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 L39: Geophysical Fluid Dynamics Stratified Flow II |
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Chair: Richard McLaughlin, UNC Chapel Hill Room: 6a |
Monday, November 25, 2019 1:45PM - 1:58PM |
L39.00001: Nonlinear dynamics of destabilized array of vortices in stratified fluid Yuji Hattori, Makoto Hirota Periodic arrays of vortices are often observed in geophysical and astrophysical fluids, in which stratification and rotation effects are important. There exist hyperbolic stagnation points in the arrays of vortices. Recently, we have discovered {\textit{strato-hyperbolic instability}} which is due to hyperbolic instability and phase shift by internal gravity waves (Suzuki et al., J. Fluid Mech. \textbf{854}, (2018) 293--323). The next step is to investigate nonlinear dynamics of a destabilized array of vortices in stratified fluid. We are particularly interested in evolution of strato-hyperbolic instability modes. Direct numerical simulation of the incompressible Navier-Stokes equations for stratified fluid under the Boussinesq approximation is performed. The results show that strato-hyperbolic instability modes of high wavenumber develop and become turbulent only in the neighborhood of the heteroclinic streamlines connecting the hyperbolic points, while the core region of the vortices survives. On the other hand, hyperbolic instability modes of low wavenumber make the whole region turbulent so that most of the energy is lost eventually. [Preview Abstract] |
Monday, November 25, 2019 1:58PM - 2:11PM |
L39.00002: Low-Reynolds dynamics of a suspension of spheres in a stratified fluid Matthieu Mercier The settling dynamics of small objects in stratified fluids is important to understand the fate of the biomass in lakes or oceanic environments, for industrial applications such as waste-water disposal. More specifically, the settling of a suspension of solid particles is a fundamental problem, well-studied for a homogeneous fluid and barely investigated numerically for stratified fluids. We present experimental results on the settling dynamics at low Reynolds number of an initially homogeneous suspensions of non-Brownian particles immersed in a linearly stratified viscous fluid, due to a linear variation with depth of salt. We characterize the mean and fluctuations of these quantities for various stratification intensities, in order to quantify the influence of the stratification on settling. We compare these results with similar experiments realized in a homogeneous viscous fluid. [Preview Abstract] |
Monday, November 25, 2019 2:11PM - 2:24PM |
L39.00003: Retention of oil droplets rising in a stratified fluid: Part 1. Kinematics De Zhen Zhou, Tracy Mandel, Lindsay Waldrop, Maxime Theillard, Dustin Kleckner, Shilpa Khatri During the 2010 Deepwater Horizon oil spill, about 5 million barrels of petroleum discharged from the Macondo Well into the Gulf of Mexico. Oceanographic studies (McNutt, 2012) estimated approximately 40 percent of that oil was trapped beneath the ocean surface, primarily in regions with strong oceanic density gradients. This work aims to quantify and explain retention of an oil droplet rising through a transition between two homogeneous-density fluids. Our laboratory experiments analyzed the rising motion of single droplets penetrating a finite stratification between salt-water and freshwater. We compared droplet behavior for a range of drop densities, drop sizes, and ambient stratification profiles. We observed that droplets experienced significant slowdown as it passed through the stratification. We characterized the droplet slowdown by delineating two droplet motion timescales: entrainment time, the span of time droplet velocity was less than the upper layer terminal velocity, and retention time, the span of time the droplet was retained in the transition layer. We observed a strong relationship between the two metrics, where retention time is a function of the length of time that dense fluid is entrained and the magnitude of the droplet's slowdown. [Preview Abstract] |
Monday, November 25, 2019 2:24PM - 2:37PM |
L39.00004: Retention of oil droplets rising in a stratified fluid: Part 2. Dynamics Tracy Mandel, De Zhen Zhou, Lindsay Waldrop, Maxime Theillard, Dustin Kleckner, Shilpa Khatri During the Deepwater Horizon oil spill in 2010, about 5 million barrels of petroleum were discharged from the Macondo Well into the Gulf of Mexico. Oceanographic studies (McNutt, 2012) have estimated that approximately 40 percent of that oil was trapped beneath the ocean surface, primarily in regions with strong oceanic density gradients. The present work aims to quantify and explain retention of an oil droplet rising through a transition between two homogeneous-density fluids. Using laboratory experiments, we examined droplet behavior for a range of drop densities, drop sizes, and ambient stratification profiles. A droplet is significantly slowed by its interaction with the ambient stratification over a characteristic timescale, which coincides with the decay of the trailing column of entrained fluid. These dynamics are independent of the far-field nature of the dropletâ€™s wake. This timescale over which fluid is entrained is found to be dependent on the drop Froude number. Significant retention only occurred for $Fr < 1$, suggesting that retention is primarily a function of the ratio of the buoyancy timescale ($1/N$) to the drop motion timescale ($d/U$), and that trapping dynamics are dominated by the effects of stratification. [Preview Abstract] |
Monday, November 25, 2019 2:37PM - 2:50PM |
L39.00005: Cluster formation and self-assembly in stratified fluids: a novel mechanism for particulate aggregation Richard McLaughlin, Roberto Camassa, Daniel Harris, Robert Hunt, Zeliha Kilic We experimentally observe and mathematically model a new fundamental attractive mechanism we have found in our laboratory by which particles suspended within stratification may self-assemble and form large aggregates without need for short range binding effects (adhesion). This phenomenon arises through a complex interplay involving solute diffusion, impermeable boundaries, and the geometry of the aggregate, which produces toroidal flows. We show that these flows yield attractive horizontal forces between particles. The collective motion we observed experimentally appears to solve jigsaw-like puzzles on its way to organizing into a large scale disc-like shape, with the effective force increasing as the collective disc radius grows. Control experiments with two objects (spheres and oblate spheroids) isolate the individual dynamics, which are quantitatively predicted through numerical integration of the underlying equations of motion. With this two-body information, we present simulations with hundreds of spheres which reproduce many of the features of our self-assembly experiments. [Preview Abstract] |
Monday, November 25, 2019 2:50PM - 3:03PM |
L39.00006: Cluster formation and self-assembly in stratified fluids: Particle imaging velocimetry and modified Stokesian dynamics Robert Hunt, Roberto Camassa, Daniel Harris, Zeliha Kilic, Richard McLaughlin We report on a new fundamental attractive mechanism we have found at the UNC Joint Fluids laboratory by which particles suspended within stratification may self-assemble and form large aggregates without need for short range binding effects (adhesion). This phenomenon arises through a complex interplay involving solute diffusion, impermeable boundaries, and the geometry of the aggregate, which produces nontrivial fluid flows. Numerical simulations are directly compared with particle imaging velocimetry for a single oblate spheroid and are shown to agree both qualitatively and quantitatively with PIV data. Numerical simulations with two spheres at an array of fixed separation distances allow for the calculation of an effective force between pairs of particles. With this two-body information, we extend to multiple bodies by modifying a Stokesian dynamics solver to include these forces. Simulations with hundreds of spheres reproduce many of the features of our self-assembly experiment. [Preview Abstract] |
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