Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session R27: Geophysical Fluid Dynamics: Stratified Flows I |
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Chair: Daniel Lecoanet, Northwestern Room: 151B |
Monday, November 20, 2023 1:50PM - 2:03PM |
R27.00001: Internal Waves and Rotating Turbulence Daniel Lecoanet, Evan H Anders, Kyle Augustson Many natural systems in which waves and turbulence interact---including oceans, atmospheres, liquid-metal planetary cores, and stars---are also rotating. Rotation modifies the properties of internal waves, as well as turbulence. Here we study the interaction of rotating turbulence and internal waves by analyzing 3D simulations of a turbulent convective layer adjacent to a stably-stratified layer. The convection is driven by internal heating, and we run simulations with a range of rotation rates corresponding to different convective Rossby numbers. When rotation and gravity are aligned with each other (i.e., near the pole), we find the convective turbulence excites internal waves in the stratified layer in the same way independent of Rossby number. The only effect of rotation is to place a lower limit on the allowed wave frequencies. The picture is more complicated when rotation and gravity are perpendicular (i.e., near the equator). In this case, the rotating turbulence is more efficient at exciting waves, provided the simulations are sufficiently turbulent that the convection three-dimensionalizes at small scales. Finally, we will compare these simulations results to heuristic theories of rotating convection and wave generation. |
Monday, November 20, 2023 2:03PM - 2:16PM |
R27.00002: Stability, transient perturbation, separatrix and search for chaotic dynamics in non-linear double triadic resonances : the case of four internal gravity waves Jean-Marc Chomaz, Sabine Ortiz Triadic instability is a generic mechanism by which a primary wave of finite amplitude is destabilized by two secondary waves forming a resonant triad. For gravity wave in the ocean (Phillips, 1967) the resonant triads form continuous families represented in two-dimension as lines in the wave vector space of the secondary waves. We show here that the crossing of two of these lines results in a double triadic resonance where the instability is reduced for initially unstable triads and increased for stable triad. Using detuning (McEwan. & Plumb, 1977), we show that this double triadic interaction expends from a singular point to a finite region scaling with the Froude number F based on the primary wave amplitude. For long wave perturbations double triad interaction should be taken into account in an extanded region close to zero scaling as F^{1/2} since several resonant branches originate from wave number zero with the same slope. The nonlinear evolution is also modified by the exact or detuned double triadic interaction and, using the invariants discussed in Bustamante & Kartashova (2009) for this nearly resonant four waves interaction, we explore the possibility of complete transfer of energy from the initial wave to resonant waves and search for chaotic dynamics. |
Monday, November 20, 2023 2:16PM - 2:29PM |
R27.00003: Stability of stratified and highly-diffusive Taylor-Couette flow Junho Park Understanding the role of thermal diffusion is important for naturally-occurring systems where fluid flow is coupled with heat transfer. The thermal diffusion is characterized by the Prandtl number Pr=nu/kappa, the ratio between fluid kinematic viscosity nu and thermal diffusivity kappa. The Prandtl number varies; for instance, Pr of O(1) for the air, Pr of O(10^{-2}) in the liquid metal core of the Earth, or Pr of O(10^{-6}) in the interior of the Sun and stars. In stably stratified flows, it has been known that high thermal diffusivity at low Pr suppresses the stratification effect. Our study aims to explore the effect of thermal diffusion in the context of Taylor-Couette (TC) flow with axial stratification. We first conduct linear stability analysis with a particular focus on Prandtl-number dependence and demonstrate how the stability of stratified TC flow is modified with high thermal diffusivity. We will also describe a self-similar behavior observed for this TC flow. Furthermore, there will be discussion on the effect of fast thermal diffusion in nonlinear instability and secondary instability using generalized quasi-linear approximation and 2D global stability analysis. |
Monday, November 20, 2023 2:29PM - 2:42PM |
R27.00004: Spectral and modal characteristics of stably stratified turbulent channel flows with differential diffusion effects Steven Thompson, Reetesh Ranjan The presence of density stratification adds to the complexity of turbulence by affecting the small-scale mixing, the large-scale circulation, and the inter-scale interactions. The density in underwater naval flows depends upon temperature and salinity scalars. The differences in the molecular diffusivity of these scalars lead to the occurrence of the differential diffusion phenomenon. A key feature of such flows is the presence of internal waves, which affect the spatio-temporal dynamics in these flows. Therefore, an improved understanding of the effects of differential diffusion on the characteristics of the internal waves is required for reliable modeling of such flows. In this study, the effects of differential diffusion on the spectral and modal characteristics of internal waves in stably stratified turbulent channel flows are examined by considering direct numerical simulation datasets at a frictional Reynolds number of 395 and the frictional Richardson number of 60. The two-dimensional spectra of fluctuations in the vertical velocity, density, momentum flux, and buoyancy flux at different wall-normal planes are obtained to examine the effects of buoyancy and shear-generated turbulence. The presence of internal waves is inferred in terms of the phase relationship between the vertical velocity and density fluctuations. The spatial structure of the internal waves is examined using the dynamic mode decomposition technique to obtain the dynamically relevant flow structures. |
Monday, November 20, 2023 2:42PM - 2:55PM |
R27.00005: Progress towards the Simulation of Very High Reynolds Number Stratified Sphere Wakes Peter J Diamessis, Nidia Reyes-Gil, Greg N Thomsen, Kristopher Rowe We report our latest progress towards implicit Large Eddy Simulations (ILES) of stratified sphere wakes at body-based Reynolds numbers of Re = 1.6 × 10^{6} and internal Froude numbers Fr = 4. Such a value of Re enables a sufficiently long window of operation of the highly energetic and relatively unexplored Strongly Stratified Regime (SSR ; de Bruyn Kops and Riley J. Fluid Mech. 2019), extending as far as Nt ≈ 500. |
Monday, November 20, 2023 2:55PM - 3:08PM |
R27.00006: Analysis and modeling of slow and rapid pressure correlations in stably stratified turbulence Young R Yi, Jeffrey R Koseff, Elie R Bou-Zeid In many turbulent flows, pressure fluctuations play a key role of redistributing the more energetic (normal) Reynolds stresses to the less energetic ones. For stably stratified turbulence, the damping of vertical fluctuations by the buoyancy force implies that this redistribution process will play a significant role in the generation of vertical Reynolds stresses, and therefore have important effects on the vertical buoyancy flux as well. In this study, we consider four datasets of direct numerical simulations (DNS) of stably stratified turbulence (one without shear forcing and three with shear forcing) across a wide range of stratification strengths. The three shear forcing datasets correspond to different turbulence generation scenarios: (i) vertically sheared mean horizontal flow, (ii) horizontally sheared mean vertical flow, (iii) laterally sheared horizontal flow. We decompose the pressure fluctuations into the slow and rapid components and analyze their contributions to the pressure-strain correlations and the pressure scrambling terms, which appear in the Reynolds stress and buoyancy flux budgets, respectively. For example, the dataset without shear forcing has a single slow and rapid pressure component associated with nonlinear and buoyancy effects, and we observe the following changes as stratification is increased: (i) the pressure-strain correlations are dominated by the slow component even as the rapid component becomes finite and switches signs; (ii) the pressure scrambling term undergoes a sign change as it becomes dominated by the rapid component and acts a source of vertical buoyancy flux. The other three datasets involve an additional rapid pressure component due to shear forcing, resulting in different behaviors with increasing stratification. Lastly, we test the performance of commonly used closures such as Rotta's return-to-isotropy model and evaluate the model coefficients as a function of stratification strength with the goal of improving subgrid-scale models used in large-scale models. |
Monday, November 20, 2023 3:08PM - 3:21PM |
R27.00007: Analysis of the Energetics of mode-1 internal waves interacting with topographic ridges of varying slope and height Matthew Klema, Karan Venayagamoorthy This research presents a detailed analysis of 40 two-dimensional high-resolution computational fluid simulations as part of a parametric study examining the interaction and energetics of mode-1 internal waves with oceanic ridges. The main parameters are internal wave forcing velocity amplitude, U_{0}, ridge height, h and ridge slope, γ. Flows initialized with a high Froude number (Fr=U0/c_{ph}, where c_{ph} is internal wave celerity) show results with increased turbulence, dissipation and non-linear structures such as internal boluses. Energy flux budget analysis shows that ridges with critical slopes concentrate internal wave beams upon interaction with the topographic ridge, resulting in increased turbulence. When ridges are characterized by super-critical slopes and a large ridge height more wave energy is reflected, while more energy is transmitted by ridges with a small topographic height and/or subcritical slopes. Without explicit resolution of all turbulent scales non-linear structures that are important for understanding these types of flows can be realistically modeled. Analysis of these interactions at intermediate scales (~100 m), below the field scale (≥1000 m) and above the laboratory scale (≤10 m) is not currently present in the literature allowing for this work.The utility of fundamental process models used in informing physically realistic models and field measurements are presented to improve understanding and prediction of mixing in stratified geophysical flows. |
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