Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session Q39: Geophysical Fluid Dynamics Stratified Flow III |
Hide Abstracts |
Chair: Alexis Kaminski, University of Washington Room: 6a |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q39.00001: 3D measurements of inclined vortex rings interacting with a density stratification Johan Pinaud, Julie Albagnac, Pierre Brancher, Sebastien Cazin, Zeinab Rida Vortex rings are coherent vortical structures that dominate the dynamics of numerous flows as they are generated each time an impulsive jet occurs in a homogeneous fluid. They are also considered as elementary bricks of turbulence. Their faculty to propagate along their revolution axis by self-induction confers to such structures interesting transport properties, namely, transport of momentum, mass and heat. They are therefore often qualified as good candidates for mixing. From this perspective, the present study addresses the interaction of a vortex ring with a density stratification in order to get a better understanding of the subsequent mixing mechanisms. A new 3D time-resolved technique is used and gives a highlight at short timescale on the 3D vorticity reorganization and at larger timescale on the 3D patterns of internal gravity waves forced by the impacting/penetrating vortex. The influence of the Reynolds number of the vortex ring and its angle of attack relative to isopycnals will be detailed. [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q39.00002: Dynamics of a reactive spherical particle falling in a linearly stratified layer Ludovic Huguet, Victor Barge-Zwick, Michael Le Bars The behavior of a particle falling in a stratified layer has already been studied for regimes of small Reynolds $Re$ or Froude $Fr$ numbers. However, the dynamics of a reactive particle have been unexplored, especially for regimes of interest for geophysical applications (large $Re$ and $Fr$ numbers). In a large water tank with linear stratification, reactive spheres made of a mixture of ice and hydrohalite solidifying below $-21.1^\circ$C. are released from the top and melt while they sink. PIV is used to track their falls and the dynamics of the surrounding environment. Results are compared with non-reactive plastic spheres. For large Reynolds and Froude number, the added drag (compared to a sphere in a homogeneous fluid) of the plastic spheres due to the stratification is proportional to $(Re/Fr^2)^{0.5}$. For the reactive spheres, the added drag is much larger, suggesting to a strong modification of the wake due to the melting. We also characterize the generation of internal waves and the associated radiated energy. While increasing with radius for plastic spheres, the ratio of wave energy compared to the initial potential energy of the spheres is constant over the explored range for reactive ones. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q39.00003: Analysis of one-dimensional models for exchange flows under strong stratification Herman Clercx, Steven Kaptein, Vincenzo Armenio, Matias Duran Matute One-dimensional models of exchange flows driven by horizontal density gradients are well known for performing poorly in situations with weak turbulent mixing. The main issue with these models is that the horizontal density gradient is usually imposed as a constant, leading to non-physically high stratification known as runaway-stratification. Here, we propose two new parametrizations of the horizontal density gradient leading to one-dimensional models able to tackle strongly stratified exchange flows at high and low Schmidt number values. The models are extensively tested against results from laminar two-dimensional simulations and are shown to outperform the models using the classical constant parametrization for the horizontal density gradients. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q39.00004: Flavours of Stratified Shear Flows: Algorithmic Detection Hesam Salehipour, Tom Eaves It remains unknown how to relate our understanding of turbulent mixing in stratified shear flows (based on direct numerical simulation (DNS)) to oceanic measurements of 1D profiles of density and velocity. In particular, there are various pathways to turbulence in stratified shear flows which behave in categorically different ways, but a method of distinguishing between these pathways in 1D microstructure measurements has yet to be proposed. Recently, a coordinate-free algorithmic classification scheme for the nonlinear states which result from the saturation of each instability has been proposed by Eaves and Balmforth (JFM 860, 2019), and hence is ideally suited for examining 1D profiles. This study was mostly restricted to relatively idealised steady, 2D flows; however, some aspects of this scheme were seen to carry over to non-steady, 3D flows after coarse-graining of the flow profiles. In this talk, we will investigate when the classification scheme works for realistic flows. In particular, we shall examine a large number of 1D profiles obtained from DNS of both Kelvin--Helmholtz and Holmboe instabilities to investigate the performance of this scheme over a large range of Reynolds and Richardson numbers, in addition to the type of instability and the `age’ of the flow. [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q39.00005: Overturning Structures in Symmetric and Asymmetric Shear Instabilities Alexis Kaminski, Jason Olsthoorn, Daniel Robb, Eric D'Asaro Turbulent mixing plays an important role in setting the distribution of heat, salt, and other biogeochemical tracers in the ocean, and quantifying the turbulent fluxes of these tracers is therefore a key question. A common approach is to look for signatures of turbulent overturns in one-dimensional profiles of temperature or other tracers and subsequently infer details of the accompanying fluxes and mixing. Often, these observed profiles are interpreted in the context of classical Kelvin-Helmholtz instability, in which shear drives the formation of a large overturn that subsequently triggers transition to turbulence. However, both the linear shear instability and nonlinear flow evolution can depend sensitively on the details of the background shear and stratification. Here we present the results of a series of direct numerical simulations of stratified shear instabilities with symmetric and asymmetric initial conditions, i.e. with either coincident or vertically offset profiles of shear and stratification. Motivated by recent Lagrangian float observations of the ocean transition layer, we examine the vertical structure of the resulting overturns. We describe both the size and stratification of these structures, and discuss implications for the associated buoyancy flux. [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q39.00006: Wake identification of stratified flows using Dynamic Mode Decomposition Chan-ye Ohh, Geoffrey 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 \textit{Phys. Fluids} 16, 298\--305, 2004). Though this result was established for certain statistical quantities, it is less clear that there is no pattern remaining, and the process by which it is lost is also not established. Here we investigate how to identify, in principle, the various known regimes of stratified flow using a more sophisticated method, Dynamic Mode Decomposition (DMD), on 3D wake data from tomographic PIV and second-order DNS at low Re ($200\leq\textrm{Re}\leq 1000$) and low Fr ($0.5\leq\textrm{Fr}\leq 8$). Within the large set of modes, the dominant dynamic modes can be ranked and categorized into known regimes from a mode selection algorithm. The identification process is further refined and tested for spatially and temporally limited wake measurements. [Preview Abstract] |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q39.00007: Critical Reflection Dissipates Internal Wave Energy Bruce Rodenborn, Clayton Bell, Charlotte Mabbs Ocean measurements show that continental slopes are eroded to the critical angle of local internal waves (Cacchione et al., Science 296, 2002). However, the feedback mechanism modifying the slopes over geologic time is not clearly understood. Other work shows that tidal motion over topography creates strong boundary flows but weak tidal conversion (Dettner et al, Phys., Fluids, 25, 2013). We find a similar boundary response in internal waves reflecting from critical slopes; most of the internal wave energy is dissipated near to the boundary. We use experiments with a Reynolds number, $Re \sim 1000$, and numerical simulations that solve the full Navier-Stokes equations in the Boussinesq limit with $Re\sim 10^3 - 10^5$. Our data show that the rate of energy dissipation at the critical angle remains high even when viscous effects are minimized in the simulations. We also present laboratory data showing reflection from a turning depth and find high rates of energy dissipation though the no slip boundary condition is not present. The data suggest that critical reflection dissipates energy from the internal wave field at high Reynolds numbers, and therefore, may contribute to the erosion of continental slopes. [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q39.00008: Internal Wave Breaking in an Isothermal Atmosphere Daniel Lecoanet, Yubo Su, Dong Lai We present a series of two-dimensional numerical simulations of internal wave breaking in an isothermal atmosphere. Waves with a fixed frequency and wavenumber are continuously excited at the bottom of the atmosphere. As they propagate upward, their amplitude increases until they become nonlinear and break. The waves deposit their momentum and spin-up the upper layers of the atmosphere until the mean velocity is equal to the waves' phase velocity. Waves continue to break at the shear layer at the bottom of the spun-up fluid, further depositing their momentum. We find the shear layer descends as ~exp(-t), as the momentum flux is constant, but the density of the atmosphere increases exponentially with depth. For simulations with sufficiently high Reynolds number, we find about 50\% of the wave flux is reflected by the shear layer, 10\% is transmitted into waves in the spun-up fluid, and 40\% is absorbed. [Preview Abstract] |
Tuesday, November 26, 2019 9:29AM - 9:42AM |
Q39.00009: Comparison of Internal Wave Kinetic Energy Estimates in Synthetic Schlieren and Particle Image Velocimetry Kyle Hakes, Allison Lee, Annie Wesolek, Julie Crockett Synthetic Schlieren (SS) and Particle Image Velocimetry (PIV) are commonly employed experimental methods for investigating internal wave generation and propagation. PIV allows direct calculations of velocity and therefore kinetic energy, but at a relatively high setup cost. SS is a less expensive experimental method that is generally more easy to implement, but is not a direct measurement of velocity but instead natural frequency changes. This can be used to estimate kinetic energy but calculations are subject to the WKB approximation. Experiments were performed to investigate when the kinetic energy estimates from SS are a good approximation of the kinetic energy calculated from PIV for the same experiment. Internal waves generated by tidal flow over 4 different topographies in 4 different density profiles are explored via both SS and PIV methods for comparison. Preliminary findings indicate SS and PIV match well overall far from regions where WKB assumptions fail, and show that their ability to generate similar results depends on the shape of the topography and density profile. [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. |
© 2020 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
1 Research Road, Ridge, NY 11961-2701
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700