Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session L17: Geophysical Fluid Dynamics: Gravity Currents |
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Chair: Claudia Cenedese, Woods Hole Oceanographic Institution Room: 2002 |
Monday, November 24, 2014 3:35PM - 3:48PM |
L17.00001: Entrainment in a density-driven current flowing down a rough slope in a rotating fluid Claudia Cenedese, Luisa Ottolenghi, Claudia Adduce Dense oceanic overflows mix with surrounding waters along the descent down the continental slope. The amount of entrainment dictates the final properties of these overflows, and thus is of fundamental importance to the understanding of the formation of deep water masses. We will discuss laboratory experiments investigating the influence of bottom roughness on entrainment in a dense current flowing down a sloping bottom in a rotating homogeneous fluid. The bottom roughness has been idealized by an array of cylinders. Both spacing (sparse vs. dense configuration) and height of the roughness elements compared with the height of the current have been varied. The presence of roughness elements has been observed to enhance entrainment for low values of the Froude number (\textit{Fr}). This suggests that if a dense current is vigorously entraining via shear-induced entrainment at the interface between the dense and ambient fluids (i.e. large \textit{Fr}) the additional entrainment occurring via the turbulence generated by roughness elements at the bottom boundary is negligible. However, for low \textit{Fr}, when the entrainment at the interface between the dense and ambient fluids is low, the additional entrainment due to bottom roughness elements dominates. As in the case of a smooth bottom, we observed a strong dependence of the entrainment on the Reynolds number. Furthermore, density measurements indicate that stratification within the dense current is enhanced when the roughness elements occupy a large portion of the current, especially for the dense roughness configuration. [Preview Abstract] |
Monday, November 24, 2014 3:48PM - 4:01PM |
L17.00002: Experimental and Numerical Studies of Oceanic Overflow Thomas Gibson, Fred Hohman, Theresa Morrison, Shanon Reckinger, Scott Reckinger Oceanic overflows occur when dense water flows down a continental slope into less dense ambient water. The resulting density driven plumes occur naturally in various regions of the global ocean and affect the large-scale circulation. General circulation models currently rely on parameterizations for representing dense overflows due to resolution restrictions. The work presented here involves a direct qualitative and quantitative comparison between physical laboratory experiments and lab-scale numerical simulations. Laboratory experiments are conducted using a rotating square tank customized for idealized overflow and a high-resolution camera mounted on the table in the rotating reference frame for data collection. Corresponding numerical simulations are performed using the MIT general circulation model (MITgcm) run in the non-hydrostatic configuration. Resolution and numerical parameter studies are presented to ensure accuracy of the simulation. Laboratory and computational experiments are compared across a wide range of physical parameters, including Coriolis parameter, inflow density anomaly, and dense inflow volumetric flow rate. The results are analyzed using various calculated metrics, such as the plume velocity. [Preview Abstract] |
Monday, November 24, 2014 4:01PM - 4:14PM |
L17.00003: Sensitivity of resolution and vertical grid types on 3D overflow simulations using mpas-ocean Shanon Reckinger, Mark Petersen, Scott Reckinger The Model for Prediction Across Scales (MPAS) is a climate model framework that supports unstructured, variable resolution grids. Since a primary issue in ocean modeling is the treatment of the vertical coordinate, MPAS-Ocean has been developed to allow for a variety of options in the vertical coordinate choice. The representation of overflows has been shown to be difficult at horizontal resolutions coarser than a few kilometers. Therefore, the combination of the unstructured horizontal grid and the variety of vertical grid choices available with MPAS-Ocean provides a unique approach. MPAS-Ocean is used to simulate an idealized density driven overflow using the dynamics of overflow mixing and entrainment (DOME) setup. Numerical simulations are carried out at a variety of resolutions to compare the accuracy and computational cost of increasing the vertical versus the horizontal resolution. Additionally, various vertical grid types are studied including z-level, z-level with partial bottom cells, and sigma coordinates. Entrainment and transport metrics are calculated and analyzed in order to compare the results from the various grid setups. [Preview Abstract] |
Monday, November 24, 2014 4:14PM - 4:27PM |
L17.00004: Mixing Induced by Slope and Valley Flow Collisions in Complex Terrain H.J.S. Fernando, C. Hocut, Q. Zhong Collision of slope and valley flows at night in complex terrain air basins lead to powerful, recurring turbulence generating events. The contributions of these collisions to turbulent mixing in complex terrain basins has been studied using data taken during the field experiments of Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program as well as laboratory measurements conducted under controlled conditions using counter flowing gravity currents in which detailed turbulence observations were made using LDV/PLIF. These collisions cause localized instabilities, which, together with turbulence generated by impingement of fronts on one another generate a turbulence field that decay rapidly under local stable stratification. Buoyancy fluxes measured during laboratory experiments are parameterized using suitable dimensionless parameters that characterize the nature of gravity currents. The laboratory results are compared with field measurments. [Preview Abstract] |
Monday, November 24, 2014 4:27PM - 4:40PM |
L17.00005: Lateral spreading in a steady turbulent density current from an isolated source Andrew Wells, Josh Vivian Turbulent buoyancy-driven flows on slopes occur in a range of environmental settings, such as dense ocean overflows, atmospheric katabatic winds, meltwater flows under ice shelves, or discharge of industrial effluents. A convenient modelling approach for dense currents from isolated sources considers a so-called ``streamtube approximation,'' averaging over the cross-section of the current to yield an effectively one-dimensional model for the evolution of flow along a streamline. However, such modelling approaches typically parameterise any changes in current width, rather than directly predicting the dynamics of lateral spreading. To build insight into the relevant dynamics, we consider steady density currents flowing down a planar slope, supplied by a continuous buoyancy flux from an isolated source. A model is developed to describe the downslope evolution of flow averaged over the width and depth of the current, including a new dynamical treatment of lateral spreading. We analyse theoretical and numerical solutions, before comparing to laboratory experiments with a dense saline current flowing down a slope. [Preview Abstract] |
Monday, November 24, 2014 4:40PM - 4:53PM |
L17.00006: Stratification established by peeling detrainment from gravity currents: laboratory experiments and models Charlie Hogg, Stuart Dalziel, Herbert Huppert, Jorg Imberger Dense gravity currents feed fluid into confined basins in lakes, the oceans and many industrial applications. Existing models of the circulation and mixing in such basins are often based on the currents entraining ambient fluid. However, recent observations have suggested that uni-directional entrainment into a gravity current does not fully describe the mixing in such currents. Laboratory experiments were carried out which visualised peeling detrainment from the gravity current occurring when the ambient fluid was stratified. A theoretical model of the observed peeling detrainment was developed to predict the stratification in the basin. This new model gives a better approximation of the stratification observed in the experiments than the pre-existing entraining model. The model can now be developed such that it integrates into operational models of lakes. [Preview Abstract] |
Monday, November 24, 2014 4:53PM - 5:06PM |
L17.00007: Gravity currents penetrating into a sheared and stratified ambient fluid Mohamad M. Nasr-Azadani, Amin Khodkar, Eckart Meiburg We have developed a circulation-based theoretical model to study gravity currents penetrating into a sheared and stratified ambient fluid. Unlike previous theories, our circulation model, which employs the vorticity equation, does not require any assumptions regarding headloss along a specific streamline in the flow. Our theoretical framework enables us to identify the existence of gravity currents penetrating into ambient environments having arbitrary velocity and/or density profiles across the channel height by means of a headloss analysis. First, we investigated a two-layered free stream configuration, where we observed excellent agreement between our theoretical model and DNS results. For various shear magnitudes, we demonstrated the existence of gravity currents of more than half of the channel height, which are not physically possible in the classical gravity current setup without any shear. Furthermore, we investigated the influence of stratification on the behavior of gravity currents. We identified the regions of physically feasible solutions. We also observed situations that produced internal waves and/or rarefaction waves. [Preview Abstract] |
Monday, November 24, 2014 5:06PM - 5:19PM |
L17.00008: Gravity current flow over topography with a two-layer stratified ambient Mitch Nicholson, Morris Flynn We report upon a laboratory experimental study of dense lock-released gravity currents propagating through a two-layer ambient and over sinusoidal topography of amplitude $A$ and wavelength $\lambda$ or $2\lambda$. Particular emphasis is placed on a Boussinesq flow regime and the initial (or ``slumping'') stage of motion. Because of the presence of the topography, the height of both the lower layer and the channel varies in the downstream direction. In contrast to the flat-bottom case, the gravity current front therefore accelerates and decelerates as it respectively flows up- and downhill. Overall, the topography has a retarding effect on the average front speed, $U_{\mathrm{avg}}$, whose variation with $A$, the layer densities and the interface height is described. The topography also alters the structure of the gravity current head by inducing large-scale vortices in regions characterized by a substantial shear flow. As in the flat-bottom case, the forward advance of the gravity current can excite a downstream-propagating interfacial wave. We identify the parametric region corresponding to wave generation. [Preview Abstract] |
Monday, November 24, 2014 5:19PM - 5:32PM |
L17.00009: Gravity currents in non-rectangular cross-area channels with stratified ambient Marius Ungarish The propagation of a high-Reynolds-number gravity current (GC) in a horizontal channel along the horizontal coordinate $x$ is considered. The current is of constant density, $\rho_c$, and the ambient has a linear stable stratification, from $\rho_b$ at the bottom $z=0$ to $\rho_o$ at $z=H$. The cross-section of the channel is given by the general $-f_1(z)\le y \le f_2(z)$ for $0 \le z \le H$. A shallow-water model is developed for the solution of a GC of fixed volume released from a lock on the bottom ($\rho_c \ge \rho_b$). The dependent variables are the position of the interface, $h(x,t)$, and the speed (area-averaged), $u(x,t)$, where $t$ is time. The cross-section geometry enters the formulation via the width of the channel $f(z) = f_1(z) + f_2(z)$. For a given $f(z)$, the free input parameters of are the height ratio $H/h_0$ of ambient to lock and the stratification parameter $S = (\rho_b - \rho_o)/(\rho_c - \rho_o)$. The equations of motion are a hyperbolic PDE system. The initial motion displays a ``slumping'' stage with constant speed, calculated analytically. An analytical solution for the long-time self-similar propagation is also available for special cases. The model is a significant generalization of the rectangular-channel analysis. [Preview Abstract] |
Monday, November 24, 2014 5:32PM - 5:45PM |
L17.00010: Gravity Currents Propagating Up a Slope in a Uniform Ambient and a Two-Layer Stratified Ambient Depth Larissa Marleau, Morris Flynn, Bruce Sutherland Bottom propagating gravity currents resulting from full- and partial-depth lock-release experiments are investigated as they propagate up a slope within a uniform and a two-layer stratified ambient. For the former case we adapt the theory of Shin et al. (J. Fluid Mech., 521, 2004) and derive a relationship for the deceleration of the front as a function of the slope angle and gravity current density. Experimental results show that the shape of the gravity current is self-similar as it decelerates over relatively steep slopes. The evolution of a gravity current in a two-layer ambient is complicated by the excitation of and interaction with a solitary wave. If the initial gravity current speed is subcritical to the wave speed, the current eventually stops abruptly while the wave continues at constant speed. If supercritical, turbulence at the current head is suppressed as it approaches the interface at the leading edge of the wave and the is re-established as the head becomes in direct contact with the upper layer. [Preview Abstract] |
Monday, November 24, 2014 5:45PM - 5:58PM |
L17.00011: Sediment-laden density currents propagating down slopes into stratified ambient Senthil Radhakrishnan, Kevin Schmidmayer, Eckart Meiburg Intrusions can form when sediment-laden gravity currents propagate down the continental slope into the density stratified ambient ocean. As the particles settle from the initially bottom propagating sediment-laden current, its bulk density decreases, and it eventually lifts off the ground to propagate as an intrusion current. Numerical simulations are performed to study such currents in the lock-exchange configuration. The flow characteristics of the currents, such as their front speed, their lift-off location and their deposit profiles are analyzed as functions of particle size, ambient strength and Reynolds number. As a general trend, currents with larger particles lift off earlier to form intrusions, and they propagate closer to the top surface as compared to currents with smaller particles. We furthermore compare our simulation results with laboratory experiments of Snow and Sutherland (2014). [Preview Abstract] |
Monday, November 24, 2014 5:58PM - 6:11PM |
L17.00012: Fluid-Mud Gravity Currents through Vegetation Firat Testik, Nazli Yilmaz This study was to investigate the effects of emergent stiff aquatic vegetation on the anatomy and propagation dynamics of fluid mud gravity currents. Fluid mud bottom gravity currents propagating through vegetated areas may form during coastal dredge disposal operations. Such currents have distinct anatomical and propagation characteristics. To study these non-Newtonian flows, a set of laboratory experiments were conducted with constant-flux release fluid mud (Kaolinite clay mixed with tap water) gravity currents propagating through a vegetated section of a laboratory tank. Emergent aquatic vegetation was simulated using stiff plastic rods of selected patterns. In the experiments, wide ranges of vegetation densities and fluid mud mixture concentrations were used. The experimental gravity currents experienced a drag-dominated propagation phase that was different than the typical propagation phases observed in the absence of vegetation. In this propagation phase, the gravity current exhibited a well-defined triangular / wedge profile. In the talk, these distinct gravity current characteristics associated with the vegetation effects will be discussed along with the underlying physical explanations and developed parameterizations. [Preview Abstract] |
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