Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session M12: Multiphase Flows: Aerodynamics and Hydrodynamics |
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Chair: Ali Tohidi, University of Maryland at College Park Room: C123 |
Tuesday, November 22, 2016 8:00AM - 8:13AM |
M12.00001: Experimental \& Numerical Modeling of Non-combusting Model Firebrands' Transport Ali Tohidi, Nigel Kaye Fire spotting is one of the major mechanisms of wildfire spread. Three phases of this phenomenon are firebrand formation and break-off from burning vegetation, lofting and downwind transport of firebrands through the velocity field of the wildfire, and spot fire ignition upon landing. The lofting and downwind transport phase is modeled by conducting large-scale wind tunnel experiments. Non-combusting rod-like model firebrands with different aspect ratios are released within the velocity field of a jet in a boundary layer cross-flow that approximates the wildfire velocity field. Characteristics of the firebrand dispersion are quantified by capturing the full trajectory of the model firebrands using the developed image processing algorithm. The results show that the lofting height has a direct impact on the maximum travel distance of the model firebrands. Also, the experimental results are utilized for validation of a highly scalable coupled stochastic \& parametric firebrand flight model that, couples the LES-resolved velocity field of a jet-in-nonuniform-cross-flow (JINCF) with a 3D fully deterministic 6-degrees-of-freedom debris transport model. The validation results show that the developed numerical model is capable of estimating average statistics of the firebrands' flight. [Preview Abstract] |
Tuesday, November 22, 2016 8:13AM - 8:26AM |
M12.00002: Simultaneous velocity measurements of particle and gas phase in particle-laden co-flowing pipe jets Isaac Saridakis, Timothy Lau, Lyazid Djenidi, Graham Nathan Simultaneous planar velocity measurements of both the carrier gas and particles are reported of well-characterized particle-laden co-flowing pipe jets. It is proposed to present measurements that were obtained through application of a median-filter discrimination technique to separate the Particle Image Velocimetry (PIV) signals of the 0.5$\mu $m diameter fluid tracers from those of the larger particles of diameter 20$\mu $m and 40$\mu $m. Instantaneous particle and fluid planar velocity distributions were measured for three Reynold's numbers ranging from 10,000 to 40,000 and five Stokes numbers from 1 to 22, at a jet bulk fluid velocity to co-flow velocity ratio of 12. Selected results will be presented which show that the slip velocity is dependent on the local Stokes number. These are the first simultaneous carrier gas and particle velocity measurements in particle-laden jets and provide new understanding of fluid-particle interactions. [Preview Abstract] |
Tuesday, November 22, 2016 8:26AM - 8:39AM |
M12.00003: Pairwise Interaction Extended Point Particle (PIEP) Model for a Random Array of Spheres. Georges Akiki, Thomas Jackson, Sivaramakrishnan Balachandar This study investigates a flow past random array of spherical particles. The understanding of the governing forces within these arrays is crucial for obtaining accurate models used in particle-laden simulations. These models have to faithfully reflect the sub-grid interactions between the particles and the continuous phase. The models being used today assumes an average force on all particles within the array based on the mean volume fraction and Reynolds number. Here, we develop a model which can compute the drag and lateral forces on each particle by accounting for the precise location of few surrounding neighbors. A pairwise interaction is assumed where the perturbation flow induced by each neighbor is considered separately, then the effect of all neighbors are linearly superposed to obtain the total perturbation. Fax\'{e}n correction is used to quantify the force perturbation due to the presence of the neighbors. The single neighbor perturbations are mapped in the vicinity of a reference sphere and stored as libraries. We test the Pairwise Interaction Extended Point-Particle (PIEP) model for random arrays at two different volume fractions of $\phi \quad =$ 0.1 and 0.21 and Reynolds number in the range 16 $\le $ \textit{Re} $\le $ 170. The PIEP model predictions are compared against drag and lift forces obtained from fully-resolved DNS performed using immersed boundary method. We observe the PIEP model prediction to correlate much better with the DNS results than the classical mean drag model prediction. [Preview Abstract] |
Tuesday, November 22, 2016 8:39AM - 8:52AM |
M12.00004: Generalized Fax\'en's theorem: Evaluating first-order (hydrodynamic drag) and second-order (acoustic radiation) forces on finite-sized rigid particles, bubbles and droplets in arbitrary complex flows Subramanian Annamalai, S Balachandar In recent times, study of complex disperse multiphase problems involving several million particles (e.g. volcanic eruptions, spray control etc.) is garnering momentum. The objective of this work is to present an accurate model (termed generalized Fax\'en's theorem) to predict the hydrodynamic forces on such inclusions (particles/bubbles/droplets) without having to solve for the details of flow around them. The model is developed using acoustic theory and the force obtained as a summation of infinite series (monopole, dipole and higher sources). The first-order force is the time-dependent hydrodynamic drag force arising from the dipole component due to interaction between the gas and the inclusion at the microscale level. The second-order force however is a time-averaged differential force (contributions arise both from monopole and dipole), also known as the acoustic radiation force primarily used to levitate particles. In this work, the monopole and dipole strengths are represented in terms of particle surface and volume averages of the incoming flow properties and therefore applicable to particle sizes of the order of fluid length scale and subjected to any arbitrary flow. Moreover, this model can also be used to account for inter-particle coupling due to neighboring particles. [Preview Abstract] |
Tuesday, November 22, 2016 8:52AM - 9:05AM |
M12.00005: Fully-resolved simulation of particle rotation in a turbulent flow Yayun Wang, Adam J. Sierakowski, Andrea Prosperetti Some results on the statistics of particle rotation induced by hydrodynamic stresses in a weakly turbulent flow, with a Taylor Reynolds number of about 32, are presented. Two particle Reynolds numbers, 80 and 150, and two different particle moments of inertia, are considered.The particle center is held fixed so that the particle statistics can be compared with those of the fluid vorticity in the absence of the particle. It is found that the particle essentially responds only to vortex structures with a scale comparable to, or larger than, its diameter, thus acting as a low-pass filter for the incident turbulent vorticity. An analysis of the flatness of the PDF's of the angular acceleration and angular velocity shows that the former is mildly non-Gaussian, while the latter is very close to Gaussian. The numerical method used, Physalis, is particularly suited for this problem due to the high accuracy with which the couple is calculated. [Preview Abstract] |
Tuesday, November 22, 2016 9:05AM - 9:18AM |
M12.00006: Angular velocity of a spheroid log rolling in a simple shear at small Reynolds number Jan Meibohm, Fabien Candelier, Tomas Rosen, Jonas Einarsson, Fredrik Lundell, Bernhard Mehlig We analyse the angular velocity of a small neutrally buoyant spheroid log rolling in a simple shear. When the effect of fluid inertia is negligible the angular velocity $\vec \omega$ equals half the fluid vorticity. We compute by singular perturbation theory how weak fluid inertia reduces the angular velocity in an unbounded shear, and how this reduction depends upon the shape of the spheroid (on its aspect ratio). In addition we determine the angular velocity by direct numerical simulations. The results are in excellent agreement with the theory at small but not too small values of the shear Reynolds number, for all aspect ratios considered. For the special case of a sphere we find $\omega/s = -1/2+0.0540\, \text{Re}^{3/2}$ where $s$ is the shear rate and $\text{Re}$ is the shear Reynolds number. This result differs from that derived by Lin et al. [J. Fluid Mech. 44 (1970) 1] who obtained a numerical coefficient roughly three times larger. [Preview Abstract] |
Tuesday, November 22, 2016 9:18AM - 9:31AM |
M12.00007: Influence of lubrication forces in direct numerical simulations of particle-laden flows Rohit Maitri, Frank Peters, Johan Padding, Hans Kuipers Accurate numerical representation of particle-laden flows is important for fundamental understanding and optimizing the complex processes such as proppant transport in fracking. Liquid-solid flows are fundamentally different from gas-solid flows because of lower density ratios (solid to fluid) and non-negligible lubrication forces. In this interface resolved model, fluid-solid coupling is achieved by incorporating the no-slip boundary condition implicitly at particleâ€™s surfaces by means of an efficient second order ghost-cell immersed boundary method. A fixed Eulerian grid is used for solving the Navier-Stokes equations and the particle-particle interactions are implemented using the soft sphere collision and sub-grid scale lubrication model. Due to the range of influence of lubrication force on a smaller scale than the grid size, it is important to implement the lubrication model accurately. In this work, different implementations of the lubrication model on particle dynamics are studied for various flow conditions. The effect of a particle surface roughness on lubrication force and the particle transport is also investigated. This study is aimed at developing a validated methodology to incorporate lubrication models in direct numerical simulation of particle laden flows. [Preview Abstract] |
Tuesday, November 22, 2016 9:31AM - 9:44AM |
M12.00008: Numerical investigation of drag characteristics of spherical particles under non-isothermal conditions Jungwoo Kim, Yeong Eun Yim In predicting particle-laden flows related to particle transport and dispersion, better understanding and accurate parameterization of the hydrodynamic forces on the particles are one of the important subjects. Heat transfer between dispersed particle and fluid is often observed in nature and engineering applications. However, existing analytical expressions and empirical correlations used in point particle approaches are made based on the assumption that the particle and surrounding ambient flow are under thermal equilibrium conditions. So, the effect of thermal non-equilibrium state of particle motion remains an unresolved issue. Therefore, we perform three-dimensional numerical simulations for the flow around a finite-sized spherical particle in order to investigate its drag characteristics under non-isothermal conditions (heated or cooled particles). In this study, the working fluids are considered to be water and air as typical cases of liquids and gases. The heated particle experiences larger drag in air and smaller drag in water than that in the isothermal case. On the other hand, the impact of cooling is to decrease drag in air and to increase it in water. These behaviors of the drag coefficient in air and water mainly depend on the variation of the viscosity in terms of the temperature. Those results would provide useful information in understanding the particle motion in heated or cooled conditions. [Preview Abstract] |
Tuesday, November 22, 2016 9:44AM - 9:57AM |
M12.00009: Drag of Spherical Particles in a Periodic Lattice: Heat Transfer, Buoyancy and Non-Boussinesq Effects Swetava Ganguli, Sanjiva Lele What are the forces that act on a particle as it moves in a fluid? How do they change in the presence of significant heat transfer from the particle, a variable density fluid or gravity? Last year, we demonstrated and quantified these effects on a single particle. We further our study by adding interactions between particles placed in a periodic lattice whose parameters we control. Our particle resolved simulations use a fully unstructured, node-based, low-Mach variable density solver to study the low-Mach response. Let the Boussinesq parameter $\lambda $ as the ratio of the difference of the particle temperature and the far-field fluid temperature to the far-field fluid temperature. The heating of the fluid near the particle affects the drag significantly which can be~characterized in a parameter space where the variation in Reynolds number, $\lambda $ and Froude number can be collapsed to a single parameter. Despite the large drag changes, the pressure and viscous fractional contributions do not vary with $\lambda $. In the low Re limit, a semi-analytical low Mach perturbation expansion has significant explanatory power. For a single particle, these variations can be captured with 95{\%} accuracy by developing correlations based on physical insights from the semi-analytical model. When particles are placed within a lattice, depending on the lattice parameter, the individual wakes of the particles interact and the drag increases or decreases based on the lattice position. [Preview Abstract] |
Tuesday, November 22, 2016 9:57AM - 10:10AM |
M12.00010: Heat transfer in suspensions of rigid particles Luca Brandt, Mehdi Niazi Ardekani, Omid Abouali We study the heat transfer in laminar Couette flow of suspensions of rigid neutrally buoyant particles by means of numerical simulations. An Immersed Boundary Method is coupled with a VOF approach to simulate the heat transfer in the fluid and solid phase, enabling us to fully resolve the heat diffusion. First, we consider spherical particles and show that the proposed algorithm is able to reproduce the correlations between heat flux across the channel, the particle volume fraction and the heat diffusivity obtained in laboratory experiments and recently proposed in the literature, results valid in the limit of vanishing inertia. We then investigate the role of inertia on the heat transfer and show an increase of the suspension diffusivity at finite particle Reynolds numbers. Finally, we vary the relativity diffusivity of the fluid and solid phase and investigate its effect on the effective heat flux across the channel. The data are analyzed by considering the ensemble averaged energy equation and decomposing the heat flux in 4 different contributions, related to diffusion in the solid and fluid phase, and the correlations between wall-normal velocity and temperature fluctuations. Results for non-spherical particles will be examined before the meeting. [Preview Abstract] |
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