Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session R7: Geophysical: General VI |
Hide Abstracts |
Chair: John Taylor, University of Cambridge Room: 24C |
Tuesday, November 20, 2012 1:00PM - 1:13PM |
R7.00001: Mixing efficiency of turbulent stratified flows: Not all flows are created equal Alberto Scotti, Brian White Small scale mixing in the stratified interior of the ocean is a fundamental, yet poorly characterized, controlling factor of the global Meridional Overturning Circulation (MOC). In the oceanographic community, the mixing efficiency is usually assumed to be 20\%. In this talk, we use DNS datasets to calculate the mixing efficiency in different class of flows. The mixing efficiency is calculated using the actual irreversible diapycnal flux of buoyancy (Winters and D'Asaro, 1996; Scotti et al., 2006) instead of the more customary turbulent diapycnal fluxes. This avoids potential issues of contamination of the latter from reversible processes (e.g., internal waves). For flows in which mixing is solely mechanically driven our profiles of mixing efficiency vs. turbulent intensity parameter agree well with the profiles previously established in the literature. However, for flows in which mixing is driven in part or fully by a thermodynamically induced excess of available potential energy, we obtain profiles characterized by much higher values of mixing efficiencies. Applications of these results to the MOC are discussed. Note: The DNS datasets of turbulent stratified channel flow was provided courtesy of M. Garcia-Villalba and J. C. del Alamo. [Preview Abstract] |
Tuesday, November 20, 2012 1:13PM - 1:26PM |
R7.00002: Instability Mechanisms in a Stratified and Rotating Shear Layer with Horizontal Shear Eric Arobone, Sutanu Sarkar One of the least understood scales of the ocean is the submesoscale. Here, rotation is important but does not necessarily control the dynamics. Instabilities and nonlinear cascades are possible in this regime while the influence of stable stratification is significant. Previous work by the authors revealed augmentation of the vertical wavenumber band associated with linear barotropic instability near the state of zero centerline absolute vorticity for strongly stratified flow. Enstrophy budgets from nonlinear simulations show a marked transition corresponding to the changing in sign of centerline absolute vorticity. We performed numerical experiments to examine the effect of both rotation and stratification on coherent dynamics with environmental parameters appropriate for submesoscale flows. Coherent structure evolution will be explored to understand the effect of stratification and rotation, in combination, on possible instability mechanisms, e.g. elliptic, zigzag, inertial, and barotropic instabilities. Additionally, physical mechanisms driving the flow evolution will be discussed with the aid of enstrophy budgets and flow visualizations. [Preview Abstract] |
Tuesday, November 20, 2012 1:26PM - 1:39PM |
R7.00003: Transition in Energy Spectra and Vortex Structures in Stably Stratified Turbulence Yoshifumi Kimura, Jackson Herring The power-law transition in the energy spectrum and relating vortex formation for stably stratified turbulence are investigated using the pseudo-spectral DNS of the Navier-Stokes equation under the Boussinesq approximation with $2048^3$ grid points. From the zero total energy initial condition, integrated horizontal forcing is imposed in a narrow wave number band centered at k=5. At the first stage of the development, the horizontal energy spectra show a steep power-law ($\sim k_{\perp}^{4-5}$). By this time, we observe that many wedge vortices are produced and they move horizontally (like dipoles) in random directions. This stage lasts a long period of time, and then the tail part of the spectrum begins to rise to show the Kolmogorov-type slope ($k_{\perp}^{-5/3}$). During this stage of time, the wings of the wedges become thinner and thinner while translating, and finally detach to be almost independent vortex layers. This thinning mechanism makes the vertical shear stronger and eventually local Richardson number small to develop Kelvin-Helmholtz billows. The relation between the horizontal breaking of the Kelvin-Helmholtz billows and the observation of the Kolmogorov-type slope will be discussed. [Preview Abstract] |
Tuesday, November 20, 2012 1:39PM - 1:52PM |
R7.00004: Large Eddy Simulations of Kelvin-Helmholtz Instabilities in Stratified Ocean Flows Dana Brown, Louis Goodman, Mehdi Raessi Numerical simulations of turbulence in the ocean environment are used to supplement and enhance understanding of observational data. Here, using the NGA framework (Dejardins et al., JCP 2008), direct numerical simulations (DNS) and large eddy simulations (LES) of Kelvin-Helmholtz instabilities are employed to study turbulence in presence of density stratification. Kelvin-Helmholtz instabilities have been shown to be a common source of turbulence in the ocean. Past DNS studies of Kelvin Helmholtz instabilities have compared favorably with observational data, but were limited to moderate Reynolds numbers. Here, LES is used to solve the filtered incompressible NS equations at a higher Reynolds number, Re = 10,000. The effect of increased Reynolds number on the turbulence behavior is examined in terms of velocity spectra and energy budgets. [Preview Abstract] |
Tuesday, November 20, 2012 1:52PM - 2:05PM |
R7.00005: Instabilities of pancake vortices modelled by rotating ellipsoids in a stratified fluid Patrice Meunier It is now well known that oceans contain very energetic vortices with a long lifetime. However, it is still unclear how these vortices destabilise and how much energy and mixing they can provide at different scales. We investigate here the destabilisation of an axisymmetric vortex in a stratified and non-rotating environment. The vortex is modeled by a rotating ellipsoid with various diameters and heights. The flow is visualised by shadowgraph, synthetic schlieren and Particle Image Velocimetry. Two types of instabilities have been observed, one being located on the side of the ellipsoid and the other being located at the top and bottom. The first instability is linked to the radiative instability, which is well known in the case of a rotating cylinder, and which emits internal waves with an azimuthal wave number equal to 1. The second instability generates an axisymmetric layering pattern which is reminiscent of the double diffusive instability (between angular momentum and density), observed and described theoretically in a rotating environment. This second instability might be responsible for the layering pattern found above oceanic vortices, which probably leads to a large localised mixing. [Preview Abstract] |
Tuesday, November 20, 2012 2:05PM - 2:18PM |
R7.00006: Unstable modes of a sheared pycnocline above a stratified layer Scott Wunsch, Kurt Keller Internal waves incident on a sheared ocean pycnocline are studied using analytic and numerical methods. Linear analysis of the unstable modes of a sheared ocean pycnocline is used to demonstrate interactions between internal waves and shear instabilities. A new analytic solution for an asymmetric shear layer over a stratified layer is presented, illustrating modes which couple to internal waves, in addition to the well-known Holmboe modes. The robustness of these solutions is demonstrated using numerical methods for realistic shear profiles. Fully nonlinear numerical simulations illustrate the growth of these modes and demonstrate the excitation of shear instabilities by incident internal waves. The results may have implications for internal wave interactions with the ocean pycnocline and the local generation of internal solitary waves. [Preview Abstract] |
Tuesday, November 20, 2012 2:18PM - 2:31PM |
R7.00007: Inertial instability of oceanic submesoscale vortices: linear analysis, marginal stability criterion and laboratory experiments Ayah Lazar, Alexandre Stegner, Eyal Heifetz Inertial instability is a possible mechanism for vertical mixing in the submesoscale ocean. The stability of axisymmetric oceanic-like vortices to inertial perturbations is investigated by linear stability analysis, taking into account the thickness and the stratification of the thermocline, as well as the vertical eddy viscosity. Numerical analysis reveals that the instability is insensitive to the vorticity profile if the intensity of the vortex is characterized by the vortex Rossby number (instead of the local normalized vorticity). This allows extending our analytical solutions for the Rankine vortex to a wide variety of oceanic cases, including results such as the analytic dispersion relation, and the marginal stability criterion. This suits oceanic conditions better than the widely used generalized Rayleigh criterion. Comparison with oceanographic data shows that our criterion permits cases that contradict the common oceanographic hypothesis for inertial instability. For instance, intense submesoscale anticyclones may be stable even with a core region of negative absolute vorticity. We corroborate our findings with large-scale laboratory experiments and find a signature of the instability on the mean-flow, which could be used in future oceanographic measurements. [Preview Abstract] |
Tuesday, November 20, 2012 2:31PM - 2:44PM |
R7.00008: The Easily Excitable Baroclinic Critical Layers in Rotating, Horizontally Shearing, Vertically Stratified Flows and Their Roll-up into Vortices Philip Marcus, Suyang Pei, Chung-Hsiang Jiang, Pedram Hassanzadeh Baroclinic critical layers can occur in rotating, vertically-stratified, uni-directional shear flows. They are special cases of neutrally stable eigenmodes. Baroclinic critical layers have logarithmic singularities in density and vertical velocity. They differ from barotropic critical layers associated with Kelvin's cats-eyes in constant-density, uni-directional shear flows, which form at locations where the shear flow velocity matches the eigenmode's phase speed and have singularities only in stream-wise velocities. Baroclinic critical layers are excited with no special tuning of parameters by perturbations from vortices or waves. Unlike barotropic critical layers the amplitudes of baroclinic layers become large by drawing energy from the background shear. The large vertical velocities in the critical layers, coupled with the Coriolis parameter create large-amplitude vortex layers. These layers often roll-up into large coherent vortices. The baroclinic critical layers' growth and roll-up are robust: they form in cylindrical and Cartesian geometries, in Boussinesq fluids and ideal gases, and in flows with uniform and non-uniform shear and vertical stratification. However, they do not form in numerical calculations with insufficient spatial resolution or large grid dissipation. [Preview Abstract] |
Tuesday, November 20, 2012 2:44PM - 2:57PM |
R7.00009: Self-Similar, Self-Replicating, Critical Layers and Vortices in Rotating, Horizontally Shearing, Vertically-Stratified Flows Suyang Pei, Chung-Hsiang Jiang, Philip Marcus, Pedram Hassanzadeh In a rotating,uni-directional flow with a vertical Brunt-V\"ais\"al\"a frequency $N(z)$ and horizontal shear $\sigma$, baroclinic critical layers (a form of neutrally stable eigenmode) form at cross-stream locations that are functions of $N$ and the eigenmode's stream-wise wavenumber and temporal frequency. The critical layers, which are easily excited by waves or vortices, grow in amplitude, roll-up and create new vortices at the critical layers. These vortices, in turn create new critical layers. In flows with uniform $\sigma$, the process of excitation, critical layer growth, roll-up and vortex creation can self-similarly self-replicate so that the entire 3D computational domain fills with a spatially periodic lattice of large-amplitude vortices. This self-replication occurs in flows that are linearly stable (in particular, they are convectively and centrifugally stable with a uni-directional flow with no inflection points). Thus, a small, but finite-amplitude perturbation in the form of a wave or vortex fills the entire flow with large-amplitude coherent structures. This phenomenon was serendipitously discovered in calculations of linearly stable Keplerian disks and of planetary vortices in zonal flows, but also applies to large Reynolds number lab flows such as Couette flow. [Preview Abstract] |
Tuesday, November 20, 2012 2:57PM - 3:10PM |
R7.00010: Turbulence, submesoscales, and the spin down of ocean fronts John Taylor Ocean fronts, regions of strong horizontal density gradients, are important for many processes in the ocean, including heat transport, CO2 uptake, water mass formation, and biological productivity. Unlike non-rotating flow, where a horizontal density gradient would lead to gravitational slumping, many ocean fronts are balanced by an along-front flow known as the ``thermal wind.'' However, this equilibrium is unstable to a variety of instabilities, some of which generate $O$(1-10km) features known as submesoscales, a major focus of recent work. Despite recent progress, several important questions remain, including: How do submesoscales interact with boundary layer turbulence, and how effective are they at transferring energy to smaller scales, thereby completing a down-scale route to dissipate frontal energy. Here, we will discuss two areas of research that address these questions. First, horizontal straining by submesoscale eddies enhances horizontal density gradients. When forced by surface cooling, these regions generate localized pockets of intense turbulence. Second, wind forcing can generate time-dependent currents that trigger ``symmetric instability,'' which is efficient at extracting frontal kinetic energy and enhances small-scale turbulent dissipation. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700