin Park In the present study, the two-phase flow turbulence in upward turbulent bubbly pipe flows (at the Reynolds number of 5300) is invesgitated, especially focusing on the changes in flow structures with bubbles depending on the length scales. For the scalewise investigation, we perform the wavelet multi-resolution analysis on the velocity fields at three streamwise locations, measured with high-speed two-phase particle image velocimetry technology. While we intentaionlly introduce asymmetrically distributed bubbles at the pipe inlet, the mean volume void fraction is varied from from 0.3\% to 1.86\% and the considered mean bubble diameter is roughly maintained at 3.8~mm. With the present condition, turbulence enhancement is achieived for most cases but the turbulent suppression is also captured near the wall for the smallest void fraction case. Comparing the scalewise energy contribution, it is understood that the flow structures with length scales between bubble radius and bubble wake size are enhanced due to bubbles, resulting in the turbulence enhancement. On the other hand, flow structure with smaller length scales (mostly existing near the wall) may decrease depending on the bubble condition, which may be one of the explanations in turbulence suppression with bubbles. [Preview Abstract] |
Tuesday, November 24, 2015 9:05AM - 9:18AM |
M22.00006: Bubble-induced turbulence study in homogeneous turbulent flow using DNS approach Jinyong Feng, Igor Bolotnov The effect of a single bubble on the energy transfer to a homogeneous turbulent flow using DNS approach is investigated for various conditions. The single-phase turbulence is numerically generated by pressure-gradient driven uniform flow through a fully resolved turbulence generating grid. The turbulent intensity measured is uniform normal to the flow direction. The decay rate of the turbulent kinetic energy is validated against analytical power law. The collected instantaneous velocity is used as inflow condition for single-bubble simulations to study the bubble-induced turbulence (BIT). In interface-resolved two-phase simulation the bubble is kept at fixed positions by using a proportional-integral-derivative controller. This simulation set allows estimating the turbulent kinetic energy before and after the bubble, quantifying the BIT. Effects of bubble deformability, velocity and turbulent intensity are separately studied. We observe that for a nearly spherical bubble, the bubble-induced turbulence is positive, increasing the level of turbulent kinetic energy in the liquid phase. BIT is influenced by the other studied parameters and the presented work will contribute to the closure BIT model development in multiphase computational fluid dynamics modeling. [Preview Abstract] |
Tuesday, November 24, 2015 9:18AM - 9:31AM |
M22.00007: Influence of bubble clusters over the turbulent structure in upward bubbly channel flows Yoshito Sekiguchi, Wenhao Zhang, Hiroaki Nakanishi, Jun Sakakibara, Shu Takagi We conducted the PIV measurement of upward, turbulent bubbly channel flows. In our experiment, bubbles do not coalesce and become mono-dispersed 1 mm spherical shape due to surfactants in the liquid phase. Adding the surfactant in some specific conditions, these bubbles are attracted toward the wall by the shear induced lift force and form bubble clusters. While they flow near wall, the Reynolds stress of the liquid phase near wall comes close to zero [Takagi, S. and Matsumoto, Y., \textit{Annu. Rev. Fluid Mech. }(2011)]. This suggests that the turbulent structure change dramatically due to bubble clusters. For the further investigation of the turbulent structure, we constructed the measurement system of Scanning Stereoscopic PIV (SSPIV) which can visualize the three-dimensional velocity field. Using this system, we acquire the velocity field and extracted the large scale vortices which dominate the turbulent structure. Also, we constructed another measurement system for tracking the bubble cluster's flow. Through the simultaneous measurement of vortices and bubble cluster, we analyze the influence of bubble cluster over the turbulent structure. The results will be discussed in the presentation. [Preview Abstract] |
Tuesday, November 24, 2015 9:31AM - 9:44AM |
M22.00008: Numerical simulations of bubbly Taylor-Couette turbulence in co- and counter rotating regime Vamsi Spandan, Roberto Verzicco, Detlef Lohse Two-phase Taylor-Couette (flow between two co-axial independently rotating cylinders) is simulated using a two-way coupled Euler-Lagrange approach in which the bubbles are treated as point particles with effective forces such as drag, lift, added mass and buoyancy acting on them. The momentum equations for the fluid and the bubbles are solved in the frame of reference of the outer cylinder. While it is already known that when the outer cylinder is stationary, within a certain Taylor number range ($Ta \sim 10^6-10^8$) the bubbles disrupt the plume ejection regions and the coherent vortical structures leading to drag reduction, their effect and arrangement in the gap-width when both cylinders are rotating is still unknown. In this study we focus on studying the effect of bubbles on the angular velocity transport for various rotation rates of the cylinders. We find that the net percentage drag reduction persists even with a rotating outer cylinder, but is there a optimum for various rotation rates ? How does the spatial distribution of bubbles vary with in the co- and counter rotating regime ? These are some questions we attempt to answer in this work. [Preview Abstract] |
Tuesday, November 24, 2015 9:44AM - 9:57AM |
M22.00009: Two-dimensional Turbulence in Symmetric Binary-Fluid Mixtures: Coarsening Arrest by the Inverse Cascade Prasad Perlekar, Nairita Pal, Rahul Pandit We study two-dimensional (2D) binary-fluid turbulence by carrying out an extensive direct numerical simulation (DNS) of the forced, statistically steady turbulence in the coupled Cahn-Hilliard and Navier-Stokes equations. In the absence of any coupling, we choose parameters that lead (a) to spinodal decomposition and domain growth, which is characterized by the spatiotemporal evolution of the Cahn-Hilliard order parameter $\phi$, and (b) the formation of an inverse-energy-cascade regime in the energy spectrum $E(k)$, in which energy cascades towards wave numbers $k$ that are smaller than the energy-injection scale $k_{inj}$ in the turbulent fluid. We show that the Cahn-Hilliard-Navier-Stokes coupling leads to an arrest of phase separation at a length scale $L_c$, which we evaluate from $S(k)$, the spectrum of the fluctuations of $\phi$. We demonstrate that (a) $L_c \sim L_H$, the Hinze scale that follows from balancing inertial and interfacial-tension forces, and (b) $L_c$ is independent, within error bars, of the diffusivity $D$. We elucidate how this coupling modifies $E(k)$ by blocking the inverse energy cascade at a wavenumber $k_c$, which we show is $\simeq 2\pi/L_c$. We compare our work with earlier studies of this problem. [Preview Abstract] |
Tuesday, November 24, 2015 9:57AM - 10:10AM |
M22.00010: Suppression of turbulent energy cascade due to phase separation in homogenous binary mixture fluid Youhei Takagi, Sachiya Okamoto When a multi-component fluid mixture becomes themophysically unstable state by quenching from well-melting condition, phase separation due to spinodal decomposition occurs, and a self-organized structure is formed. During phase separation, free energy is consumed for the structure formation. In our previous report, the phase separation in homogenous turbulence was numerically simulated and the coarsening process of phase separation was discussed. In this study, we extended our numerical model to a high Schmidt number fluid corresponding to actual polymer solution. The governing equations were continuity, Navier-Stokes, and Chan-Hiliard equations as same as our previous report. The flow filed was an isotropic homogenous turbulence, and the dimensionless parameters in the Chan-Hilliard equation were estimated based on the thermophysical condition of binary mixture. From the numerical results, it was found that turbulent energy cascade was drastically suppressed in the inertial subrange by phase separation for the high Schmidt number flow. By using the identification of turbulent and phase separation structure, we discussed the relation between total energy balance and the structures formation processes. [Preview Abstract] |
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