Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session H2: Particle-Laden Flows V: DNS and Non-Spherical Particles |
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Chair: Ismail Celik, West Virginia University Room: 324 |
Monday, November 25, 2013 10:30AM - 10:43AM |
H2.00001: The effect of particle rotation in multi-particle flow simulations Adam Sierakowski, Andrea Prosperetti In multi-particle flow simulations, particle rotation is difficult to calculate and is often imprecisely accounted for or ignored altogether. We examine the effect of these procedures on the overall flow characteristics through large systems of particles when the particle center is fixed and either allowed or not to rotate. We use a newly developed GPU-centric implementation of the Physalis method for the solution of the Navier-Stokes equations in the presence of finite-size spheres. We investigate periodic systems of more than 100 randomly-distributed particles at Reynolds numbers up to 100. By considering flow characteristics such as mean velocity and pressure drop, we shed light on the importance of including particle rotation effects in large particle-flow simulations. [Preview Abstract] |
Monday, November 25, 2013 10:43AM - 10:56AM |
H2.00002: Numerical Investigation of Cloud Droplet Growth via Collision Coalescence: One Step Approach Hossein Parishani, Orlando Ayala, Bogdan Rosa, Lian-Ping Wang Growth of inertial particles and droplets in a turbulent flow is a critical step in a wide range of applications. It is known that cloud turbulence could make a substantial impact on the growth of cloud droplets by collision and coalescence. Reade and Collins (2000) performed a DNS study of one step growth of coagulating particles in turbulence. They found that the limiting solutions of zero or infinite St numbers are not capable of describing the dynamics of finite-inertia particles. In this talk we extend their work to include the effects of gravity and flow Reynolds number on growth of droplets in turbulence. Starting from an initially monodisperse distribution of particles, we study how turbulent collision coalescence affects the particle size distribution. The simulations are performed with a 256$^3$ grid resolution and $O(10^6)$ droplets of radii ranging from 10 to 60 microns. We obtain particle size distributions for a range of flow Reynolds numbers to study the effect of flow Reynolds number on the size distribution of inertial particles. The one-step results are compared to those from the kinetic collection equations using gravitational and turbulent collision kernels. [Preview Abstract] |
Monday, November 25, 2013 10:56AM - 11:09AM |
H2.00003: Effect of ambient flow inhomogeneity on drag forces on a sphere at finite Reynolds numbers Jungwoo Kim, S. Balachandar, Hyungoo Lee For studies on particle-laden flows involving particle transport and dispersion, the prediction capability of hydrodynamic forces on the particle in a non-uniform flow is one of the central issues. However, existing analytical expressions and empirical correlations are mainly made based on the homogeneous flow conditions such as uniform or uniform shear flows. Therefore, the objective of this study is to investigate the effect of flow inhomogeneity on drag forces on a sphere at finite Reynolds numbers. To do so, we perform direct numerical simulations of flow over a sphere in an inhomogeneous flow. In this study, we consider three different kinds of the inhomogeneous flows: cosine, hyperbolic cosine and hyperbolic secant profiles. The Reynolds number of the sphere based on the freestream velocity and sphere diameter is 100. The present simulations show that the quasi-steady drag forces in inhomogeneous flows are reasonably estimated by standard drag law based on the relative velocity if the fluid velocity seen by the particle is evaluated by surface average. The results support Loth and Dorgan (2009)'s proposed formula. In the final presentation, the effect of ambient flow inhomogeneity on drag forces would be presented in more detail. [Preview Abstract] |
Monday, November 25, 2013 11:09AM - 11:22AM |
H2.00004: Particle dispersion in stably stratified open channel flow Salvatore Lovecchio, Francesco Zonta, Alfredo Soldati Many geophysical flows are influenced by stable stratification effects. In terrestrial water bodies, the vertical distribution of temperature produces a thermocline (a region where large gradients occur) which strongly influences mixing. In this study we analyse the effect that the formation of the thermocline has on particle dispersion in stably stratified turbulence in an open channel flow using Direct Numerical Simulation and Lagrangian Particle Tracking. The parameter that characterizes the physical problem is given by the ratio $Gr/Re_{\tau}^2$, where $Gr$ is the Grashof number and $Re_{\tau}$ the Reynolds number. This parameter represents the relative importance of buoyancy and inertia (namely of stratification). We perform a parametric study, considering different stratification levels (i.e, different values of $Gr/Re_{\tau}^2$) and particles with different inertia. Preferential concentration is quantified using the correlation dimension and Voronoi diagrams. Results indicate that the thermocline in the upper flow layers influences the dynamics of the coherent flow structures by reducing the frequency with which upwelling/downwelling motions of fluid are formed. This in turn decreases particle dispersion and segregation at the flow surface. [Preview Abstract] |
Monday, November 25, 2013 11:22AM - 11:35AM |
H2.00005: Inertial Range Scaling in Rotations of Long Rods in Turbulence Greg Voth, Shima Parsa We measure the rotational statistics of neutrally buoyant rods with lengths $2.8 < l/\eta <72.9 $ in turbulence. For particles with length in the inertial range, we derive a scaling relationship for the mean square rotation rate, $\langle \dot{p}_i \dot{p}_i \rangle \propto l^{-4/3}$ and show that measurements approach this scaling. Deviations from the proposed scaling are explained as the effect of dissipation range scales. The correlation time of the Lagrangian autocorrelation of rod rotation rate scales as the turn over time of eddies of the size of the rod. Measuring rotational dynamics of single long rods provides a new way to access the dynamics of turbulence at fixed spatial scale in a frame advected with the flow. [Preview Abstract] |
Monday, November 25, 2013 11:35AM - 11:48AM |
H2.00006: Alignment of vorticity and rods with Lagrangian fluid stretching in turbulence Rui Ni, Greg Voth Stretching in continuum mechanics is naturally described using the Cauchy-Green strain tensors. These tensors quantify the stretching experienced in a Lagrangian reference frame, which provides a powerful way to study interesting processes that involve stretching, such as vortex stretching and alignment of anisotropic particles. We integrate the velocity gradient tensor from direct numerical simulation of isotropic turbulence to obtain the Cauchy-Green strain tensor. We find that the preferential alignment between anisotropic particles and vorticity is because both of them tend to align with the strongest stretching direction, defined by the maximum eigenvector of the left Cauchy-Green strain tensor. In particular, anisotropic particles approach almost perfect alignment with the strongest stretching direction. The alignment of vorticity with the stretching direction is weaker, but still much stronger than previously observed alignment of vorticity with the eigenvectors of the velocity gradient tensor. [Preview Abstract] |
Monday, November 25, 2013 11:48AM - 12:01PM |
H2.00007: On the compaction of fibers in a flow and the formation of sea balls Patrice Le Gal, Gautier Verhille Sea balls found on Mediterranean beaches are made of Posidonia fibers which aggregate due to the sea motions. To understand the mechanism of aggregation and compaction of these structures, we have studied the distribution of sizes and masses of these balls. We show that the pdfs are very close to log-normal distributions which suppress some formation mechanisms such as fibers aggregation one by one (that would give Poisson distributions) or random clustering or fragmentation (that would lead to Gamma distributions). Then, we present an experimental investigation on the dynamic of aggregation of fibers by waves generated in a tank. This experiment underlines the importance of the fiber rigidity. [Preview Abstract] |
Monday, November 25, 2013 12:01PM - 12:14PM |
H2.00008: Rotation of rigid fibers in wall shear turbulence Cristian Marchioli, Alfredo Soldati In this paper we examine the rotation of rigid fibers with different elongation and inertia in turbulent channel flow, focusing on the effect of local shear and turbulence anisotropy. Statistics of the fiber angular velocity, $\Omega$, are extracted from DNS of turbulence at shear Reynolds number $Re_{\tau}=150$ coupled with Lagrangian tracking of prolate ellipsoidal fibers with Stokes number $1 < St < 100$, and aspect ratio $1 < \lambda < 50$. Results for mean and fluctuating angular velocities show that elongation is important for fibers with small inertia ($St \le 5$ in the present study). At larger inertia, elongation has an impact on rotation only in the streamwise and wall-normal directions. In the channel center, the Lagrangian autocorrelation coefficients of $\Omega$ and corresponding rotational turbulent diffusivities match the exponential behavior predicted by the theory of homogeneous dispersion. Also, the PDF of fiber angular velocities is generally close to Gaussian, indicating that fiber rotation away from solid walls can be modeled as a Ornstein-Uhlenbeck diffusion process at stationary state. In the strong shear region, fiber anisotropy adds to flow anisotropy to induce strong deviations on fiber rotational dynamics with respect to spherical particles. [Preview Abstract] |
Monday, November 25, 2013 12:14PM - 12:27PM |
H2.00009: Surface singularities of nanorod laden droplets in magnetic field Konstantin Kornev, Alexander Tokarev, Wah-Keat Lee Magnetic nanorods are attractive materials enabling assembly, ordering, control, and reconfiguration of different magnetic lattices within milliseconds in milliTesla magnetic fields. In this talk we will show a new physical principle of self-assembly of magnetic nanorods into singular cusps at the droplet surface. These singularities can be formed on demand not deforming the entire droplets by taking advantage of the magneto-static interactions between nanorods in non-uniform magnetic field. Using X-ray phase contrast imaging and scaling analysis we will explain the behavior of magnetic nematics and their interactions with the droplet surface. [Preview Abstract] |
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