Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session E21: Particle-laden Flows II |
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Chair: Changhoon Lee, Yonsei University Room: 324-325 |
Sunday, November 20, 2011 4:40PM - 4:53PM |
E21.00001: Inertial particle clustering in local linear flow: Novel satellite particle simulations Baidurja Ray, Lance Collins At spatial scales below the Kolmogorov length-scale, a turbulent flow around an inertial particle can be adequately approximated by a locally linear flow field, with the velocity at any point being determined solely by the velocity gradient at the particle location. Here, we describe novel satellite particle simulations, where inertial particle clustering is studied at length-scales below the Kolmogorov scale ($\eta$). Such simulations allow us to isolate the effect of the sub-Kolmogorov scales on particle statistics. We show that such simulations capture the correct qualitative behavior of the radial distribution function (RDF, a measure of particle clustering) with Stokes number ($St$) and separation distance ($r/\eta$) and accurately predict the power ($c_1$) in the power-law behavior of the RDF ($g(r/\eta)=c_0(r/\eta)^{-c_1}$) for $St \le 0.4$. We also test a drift-diffusion model for particle motion and show that it correctly predicts the power-law for $St \le 0.4$. We also investigate the effect of velocity filtering and show that the satellite simulations are able to capture the effect of filtering on the slope $c_1$ of the RDF. Our results provide insight on the effect of small-scales on inertial particle statistics and guide us towards modeling the role of sub-Kolmogorov scales on particle motion. [Preview Abstract] |
Sunday, November 20, 2011 4:53PM - 5:06PM |
E21.00002: Computational and experimental analysis of particle clustering in a shearless turbulent mixing layer Peter Ireland, Garrett Good, Zellman Warhaft, Lance Collins Entrainment, the drawing in of external fluid by a turbulent flow, is ubiquitous to both industrial and natural turbulent processes. This mechanism is particularly important in atmospheric clouds, where the entrainment of dry air by turbulence can affect precipitation mechanisms. We use parametrically matched wind-tunnel experiments and direct numerical simulations with inhomogeneous particle seeding to explore particle clustering in a shearless turbulent mixing layer. We find high degrees of clustering, both visually and statistically, even for particles with negligible inertia. These particle clusters have characteristic sizes on the order of the integral lengthscale of the turbulence and are thus much larger than those resulting from particle inertia. The degree of clustering at a particular location generally decreases as the mixing layer evolves and depends on both the turbulent kinetic energy ratio in the mixing layer and the magnitude and orientation of gravity. We observe the same qualitative trends in both the experiments and the simulations. We anticipate that a better understanding of particle clustering in entraining flows will lead to, among other things, improved cloud evolution predictions and more accurate climate models. [Preview Abstract] |
Sunday, November 20, 2011 5:06PM - 5:19PM |
E21.00003: Computational and experimental study of charged inertial particles in turbulence Jiang Lu, Hansen Nordsiek, Raymond Shaw We have investigated the behavior of electrically charged, inertial particles in homogeneous, isotropic turbulence. Specifically, a drift-diffusion theory describing the scale dependent radial distribution function (RDF) for the particles is evaluated computationally and experimentally. The experiments were carried out in a laboratory chamber that generates nearly homogeneous, isotropic turbulence, and seeded with uniformly charged water droplets. We also report results from a direct numerical simulation (DNS) of turbulence in a periodic box using the pseudospectral numerical method. The numerical study explicitly calculates the interactions of electrically charged inertial particles in homogeneous, isotropic turbulence. Conditions are selected to investigate the effects of mutual electrostatic repulsion of particles on their dynamics, sufficiently strong so as to mimic behavior of a nonideal gas: Coulomb interactions lead to short range repulsion overcoming inertial clustering below a shielding length as seen by a strong reduction in the RDF, but turbulence is sufficiently intense so as to suppress long range correlations (e.g., analogous to thermal energy leading to melting of a Coulomb crystal). [Preview Abstract] |
Sunday, November 20, 2011 5:19PM - 5:32PM |
E21.00004: Experimental and Numerical investigation of a droplet-laden turbulent flow: preferential concentration due to turbulence and its influence on droplet collisions and growth Alberto Aliseda, Colin Bateson, Orlando Ayala, Hossein Parishani, Lian-Ping Wang, Bogdan Rosa We have conducted a multi-laboratory investigation of the dynamics of small inertial droplets ($St \approx 0.1-1$) immersed in homogeneous isotropic turbulence. We compare experimental results from a wind tunnel experiment with Direct Numerical Simulations of slowly decaying homogeneous isotropic turbulence laden with spherical droplets. The Reynolds number ($Re_{\lambda}$) in both cases is of the order of 200 and the particle distribution is polydisperse with droplets in the $10-30~\mu m$ range. We compare the one-dimensional Radial Distribution Function from the experiments to the 1D, 2D and 3D RDFs from the simulations to validate the numerical treatment of the droplet dynamics in close proximity, and to develop methods to extrapolate the experimental measurements to 3D. We also compare the relative velocity of a pair of droplets, obtained along a line or in a plane from PDPA and PIV measurements, to the equivalent statistics obtained from the 3D velocity fields in the DNS. These are key components of the droplet collision kernel necessary to calculate turbulence-induced collision-coalescence and droplet growth. [Preview Abstract] |
Sunday, November 20, 2011 5:32PM - 5:45PM |
E21.00005: Highly scalable parallel implementation of turbulent collision of aerodynamically interacting cloud droplets Hossein Parishani, Orlando Ayala, Lian-Ping Wang, Bogdan Rosa, Wojciech Grabowski Hybrid direct numerical simulation (HDNS) has advanced our understanding of turbulent collision-coalescence of cloud droplets. In this approach, the background fluid turbulence is simulated by a pseudospectral method and disturbance flows of droplets are treated analytically. To better realize its potential on PetaScale computers with $\sim$100,000 processors, here we implement and test a parallel implementation using two-dimensional domain decomposition. The purpose is to increase both the range of flow scales and the number of droplets realizable in the simulations, so the dependence of collision statistics on flow Reynolds number and droplet size can be explored. We expect that the 2D domain-decomposition HDNS code can be used to produce statistics of aerodynamically-interacting droplets with Taylor microscale flow Reynolds number $R_\lambda$ up to $\sim 1000$ and a system of ${\cal O}$(10$^7$) polydisperse droplets. We will present the implementation details as well as results of turbulent collision statistics (e.g., collision kernel, radial distribution function, relative velocity statistics) of sedimenting cloud droplets from our latest high-resolution HDNS. [Preview Abstract] |
Sunday, November 20, 2011 5:45PM - 5:58PM |
E21.00006: The effect of dissipation intermittency on the turbulent collision statistics of cloud droplets Lian-Ping Wang, Wojciech Grabowski Atmospheric cloud turbulence has a Taylor microscale flow Reynolds number typically in range of $10^4$ to $10^5$, therefore, the local fluid acceleration and local dissipation rate are highly intermittent. The collision rate of cloud droplets and related pair statistics (i.e., the radial relative velocity and radial distribution function) are affected by these local flow intermittency, therefore, it has been speculated that the flow intermittency might have a significant impact on the turbulent collision of cloud droplets. In this talk, we argue, however, that this speculation may not hold for several reasons. First, collision is a binary interaction and its average statistics are governed by second-order statistical moments, the average impact of intermittency is not significant for these lower order statistics. Second, while the intermittent regions of high flow acceleration and high local dissipation rates do occur, they occupy a very insignificant volume with short lift time. Third, for given droplet sizes, the range of scales governing the pair statistics is limited. An analysis is developed to assess the effect of flow intermittency on the average collision kernel, showing that the flow intermittency does not have a significant effect. Other related works and results from direct numerical simulations will also be used to support this finding. [Preview Abstract] |
Sunday, November 20, 2011 5:58PM - 6:11PM |
E21.00007: A Lagrangian clustering analysis of inertial particles in turbulence using three-dimensional Vorono\"i tessellations Yoshiyuki Tagawa, Julian Martinez Mercado, Vivek N. Prakash, Enrico Calzavarini, Chao Sun, Detlef Lohse Three-dimensional Vorono\"i analysis is used to quantify the clustering of inertial particles in homogeneous isotropic turbulence using data sets from numerics in the point particle limit and one experimental data set. We study the clustering behavior at different density ratios, particle response times (i.e. Stokes numbers St) and several Taylor-Reynolds numbers. The Probability Density Functions (PDFs) of the Vorono\"i cell volumes of light and heavy particles show a different behavior from that of randomly distributed particles, implying that clustering is present. The results are consistent with previous investigations. The small Vorono\"i volumes of light particles correspond to regions of higher enstrophy than those of heavy particles, indicating that light particles cluster in higher vorticity regions. The Lagrangian temporal autocorrelation function of Vorono\"i volumes shows that the clustering of light particles lasts much longer than that of heavy or neutrally buoyant particles. Due to inertial effects arising from the density contrast with the surrounding liquid, light and heavy particles remain clustered for much longer times than the flow structures which cause the clustering. [Preview Abstract] |
Sunday, November 20, 2011 6:11PM - 6:24PM |
E21.00008: Inertial particles in turbulent convection Valentina Lavezzo, Herman J.H. Clercx, Federico Toschi Particle re-suspension mechanism in a non-isothermal flow has direct relevance for many industrial and environmental applications, where the properties of the flow (heat transfer, turbulence, etc.) can be modified by the presence of particles. A Lattice Boltzmann Method coupled with Lagrangian particle tracking is used to investigate the behaviour of inertial particles released in a horizontally periodic turbulent Rayleigh-B\'{e}nard convection cell. In particular, we focus on the effects of plume formation and turbulent structures on particle re-suspension in the domain. Different Froude numbers are considered to evaluate the influence of inertia and gravity on particle entrainment and dispersion. A two-way coupling approach has been included to estimate the impact of the presence of particles on the heat exchange between the two walls. Statistics revealed a close relationship between particle behaviour and fluid thermal structures and therefore a strong variation of the temperature field, when a back reaction of the particles onto the fluid is considered. Mean and higher order statistics on particle and fluid velocity and temperature fields are also presented. [Preview Abstract] |
Sunday, November 20, 2011 6:24PM - 6:37PM |
E21.00009: Effect of gravity on the dispersion of heavy particles Yongnam Park, Yongrak Jung, Changhoon Lee The effect of gravity on the dispersion of heavy particles is investigated in direct numerical simulation of forced isotropic turbulence at moderate Reynolds numbers. The integral time scales of fluid seen by particle show different behavior from that of Lagrangian fluid for wide range of Stokes number and magnitude of gravity. Autocorrelation functions of velocity of heavy particles become reduced when stronger gravity is applied, implying the reduction of dispersion. Integral time scales of fluid seen by particle also get smaller when gravity is stronger. Compared to the direction of gravity, normal direction velocities show shorter correlation. Settling velocity of heavy particle gets smaller when the Stokes number goes to zero or gravity gets smaller. Gravity enhances the preferential concentration of heavy particle and different type of large-scale nonuniform distribution of particles is clearly observed for the Stokes number of 10 when strong gravity is exerted. The preferential concentrations by strong gravity are not related with the turbulence structure and relevant physical explanation will be discussed in the meeting. [Preview Abstract] |
Sunday, November 20, 2011 6:37PM - 6:50PM |
E21.00010: Anisotropy in Pair Dispersion of Inertial Particles in Turbulent Channel Flow Enrico Pitton, Cristian Marchioli, Alfredo Soldati, Valentina Lavezzo, Federico Toschi The rate at which two particles separate in turbulent bounded flows is crucial to predict the inhomogeneities of particle spatial distribution and to characterize mixing. In this paper we examine the role of mean shear and small-scale turbulent velocity fluctuations on pair separation. To this aim an Eulerian-Lagrangian approach based on pseudo-spectral DNS of turbulent channel flow is used. Pair separation statistics were computed for particles with different inertia (and for tracers) released from different regions of the channel. Results confirm that shear-induced effects predominate when the pair separation distance becomes comparable to the largest scale of the flow, and also reveal the fundamental role played by particles-turbulence interaction at the small scales in triggering separation during the initial stages of pair dispersion. As a result, pair dispersion in non-homogeneus anisotropic turbulence has a superdiffusive nature and may generate non-Gaussian number density distributions of both particles and tracers. These features persist even when statistics are shown free of unidirectional convection, and exhibit strong dependency on particle inertia. [Preview Abstract] |
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