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 M32: Geophysical Fluid Dynamics: Mixing and Sediment TransportGeophysical
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Chair: Oliver Fringer, Stanford University Room: 104 |
Tuesday, November 21, 2017 8:00AM - 8:13AM |
M32.00001: ABSTRACT WITHDRAWN |
Tuesday, November 21, 2017 8:13AM - 8:26AM |
M32.00002: The effect of existing turbulence on stratified shear instability Alexis Kaminski, William Smyth Ocean turbulence is an essential process governing, for example, heat uptake by the ocean. In the stably-stratified ocean interior, this turbulence occurs in discrete events driven by vertical variations of the horizontal velocity. Typically, these events have been modelled by assuming an initially laminar stratified shear flow which develops wavelike instabilities, becomes fully turbulent, and then relaminarizes into a stable state. However, in the real ocean there is always some level of turbulence left over from previous events, and it is not yet understood how this turbulence impacts the evolution of future mixing events. Here, we perform a series of direct numerical simulations of turbulent events developing in stratified shear flows that are already at least weakly turbulent. We do so by varying the amplitude of the initial perturbations, and examine the subsequent development of the instability and the impact on the resulting turbulent fluxes. [Preview Abstract] |
Tuesday, November 21, 2017 8:26AM - 8:39AM |
M32.00003: The effect of different methods to compute N on estimates of mixing in stratified flows Oliver Fringer, Robert Arthur, Subhas Venayagamoorthy, Jeffrey Koseff The background stratification is typically well defined in idealized numerical models of stratified flows, although it is more difficult to define in observations. This may have important ramifications for estimates of mixing which rely on knowledge of the background stratification against which turbulence must work to mix the density field. Using direct numerical simulation data of breaking internal waves on slopes, we demonstrate a discrepancy in ocean mixing estimates depending on the method in which the background stratification is computed. Two common methods are employed to calculate the buoyancy frequency N, namely a three-dimensionally resorted density field (often used in numerical models) and a locally-resorted vertical density profile (often used in the field). We show that how N is calculated has a significant effect on the flux Richardson number Rf, which is often used to parameterize turbulent mixing, and the turbulence activity number Gi, which leads to errors when estimating the mixing efficiency using Gi-based parameterizations. \newline [Preview Abstract] |
Tuesday, November 21, 2017 8:39AM - 8:52AM |
M32.00004: Mixing in stratified fluids induced by bubble swarms Abril Amezcua-Montiel, Angel Ruiz-Angulo, Roberto Zenit, B. Subramanian, Paolo Luzzatto-Fegiz, Marco Carminati The mixing dynamics induced by bubble swarms rising across a sharp stably-stratified density interface are studied experimentally. In the middle of a column, an interface separates two Newtonian-miscible-liquids: fresh water and brine. The bubble swarm is injected from the bottom with a bank of small capillaries. When the bubbles cross the interface, they drag denser fluid into the upper lighter fluid and then some denser fluid returns to the lower layer (Diaz-Damacillo et al., 2015). This process induces mixing. We record the bubble with a high speed camera and track the temporal evolution of the fluids conductivity with a \textit{Conduino} (Carminati and Luzzatto-Fegiz, 2017). We obtain the mixing coefficient , ${D_b}$, by fitting the concentration profiles to a simplified advection-diffusion equation, $\partial c / \partial t = D_b \nabla^2 c$. Experiments are conducted for a range of gas volume fractions and density contrasts between the fluids. Finally, we discuss the implications of our results for lake and ocean mixing. [Preview Abstract] |
Tuesday, November 21, 2017 8:52AM - 9:05AM |
M32.00005: Direct numerical simulations of fluvial bedforms under turbulent flow. Nadim Zgheib, J.J. Fedele, D.C.J.D. Hoyal, M.M. Perillo, S. Balachandar We examine fluvial bedforms using bed-flow coupled direct numerical simulations in a turbulent open channel at a shear Reynolds number of Re$_{\mathrm{\tau }}=$ 180 \begin{figure}[htbp] \centerline{\includegraphics[width=0.63in,height=0.17in]{250720171.eps}} \label{fig1} \end{figure} . The back coupling from the temporally and spatially evolving bed to the flow is enforced via the immersed boundary method. Using the near-bed flow field, we provide evidence on the role of locally intense near-bed vortical structures during the early stages of bed formation, from the emergence of quasi-streamwise streaks to the formation of incipient bedform crestlines. Additionally, we take a new look at a number of defect-related bedform interactions, including lateral linking, defect and bedform repulsion, merging, as well as defect creation and show that the underlying mechanisms, in these flow-aligned interactions, are very similar. Consequently, the interactions are labelled differently depending on the geometry of interacting structures and the outcome of the interaction. We compare our results to published experimental data and demonstrate the importance of neighbouring structures, especially upstream neighbours, on bedform dynamics and wave coarsening. We also show that not only do bedforms attain a self-similar shape, but that the bed shear stress becomes self-similar as well.. [Preview Abstract] |
Tuesday, November 21, 2017 9:05AM - 9:18AM |
M32.00006: ABSTRACT WITHDRAWN |
Tuesday, November 21, 2017 9:18AM - 9:31AM |
M32.00007: A probabilistic description of sediment rest and motion regimes under varying shear stress Mingxiao Liu, Michele Guala The kinematics of sand grain particles is investigated experimentally to provide a statistical description of bedload transport under varying shear stress, close to critical mobility conditions. In particular, we focus on the continuous sequence of particle steps and rests to provide a Lagrangian statistical description of particle kinematics. Within the limitations of the spatio-temporal domain of our submerged camera, we are able to identify the probability distributions of the particle step time and length, velocity, acceleration, and waiting time. The tail thickness of these distributions allow us to identify quantities, characterized by an exponential distribution, and exhibiting well converged mean values, as opposed to, e.g. the waiting (or rest) time, characterized by a power law distribution and converging very slowly. The experimental results shown here for four different shear stress conditions highlight the importance of the waiting time distribution in the stochastic description and modeling of particle transport. A distinction between active and deep waiting times is also discussed, and related to the effect of turbulent structures at the wall. [Preview Abstract] |
Tuesday, November 21, 2017 9:31AM - 9:44AM |
M32.00008: A comparison of microspheres and sediment drag using a Visual Accumulation Tube John Garcia, Bruce J Harrison, Michael Hargather A Visual Accumulation Tube (VAT) is an instrument used to measure particle sizes in a collected sand sample based on terminal velocity and Stokes Law. The particles of interest have diameters between 1 and 1000 micrometers and samples, which are typically collected from rivers as suspended sediment, can be as small as 0.05 grams dry weight. A modernized VAT was constructed at New Mexico Tech consisting of a stepper motor to release particles into a 1.2 meter distilled water column, a video camera to collect settling data, and an automated MATLAB routine to extract particle sizes from the video recordings. Fundamental experiments were performed to validate particle size measurements and Stokes Law drag assumptions for spherical particles of a known size and coarse-grained sand samples of known size distributions. [Preview Abstract] |
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