Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session G35: Geophysical Fluid Dynamics: Stratified Flows II |
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Chair: Scott Wunsch, Sandia National Lab/Johns Hopkins University Room: Georgia World Congress Center B407 |
Monday, November 19, 2018 10:35AM - 10:48AM |
G35.00001: Thermohaline staircase dynamics in the presence of nonlinear internal waves Scott Wunsch Thermohaline staircases, generated by double-diffusive convection, are found in many regions of the ocean. Here, the potential impact of oceanic internal waves on the dynamics of these staircases is explored using weakly nonlinear theory. Recent results have shown that nonlinear self-interaction of internal waves at a density interface generates smaller scale harmonic waves (Diamessis et al. Dyn. Atm. Oceans 2014; Wunsch JFM 2017). This process applies to internal waves incident upon a thermohaline staircase. Energy transferred to harmonic waves may be trapped within the staircase, enhancing shear and potentially altering transport and/or layer sizes. Rotation is included, and variations with latitude are considered. |
Monday, November 19, 2018 10:48AM - 11:01AM |
G35.00002: Internal gravity waves generated by turbulent convection: spectral characteristics and non-linear interactions Pierre Léard, Michael Le Bars, Patrice Le Gal We investigate flows in a stratified layer adjacent to a turbulent convective layer. Such configurations are found in planetary atmospheres, in stars and potentially in the Earth core. The dynamics of the stratified layer have long been neglected. However, it requires a systematic investigation: the convective layer generates internal gravity waves (IGW) which transport energy and momentum. Their non-linear interactions can drive large-scale flows (e.g. Quasi Biennial Oscillation in the Earth atmosphere). It is thus crucial to understand how much of the convection energy is transported by the waves, with which spectral signature and how large-scale flows appear. To investigate this experimentally, we use a peculiar feature of water: its density maximum is at 4°C. By cooling down the bottom of a tank at 0°C and heating up the top at a given temperature, the requested two-layer configuration spontaneously appears, with the stratified layer situated above the convective layer. Here, I will introduce the first results of our ongoing study using PIV: convection and IGW spectra, and preliminary observations of mean flow reversals in the stratified layer. Comparisons with a 1D analytical model, extending the classical Lindzen & Holton (1968) model will also be discussed. |
Monday, November 19, 2018 11:01AM - 11:14AM |
G35.00003: Which Internal Waves are Excited by Turbulent Flows? Louis-Alexandre Couston, Daniel Lecoanet, Benjamin Favier, Michael Le Bars Many geophysical and astrophysical fluids, including planetary atmospheres, stars and oceans, have turbulent flows adjacent to stably-stratified fluid layers. Because internal waves in stably-stratified fluids can drive large-scale flows, increase scalar mixing and are sometimes easier to observe than turbulent motions, two important questions for these fluids are: How much energy goes from the turbulence into internal waves in the stable layer? What kind of waves are generated most efficiently (i.e. what wavenumbers and frequencies)? In this talk we will answer these two questions by presenting a theoretical prediction for the energy flux spectrum of internal waves generated by adjacent turbulent convection and comparing it with results from 3D direct numerical simulations (DNS) of a generic convective--stably-stratified fluid model. We will show that DNS and theory agree well for the range of strong turbulence-strong stratification parameters tested, giving some confidence in the analytical expression for the energy flux spectrum of the waves. Our results should help estimate the effects that internal waves have on the dynamics and evolution of geophysical and astrophysical fluids. |
Monday, November 19, 2018 11:14AM - 11:27AM |
G35.00004: The structure and origin of confined Holmboe waves Adrien Lefauve, J. L. Partridge, Q. Zhou, C. P. Caulfield, Stuart B Dalziel, Paul F Linden We describe and elucidate the origin of confined Holmboe waves (CHWs), which are apparently previously unreported long-lived coherent structures in a sustained stratified shear flow generated in the laboratory by exchange flow through an inclined square duct. We use a combination of (i) novel time-resolved, volumetric measurements of the three-component velocity and density fields simultaneously; (ii) a linear stability analysis solving for three-dimensional perturbations about the two-dimensional streamwise-averaged experimental flow. We show that the lateral confinement by the duct walls is an important ingredient of the confined Holmboe instability, which gives rise to the CHW, with implications for many inherently confined geophysical flows. Our results suggest that the CHW is an example of an experimentally observed, inherently nonlinear, robust, long-lived coherent structure which has developed from a linear instability. We conjecture that the CHW is a promising candidate for a class of exact coherent states underpinning the dynamics of more disordered, yet continually forced stratified shear flows. |
Monday, November 19, 2018 11:27AM - 11:40AM |
G35.00005: Numerical simulations of turbulent bursts in large amplitude internal Kelvin waves Marek Stastna, David Deepwell Rotating stratified adjustment can lead to the generation of large amplitude internal Kelvin waves. We have shown that for mode-2 waves in single pycnocline stratifications, the Kelvin wave may be breaking, with a nearly trapped core coexisting with billows along the edge of the pycnocline (Deepwell & Stastna, NPG, 2018). These are particularly prominent for early times, before the Kelvin wave-Poincare wave resonance begins to compete for the leading wave’s energy. We present three-dimensional characterizations of the billows and their breakdown, including the structure of the enstrophy and viscous dissipation fields. We then outline how the vortex stretching/tilting, baroclinic and viscous terms in the budget of enstrophy evolve, and contrast these results with the literature on rotating, stratified turbulence. |
Monday, November 19, 2018 11:40AM - 11:53AM |
G35.00006: Near wake structures in a stably stratified fluid using Dynamic Mode Decomposition Chan-ye Ohh, Xinjiang Xiang, Trystan J Madison, Geoffrey R Spedding Early experiments suggest that early wake information including body geometry and initial conditions in a linearly stratified fluid is lost during the wake evolution process (Meunier and Spedding Phys. Fluids 16, 298-305, 2004). However, limited by the experimental techniques in these early experiments, details of possible pattern memory remain elusive. The current project seeks to evaluate the near wake structures of various body shapes in a stratified fluid using Dynamic Mode Decomposition (DMD) on both experimental and numerical wake data at low Re (200 ≤ Re ≤ 1000) and low Fr (0.5 ≤ Fr ≤ 8). The dominant dynamic modes depend systematically on both Re and Fr. The energy contributions of different modes are further investigated to understand the complex, near wake evolution and identify any retained early wake information. |
Monday, November 19, 2018 11:53AM - 12:06PM |
G35.00007: Regularization and eddy-viscosity sub-grid scale (SGS) models for large eddy simulations (LES) of stably-stratified flows Kiran Jadhav, Abhilash Chandy Stratified flows involve a density variation in the vertical direction, and have wide applications in many of the phenomena occurring in a variety of geophysical flows. Regularization-based sub-grid scale (SGS) models such as Leray-α, LANS-α and Clark-α models and eddy viscosity-types approaches such as non-dynamic and dynamic Smagorinsky models are assessed through comparisons with direct numerical simulations (DNS) of a Taylor-Green vortex problem in a stably stratified Boussinesq fluid from Rahimi and Chandy, J. Turbulence, 2015. 1283 LES calculations are presented for a Froude number, Fr, of 1 and a Reynolds number, Re, of 1600, using a fast Fourier transform (FFT)-based pseudo-spectral methodology formulated for triple-periodic domains. Features investigated include temporal variations of the energy spectrum cascade, turbulent kinetic and potential energies, local Froude numbers, enstrophy, vertical shearing of the velocities, and dissipation of kinetic and potential energy. The results from these computations demonstrated the capability of LES to effectively predict the cascade of energy for high-Re and suppression of vertical motion under stratification. They also showed the eddy-viscosity-based models performing better in comparison to regularization models. |
Monday, November 19, 2018 12:06PM - 12:19PM |
G35.00008: Exact coherent states in a quasi-linear model of strongly stratified Kolmogorov flow Gregory P. Chini, Guillaume Michel, Keith A. Julien, Colm-cille Caulfield Strongly stratified turbulent flows are characterized by sufficiently large values of the Reynolds number Re that the buoyancy Reynolds number Reb ≡ Re Fr2 > 30 (or more) as the Froude number Fr-->0. In this extreme parameter regime, the flow is dominated by highly anisotropic structures that have horizontal scales much larger than their vertical scales. Owing to their relative horizontal motion, these structures are susceptible to stratified shear instabilities that drive spectrally non-local energy transfers. A quasi-linear (QL) model that captures these features of strongly stratified shear flows is derived via asymptotic analysis of the non-rotating Boussinesq equations. The model is used to investigate the mixing efficiency of certain exact coherent states (ECS) in strongly stratified 2D Kolmogorov (i.e. sinusoidally-forced) flow. The ECS are computed using a new methodology for numerically integrating multiple time-scale QL systems strictly on the "slow" time scale of the mean flow. |
Monday, November 19, 2018 12:19PM - 12:32PM |
G35.00009: Abstract Withdrawn
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Monday, November 19, 2018 12:32PM - 12:45PM |
G35.00010: Turbulent mixing in stratified centrifugally unstable Taylor-Couette Flow Kanwar Nain Singh, Jamie Partridge, Stuart B Dalziel, Colm-Cille P Caulfield
We investigate the mixing in an axially stably stratified turbulent Taylor-Couette (STC) flow. STC is prone to the `stratio-rotational instability’ (SRI) even when the flow is centrifugally stable. Conversely, for the centrifugally unstable flow at high rotation rates, the flow spontaneously forms layers for an initially linear stratification, separated by robust interfaces. It was previously observed by Oglethrope et al. (2013) that the flux across the interface in a two-layered experiment with only inner cylinder rotating at a constant frequency Ω, stays constant with reducing Richardson number (Ri∝ Δρ_interface), until a critical Ri where the flux starts to increase. Using particle-image-velocimetry (PIV), we demonstrate that this enhanced flux is due to the onset of micro-splashing of fluid from near the inner cylinder onto the region of outer cylinder near the interface. Also, these interfaces appear to support m = 1, inherently nonlinear perturbations with a characteristic period which depends only on Ω. Using PIV and laser-induced fluorescence (LIF), we characterise the flow behaviour, and in particular the mixing mechanisms due to these coherent perturbations.
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