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 M1: Geophysical: Ocean III |
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Chair: Bruce Sutherland, University of Alberta Room: 22 |
Tuesday, November 20, 2012 8:00AM - 8:13AM |
M1.00001: A model for internal bores in continuously stratified fluids Brian White Internal bores, propagating horizontal jumps that connect regions of varying density structure, are generated in the ocean by stratified tidal flow over topography, river plumes, and the breaking of internal waves on a slope. In an undular or soli-bore, internal waves are radiated from the jump and in some cases strong shear and turbulent mixing may occur. Theories for the propagation speed and energy flux through an internal jump have been based on idealized two-layer stratification and require assumptions about the distribution of energy dissipation between internal layers. We discuss a theoretical model for an internal jump in a continuously stratified fluid that applies the Dubreil-Jacotin-Long (DJL) equation (a model for nonlinear solitary waves for which an energy-conserving bore is a well-known limiting solution) and adds a term for depth-dependent dissipation to calculate the velocity and density structure across the jump. The applicability of the continuous model is explored with two- and three-dimensional Navier Stokes calculations of internal bores formed from dam-break initial conditions, focusing on the influence of ambient stratification on energy dissipation and turbulent mixing. [Preview Abstract] |
Tuesday, November 20, 2012 8:13AM - 8:26AM |
M1.00002: Mixing by internal waves impinging on a slope Vamsi Krishna Chalamalla, Sutanu Sarkar Direct and large eddy simulations are performed to study the mixing that occurs when internal waves interact with critical and near-critical sloping bottom. Different horizontal wave lengths up to $\mathcal{O} \sim$ 1 km are considered at a moderate value of Froude number. The pathway from the input wave energy to the irreversible mixing of density field is explored. Diagnostics such as turbulent kinetic energy and density variance budget are used to obtain the phasing of turbulence and associated mixing. Energy transfer to higher harmonics and subharmonics is also quantified. [Preview Abstract] |
Tuesday, November 20, 2012 8:26AM - 8:39AM |
M1.00003: Transport by Internal Waves Near the Boundary of a Lake Chris Rehmann, Danielle Wain Because fluxes in stratified water bodies are often controlled by turbulence and mixing at sloping boundaries, determining how the mixed fluid moves from the boundary to the interior is important for estimating basin-wide transport of heat and other scalars. We conducted a tracer-release experiment in a lake that illustrates the importance of advection and dispersion driven by internal waves. Motivated by those observations, we developed an analytical model of transport by internal waves. For tracer releases near the boundary, the velocity field resulting from vertical mode-2 waves consists of oscillatory strain, the oscillating analog of flow near a stagnation point. The horizontal and vertical length scales of the tracer cloud oscillate about the values in the base case with no waves, and the deviation from the base case depends on the ratio of the amplitude of the isotherm displacements and the water depth, as well as the phase at which the tracer was injected. The effect of different modes and the implications for basin-scale transport will also be discussed. [Preview Abstract] |
Tuesday, November 20, 2012 8:39AM - 8:52AM |
M1.00004: Gravity Currents and Internal Solitary Waves Approaching Slopes Bruce Sutherland, Delyle Polet, Greg Ivey The theory for the speed of gravity currents and solitary waves usually assumes the depth of the ambient is constant. An adaptation of WKB theory predicts that gravity currents decelerate at a constant rate as they approach as uniform, though small slope. This is bourne out in laboratory experiments of surface gravity currents propagating over a bottom slope even when the rise of the slope is comparable to the head height over its length. However, the deceleration is found to differ by an order of magnitude. The mathematics used to describe internal solitary waves is qualitatively different from that used to describe gravity currents. And yet their dynamics upon approaching a slope is similar. Of particular interest for the off-shore oil industry is where the separation point of the wave trough impacting upon the slope occurs. On the basis of experiments this is predicted as it depends upon the ambient fluid and slope. [Preview Abstract] |
Tuesday, November 20, 2012 8:52AM - 9:05AM |
M1.00005: Three-dimensional structure of vortex shedding beneath internal solitary waves of depression in a two-layered system Payam Aghsaee, Leon Boegman Field observations show bed sediment re-suspension occurs in the wake of internal solitary waves (ISWs) of depression traveling over flat and sloping ocean topography. Previous studies suggest that near-bed vortex shedding elevates the near-bed shear and Reynolds stress fields leading to re-suspension. However this work has been limited to investigating the two-dimensional (2D) flow structure and planar PIV results are inconsistent with 2D DNS; vortices ascend higher in the watercolumn in 2D relative to 3D. In this study we present the first three-dimensional (3D) acoustic (ADV) and optical (stereo PIV) observations of vortex shedding beneath ISWs. We show that vortex shedding occurs for smaller values of Reynolds number and pressure gradient parameters in 3D, compared to 2D DNS. The ADV profile data shows transverse velocity fluctuations to be of the same order as horizontal and vertical ones, and this contributes to faster energy dissipation of vortices in 3D relative to 2D simulations, thus limiting the height to which vortices ascend into the watercolumn and potentially transport sediment. [Preview Abstract] |
Tuesday, November 20, 2012 9:05AM - 9:18AM |
M1.00006: Mixing by internal waves Philippe Odier, Baptiste Bourget, Sylvain Joubaud, Thierry Dauxois A key-ingredient in oceanic dynamics is the mixing between waters of different densities. Internal gravity waves, ubiquitous in the ocean, are expected to contribute to these mixing processes. We performed a preliminary experimental study of the mixing induced by a vertical mode-1 or mode-2 internal wave propagating in a linearly stratified fluid. We use a conductivity probe to follow the evolution with time of the density profile, while the wave is continuously generated in the fluid. We observe that a mixing layer is formed at the location of the velocity minima (maximum shear). By measuring the evolution with time of the local Brunt-V\"ais\"al\"a frequency in this region, we are able to derive a mixing velocity. We notice that this mixing velocity increases with the amplitude of the forcing, as expected. We also observe a strong increase of this velocity when the internal wave becomes unstable via a triad resonant interaction (Parametric Subharmonic Instability, see associated abstract by B. Bourget), resulting in the growth of two daughter waves of smaller wave length and frequency. [Preview Abstract] |
Tuesday, November 20, 2012 9:18AM - 9:31AM |
M1.00007: Experimental Study of Parametric Subharmonic Instability in Stratified Fluids Baptiste Bourget, Sylvain Joubaud, Philippe Odier, Thierry Dauxois Internal waves are believed to be of primary importance as they affect ocean mixing and energy transport. Several processes can lead to the breaking of internal waves and they usually involve non linear interactions between waves. In this work, we study experimentally the Parametric Subharmonic Instability, which provides an efficient mechanism to transfer energy from large to smaller scales. It consists in the destabilization of a primary wave and the spontaneous emission of two secondary waves, of lower frequencies and different wave vectors. We observe that the instability displays a different behavior if the primary wave is a monochromatic vertical mode-1 or a plane wave. Moreover, using a time-frequency analysis, we are able to observe the time evolution of the secondary frequencies. Using a Hilbert transform method we measure the different wave vectors and compare with theoretical predictions. As will be shown further, this instability plays a role in the mixing processes of stratified fluids (see abstract from P. Odier). [Preview Abstract] |
Tuesday, November 20, 2012 9:31AM - 9:44AM |
M1.00008: Reflection of an internal gravity wave beam off a horizontal free-slip surface Qi Zhou, Peter Diamessis The reflection of a planar finite-amplitude internal gravity wave beam off a free-slip flat horizontal surface is investigated numerically in a uniformly stratified Boussinesq fluid. Nonlinear effects such as mean currents and harmonics are observed in the wave reflection zone. Mean currents form a stationary, vertically oscillatory, layered structure under the free-slip reflecting surface. The vertical wavelength of the mean-flow layers equals half of the vertical wavelength of the reflecting wave. An empirical predictive model for the steady-state mean flow strength, based on the degree of wave nonlinearity and hydrostaticity, is proposed and subsequently compared to the weakly-nonlinear theory by Tabaei \textit{et al.}, \textit{J. Fluid Mech.}, 2005, \textit{vol.} 526, \textit{pp.} 217-243. Both propagating and evanescent superharmonics are observed, and for waves with steepness of $O$(5\%), subharmonic instabilities can occur in the late-time of reflection. Other complications to the basic set-up, such as addition of a subsurface mixed layer and spanwise localization of beam, will also be discussed. [Preview Abstract] |
Tuesday, November 20, 2012 9:44AM - 9:57AM |
M1.00009: Internal waves in the Petacalco canyon, Mexico Angel Ruiz-Angulo, Jorge Zavala-Hidalgo On the Mexican coastline, specifically on the Pacific side, there are many submarine canyons. One of the key rolls of coastal submarine canyons is that deep water from adjacent oceanic regions is brought up to the shelf with lots of nutrients enhancing primary production. The head of the Petacalco canyon is located in the Petacalco Bay, in the Pacific Ocean (ca. 17.5N and 102W). During, previous CTD surveys in the area, strong upwelling has been noticed, based on those observations a later survey was designed covering the Petacalco canyon with much larger spatial resolution. Along with those measurements, two thermistor arrays were deployed on the SW crest of the canyon at depths of approximately 60 [m]. The observations, from the thermistor arrays, show large temporal temperature variations with a semi-diurnal frequency. Those variations suggest the presence of internal waves traveling along the canyon axis, if the incidence angle of the internal wave matches the topographic slope results on breaking of internal waves enhancing mixing. This condition occurs at several locations along the canyon axis producing enough mixing of deep oceanic waters with continental waters, increasing the abundance of nutrients in the surrounding region. [Preview Abstract] |
Tuesday, November 20, 2012 9:57AM - 10:10AM |
M1.00010: Settling dynamics of a non-neutrally buoyant particle in stratified fluids Amin Doostmohammadi, Sadegh Dabiri, Arezoo Ardekani Sedimentation is considered as one of the most important phenomena in characterizing the geochemistry of atmosphere and upper ocean. The vertical variations of temperature and salinity in these environmental systems can have a large impact on settling dynamics of suspended particles. Although the drag increase of the settling particles has been well documented in the recent decade, the fundamental fluid dynamics of unsteady particle-fluid interaction in the presence of density gradients is yet to be explored. Most of the experimental studies have focused on settling in sharp stratified fluids and the numerical works have been limited to steady state flows around axisymmetric neutrally buoyant particles in a linear stratified fluid. We implement a direct numerical simulation of the particle descent in both continuous and sharp stratified fluids to unveil the time dependent response of a non-neutrally buoyant particle. The relative importance of inertia, buoyancy, viscosity and diffusivity is characterized for a wide range of pertinent parameters. Moreover, the quantified investigation of the stratification effects on partial drift volume and time dependent added-mass force will shed light on recent arguments about the importance of drift mechanism in biogenic ocean mixing. [Preview Abstract] |
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