10:45
Complex and Acticve Folws
10:45
15 mins
#75
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Effects of large-scale turbulence on the preferential concentration of elongated gyrotactic swimmers
Filippo De Lillo, Guido Boffetta, Matteo Borgnino, Massimo Cencini
Abstract: We consider the case of gyrotactic cells elongated
along the fore-aft axis defined by the direction of
swimming. In this case, previous theoretical and numerical results predict lower
small-scale clustering overall, except in the case of negligible gyrotactic
effect. On the other hand, preferential sampling of the vertical component of
velocity is expected to change sign for some critical swimming speed at large enough cell eccentricities. We show, by means of direct
numerical simulations, that such reversal is governed by the ratio of the
swimming speed to the large scale velocity of the flow, at
variance with several other observables which are correctly described by
considering only small scale quantities. This phenomenon highlights the relevance of the scale separation typical of turbulence for the dynamics of this kind of swimmers.
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11:00
15 mins
#39
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SURFACING AND CLUSTERING OF GYROTACTIC MICRO-SWIMMERS IN FREE-SURFACE TURBULENCE
Harshit Bhatia, Cristian Marchioli, Alfredo Soldati
Abstract: We investigate surfacing and clustering of micro-swimmers in flat shear-free surface of open channel flow in which flow is turbulent. Micro-swimmers are modeled as self-propelling cells with preferential swimming direction (vertically upwards) which helps them to swim towards sun lit free-surface to activate photosynthesis. This configuration mimics the dynamics of micro-organisms in environmental situations where the surface waves and ripples are smooth or absent.
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11:15
15 mins
#195
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CONTROLLING ACTIVE SPINNERS USING VORTEX LATTICES
Hua Xia, Jean-Baptiste Gorce, Nicolas Francois, Horst Punzmann, Michael Shats
Abstract: Active colloids are an emergent class of out-of-equilibrium materials demonstrating complex collective behavior. Microscopic particles energized by external fields exhibit a wealth of fascinating phenomena, yet mechanisms of control and manipulation of active particles remain lacking. It has been recently shown that vortex lattices can be generated using two orthogonal standing surface waves on the liquid-air interface. Such driven vortex lattice allows controlling transport of the surface matter, such as active microparticles. In this experiment, we investigate the interaction of the magnetically driven active spinners with the vortex lattice generated by rotating surface waves.
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11:30
15 mins
#68
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ROTATION RATE AND PREFERENTIAL ALIGNEMENT OF RODS IN CONVECTIVE TURBULENCE FROM EXPERIMENTS AND SIMULATIONS
Enrico Calzavarini, Linfeng Jiang, Chao Sun
Abstract: The rotational dynamics of small anisotropic material particles (e.g. fibers or disks) in turbulent flows has been the focus of a series of recent studies, see e.g. [1]. Experiments as well as numerical simulations have highlighted their complex behaviour, which is inherited from the non-trivial dynamics of the velocity gradient tensor along the particle trajectories [2]. Preferential alignement of particles with intrinsic orientations of the small scale turbulence structures have been observed. For exemple prolate particles preferentially align with the vorticity direction [3]. However, while the phenomenology of orientations is now clear for homogeneous and isotropic turbulent flows, much less explored remains the case of non-homogeneous turbulent flows [1]. A recent step in this direction has been in made in [4] where fibers have been numerically tracked in a turbulent channel flow and in [5] where fibers have been experimentally tracked in a high-Reynolds number Taylor-Couette flow.
In this study we extend the investigation of non-isotropic particles to the case of turbulent convection, which is both an inhomogeneous and anisotropic flow. This is performed by means of combined experimental and numerical efforts, where nearly inertialess anisotropic particles (rods and disks) are tracked in a cubic Rayleigh-Bénard (RB) cell. We show the relevant role of the large scale circulation flow in determining the preferential orientation despite the developed level of turbulence present in the system. The correlation between orientation and temperature gradient field is further investigated.
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11:45
15 mins
#230
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Kinematics of Large Buoyant Ellipsoids Rising in a Quiescent Fluid
Jelle Will, Varghese Mathai, Dominik Krug, Sander Huisman, Detlef Lohse, Chao Sun
Abstract: Many relevant natural and industrial flows contain some form of particles dispersed in the fluid. Consider for instance transport and dispersion of pollutants in the oceans or the atmosphere. Up to this now most of the research on such flows has focused on the idealized case of spherical particles; only recently the effect of particle anisotropy is starting to be appreciated in these type of systems [3]. Here we experimentally investigate to what degree particle drag, rise-velocity, oscillation frequency, and alignment are affected by the shape anisotropy for a single particle rising in a still fluid. In this work we fix the particle buoyancy force of the biaxial ellipsoidal particles, Galileo number ($\textit{Ga} = \sqrt{|1-\Gamma|gD^3}/\nu = 6300$) where $\Gamma$ is the particle density with respect to that of the fluid, $D$ is the volume equivalent diameter, and $\nu$ is the fluid viscosity. We only change the aspect ratio ($\chi = h/d$, height of the particle over the circular diameter) by deforming a spherical particle into respectively prolate (needles) and oblate (disks) ellipsoids. The aspect ratio is varied from 0.20 to 5.00. We experimentally measure full three-dimensional translations and rotations of the particles using an algorithm developed to track anisotropic shapes.\\
We observe a wide variety of behavior which can be classified into different regimes based on the kinematics of the trajectories. The behavior ranges from flutter to tumbling to cylinder like vortex shedding and finally helical spiraling. Further it is noted that the Reynolds number (and rise-velocity) of the particles is strongly dependent on the aspect ratio. Our results are compared to work done by Fernandes et al. [1] and [2], we find an interesting similarity in the oscillation frequencies of the particles despite the large difference in $Ga$. On the other hand the variation of Reynolds number, as found in this work, was not observed in prior work by [1] and [2], at lower $Ga$.
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12:00
15 mins
#510
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Phase Transitions to Condensate Formation in two-dimensional turbulence
Moritz Linkmann, Manuel Hohmann, Guido Boffetta, Cristina M Marchetti, Bruno Eckhardt
Abstract: Two-dimensional (2d) and quasi-2d flows occur at macro- and mesoscale in a variety of physical systems. Examples in- clude stratified layers in Earth’s atmosphere and the ocean, soap films and more recently also dense bacterial suspensions, where the collective motion of microswimmers induces patterns of mesoscale vortices [1]. A characteristic feature of 2d turbulence is the occurrence of an inverse energy cascade [2]. In absence of large-scale friction the inverse energy cascade results in the formation of large-scale coherent structures, so-called condensates. We here study the formation of the condensate as a function of the kind and amplitude of the forcing. Direct numerical simulations show that the condensate does not appear gradually but in a phase transition. For prescribed energy dissipation the transition is second order (fig. 1, left panel); for active matter, where the forcing is due to a small-scale instability [3, 4], the transition is first order [5], see fig. 1, right panel. The phase transition separates two markedly different types of 2d turbulence: in turbulence with a condensate, energy input is predominantly balanced by dissipation in the condensate and intermediate scales follow an inertial cascade; without a condensate dissipation is spread over the intermediate scales and the properties of the energy transfer are noticeably different and non-universal.
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12:15
15 mins
#46
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Orientation of non-spherical swimming particles in turbulence
Massimo Cencini, Guido Boffetta, Matteo Borgnino, Filippo De Lillo, Kristian Gustavsson, Bernhard Mehlig
Abstract: We study swimming ellipsoidal particles, relevant to motile microorganisms with non-spherical shapes, e.g. bacteria, or artificial microswimmers that
can be engineered with different shapes, in turbulent flows. By means of direct numerical simulations, we show that for turbulent flows of moderate intensity, rod-(disk-)like particles tend to swim on average along (against) the fluid flow. The origin of this alignment is kinematical and is analytically rationalised by mean of perturbative calculation expanding in the limit of small Kubo number. The dependence in Reynolds of the alignment is also studied.
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