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 ZC28: Geophysical Fluid Dynamics: Rotating Flows, Mesoscale Dynamics, Transport and Mixing |
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Chair: Qi Zhou, University of Calgary Room: 251 F |
Tuesday, November 26, 2024 12:50PM - 1:03PM |
ZC28.00001: Transport By Oceanic Mesoscale Eddies Mehrnoush Kharghani, Benjamin A Storer, Shikhar Rai, Hussein Aluie Mass transport in the ocean plays a crucial role in regulating Earth’s climate and natural marine resources. |
Tuesday, November 26, 2024 1:03PM - 1:16PM |
ZC28.00002: Diffusiophoretic dispersion of microplastics in oceanic surface flows Mobin Alipour, Amir A Pahlavan Microplastic spreading in the oceans poses a significant global biogeochemical threat. Understanding its transport pathways is therefore of essence in our efforts to predict and mitigate this problem. Before these particles can reach the deep oceanic layers through sedimentation and vertical mixing, surface currents can transport them over vast distances. Through table-top experiments and numerical simulations, designed to mimic the nearly two-dimensional conditions of large-scale oceanic flows, we demonstrate that even slight salinity gradients, which are prevalent in the oceans, can profoundly influence the dispersion of particles in both ordered and chaotic flows. These solute gradients lead to diffusiophoretic migration of particles across the flow streamlines, resulting in either enhanced dispersion or trapping. Our findings suggest that the interaction between flow and salinity gradients can play a crucial role in modulating the dispersion of contaminants and microplastics in marine environments. |
Tuesday, November 26, 2024 1:16PM - 1:29PM |
ZC28.00003: ABSTRACT WITHDRAWN
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Tuesday, November 26, 2024 1:29PM - 1:42PM |
ZC28.00004: Abstract Withdrawn
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Tuesday, November 26, 2024 1:42PM - 1:55PM |
ZC28.00005: Rotating convection in a dimpled sphere Tobias Oliver, Eric G Blackman, John A Tarduno, Michael A Calkins Rotating convection in a spherical geometry is commonly used as a model for understanding geophysical and astrophysical fluid systems. However, real systems likely deviate from spherical symmetry. The boundary between Earth’s liquid core and mantle, for instance, is thought to be characterized by inverted mountains that protrude into the core. This non-axisymmetric topography dynamically couples the core and mantle and leads to angular momentum transport between the two systems. Here, we study rotating convection in a spherical shell with a small deformation on the outer boundary—a dimple—which breaks the axial symmetry about the rotation axis. A suite of numerical simulations are used to investigate the influence of topography on the convective dynamics, as well as the scaling behavior of the pressure torques. The simulations show that the torque scales linearly with the topographic amplitude, which can be explained by simple scaling arguments. When extrapolated to the conditions of the outer core, these results suggest that topographic coupling at the core mantle boundary is likely important in explaining the observed variations in Earth’s length of day.
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Tuesday, November 26, 2024 1:55PM - 2:08PM |
ZC28.00006: Direct numerical simulations of rotating Rayleigh-Taylor instability under the influence of magnetic fields. Narinder Singh, Anikesh Pal The combined effects of the imposed vertical mean magnetic field (B0) and rotation on the heat transfer phenomenon driven by the Rayleigh-Taylor (RT) instability are investigated using DNS. In the hydrodynamic (HD) case (B0 = 0), as the rotation rate f increases from 4 to 8, the Coriolis force suppresses the growth of the mixing layer height (h) and the vertical velocity fluctuations (u′3), leading to a reduction in the heat transport, characterized by the Nusselt number (Nu). In non-rotating magnetohydrodynamic (MHD) cases, we find a significant delay in the onset of RT instability with increasing B0 (= 0.1, 0.15, 0.3), consistent with the linear theory in the literature. The imposed B0 forms vertically elongated thermal plumes that exhibit larger u′3 and efficiently transport heat between the bottom hot fluid and the upper cold fluid with limited horizontal mixing. Therefore, due to higher u′3, we observe an enhancement in heat transfer in the initial regime of unbroken elongated plumes in non-rotating MHD cases compared to the corresponding HD case. In the mixing regime of broken small-scale structures, the flow is collimated along the vertical magnetic field lines due to the imposed B0, resulting in a decrease in u′3 and an increase in the growth of h compared to the non-rotating HD case. This increase in h enhances heat transfer in the mixing regime of non-rotating MHD over the corresponding HD case. When rotation is added along with the imposed B0, the growth and breakdown of vertically elongated plumes are inhibited by the Coriolis force, reducing h and u′3. Consequently, heat transfer is also reduced in the rotating MHD cases compared to the corresponding non-rotating MHD cases. Interestingly, the heat transfer in the rotating MHD cases remains higher than in corresponding rotating HD cases due to the vertical stretching and collimation of flow structures along the vertical magnetic field lines. This also suggests that the mean magnetic field mitigates the instability- suppressing effect of the Coriolis force. The turbulent kinetic energy budget reveals the conversion of the turbulent kinetic energy, generated by the buoyancy flux, into turbulent magnetic energy. |
Tuesday, November 26, 2024 2:08PM - 2:21PM |
ZC28.00007: Rotating spherical convection with strong zonal flows Michael A Calkins, Justin Nicoski We analyze the results of direct numerical simulations of rotating convection in spherical shell geometries with stress-free boundary conditions, which develop strong zonal flows. Both the Ekman number and the Rayleigh number are varied. We find that the asymptotic theory for rapidly rotating convection can be used to predict the Ekman number dependence of each term in the governing equations, along with the convective flow speeds and the dominant length scales. Using a balance between the Reynolds stress and the viscous stress, together with the asymptotic scaling for the convective velocity, we derive an asymptotic prediction for the scaling behavior of the zonal flow with respect to the Ekman number, which is supported by the numerical simulations. We do not find evidence of distinct asymptotic scalings for the buoyancy and viscous forces and, in agreement with previous results from asymptotic plane layer models, we find that the ratio of the viscous force to the buoyancy force increases with Rayleigh number. Thus, viscosity remains non-negligible and we do not observe a trend towards a diffusion-free scaling behavior within the rapidly rotating regime. |
Tuesday, November 26, 2024 2:21PM - 2:34PM |
ZC28.00008: Characterization of magnetostrophic liquid metal convection Tao Liu, Yufan Xu, Jonathan M Aurnou In magnetostrophic convection, the fluid dynamics are dominated by the Elsasser number, $\Lambda$, being of order 1, which describes the ratio of the Lorentz and Coriolis forces. Linear theory predicts magnetostrophic $\Lambda \approx 1$ convection will develop via nearly system-scale modes, leading geophysicists to argue that these modes exist in planetary cores and generate their large-scale dynamo fields. To date, however, magnetostrophic convective modes have yet to be unambiguously identified in laboratory experiments, nor have they been clearly found in liquid metal ($Pm < Pr < 1$) planetary dynamo simulations. Thus, the characterization of magnetostrophic modes is crucial for understanding how dynamo processes operate in deep planetary interiors. To detect and characterize the magnetostrophic regime, we use ultrasonic doppler velocimetry (UDV) and multi-point thermometry to measure the velocities ($Re$) and the global heat transfer efficiency ($Nu$) in a cylindrical rotating magnetoconvection experiment of aspect ratio $\Gamma = D/H=2$ that is filled with liquid gallium ($Pr = 0.027; \ Pm \simeq 10^{-6}$). We will present magnetostrophic experimental results in a range of Rayleigh numbers $10^5 < Ra < 10^7$, and Ekman numbers $5\times 10^{-5} < Ek < 10^{-4}$. We will present vertical and horizontal chord velocity measurements, in which we are seeking to detect the magnetostrophic modes and separate them out from the oscillatory and geostrophic modes that all co-exist in these parameter ranges. |
Tuesday, November 26, 2024 2:34PM - 2:47PM |
ZC28.00009: Laboratory Experiments on Rigid Particle Accumulation in an Ekman-driven Can Flow Michael Dotzel, Claudia Cenedese, Jim McElwaine, Irina Rypina, Lawrence Pratt In the Maxey-Riley regime, rigid particles have the propensity for accumulation around certain flow features. Recently, Rypina et al 2024 investigate this phenomenon for small buoyant spherical particles inside an analytical phenomenological analogue of a three-dimensional, Ekman-driven eddy flow, and observed particle accumulation around limit cycles close to the corresponding periodic orbits of fluid parcels. They also present simple theoretical arguments based on vorticity—strain considerations for why this attraction occurs. Motivated by an interest in observing this accumulation phenomenon firsthand, we conducted laboratory experiments seeding buoyant spherical particles within a cylindrical tank full of glycerin forced by a rotating lid. Experiments were carried out in an axially symmetric cylinder flow, as well as in a non-symmetric configuration with tilted lid. Preliminary findings indicate the existence of a central region where particles are retained and accumulating within for over a week, and an outer region spanning the periphery and central axis that becomes rapidly devoid of particles. We compare our observations with a numerical model of the can flow, and provide estimates of the particle accumulation rate within the can interior. |
Tuesday, November 26, 2024 2:47PM - 3:00PM |
ZC28.00010: Deformation and breakdown of vortex structure due to rotational-hyperbolic instability of 2D Taylor-Green vortices in rotating fluids Naoya Ueno, Makoto Hirota, Yuji Hattori To understand planetary-scale vortex dynamics, instabilities in rotating and stratified fluids are important. This study focuses on the effects of rotation on 2D Taylor-Green vortices (TGVs) which are periodic vortical cells with stagnation points at the corners. The objective is to elucidate changes and breakdown processes of this vortex structure due to nonlinear development of unstable modes influenced by rotation. Direct numerical simulations are performed using the Fourier spectral method, where one of the rotational-hyperbolic (RH) modes is selected as an initial disturbance. The RH modes are obtained by linear stability analysis (Hattori & Hirota, JFM, 2023), which mainly occur in the vortical cells where the flow is rotating in the same direction as the system (cyclonic cells). These RH modes can be classified by the radial and azimuthal mode numbers (n,m) around the vortex axis. When an m=1 mode grows, TGVs break down destroying the cell structure and significantly weakened TGVs remain eventually. When an m=2 mode grows, the cell structure is preserved and each vortex in the cyclonic cells is deformed into four vortex tubes. Increasing the rotation speed tends to reduce the loss of vortex energy caused by the RH modes, indicating the stabilizing effect of rotation. |
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