Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session E25: Turbulence: Multiphase Flow |
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Chair: Igor Bolotnov, North Carolina State University Room: 2005 |
Sunday, November 23, 2014 4:45PM - 4:58PM |
E25.00001: Drag Modification by Micro-bubbles in Taylor-Couette Turbulence: A Numerical Approach Vamsi Spandan , Rodolfo Ostilla-Monico , Roberto Verzicco , Detlef Lohse We simulate two phase Taylor-Couette (flow between two co-axial independently rotating cylinders) using the Euler-Lagrange approach in which bubbles are treated as point particles with effective forces such as drag, lift and added mass acting on them. The outer cylinder is stationary, while the inner cylinder is rotated to reach a Reynolds number $Re\sim10^4$ with almost $10^5$ bubbles dispersed into the carrier phase.Two-way coupling is implemented between the dispersed phase and the carrier phase allowing us to study the effect of these point like bubbles on the overall structure of the flow. The two-way coupling is implemented through a unique forcing scheme where the back reaction from a single bubble is spread out over a finite computational volume rather than a finite number of nodes as previously done in literature, which ensures grid independent results. We observe that the bubbles are responsible for disrupting the coherent vortical structures in the carrier flow ultimately resulting in drag modification. In addition we also study the spatial distribution and effect of neutrally buoyant particles dispersed into the flow. [Preview Abstract] |
Sunday, November 23, 2014 4:58PM - 5:11PM |
E25.00002: Direct numerical simulation of turbulent core-annular flow in a vertical pipe Kiyoung Kim , Haecheon Choi The core-annular flow has been considered as a useful tool to effectively transport highly viscous oil by having lower viscous fluid such as water near the pipe surface. There have been several studies to investigate turbulent core-annular flows but most of them have been conducted experimentally. We solve the three-dimensional Navier-Stokes equations in a cylindrical coordinate and use the level-set method for interface tracking between two fluids (oil and water). A few different flow parameters such as the superficial velocity of fluids and mean pressure gradient are considered in a vertical pipe. The results show that the oil core region is nearly a plug flow and the water region experiences high shear rates, which generate turbulence structures different from those of single phase flow. The interface wave suppresses the near-wall coherent structures but produces complex fluid motions caused by its interaction with the wall. The phenomenon of maximum drag reduction and the effect of water turbulence on total drag will be discussed at the presentation. [Preview Abstract] |
Sunday, November 23, 2014 5:11PM - 5:24PM |
E25.00003: Numerical study of bubble generation in a turbulent two-phase Couette flow Andrey Ovsyannikov , Ali Mani , Parviz Moin , Dokyun Kim The objective of this work is to develop an understanding bubble generation mechanism due to interactions between free surfaces and turbulent boundary layers as commonly seen near ship walls. To this end, we have focused on a canonical problem that involves Couette flow between two vertical parallel walls with an air-water interface in between. We have considered flow at Reynolds number of 8000 and Froude number of 3.6, both based on half domain dimension and water properties. Our calculations resolve both Kolmogorov lengths and the Hinze scale. Additionally, a conservative VOF method coupled to a subgrid Lagrangian breakup model is used to represent the ligament breakup phenomena and their resulting bubbles and drops. We will present results from these calculations revealing bubble formation rates, bubble size distribution, and effects of bubbles on modulation of turbulence [Preview Abstract] |
Sunday, November 23, 2014 5:24PM - 5:37PM |
E25.00004: Single deformable bubble interaction with turbulence in uniform and shear flows Jinyong Feng , Igor Bolotnov Combined direct numerical simulation (DNS) and interface tracking method (ITM) approach is utilized to study the effect of bubble deformability on the bubble-induced turbulence. Set of simulations is performed with 5mm diameter bubble in laminar and turbulent flows. Uniform shear and constant mean velocity profiles are used to perform evaluation of bubble-induced turbulence in various cases. The simulation capabilities allow estimating the turbulent kinetic energy before and after the bubble thus providing the information about bubble's influence on the liquid turbulence. The effect of bubble deformability is studied by separately changing the surface tension parameter. The bubble is controlled in one location of the domain using external forces. The force evolution is managed by proportional-integral-derivative (PID) controller. The steady-state values of the lateral and stream-wise forces result in the lift and drag force estimates on the bubble. DNS approach allows for comprehensive, well-defined studies of bubble-induced turbulence and interfacial forces by separately varying bubble's deformability, relative velocity, level of turbulence and local shear. This work presents new opportunities for the development of multiphase computational fluid dynamics closure laws. [Preview Abstract] |
Sunday, November 23, 2014 5:37PM - 5:50PM |
E25.00005: The evaporation of dense sprays as a mixing process Alois de Rivas , Emmanuel Villermaux A dense spray of micron-sized droplets (water or ethanol) is formed in air by a pneumatic atomizer in a closed chamber, and is then conveyed through a nozzle in ambient air, forming a plume whose extension depends on the relative humidity of the diluting medium. We focus on the dry ambient medium, and large plume Reynolds number limit. Standard shear instabilities develop at the plume edge, forming the stretched lamellar structures familiar with passive scalars, except that these vanish in a finite time, because individual droplets evaporate at their border. Experiments also demonstrate that the lifetime of an individual droplet embedded in a lamellae is much larger than expected from the usual $d$-square law for an isolated droplet. By analogy with the way mixing times are understood from the convection-diffusion equation for passive scalars, we show that the lifetime of a lamellae stretched at a rate $\gamma $ is $t_v = \frac{1}{\gamma} \, \ln \left(\frac{1 + \phi}{\phi}\right)$ where $\phi$ is a parameter which incorporates the thermodynamic and diffusional properties of the vapor in the diluting phase. The droplets field thus behaves as a -non conserved- passive scalar. [Preview Abstract] |
Sunday, November 23, 2014 5:50PM - 6:03PM |
E25.00006: Suppression of self-organized structure coarsening in homogenous isotropic turbulence Youhei Takagi Self-organized structure by spinodal decomposition is often seen in quenched binary mixture. Complex network structure is formed through coarsening process of self-organized structure when the phase separation due to spinodal decomposition proceeds. The phase separation governed by the Cahn-Hilliard equation have been well investigated for stationary fluid in previous studies, however, the turbulent effect on the formation of structures was not fully discussed. In this study, we carried out a numerical simulation for homogenous isotropic turbulence with phase separation, the relation between turbulent vortex formation and self-organized structure coarsening. The governing equations are incompressible Navier-Stokes equation considering phase separation force and Cahn-Hilliard equation with the chemical potential based on the Landau-Ginzburg free energy. From the identification and visualization of turbulent structures, it was found that the local entrainment of small eddy structure suppressed the coarsening process of self-organized structure. The energy used in phase separation was related to the initial process of vortex sheet-tube transition in turbulent flow, and the energy cascade from large turbulent structure to small eddy was different from that without phase separation. [Preview Abstract] |
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