Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session D34: Geophysical Fluid Dynamics: Stratified Flows IGeophysical

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Chair: Peter Diamessis, Cornell University Room: 102 
Sunday, November 19, 2017 2:15PM  2:28PM 
D34.00001: Energetics of the Internal Wave Radiation from a Stratified Turbulent Wake Kristopher Rowe, Peter Diamessis, Qi Zhou The study of the turbulent wake generated by a bluff body moving through a stably stratified fluid has important applications for naval hydrodynamics as well as geophysical flows around topography. Significant progress has been made in terms of investigating the structure and dynamics of the turbulent wake core and the associated near and farfield spectral properties of the wakeradiated internal gravity wave (IGW) fields, namely in the context of high Reynolds stratified turbulence within the wake itself. Nevertheless, little has been done to quantify the amount of energy and momentum radiated away by the IGWs generated by the wake. Through analysis of a broad Large Eddy Simulation dataset, spanning values of bodybased Reynolds $Re=~5\times 10^3,~10^5$, and $4\times10^5$, and Froude numbers $Fr=~4,~16$ and $64$, we compute the energy and momentum fluxes of IGWs radiated by the stratified turbulent wake of a towed sphere and explore the relevant parametric dependence. The analysis further aims to determine the potential of the IGWs as a sink for energy and momentum relative to the dissipation of turbulent kinetic energy in the wake itself. Finally, we discuss the implications that our findings have for wake meanflow selfsimilarity and turbulence closure models. [Preview Abstract] 
Sunday, November 19, 2017 2:28PM  2:41PM 
D34.00002: Stratified turbulence diagnostics for highReynoldsnumber momentum wakes Peter Diamessis, Qi Zhou We analyze a largeeddy simulation (LES) dataset of the turbulent wake behind a sphere of diameter $D$ translating at speed $U$ in a linearly stratified Boussinesq fluid with buoyancy frequency $N$. These simulations are performed at Reynolds numbers $Re\equiv UD/\nu \in\{5\times10^3$, $10^5, 4\times10^5\}$ and various Froude numbers $Fr \equiv 2U/(ND)$. The recently obtained data at $Re= 4\times10^5$, the highest $Re$ attained so far in either simulation or laboratory, and $Fr\in\{4,16\}$ enable us to systematically investigate the effects of Reynolds number on this prototypical localized stratified turbulent shear flow. Our analysis focuses on the time evolution of various diagnostics of stratified turbulence, such as the horizontal and vertical integral length scales, turbulent kinetic energy and its dissipation rate $\varepsilon$, and the local rate of shear between the spontaneously formed layers of vorticity within the largerscale quasihorizontal flow structures. This leads to a discussion of the transitions between distinct stratified flow regimes (Brethouwer \textit{et al.} 2007) in the appropriately defined phase diagram, and we highlight the dynamical role of the Gibson number $Gi=\varepsilon/(\nu N^2)$, and its dependence on the bodybased Reynolds number $Re$. [Preview Abstract] 
Sunday, November 19, 2017 2:41PM  2:54PM 
D34.00003: Comparing laboratory and numerical experiments on stratified wakes of bluff body Trystan Madison, Xinjiang Xiang, Geoffrey Spedding Turbulent wakes in stratified flows eventually reach a strongly stratified limit. In this limit, pattern geometry and information are preserved for long times with implications for detection of the wake signature. There is interest in, but little information on, how and whether pattern information originating at the body persists through this late time state. It is now possible to run laboratory experiments and affordable numerical approximations to estimate and predict the effects that initial conditions have on near and far wakes. Presented here is a comparison of recent laboratory and numerical experiments where parameterizations of near wakes are tested and sought for a number of $Fr$, $Re$, and varying geometries. Cases that are generalizable are identified. [Preview Abstract] 
Sunday, November 19, 2017 2:54PM  3:07PM 
D34.00004: Conversion of Internal Waves into NonDispersive Waves: Part I Background & Theory Daniel Lecoanet, Geoffrey Vasil, Jim Fuller, Matteo Cantiello, Keaton Burns The character of internal waves changes with variations of the background in which they propagate. This is especially important in media that support other types of wave modes. If this occurs, the internal wave can interact with other waves, leading to reflection, transmission, and conversion. We study the propagation of internal gravity waves in a magnetized fluid which supports nondispersive magnetic waves. This problem may be important for interpreting observations of stars. Because the group and phase velocity of internal waves are perpendicular (whereas they are parallel for nondispersive waves), we argue that we expect conversion between internal and magnetic waves where the magnetic field exceeds a critical amplitude. [Preview Abstract] 
Sunday, November 19, 2017 3:07PM  3:20PM 
D34.00005: Conversion of Internal Waves into NonDispersive waves: Part II Simulations Geoffrey Vasil, Daniel Lecoanet, Jim Fuller, Mattaeo Cantiello, Keaton Burns The character of internal waves changes with variations of the background in which they propagate. We present Boussinesq simulations of internal gravity waves in a magnetized fluid, which also supports nondispersive magnetic waves. The simulations show that internal gravity waves are strongly altered as they propagate into regions of strong magnetic field. Theoretically, we expect complete conversion from internal gravity waves into magnetic waves in regions of strong field. We confirm this by comparing the simulation with a phaseintegral approximation to the solution in terms of Mathieu functions. The approximate solution breaks down near critical layers where dissipation is expected to be important. [Preview Abstract] 
Sunday, November 19, 2017 3:20PM  3:33PM 
D34.00006: Influence of Internal Waves on Transport by a Gravity Current Jeffrey Koseff, Charlie Hogg, Raphael Ouillon, Nicholas Ouellette, Eckart Meiburg Gravity currents moving along the continental slope can be influenced by internal waves shoaling on the slope resulting in mixing between the gravity current and the ambient fluid. Whilst some observations of the potential influence of internal waves on gravity currents have been made, the process has not been studied systematically. We present laboratory experiments, and some initial numerical simulations, in which a gravity current descends down a sloped boundary through a pycnocline at the same time as an internal wave at the pycnocline shoals on the slope. Measurements of the downslope mass flux of the gravity current fluid in cases with different amplitudes of the incident internal wave will be discussed. For the parameter regime considered, the mass flux in the head of the gravity current was found to reduce with increasingly larger incident amplitude waves. This reduction was effectively caused by a ``decapitation" process whereby the breaking internal wave captures and moves fluid from the head of the gravity current back up the slope. The significance of the impact of the internal waves on gravity current transport, strongly suggests that the local internal wave climate may need to be considered when calculating gravity current transport. [Preview Abstract] 
Sunday, November 19, 2017 3:33PM  3:46PM 
D34.00007: Dense Gravity Currents with Breaking Internal Waves Yukinobu Tanimoto, Charlie Hogg, Nicholas Ouellette, Jeffrey Koseff Shoaling and breaking internal waves along a pycnocline may lead to mixing and dilution of dense gravity currents, such as cold river inflows into lakes or brine effluent from desalination plants in nearcoastal environments. In order to explore the interaction between gravity currents and breaking interfacial waves a series of laboratory experiments was performed in which a sequence of internal waves impinge upon a shelfslope gravity current. The waves are generated in a twolayer thininterface ambient water column under a variety of conditions characterizing both the waves and the gravity currents. The mixing of the gravity current is measured through both intrusive (CTD probe) and nonintrusive (Planarlaser inducted fluorescence) techniques. We will present results over a full range of Froude number (characterizing the waves) and Richardson number (characterizing the gravity current) conditions, and will discuss the mechanisms by which the gravity current is mixed into the ambient environment including the role of turbulence in the process. [Preview Abstract] 
Sunday, November 19, 2017 3:46PM  3:59PM 
D34.00008: Homogeneous internal wave turbulence driven by tidal flows Thomas Le Reun, Benjamin Favier, Michael Le Bars We propose a novel investigation of the stability of strongly stratified planetary fluid layers undergoing periodic tidal distortion in the limit where rotational effects are negligible compared to buoyancy. With the help of a local model focusing on a small fluid area compared to the global layer, we find that periodic tidal distortion drives a parametric subharmonic resonance of internal. This instability saturates into an homogeneous internal wave turbulence pervading the whole fluid interior: the energy is injected in the unstable waves which then feed a succession of triadic resonances also generating small spatial scales. As the timescale separation between the forcing and BruntVäisälä is increased, the temporal spectrum of this turbulence displays a 2 power law reminiscent of the Garrett and Munk spectrum measured in the oceans (Garett \& Munk 1979). Moreover, in this state consisting of a superposition of waves in weak nonlinear interaction, the mixing efficiency is increased compared to classical, Kolmogorovlike stratified turbulence. This study is of wide interest in geophysical fluid dynamics ranging from oceanic turbulence and tidal heating in icy satellites to dynamo action in partially stratified planetary cores as it could be the case in the Earth. [Preview Abstract] 
Sunday, November 19, 2017 3:59PM  4:12PM 
D34.00009: Internal wave energy flux from density perturbations in nonlinear stratifications Frank M. Lee, Michael R. Allshouse, Harry L. Swinney, P. J. Morrison Tidal flow over the topography at the bottom of the ocean, whose density varies with depth, generates internal gravity waves that have a significant impact on the energy budget of the ocean. Thus, understanding the energy flux ($\bm{J} = p \, \bm{v}$) is important, but it is difficult to measure simultaneously the pressure and velocity perturbation fields, $p$ and $\bm{v}$. In a previous work, a Green'sfunctionbased method was developed to calculate the instantaneous $p$ $,\bm{v},$ and thus $\bm{J}$, given a density perturbation field for a constant buoyancy frequency $N$. Here we extend the previous analytic Green's function work to include nonuniform $N$ profiles, namely the tanhshaped and linear cases, because background density stratifications that occur in the ocean and some experiments are nonlinear. In addition, we present a finitedifference method for the general case where $N$ has an arbitrary profile. Each method is validated against numerical simulations. The methods we present can be applied to measured density perturbation data by using our MATLAB graphical user interface EnergyFlux. [Preview Abstract] 
Sunday, November 19, 2017 4:12PM  4:25PM 
D34.00010: Spontaneous generation and reversals of mean flows in a convectivelygenerated internal gravity wave field LouisAlexandre Couston, Daniel Lecoanet, Benjamin Favier, Michael Le Bars We investigate via direct numerical simulations the spontaneous generation and reversals of mean zonal flows in a stablystratified fluid layer lying above a turbulent convective fluid. Contrary to the leading idealized theories of mean flow generation by selfinteracting internal waves, the emergence of a mean flow in a convectivelygenerated internal gravity wave field is not always possible because nonlinear interactions of waves of different frequencies can disrupt the mean flow generation mechanism. Strong mean flows thus emerge when the divergence of the Reynolds stress resulting from the nonlinear interactions of internal waves produces a strong enough antidiffusive acceleration for the mean flow, which, as we will demonstrate, is the case when the Prandtl number is sufficiently low, or when the energy input into the internal wavefield by the convection and density stratification are sufficiently large. Implications for mean zonal flow production as observed in the equatorial stratospheres of the Earth, Saturn and Jupiter, and possibly occurring in other geophysical systems such as planetary and stellar interiors will be briefly discussed. [Preview Abstract] 
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