Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session BT: Multiphase Flows II |
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Chair: Vladimir Ajaev, Southern Methodist University Room: Salt Palace Convention Center Ballroom FH |
Sunday, November 18, 2007 10:34AM - 10:47AM |
BT.00001: Experimental Investigation of Lagrangian Statistics of Motion of Diesel Oil Droplets and Fluid Particles in Isotropic Turbulence Balaji Gopalan, Edwin Malkiel, Joseph Katz Lagrangian motion in isotropic turbulence of slightly buoyant diesel oil droplets (specific gravity 0.85 and size 0.6-1.1 mm) and almost neutrally buoyant, 50 $\mu $m tracer particles are studied using high speed, in-line digital holographic cinematography. Droplets and particles are injected into a 50x50x70 mm$^{3}$ sample volume located at the center of a nearly isotropic turbulence facility, and data are obtained for Re$_{\lambda }$ of 190, 195 and 214. The turbulence is characterized by 2D PIV measurements at different planes. An automated tracking program has been used for measuring velocity time history of more than 22000 droplet tracks and 15000 particle tracks. Analysis compares probability density functions (PDF) of Lagrangian velocity and acceleration, spectra, as well as velocity and acceleration autocorrelation functions of droplets with those of particles. For most of the present conditions, rms values of horizontal droplet velocity exceed those of the fluid. The rms values of droplet vertical velocity are higher than those of the fluid only for the highest turbulence level. PDFs of droplet velocity have nearly Gaussian distributions, justifying use of Taylor's (1921) model to calculate diffusion parameters. The fluid particle diffusion coefficient exceeds that of the droplet primarily because the fluid diffusion timescale is higher than that of the droplet. For all droplet sizes and Reynolds numbers, the diffusion coefficient, calculated using Taylor's model, scaled by quiescent rise velocity and turbulence integral length scale, is a monotonically increasing function of the turbulence level normalized by droplet quiescent rise velocity. [Preview Abstract] |
Sunday, November 18, 2007 10:47AM - 11:00AM |
BT.00002: ABSTRACT WITHDRAWN |
Sunday, November 18, 2007 11:00AM - 11:13AM |
BT.00003: The effect of capillary condensation on optical transmission in porous silicon Vladimir Ajaev Capillary condensation in porous silicon superlattices can be used to design photonic structures with unique and easily tunable properties, as recently demonstrated by Barthelemy et al. [Nature Photonics, 1, 172 (2007)]. Potential applications of these structures range from switching devices in communications networks to optical computing. The effect of condensed liquid on the optical properties of porous silicon is usually described in terms of the volume fraction of the liquid. The latter is found from either adsorption studies in disordered porous media or approximate models of capillary condensation in macroscopic pores of circular cross-section. However, porous silicon used in optical applications is in fact an ordered structure with long non-intersecting pores of polygonal cross-section and typical cross-sectional dimensions on the order of 100 nm. We develop a model of capillary condensation in such structures and use it to interpret recent experimental studies of transmission of optical signals through porous silicon partially filled with liquid. [Preview Abstract] |
Sunday, November 18, 2007 11:13AM - 11:26AM |
BT.00004: Simulating gas-liquid flow in a micro-channel with the lattice Boltzmann method Grace Shi, Volha Lazouskaya, Yan Jin, Lian-Ping Wang The flows of water in natural soil porous media with air-water interface are important to colloid-facilitated transport of contaminants and other phenomena with groundwater as the carrier. These flows are complex in terms of the geometrical feature and physical and chemical forces involved. As first step, we here demonstrate that a gas-liquid interfacial viscous flow in a 3D micro-channel with a square cross-section can be simulated using the lattice Boltzmann method. The talk will cover the detailed ingredients of the two-phase LBE model including the proper equation of state, surface tension, and the triple-phase boundary conditions. Methods to improve the stability of the code such as using multiple relaxation times will be tested. Preliminary results will be presented and compared to parallel experimental observations using confocal laser scanning microscopy. [Preview Abstract] |
Sunday, November 18, 2007 11:26AM - 11:39AM |
BT.00005: Scaling properties of coating flows in rectangular channels Alberto de Lozar, Andrew Hazel, Anne Juel We present an experimental and numerical study of the aspect- ratio dependence of two-phase displacement flows in channels of rectangular cross-section. In square and near-square channels, the bulk features of the flow appear to be virtually independent of aspect ratio, $\alpha$, so that the capillary number, $Ca$, is the only governing parameter. Saffman-Taylor fingering, governed by the modified capillary number, $1/B \propto Ca\, \alpha^2$, is observed for $\alpha> 7$. For channels of intermediate aspect ratio, the interfacial flow depends solely on a generalised modified capillary number consistent with both limits, $\widehat{\mathrm{Ca}} = f(Ca, \alpha)$, above a threshold value $\widehat{\mathrm{Ca}}_t$.. This novel scaling has tremendous practical significance since it implies that the bulk features of any two-phase displacement flow can be inferred from those of the square channel for $\widehat{\mathrm{Ca}} > \widehat{\mathrm{Ca}}_t$. [Preview Abstract] |
Sunday, November 18, 2007 11:39AM - 11:52AM |
BT.00006: Air bubble break-ups by vertical oscillations in micro- and normal gravity environments Harunori Yoshikawa, Pascal Kurowski, Philippe Petitjeans, Farzam Zoueshtiagh, Herve Caps Dynamics of a bubble subjected to vertical oscillations is studied experimentally in micro- and normal gravity environments. A large air bubble (typically $D=1.8$ cm in volume equivalent diameter) is sealed with a surrounding liquid in a cell oscillating vertically. The bubble breaks up when the acceleration of the cell exceeds a certain value $a_{cr}$. This critical acceleration $a_{cr}$ is substantially smaller in micro-gravity environment than in normal gravity environment. In both environments, $a_{cr}$ is found to be constant for a given surrounding liquid when the cell's oscillation amplitude $A$ is large in comparison with the bubble size $D$. It is also found that $ a_{cr}$ increases rapidly with decreasing $A$ below the bubble size $D$. Influence of surrounding liquid viscosity is investigated by experiments with surrounding liquids of different viscosities. An increase of kinematic viscosity by a factor $10^2$ (from 1 cSt to 100 cSt) is found to lead a 2-2.5 times larger critical acceleration in both environments. Experimentally obtained critical accelerations are discussed, being compared with a simple model based on hydrodynamic instability of an accelerated interface. [Preview Abstract] |
Sunday, November 18, 2007 11:52AM - 12:05PM |
BT.00007: Quantitative Visualization of Two-Phase Flow in a Model of a Fuel Cell Gas Transport Channel Grant Minor, Peter Oshkai, Nedjib Djilali Two phase air-water flow in a model of a proton exchange membrane fuel cell (PEMFC) gas distribution channel is investigated experimentally using a quantitative flow imaging of the liquid phase. A rectangular PEMFC gas channel model was fabricated from polydimethylsiloxane (PDMS), glass, and carbon paper. Micro-digital-particle-image-velocimetry (micro-DPIV) techniques are used to provide qualitative and quantitative visualizations of flow inside a water droplet adhered to the bottom wall of a gas channel and exposed to an air flow within the channel. Velocity measurements within several cross-sectional planes inside a droplet placed in the channel are obtained for a range of air flow rates. Particle streak images are obtained for qualitative analysis of the flow. Relationships between air velocity in the channel, secondary rotational flow inside a droplet, droplet deformation, and contact angle hysteresis are examined. In addition, quantitative rotational flow patterns within the droplet are acquired using micro-DPIV. The resulting flow fields provide insight into the interactions between the air and water flows that occur at the gas-liquid interface. [Preview Abstract] |
Sunday, November 18, 2007 12:05PM - 12:18PM |
BT.00008: An energy-spectrum shift in the interaction between a bubble swarm and oscillating-grid turbulence analyzed via recursive PIV Takayuki Saito, Koichi Morikawa, Toshiyuki Sanada In order to elucidate the liquid-phase turbulence modulation owing to dispersed bubbles, the authors employed both methods to generate arbitrary turbulence and control the bubble size and bubble number density of the bubble swarm. For the first purpose, a method of well-controlled oscillating-grid turbulence was employed; this method easily characterized integral scale and Taylor micro scale. For the second purpose, a bubble formation method using audio speakers was employed; this method completely controlled bubble size, bubble number density and launch timing. In the present study, the swarm of zigzagging rising bubbles in 2{\%} void fraction was examined. Liquid phase velocities at two spatially-separate points were measured via two LDV probes, simultaneously. Furthermore, liquid-phase velocity field was measured via recursive PIV with a high-speed video camera. Motion of each bubble was obtained from visualization and 4-time-step tracking algorithm. From the two-point LDV data, turbulence intensity, spatial correlation, integral scale and Taylor micro scale were calculated and discussed. From the PIV results, energy spectra were obtained. On the basis of these results, interactions between the turbulence induced by the bubble swarm (i.e. dispersed bubbles) and ambient liquid-phase turbulence are quantitatively and systematically discussed. [Preview Abstract] |
Sunday, November 18, 2007 12:18PM - 12:31PM |
BT.00009: Minkowski functionals: Characterizing particle and bubble clusters in turbulent flow Detlef Lohse, Enrico Calzavarini, Martin Kerscher, Federico Toschi Even in homogeneous isotropic turbulence particles, drops, and bubbles (all from now on called ``particles'') do not distribute homogeneously, but {\it cluster}. Considerable advances in particle tracking velocimetry and in numerics now allow for the acquisition of huge data sets for particle positions and velocities in turbulence. Here we employ, next to the calculation of the Kaplan-Yorke dimension, a method developed in astrophysics for the characterization of galaxy clusters to quantitatively characterize the particle distribution in dispersed turbulent flow, namely by calculating the Minkowski functional. They contain the complete morphological information and sensitively depend on the density ratio between particle and fluid and on the Stokes number, but much less on the degree of turbulence. [Preview Abstract] |
Sunday, November 18, 2007 12:31PM - 12:44PM |
BT.00010: Numerical Investigation of Evaporating Droplets with Direct Quadrature-Based Moments of Closure Method Seyed Mohammad Jamaly, Mohammad Hasan Saidi, Akbar Ghafourian In this study, due to the weaknesses of the models with Lagrangian approaches, an attempt has been made to model the spray flow with Eulerian approach. In this regard, the quadrature-based moment closure model for the spray equation, the so-called DQMOM, is applied. This method overcomes the shortcoming of other Eulerian methods while it is in good agreement with the Lagrangian methods. After that, the model has been developed to be able to deal with the evaporating droplets. Moreover, the feasibility of applying non-linear external forces, such as drag forces, and evaporation laws for the droplets are considered and implemented. The required order for the equations in this method has been studied thoroughly as well. Finally, the solution procedure for accurate computations of multi dimension problems is presented. In general, the proposed modified DQMOM method can consider and solve all kinds of spray flows with any desirable dimension for the problem. Here, assuming one-way coupling situation with the gas-phase in an axial engine, the spray phase equations are solved by the proposed method to account for evaporating droplets. Results are compared with the methods with Lagrangian approach and the computational costs and accuracies of the methods are compared as well. [Preview Abstract] |
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