Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session E37: Particle-Laden Flows: Deformable ParticlesParticles
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Chair: Cristian Marchioli, University of Udine Room: 303 |
Sunday, November 19, 2017 4:55PM - 5:08PM |
E37.00001: Using deformable particles for single particle measurements of velocity gradient tensors Bardia Hejazi, Michael Krellenstein, Greg Voth We examine the deformation of particles made of several slender arms in 2D simple shear and 3D turbulent flow. Measurements of the bending of several arms and the rotation rate of the particle allow us to extract the full velocity gradient tensor using a single particle. Normally to obtain the same information we would have to use a high density of tracer particles. We use multiple armed (ramified) particles because a single slender fiber is not deformed in a uniform velocity gradient. Measurements in a 2D simple shear flow using a particle free to rotate about a fixed position are used to validate the technique. We then explore the use of this method to measure coarse grained velocity gradients in a turbulent flow created by a jet array in a vertical water tunnel. [Preview Abstract] |
Sunday, November 19, 2017 5:08PM - 5:21PM |
E37.00002: Dynamics of small flexible fibers in turbulent channel flow Cristian Marchioli, Diego Dotto, Alfredo Soldati In this paper we investigate the dynamics of small flexible fibers in turbulent channel flow. Our aim is to examine the effect of local shear and turbulence anisotropy on the translation and rotation of fibers with different elongation and inertia. To these aims, we use a Eulerian-Lagrangian approach based on direct numerical simulation of turbulence in the dilute regime, and we model fibers, which are longer than the Kolmogorov scale, as chains of sub-Kolmogorov rods connected through ball-and-socket joints that enable bending and twisting. Velocity, orientation and concentration statistics, extracted from simulations at $Re_{\tau}=300$, are presented to give insights into the complex fibers-turbulence interactions that arise when non-sphericity and deformability add to inertial bias. Compared to fibers that translate and rotate as rigid bodies relative to the surrounding fluid, flexible fibers exhibit a stronger tendency to accumulate in the near-wall region, where they are trapped by the same mechanisms that govern preferential concentration of spherical particles. In such region, the mean shear is strong enough to reduce bending and stretch the fibers. Preferential segregation into low-speed streaks and preferential orientation in the mean flow direction are also observed. [Preview Abstract] |
Sunday, November 19, 2017 5:21PM - 5:34PM |
E37.00003: Hydrodynamic and elastic interactions of sedimenting flexible fibers Maria L. Ekiel-Jezewska, Marek Bukowicki Dynamics of flexible micro and nano filaments in fluids is intensively investigated in many laboratories, with a perspective of numerous applications in biology, medicine or modern technology. In the literature, different theoretical models of elastic interactions between flexible fiber segments are applied. The task of this work is to examine the impact of a chosen elastic model on the dynamics of fibers settling in a viscous fluid under low Reynolds number. To this goal, we construct two trumbbells, each made of three beads connected by springs and with a bending resistance, and we describe hydrodynamic interactions of the beads in terms of the Rotne-Prager mobility tensors. Using the harmonic bending potential, and coupling it to the spring potential by the Young's modulus, we find simple benchmark solutions: stable stationary configurations of a single elastic trumbbell and a fast horizontal attraction of two elastic trumbbells towards a periodic long-lasting orbit. We show that for sufficiently large bending angles, other models of bending interactions can lead to qualitatively and quantitatively different spurious effects. We also demonstrate examples of essential differences between the dynamics of elastic dumbbells and trumbbells. [Preview Abstract] |
Sunday, November 19, 2017 5:34PM - 5:47PM |
E37.00004: A numerical study of a long flexible fiber in shear flow: dynamics and rheology Pawel Zuk, Antonio Perazzo, Janine Nunes, Howard Stone Long slender particles can span the whole spectrum of stiffness: from very flexible particles such as globular proteins to extremely rigid particles, e.g. carbon nanotubes or $\beta$-amyloid fibers. The behavior of rigid particles is well understood, however there are only few recent experimental reports about long fibers of moderate flexibility. We present a numerical study of a single long flexible fiber in a shear flow. The fiber is simulated as a bead-spring model including hydrodynamic interactions in the Rotne-Prager-Yamakawa approximation. We analyze fiber shape, motion and stress induced in the fluid under the shear flow. We find that all of these properties appear to be related to the characteristic length scale of the kinks formed in the fibers. We present a scaling law for the kink size as a function of shear rate and the fiber parameters and justify it using elastic theory. The study suggests that local properties of a single fiber may condition the behavior of concentrated suspensions. [Preview Abstract] |
Sunday, November 19, 2017 5:47PM - 6:00PM |
E37.00005: Periodic motions of flexible fibers in shear flow Agnieszka M. Slowicka, Howard A. Stone, Maria L. Ekiel-Jezewska Numerical simulations of a single open flexible fiber entrained by shear flow are performed in the absence of both inertia and Brownian motion. The bead-spring model is used to account for hydrodynamic and elastic forces exerted on the fiber segments. The motion is studied for a wide range of the fiber bending stiffness ratios (i. e. the ratios of bending to hydrodynamic forces), aspect ratios and initial configurations. The essential finding is the existence of certain universal classes of periodic, quasi-periodic and steady modes, with different characteristic evolution of fiber shapes and different values of the orientation parameter. Types of the dominating modes, their lifetime and stability depend on the fiber bending stiffness and aspect ratios. [Preview Abstract] |
Sunday, November 19, 2017 6:00PM - 6:13PM |
E37.00006: Numerical study of suspensions of deformable particles. Luca Brandt, Marco Edoardo Rosti We consider a model non-Newtonian fluid consisting of a suspension of deformable particles in a Newtonian solvent. Einstein showed in his pioneering work that the relative increase in effective viscosity is a linear function of the particle volume fraction for dilute suspensions of rigid particles. Inertia has been shown to introduce deviations from the behaviour predicted by the different empirical fits, an effect that can be related to an increase of the effective volume fraction. We here focus on the effect of elasticity, i.e. visco-elastic deformable particles. To tackle the problem at hand, we perform three-dimensional Direct Numerical Simulation of a plane Couette flow with a suspension of neutrally buoyant deformable viscous hyper-elastic particles. We show that elasticity produces a shear-thinning effect in elastic suspensions (in comparison to rigid ones) and that it can be understood in terms of a reduction of the effective volume fraction of the suspension. The deformation modifies the particle motion reducing the level of mutual interaction. Normal stress differences will also be considered. [Preview Abstract] |
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