Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session M36: Geophysical: Oceanographic VII |
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Chair: Arezoo Ardekani, University of Notre Dame Room: 407 |
Tuesday, November 26, 2013 8:00AM - 8:13AM |
M36.00001: Experiments in Stably Stratified Wakes I: Measurement and Characterization of Mean and Fluctuating Quantities Xinjiang Xiang, Trystan Madison, Prabu Sellappan, Geoffrey Spedding In a stable background density gradient, initially turbulent motions evolve into a state that is dominated by low Froude number dynamics and that can also contain persistent pattern information. Nevertheless, little quantitative information is available in the initial flow evolution when the turbulence first adjusts to the background. Here we report on experiments in a refractive index matched facility for $0.6 \leq Fr \leq 8$ and $2500 \leq Re \leq 10000$, where flow quantities behind a towed grid are examined, and appropriate measures for this early wake regime are considered. [Preview Abstract] |
Tuesday, November 26, 2013 8:13AM - 8:26AM |
M36.00002: Experiments in stably stratified wakes II: The early wake behind a sphere Trystan Madison, Xinjiang Xiang, Prabu Sellappan, Geoffrey Spedding The wake of a towed sphere has been used as a canonical case for investigating turbulence in a stratified environment, and certain late wake features compare well with numerical experiments that have no sphere. As a result empirically-established evolution laws that do not depend on initial or boundary conditions are thought to be quite general. It is just becoming possible, experimentally and numerically, to access the early stages of flow development around the sphere itself, when a much more specific and rigorous comparison of similar quantities can be made. Here the first quantitative early wake data behind a towed sphere in a laboratory experiment for $Fr = \{2,8\}$ and $Re = \{2500, 10000\}$ are presented. [Preview Abstract] |
Tuesday, November 26, 2013 8:26AM - 8:39AM |
M36.00003: Retention and entrainment effects: experiments and theory for porous spheres settling in sharply stratified fluids Shilpa Khatri, Roberto Camassa, Claudia Falcon, Richard McLaughlin, Jennifer Prairie, Brian White, Sungduk Yu Marine snow, porous aggregates composed of phytoplankton, fecal pellets, sediment, detritus and other material found in the ocean, are fundamental to the carbon flux from the surface ocean to the deep ocean. Oceanographers observe that marine snow often accumulate in layers whose location are correlated with sharp density gradients in the water column. Understanding the formation and depletion of these marine layers is important to being able to accurately model the marine carbon cycle. A first step in an ongoing investigation is to study the settling of a single porous particle through ambient density gradients. We have conducted experiments to study the settling behavior of single porous spheres in sharp and linear density gradients. Experimental data are first compared to a model based on diffusive processes. Comparisons show that the model predicts accelerations of the particle but not the retention times accurately. Entrainment of less dense fluid from above is then included in the modeling, which allows retention times to be accurately captured. Entrainment shell thickness as a function of parameters will be discussed. [Preview Abstract] |
Tuesday, November 26, 2013 8:39AM - 8:52AM |
M36.00004: Rapid distortion theory for mixing efficiency of a flow stratified by one or two scalars Chris Rehmann, Jennifer Jefferson The mixing efficiency of unsheared homogeneous turbulence in flows stratified by one or two active scalars was calculated with rapid distortion theory (RDT). For one scalar the mixing efficiency $\eta $depends on the Schmidt number and the Grashof number. For two scalars the efficiency also depends on the density ratio $R_{\rho }$, which compares the density differences caused by temperature and salt. In the one scalar case when \textit{Gr }is large, $\eta $ decreases as \textit{Sc} increases. The mixing efficiency increases with \textit{Gr} up to a maximum value, as in numerical simulations and experiments. The maximum of approximately 30{\%} for low \textit{Sc} is consistent with simulations, while the maximum of 6{\%} for heated water is consistent with laboratory measurements. However, RDT underpredicts the maximum for saltwater and the value of \textit{Gr} at which the efficiency becomes constant. For two active scalars, $\eta $ decreases as $R_{\rho }$ decreases, as in experiments. Results from simulations with low \textit{Sc} likely overestimate the efficiency of turbulence in strongly stratified flows in lakes and oceans. [Preview Abstract] |
Tuesday, November 26, 2013 8:52AM - 9:05AM |
M36.00005: Reorientation of elongated particles at density interfaces Amin Doostmohammadi, Arezoo Ardekani The settling rates of particles and organisms in oceanic environments can be considerably affected by approaching density interfaces. The presence of density gradients have been correlated to important environmental phenomena such as accumulation of marine snow particles and intense biological activities. Although many of these settling particles and organisms are of elongated shapes, the current knowledge of settling through density interfaces centers around spherical particles. Here, we uncover the role of the density gradient in changing the orientation of elongated particles. By using direct numerical simulations, we demonstrate that unlike the homogeneous fluid, the presence of density gradients tend to turn the elongated particle so that its broadside is parallel to the direction of the gravity. We provide a phenomenological description of the underlying physics by characterizing deflection of isopycnals and generation of buoyancy induced vortices. [Preview Abstract] |
Tuesday, November 26, 2013 9:05AM - 9:18AM |
M36.00006: Stratified mixing by microorganisms Gregory Wagner, William Young, Eric Lauga Vertical mixing is of fundamental significance to the general circulation, climate, and life in the ocean. In this work we consider whether organisms swimming at low Reynolds numbers might collectively contribute substantially to vertical mixing. Scaling analysis indicates that the mixing efficiency $\eta$, or the ratio between the rate of potential energy conversion and total work done on the fluid, should scale with $\eta \sim (a / \ell)^3$ as $a / \ell \to 0$, where $a$ is the size of the organism and $\ell = \left ( \nu \kappa / N^2 \right )^{1/4}$ is an intrinsic length scale of a stratified fluid with kinematic viscosity $\nu$, tracer diffusivity $\kappa$, and buoyancy frequency $N^2$. A regularized singularity model demonstrates this scaling, indicating that in this same limit $\eta \approx 1.2 \left ( a / \ell \right )^3$ for vertical swimming and $\eta \approx 0.14 \left ( a / \ell \right )^3$ for horizontal swimming. The model further predicts the absolute maximum mixing efficiency of an ensemble of randomly oriented organisms is around 6\% and that the greatest mixing efficiencies in the ocean (in regions of strong salt-stratification) are closer to 0.1\%, implying that the total contribution of microorganisms to vertical ocean mixing is negligible. [Preview Abstract] |
Tuesday, November 26, 2013 9:18AM - 9:31AM |
M36.00007: Turbulence structure of gravity and turbidity currents Senthil Radhakrishnan, Mario Schiller, Eckart Meiburg DNS of moderate Reynolds number gravity currents has shown that the vortices in the near-wall region are qualitatively similar to the vortices found in the near-wall region of boundary layers, and the Kelvin-Helmholtz billows at the interface between the high and the low density fluid are similar to the ones observed in mixing layers. In the current work, we perform a quantitative study of the turbulence structure of gravity and turbidity currents at higher Reynolds number using LES. In gravity currents, the streamwise Reynolds stresses in the near-wall region match the ones observed in boundary layers. Wall-normal Reynolds stresses are, however, damped due to stable stratification. In turbidity currents, the bulk density in the near-wall region increases due to particle settling, which results in increased damping of Reynolds stresses due to stronger stratification effects. In the interfacial region, the wall-normal Reynolds stresses in gravity currents are damped as compared to the ones observed in mixing layer. In turbidity currents, however, the Reynolds stresses in the interfacial region match the ones in mixing layers. This is a result of weaker stratification as the density difference across the interfacial region decreases due to particle settling closer to the wall. [Preview Abstract] |
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