Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session G04: Suspensions: General III |
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Chair: Alexander Zinchenko, University of Colorado Boulder Room: 203 |
Sunday, November 24, 2019 3:48PM - 4:01PM |
G04.00001: Tunable solidification of cornstarch under impact: how to make someone walking on cornstarch sink Ran Niu, Meera Ramaswamy, Christopher Ness, Abhishek Shetty, Itai Cohen Hundreds of Youtube videos with millions of views show people running on a mixture of cornstarch and water. These videos demonstrate a general phenomenon in fluid mechanics that dense shear thickening suspensions can solidify under impact. Such processes can be mimicked by impacting and pulling out a solid plate from the surface of a thickening cornstarch suspension. Here, using both experiments and simulations, we show that by applying fast oscillatory shear transverse to the primary impact or extension directions we can tune the degree of suspension solidification. The forces acting on the impacting surface can be modified by varying the dimensionless ratio of the orthogonal shear to the compression and extension flows. Simulations show that varying this parameter changes the number of particle contacts governing solidification. To demonstrate this strategy in an untethered context, we show that the sinking speed of a cylinder dropped onto the cornstarch suspension can be varied dramatically by changing this dimensionless ratio. These results suggest that applying orthogonal shear in the context of people running on cornstarch would de-solidify the suspension and cause them to sink. [Preview Abstract] |
Sunday, November 24, 2019 4:01PM - 4:14PM |
G04.00002: Dynamic Jamming Around a Cylinder Moving Through a Shear Thickening Suspension Olav Rømcke, Ivo R. Peters, R. Jason Hearst Dense suspensions exhibit a rich set of behavior, such as shear banding, rheochaos, continuous and discontinuous shear thickening, and in some extreme cases even jamming. Dynamic jamming by shear, which is the focus here, is caused by the formation of a frictional contact network between grains as stress increases. Previous works have shown, in separate experiments, that a transient jammed region can develop under pulling, pushing and shearing. How these different scenarios interact when they appear in a single system, however, is unknown. By dragging a cylinder through a suspension of cornstarch and water, we are able to track the shape of the jammed region as it propagates through the suspension using PIV on the free surface. This work makes it possible to directly compare the propagation of the jammed region, in a system where pulling, pushing and shearing coexist. [Preview Abstract] |
Sunday, November 24, 2019 4:14PM - 4:27PM |
G04.00003: Flow regimes of a neutrally buoyant suspension in the wake of a circular cylinder Raphael Maurin, Matthieu Mercier, Laurent Lacaze, Jeffrey Morris While the rheology of suspensions has been mainly studied in the Stokes regime, fluid mechanical applications such as blood flows or sediment transport at finite particle Reynolds number require a more general understanding. From this perspective, we revisit experimentally the well-known flow around a cylinder, considering a neutrally-buoyant suspension instead of a pure fluid. Varying the particle Reynolds number, $Re$, and the solid volume fraction, $\phi$, we investigate various wake structures, from the laminar creeping flow to the Karman vortex street. We characterize the corresponding stability map as a function of $Re$ and $\phi$. The presence of particles affects the shape of the wake of the cylinder in a non-trivial way, which cannot be accounted for by a modification of the effective fluid viscosity due to particle loading. Furthermore, the presence of the particles alters the critical values of the Reynolds number at which transitions in the flow regimes occur. These results can also be interpreted by the local behavior of the particles relative to the fluid near specific features of the flow around the cylinder. [Preview Abstract] |
Sunday, November 24, 2019 4:27PM - 4:40PM |
G04.00004: Fast Lagrangian-averaged transport of particles in streaming flows Mathieu Le Provost, Jeff D. Eldredge Viscous streaming is an efficient method to transport, trap or cluster inertial particles in a fluid, used in biomedicine, microfluidics... Nonetheless, current methods to simulate the long term behavior of inertial particles are computationally expensive due to the nonlinearity and stiffness of the Maxey-Riley equation when applied to the wide disparity of time scales of oscillation and mean convection in streaming flows. To handle these issues, we propose a novel framework called Fast Lagrangian Averaged Transport to efficiently compute the Lagrangian-averaged motion of inertial particles. Two key ingredients are used to get this improved performance. First, we derive an asymptotic expansion for an Eulerian inertial particle velocity field for small Stokes number about the Eulerian fluid particle velocity field. Secondly, we decompose the motion of an inertial particle into a mean (slow) and fluctuating (rapid) component derived from an Eulerian disturbance field evaluated at the mean Lagrangian position of the particle. Linearized equations for the disturbance field are derived, and solved using an Immersed Boundary Method. Our method is assessed on the transport generated by one or two weakly oscillating cylinders. Computations are up to 300 times faster with this new method. [Preview Abstract] |
Sunday, November 24, 2019 4:40PM - 4:53PM |
G04.00005: Is a sedimenting array of disks stable? Rahul Chajwa, Rama Govindarajan, Narayanan Menon, Sriram Ramaswamy We study experimentally the stokesian sedimentation (Re $\sim 10^{-4}$) of a one dimensional lattice of discs in a quasi-two-dimensional geometry with the trajectory of the centres of the disks lying in a plane. We induce initial positional perturbations over a configuration in which the disks are uniformly spaced with their separation vectors and normals aligned, and perpendicular to gravity. For various perturbation wavenumbers and interparticle separations, we find two classes of behaviour:(i) a transient wave of orientations coupled with number-density fluctuations and (ii) a clumping instability resembling that of spheres [J.M. Crowley, J.Fluid Mech. 45, 151 (1971)], decorated with orientations. We construct the equations of motion for displacements and orientations using pairwise addition of forces and torques [R. Chajwa et. al. PRL 122, 224501 (2019)]. Linear stability analysis demarcates a phase boundary between neutrally stable and unstable regimes in the plane of wavenumber and lattice spacing, consistent with our experiments. We predict non-modal growth in this plane, with a critical density of the lattice below which all wavenumbers are asymptotically stable, showing that orientable particles need not be subject to the inevitable clumping instability of spheres. [Preview Abstract] |
Sunday, November 24, 2019 4:53PM - 5:06PM |
G04.00006: Transient and steady sedimentation of flocculating non-Brownian suspensions Alexander Zinchenko Evolution to the steady state is rigorously simulated for a monodisperse non-Brownian suspension of spheres (initially unaggregated and well-mixed) with short-range van der Waals attraction and electrostatic repulsion in the realistic range of colloidal parameters. Flocculation is mostly affected by the maximum net attractive force near the secondary minimum relative to the effective gravity force. An economical high-order multipole algorithm, combined with geometry perturbation (Zinchenko A.Z. Phil. Trans. R. Soc. Lond. A(1998), v.356, 2953) to include lubrication, fully resolves hydrodynamic interactions in simulations with up to 1000 spheres in a periodic box and millions of time steps. Averaging over many initial configurations is used to predict the transient sedimentation rate U(t) for suspension volume fractions c=0.1-0.4 in a wide time range. The results are convergent and system-size independent. For particles of ~10-15 micron size, flocculation is ubiquitous and has a large effect, allowing U(t) to grow several-fold or more before it reaches the statistical steady state, as the balance is achieved between the formation and breakage of aggregates. In contrast to the precise many-body solution, the far-field Rotne-Prager approximation predicts much faster growth of U(t). [Preview Abstract] |
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