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 C01: Interact: Geophysical and Astrophysical Fluids |
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Chair: Steven Tobias, University of Leeds Room: Ballroom C |
Sunday, November 24, 2024 10:50AM - 11:20AM |
C01.00001: INTERACT FLASH TALKS - Geophysical and Astrophysical Fluids. Each Interact Flash Talk will last around 1 minute, followed by around 30 seconds of transition time. |
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C01.00002: Geophysical Fluids on Logarithmic Lattices Steven Tobias, Curtis J Saxton, Rich R Kerswell, Keaton J Burns Fluid dynamics for geophysics and astrophysics requires the modelling of nonlinear process over a vast range of spatial and temporal scales. Current (and future!) computational methods are not capable of modelling the range of scales required, so some compromises are required. In this talk I will describe using logarithmic lattices for two problems of geophysical interest, Rayleigh-Benard convection and double-diffusive convection in the fingering regime. |
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C01.00003: Large scale magnetic field generation via inertial wave dynamo Emma Kaufman, Daniel Lecoanet, Michael Le Bars, Benjamin Favier Measurements from the Galileo probe indicate a large scale magnetic field of 750 nT on Ganymede, one of Jupiter's moons. This magnetic field far exceeds the strength which could be generated by a dynamo driven by compositional and thermal convection, such as that which generates the Earth's magnetic field. This work investigates the feasibility of an inertial wave dynamo in generating large scale magnetic field in such a body. In this framework, inertial waves, those whose restoring force is the Coriolis force, are responsible for generating a large scale magnetic field providing only waves propagating in one direction are present. We utilize a novel 2D eigenvalue solver to determine the growth rate of the magnetic energy in such a system, and compare to predictions given by asymptotic theory in Moffatt (1970). |
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C01.00004: Transport by mean flows in stellar radiative zones Lydia Korre, Nicholas A Featherstone The penetration of meridional flows below the base of the convection zone of solar-type stars may play a significant role in the transport of chemical species, angular momentum and magnetic fields within their stable radiative zones. We systematically study these large-scale flows by performing a series of three-dimensional numerical simulations in a rotating spherical shell that consists of an outer convective region that lies on top of a stable zone. We vary the number of density scale heights in the convection zone, the degree of convective driving and the rotation rate, and investigate the dynamical balances and angular momentum transport established throughout the convection zone and the radiative interior within different parameter regimes. When operating in the solar-like regime, where the Eddington–Sweet timescale tES is shorter than the viscous timescale tν, as measured by the parameter σ = (tES/tν)0.5, we find that the mean flows can propagate large distances beyond the inner convective boundary into the radiative interior. We present scaling laws of the penetration depth of these mean flows below the base of the convection zone with respect to σ. Our findings may shed new light on the role that the meridional flows play in different dynamical processes occurring within stellar interiors. |
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C01.00005: Automatic Differentiation and Adjoint Looping for Geophysical Flows in Dedalus Keaton J Burns, Calum S Skene The adjoint state method is used to perform PDE-based optimization across geophysical fluid dynamics, including parameter fitting and nonlinear stability analyses for many types of flows. Classical adjoint methods require manually writing a model's adjoint equations, which becomes cumbersome and error-prone for complex models. Reverse-mode automatic differentiation performs the same computations at the compiler level and has become a mainstay of differential programming for machine learning. Here, we present a high-level automatic differentiation system that automatically computes the discrete adjoints of general spectral PDE models in the Dedalus framework. This system leverages the symbolic equation representation in Dedalus, inherently supports MPI parallelization, and does not require users to rewrite their code for a differentiable compiler. We will briefly review the implementation of the system and illustrate its capabilities with various geophysical applications, including transient growth in kinematic dynamos and optimal mixing in stratified shear flows. |
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C01.00006: Manipulating Geophysical Lagrangian Coherent Structures: Theory and Experiment Lei Fang, Xinyu Si Geophysical Lagrangian Coherent Structures (LCSs) act as transport barriers in geophysical flows, hindering proper mixing in coastal areas and potentially leading to ocean forbidden zones. Disrupting or partially breaking these LCSs in coastal regions could alleviate these forbidden zones and improve the health of coastal ecosystems. However, the significant amount of power driving geophysical flows raises key questions about the feasibility of manipulating kilometer-scale LCSs in an energetically viable manner. |
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C01.00007: Volcanic fissure localisation: Thermoviscous focussing in non-uniform geometries Jesse Taylor-West, Edward W Llewellin Volcanic fissure eruptions, which occur when a magma-filled crack intersects the surface, typically evolve from a continuous curtain of lava to one or more point-source lava fountains at distinct vents. One proposed mechanism for localisation is via a thermoviscous fingering instability, in which hot, low-viscosity magma forms finger-like preferential pathways through cooler, higher-viscosity magma, analogously to the classical Saffman-Taylor instability. Previous work has considered the flow of hot fluid driven by an imposed pressure drop through a planar fissure with walls held at a constant, colder temperature. |
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C01.00008: Developing a numerical framework for high-fidelity simulation of contrails: sensitivity analysis for hydrogen contrails Tânia Ferreira, Juan J Alonso, Catherine Gorle Contrails are a non-CO2 effect with an uncertain but high radiative forcing impact. |
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C01.00009: Marine atmospheric boundary layer response to dynamic wave model in Large Eddy Simulation Hannah Hata H Williams, Aditya K Aiyer, Luc Deike, Michael E Mueller The development of accurate and computationally efficient approaches for representing the interactions of wind and ocean waves is critical to the simulation of offshore wind farms to predict power production given real environmental conditions. In this work, neutral atmospheric boundary layers are generated using Large Eddy Simulation with a Dynamic Wave Spectrum Drag Model (Dyn-WaSp) that represents a full spectrum of waves via a drag force, the magnitude of which is solved for at each location given the phase of each constituent wave mode and the local wind speed. The Dyn-WaSp model is a wall model that separately accounts for resolved wind-waves, swell modes, and subfilter waves; it is horizontally resolved and vertically unresolved, which preserves phase dynamics without incurring significant computational cost. First- and second-order characteristics of neutral marine atmospheric boundary layers generated with the inclusion of different combinations of wave modes are examined and compared to other data such as those collected in field campaigns or generated by static roughness wave models to understand how the relative magnitude of resolved wind-waves, swell modes, and subfilter waves influence predictions. |
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C01.00010: How do oceanic currents influence the dynamics of ice floes? Luke Morad, Mobin Alipour, Amir A Pahlavan Ice floes are floating sea ice pieces with a wide range of length-scales (m-km) and shapes. They play an important role in the global climate as they modulate the sunlight energy absorbed by the oceans. Understanding the dynamics of ice floes and their coupling to the atmospheric and oceanic flows is therefore an essential ingredient in the global climate forecast. Here, we use a table-top experimental setup to study the dynamics of arbitrary-shaped objects atop a forced quasi-2D flow. Our observations show that the main features controlling the dynamics of these objects are their size relative to the mean flow eddy size and their shape. We finally discuss the implications of our observations for the dynamics of ice floes. |
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C01.00011: Global Ocean Cascades and a Direct Gyrescale-Mesoscale Energy Pathway Induced by the Atmosphere Benjamin A Storer, Michele Buzzicotti, Hemant Khatri, Stephen Griffies, Hussein Aluie Our understanding of the ocean's spatial scales and their coupling has been derived mostly from Fourier analysis in small "representative" regions, typically a few hundred kilometers in size, that cannot capture the vast dynamic range at planetary scales. Using coarse-graining on a 1/12-degree reanalysis dataset, we probe a range of spatial scales spanning more than three orders of magnitude, including mesoscales and planetary scales, and quantify both the kinetic energy (KE) spectral density and the across-scale KE transfer. Since coarse-graining preserves the temporal signal, we are able to observe in the oceanic mesoscale, a steady propagation of energy to larger scales across the seasonal cycle, indicative of a scale-local energy cascade. This 'spectral advection' signal presents a characteristic lag time of ~27 days per octave, and is present in both the KE spectra and the KE transfer. Combined with a phase-lag between the KE spectra and KE transfer signals, this suggests that energy transferred across scale L is primarily deposited at scales 4×L. We also find that the highly energetic mesoscales have a direct transfer with gyrescales (> 1000km), which is induced by the atmospheric circulation cells (Hadley, Ferrel, Polar). This gyrescale-mesoscale exchange is new and absent from standard theories of gyre circulation. |
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C01.00012: Multiscale mixing dynamics on the California Inner Shelf Sam Lewin, Alexis K Kaminski, Jack McSweeney, Amy F Waterhouse The inner shelf (the transition between the surf zone and midshelf) is home to a wide range of physical processes, including surface and internal waves, boundary layers, submesoscale features, and vortices arising from interactions with complex coastal topography. The interplay between these different processes sets the circulation, stratification, and transport across this region, leading to complex dynamics over a variety of length and time scales. Here, using data from the Inner Shelf Dynamics Experiment field campaign (Waterhouse et al., 2020), we explore the details of the turbulent mixing off the coast of central California over an intensive sampling period in September 2017. Using microstructure turbulence measurements, we show that mixing coefficients are well-predicted by the ratio of the Thorpe and Ozmidov scale and that the overall mixing is dominated by a small number of intense events. By looking at detailed timeseries, we find that these events are associated with elevated dissipation at the front of shoaling internal bores, with different bore structures arising over the course of the observational period. We finally compare our results with numerical studies of nonlinear internal waves and gravity currents propagating in stratified ambients. |
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C01.00013: Investigating the Stability of Icebergs Olivia Carr Roach, Jim McElwaine, Claudia Cenedese, Alan Condron Small icebergs roll and bob as they float in the ocean like damped harmonic oscillators. We present the results of experiments using low-density polyethylene cylinders as surrogate icebergs and use steel rods to represent a sediment layers within them. Their motion, translation and rotation, is measured at high frequency using a MEMS sensor. We show that the motion can be understood by calculating the potential energy of the system as a function of the iceberg’s depth and orientation. The water gives rise to an added mass term and damping by the radiation of surface waves, which can be calculated using the far-field expansion of a Greens function. We show good agreement between this model and the data. The importance of this result is that if waves of the right frequency are incoming, they may induce resonance and then even relatively small waves can give rise to large amplitudes oscillations and cause icebergs to capsize if they are not globally stable. |
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C01.00014: Near-Resonant Triadic Instability of Internal Waves in Non-uniform Stratification Bruce R Sutherland, Akash Kav, Dheeraj Varma, Corentin Pacary, Sylvain Joubaud One proposed mechanism for the transfer of energy from large-scale internal tides to small dissipative scales is through triadic resonant instability, whereby a pair of sibling waves interact with a parent internal tide so that they grow in amplitude, extracting energy from the parent. In uniform stratification the sibling waves can grow in pure resonance such that their frequencies and both horizontal and vertical wavenumbers add or subtract to those of the parent. However, in realistic ocean stratification, a low vertical mode internal tide does not have sinusoidal vertical structure. So the sibling waves can only be in near resonance with the parent. In this work we develop a theory for the growth of sibling waves in near resonance with a vertical mode-1 parent wave, testing its predictions against fully nonlinear numerical simulations and laboratory experiments in which the background buoyancy frequency decreases linearly with depth. Theory, simulations and experiments show that near-resonance can occur if the parent wave has frequency close to the maximum buoyancy frequency, N0. However, the maximum growth rate is negligibly small if the parent wave has frequency less than approximately 0.5 N0. |
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C01.00015: Rotational-hyperbolic instability and ring-type elliptic instability: new unstable modes of rotating stratified fluids Yuji Hattori, Makoto Hirota Periodic arrays of vortices are often observed in geophysical and astrophysical fluids, in which stratification and rotation effects are important. The linear stability of the two-dimensional (2D) Taylor-Green vortices and the Stuart vortices is studied by local and modal stability analysis. These two base flows are characterized by the existence of the hyperbolic stagnation points as well as the elliptic stagnation points. The rotational-hyperbolic (RH) instability mode has been found for the 2D Taylor-Green vortices under strong rotation. This instability is caused by the resonance due to the inertial-gravity waves. The ring-type elliptic instability mode has been found for the Stuart vortices. It occurs when strong rotation and anticyclonic rotation stabilize the elliptic instability near the vortex center selectively. Both the RH instability and the ring-type elliptic instability become dominant depending on the magnitude of rotation and stratification as shown in our recent papers (Hattori and Hirota, J. Fluid Mech. 967 (2023) A32; Hattori and Hirota, J. Fluid Mech. 987 (2024) A12). |
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C01.00016: Regional Quasi-Geostrophic Convection on the Tilted f-Plane Benjamin Miquel, Abram C Ellison, Keith A Julien, Michael A Calkins, Edgar Knobloch In planetary interiors, stellar interiors, and icy moons' underground oceans, turbulent convection is heavily constrained by rotation, which promotes anisotropy along its axis (the Taylor-Proudman theorem) and the formation of large scale structures via an inverse energy cascade mechanism. |
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C01.00017: Simulating rotating convection at very low Ekman number Adrian van Kan, Keith A Julien, Benjamin Miquel, Edgar Knobloch Geophysical and astrophysical fluid flows are typically buoyantly driven and are strongly constrained by planetary rotation at large scales. Rapidly rotating Rayleigh-Bénard convection (RRRBC) provides a paradigm for direct numerical simulations (DNS) and laboratory studies of such flows, but the accessible parameter space remains restricted to moderately fast rotation (Ekman numbers Ek ≳ 10-8), while realistic Ek for astro-/geophysical applications are significantly smaller. Reduced equations of motion, the non-hydrostatic quasi-geostrophic equations describing the leading-order behavior in the limit of rapid rotation (Ek → 0) cannot capture finite rotation effects, leaving the physically most relevant part of parameter space with small but finite Ek currently inaccessible. Here, we introduce the rescaled incompressible Navier-Stokes equations (RiNSE) – a reformulation of the Boussinesq-Navier-Stokes equations informed by the scalings valid for Ek → 0. We provide the first full DNS of RRRBC at unprecedented rotation strengths down to Ek = 10-15 and below, showing that the RiNSE converge statistically to the asymptotically reduced equations. |
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C01.00018: Turbidity Currents Containing Mixtures of Sand and Mud Alexandre Dillon Leonelli, Eckart Heinz Meiburg, S Balachandar, Perry L Johnson We conduct particle resolved direct numerical simulations of turbidity currents propagating over erodible substrates. The sediment in turbidity currents has a wide range of grain sizes, from mud, Ο(10μm), to sand of Ο(1mm) and larger. To address this multi-scale nature we employ a two-level modeling approach in which the much larger sand grains are resolved with the immersed boundary method while the finer sediment is treated using an Eulerian framework. We investigate four classes of substrate and sediment mobility; smooth wall, fixed bed, bed-load transport, and suspended-load transport. We present a detailed analysis on the effects of bed mobility, porosity, and particle-turbulence coupling on the propagation velocity, structure, and turbulence properties of turbidity currents. Finally, we demonstrate a non-monotonic increase in the friction factor as a function of bed mobility. |
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C01.00019: Turbulent Lengthscales in Asymmetric Shear Instabilities Subject to Subharmonic Vortex Pairing Shuai Meng, Alexis K Kaminski Shear-driven mixing in the upper ocean can be modeled using different varieties of stratified shear instabilities. These mixing events can be described in terms of various key turbulent lengthscales, which characterize the physical state and properties of the turbulent flow. However, there has been a relative lack of work exploring how the relationships between such lengthscales evolve for asymmetric shear instabilities, both with and without subharmonic vortex pairing. To explore this, we use direct numerical simulations of flows subject to either asymmetric Kelvin-Helmholtz or Holmboe instability, where the shear and density interfaces are vertically offset. We examine the evolution of three major lengthscales: Thorpe, Ozmidov, and Corrsin scales in both single- and double-wavelength domains. Our findings show that pairing induces a relatively sustained large overturn, indicated by the Thorpe scale, and significantly modifies the temporal evolution of the Ozmidov and Corrsin scales. We also investigate how asymmetry and pairing influence other turbulent energy and mixing quantities related to these lengthscales. |
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C01.00020: Zigzag instability of columnar Taylor-Green vortices in a strongly stratified fluid Qi Zhou, Junwei Guo, John R Taylor We investigate the dynamics of a columnar Taylor-Green vortex array under strong stratification, focusing on Froude numbers from 0.125 to 1.0, to identify and understand the primary instabilities leading to the vortices' breakdown. Linear stability analysis reveals that the fastest-growing vertical wavenumber scales with 1/Fr, while the dimensionless growth rate remains approximately constant. The most unstable eigenmode, identified as the mixed hyperbolic mode by Hattori et al. (J. Fluid Mech. vol. 909, 2021, A4), bears significant similarities to the zigzag instability first discovered by Billant and Chomaz (J. Fluid Mech. vol. 418, 2000, pp. 167-188). Direct numerical simulations confirm that the zigzag instability is crucial in amplifying initial random perturbations to finite amplitude, with the flow structure and modal growth rate consistent with the linear stability analysis. In particular, the characteristic vertical length scale of turbulence matches that of the fastest-growing linear mode. These findings underscore the broader relevance of the zigzag instability mechanism beyond its initial discovery in vortex pairs, demonstrating its role in facilitating direct energy transfer from vertically uniform vortical motions to a characteristic wavenumber inversely proportional to Fr in strongly stratified flows. |
Sunday, November 24, 2024 11:20AM - 12:50PM |
C01.00021: INTERACT DISCUSSION SESSION WITH POSTERS: Geophysical and Astrophysical Fluids After each Flash Talk has concluded, the Interact session will be followed by interactive poster or e-poster presentations, with plenty of time for one-on-one and small group discussions. |
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