10:30
Multiphase and non-Newtonian flows 2
Chair: Francesco Picano
10:30
15 mins
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DYNAMICS OF INERTIAL DISK PARTICLES IN TURBULENT CHANNEL FLOW
Niranjan Reddy Challabotla, Lihao Zhao, Helge I. Andersson
Abstract: A suspension of oblate spheroidal (disk-like) particles in turbulent channel flow has been investigated with focus on the translational and rotational particle statistics. The effects of particle aspect ratio and inertia have been explored. The disk-like particles exhibited a significant preferential orientation in the plane of the mean shear. The influence of the particle shape on the orientation and rotation diminished as translational inertia increased from Stokes number 1 to 30. Isotropization of both orientation and rotation could be observed in the core region of the channel.
Keywords: oblate spheroids, preferential orientation, shape effects, inertia
effects
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10:45
15 mins
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Inertial effects on non-spherical particle rotation on turbulent channel flow
Helge I. Andersson, Lihao Zhao, Niranjan R. Challabotla, Evan A. Variano
Abstract: We investigated the rotation of non-spherical particles (rod-like and disk-like) in turbulent channel flow with focus on inertial effects. A direct numerical simulation (DNS) with an Eulerian-Lagrangian approach was performed. A wide range of particle aspect ratios, λ, ranging from 0.01 to 50 were considered for Stokes numbers St equal to 1 and 30. In the particle reference frame, statistical results reveal the importance of shape effect on the particle rotation. The rods (λ > 1) are spinning (rotation about their symmetry axis) more than tumbling (rotation about other axes) whereas disks (λ < 1) behave oppositely. With increasing particle inertia, i.e. higher St, the preferential tumbling of the disks and the spinning of the rods are reduced. We ascribe these observations to the varying degree of alignment of the particle symmetry axis with the fluid vorticity vector.
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11:00
15 mins
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CONTACT VELOCITIES OF SMALL ELLIPSOIDS SETTLING IN TURBULENCE
Christoph Siewert, Rudie P. J. Kunnen, Wolfgang Schröder
Abstract: Collisions of small and heavy non-spherical particles settling in turbulence are very important for systems such as ice clouds and proto-planetary disks where the particle spectra evolution is strongly dependent on the collision induced growth rate. Still, the influence of the particle shape on the collision probability is virtually unknown. Building on our recent investigation on the collision rate of monodisperse suspensions of ellipsoidal particles (Siewert et al., J. Fluid Mech. 758, 686-701, 2014), we show theoretically and by direct numerical simulations that the behavior of ellipsoids subject to turbulence and gravity is different from the behavior of spheres. Due to the dependence of the particle settling velocity on the particle orientation, the relative velocity at contact is influenced by turbulence. When ellipsoids differ either by mass or shape, their contact velocity is randomized by the randomized particle orientation. For particles much heavier than the fluid these orientation dependent settling velocity differences are larger than the relative velocities directly induced by the turbulent fluctuations.
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11:15
15 mins
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Suppression of turbulent diffusion on the water surface by viscoelastic nano layer
Michael Shats, Nicolas Francois, Hua Xia, Horst Punzmann
Abstract: We study effects of thin (10 nm) layers of adsorbed proteins on the water surface hydrodynamics. We show that extremely small concentrations of protein (less than 1 ppm) form strong viscoelastic layer at the water-air interface. This layer greatly reduces single particle dispersion on the surface perturbed by Faraday waves and turns disordered surface waves into a square stationary oscillating crystal. The viscoelastic film is destroyed by minute addition of surfactant which leads to the recovery of the horizontal mobility of fluid particles and the restoration of the Faraday wave driven turbulence.
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11:30
15 mins
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Settling of finite-size particles in isotropically forced, homogeneous turbulence: interface-resolved simulations
Agathe Chouippe, Todor Doychev, Markus Uhlmann
Abstract: We have simulated the gravity-induced settling of finite-size particles in a turbulent background flow which is forced in a statistically-stationary fashion. The simulations are accurately resolving the solid-fluid interface with the aid of an immersed boundary technique [1]. The parameters of the simulation are (apart from background turbulence) identical to those of reference [2], where particle clustering was observed at a Galileo number of 178 and a solid volume fraction of 0.005. In the present case, it is found that a relative turbulence intensity of 0.24 leads to the disappearance of the clusters; as a consequence, the increase in average particle settling velocity found in [2] also vanishes.
[1] M. Uhlmann. An immersed boundary method with direct forcing for the simulation of particulate flows. J. Comput. Phys., 209(2):448–476, 2005.
[2] M. Uhlmann and T. Doychev. Sedimentation of a dilute suspension of rigid spheres at intermediate Galileo numbers: the effect of clustering upon
the particle motion. J. Fluid Mech., 752:310–348, 2014.
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11:45
15 mins
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Fully Turbulent Mean Velocity Profile for Purely Viscous non-Newtonian Fluids
Hamidrez Anbarlooei, Daniel O.A. Cruz, Atila P. Silva Freire
Abstract: The characteristic near wall behavior of turbulent flow of purely-viscous non-Newtonian fluids is discussed for both power-law (P.-L.) and Herschel-Bulkley (H.-B.) rheological models. A proper scaling is presented for H.-B. fluids to establish an analogy with power-law fluids with same flow index. To provide reference data for turbulent flow of non-Newtonian fluids, DNS simulations of power-law fluids are conducted in a rectangular channel for a large range of power-law indices ($n$ = 0.5, 0.69, 0.75, 0.9, 1, 1.2). The DNS data show that the mean velocity profile in the viscous and logarithmic layers follow expressions of the form $u^{+}=y^{+}$ and $u^{+}=2.5\,log(y^{+})+B_{n}$ respectively, where $B$ shows a logarithmic dependency on the flow index.Comparison with some experimental data shows the above formulation to be valid for Reynolds numbers (based on shear velocity) as high as 1000.
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12:00
15 mins
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Interaction between a large bubble and turbulence
Aurore Loisy, Aurore Naso, Peter Spelt
Abstract: Bubble dynamics in turbulent flows has been extensively studied using the point-bubble approximation. But when the carrier flow varies on length scales smaller than the bubble size, this approximation is no longer appropriate and one needs to resolve all the scales present in the two-phase flow. 3D Direct Numerical Simulations of turbulent bubbly flows are carried out for bubbles ten times larger than the Kolmogorov length scale. The dynamics of large bubbles subjected to homogeneous isotropic turbulence and the turbulence modulation induced by their presence are investigated by mean of conditional statistics.
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