Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session T28: Geophysical Fluid Dynamics: Stratified Flows III |
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Chair: Adam Jiankang Yang, Yale University Room: 251 F |
Monday, November 25, 2024 4:45PM - 4:58PM |
T28.00001: Reflection of internal wave beams from rough surfaces Bruce E Rodenborn, Olivia Carr Roach, Michael Allshouse Internal wave beams do not propagate long distances in the ocean, which has been attributed to nonuniform background density and other mechanisms. However, beams also reflect from solid and free surfaces shortly after generation, which reduces the energy in the reflected beams and may be an important mechanism in scattering this tidal energy. We use low Reynolds number tank experiments and numerical simulations to understand how the reflection coefficient: the ratio of the outgoing energy flux to the incoming energy flux through a surface near the reflection region, is affected by properties of the surface from which it reflects. We measure the velocity field using particle image velocimetry and determine the energy flux using the work of Lee et al. (Phys. Fluids, 26, 2014). The wave beams are separated using the Hilbert transform method of Mercier et al. (Phys. Fluids, 20, 2008) to calculate the contributions from the incoming wave beam and any harmonic waves. We find that sinusoidal topography with wavelengths less than the beam's dominant wavelength significantly reduces the reflection coefficient. We also find wave energy reflected backwards toward the source from sinusoidal topography. Our tank experiments are scaled to higher Reynolds numbers in matched numerical simulations. The simulations also show reduced reflection coefficients when the surface is roughened even when viscosity is reduced by an order of magnitude. |
Monday, November 25, 2024 4:58PM - 5:11PM |
T28.00002: Interaction between in-line spheroids settling in a linearly stratified fluid Seungwon Shin, Abdullah M Abdal, Lyes Kahouadji, Damir Juric, Jalel Chergui, Colm-Cille P Caulfield, Omar K Matar This study explores the transport of solid particles in density-stratified fluids. In oceans, particles and marine snow fall through fluids with notable density differences caused by variations in salinity and temperature. Such heterogeneity in the background fluid influences the settling or rising rates of particles, often causing them to accumulate at transitional density layers. Previous research has mainly examined spherical particles, focusing on their isolated motion, pairwise interactions, and collective transport in stratified fluids. This research, however, extends the investigation to the interaction between two spheroidal particles settling in-line in a stratified fluid.The study employs an immersed-boundary technique to conduct particle-resolvednumerical simulations in a 3D Cartesian domain. The spheroidal particles, initially at rest and vertically aligned, are placed in a quiescent, linearly-stratified fluid within a 12Dp×12Dp×24Dp computational domain, with Dp=250 μm as the equivalent spherical particle diameter. The shape of the spheroids is determined by their aspect ratio A/R=b/aa nd they are modeled using a fictitious domain method with a no-slip condition on their surfaces. The study is governed by the Galilei, Froude, and Schmidt numbers. The results demonstrate the effects of varying the stratification strength through the Froude number, the particles' aspect ratios, and the initial separation distance between the particles on the interaction dynamics between the settling spheroids.This work further considers the effect of smaller Schmidt numbers, relevant for thermally stratified fluids, and of increasing the particle size to investigate the effects of particle inertia on the settling and interaction dynamics. |
Monday, November 25, 2024 5:11PM - 5:24PM |
T28.00003: Progress towards the Simulation of Very High Reynolds Number Stratified Wakes Ioannis Pyrovolakis, Greg N Thomsen, Nidia C Reyes-Gil, Peter J Diamessis We provide an update on our efforts of pursuing implicit Large Eddy Simulations of stratified sphere wakes at body-based Reynolds numbers of Re = O(106). Such a value of Re enables a sufficiently long window of operation of the highly energetic and relatively unexplored Strongly Stratified Regime which can extend as far as Nt ≈ 500. A process-resolving simulation of this regime, which captures as wide as possible a fraction of the turbulence dynamic range, is highly costly as it requires 35 billion grid points. To this end, we leverage a high-accuracy, modal Spectral Element and Fourier-Galerkin code, efficiently implemented on 8,000 core son DoD-HPC platforms. In the first part of this presentation, we will show our first results from a stratified wake run at Re = 1.6 x 106 and internal, body-based, Froude number of Fr = 4. |
Monday, November 25, 2024 5:24PM - 5:37PM |
T28.00004: Inclined 6:1 prolate spheroid wake characteristics in a stratified background Madeleine Oliver, Geoffrey R Spedding Initially-turbulent wakes of bluff and streamlined bodies have been studied and simulated in a stratified ambient, and there are conflicting findings on the possible power-law evolution of early- and late-wake profiles. Moreover, when bodies travel at incidence, breaking the axisymmetry, there are a number of ways in which non-general properties might be found. |
Monday, November 25, 2024 5:37PM - 5:50PM |
T28.00005: Stratified wakes of spheroids at moderate pitch angle: influence of Reynolds number Sanidhya Jain, Sheel Nidhan, Sutanu Sarkar With rapid developments in the technology of aerial vehicle and under-sea exploration, the study of wakes in density-stratified fluids has become crucial for understanding vehicle dynamics and their environmental impact. We report results from LES of a 6:1 prolate spheroid placed at a moderate pitch, α = 10°. Cases at different stratification levels (Froude number, Fr = U/ND = 1.9,6,∞) are considered. Reynolds number (Re = UD/ν) of 5000 and 15000, both with laminar flow separation to form a counter-rotating streamwise vortex pair, but different wake Re are studied. |
Monday, November 25, 2024 5:50PM - 6:03PM |
T28.00006: Particle attraction to walls by diffusion induced stratified flows Richard M McLaughlin, Tyler J Britt, Roberto Camassa, Saiful I Tamim We present experiments and theoretical predictions for a new phenomena in which particulate suspended in stable stratification are attracted to vertical walls. The mechanism originates from a broken symmetry in the diffusion induced stratified flow exterior to a sphere near a vertical wall which creates an effective force of attraction arising through the viscous stress tensor. This boundary layer behavior drives particles initially within one radius of the wall to collapse in finite time. Comparisons between experiments and theory will be discussed and further contrasted with prior self-assembly phenomena which documented two spheres collapsing in finite time in a stratified water column (https://www.nature.com/articles/s41467-019-13643-y). |
Monday, November 25, 2024 6:03PM - 6:16PM |
T28.00007: Settling and dispersion of Lagrangian particles in the presence of stratified Kelvin-Helmholtz instability and turbulence Adam Jiankang Yang, Mary-Louise Timmermans, Mona Rahmani This study investigates the influence of the Kelvin-Helmholtz (KH) instability on particle settling and dispersion in stratified fluid environments, addressing gaps in our understanding of how these instabilities alter particle settling and dispersion compared to classic Stokes settling in a quiescent environment. We conducted direct numerical simulations to analyze particle motion in a Lagrangian framework, focusing on the effects of the KH instability on particles ranging in size from 20-100 micrometers in diameter. Our results show that the settling of larger particles is slower than pure Stokes settling because particles become trapped within KH billows; this effect diminishes for particles larger than about 80 micrometers. Conversely, small particles (20 micrometers) exhibit enhanced settling in the presence of KH billows, with velocities up to seven times greater than Stokes settling. These smaller particles are preferentially trapped in the downward-moving portion of a billow, which dominates their vertical motion. Additionally, we observed a significant increase in spanwise dispersion following the breakdown of a KH billow to turbulence, while vertical dispersion increases immediately upon KH billow formation and decreases with increasing particle size. This research offers new insights into the interaction between the KH instability and particle dynamics, highlighting the significant role of the instability in altering sediment transport in stratified fluids. |
Monday, November 25, 2024 6:16PM - 6:29PM |
T28.00008: Simulations of settling marine aggregates in a stratified fluid Eunji Yoo, Shilpa Khatri, Francois Blanchette Settling marine aggregates is essential in transporting dissolved carbon dioxide from the ocean surface to the deep sea. While sinking, they accumulate in thin layers where density stratifications are present, becoming nutrient hotspots for bacterial and animal activity. Here, we simulate settling aggregates in a density-stratified fluid. We assemble fractal aggregates as a collection of cubes to model a marine aggregate. In the absence of stratification, the flow around the aggregate is computed at a limit of zero Reynolds number using a boundary integral method. A term involving a volume integral is added to the boundary integral formulation to allow variable density in the ambient fluid. We couple the velocity with the advection-diffusion equation to track the density over time. We use this method to quantify how stratification affects the aggregate settling speed and residence time in a sharp stratification. |
Monday, November 25, 2024 6:29PM - 6:42PM |
T28.00009: Marangoni effects on oil droplets rising in a stratified fluid De Zhen Zhou, Dustin P Kleckner, Shilpa Khatri There are many environmental examples of droplets and particles getting trapped in regions of high density gradients, including oil plumes in the ocean and smoke stack exhaust in thermal inversions. In the former case, the gradients are due to varying salt and temperature concentrations, and subsurface trapping happens despite the fact that oil is less dense than all salt water layers. Although this trapping is usually attributed to entrained fluid, interfacial tension gradients are also present. The resulting Marangoni force from the interfacial tension gradient generates additional flows that may influence droplet trapping, but this effect has received little experimental attention until recently. When a droplet of lower density rises through a stratified ambient fluid, it often gets trapped in regions with a high density gradient. We present experimental results that quantify the effect of interfacial tension gradients on droplet slowdown in stratified fluids at the intermediate Reynolds regime and low Froude number regime. |
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