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 S39: Geophysical Fluid Dynamics Stratified Flow IV |
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Chair: Sina Khani, University of Washington, API Room: 6a |
Tuesday, November 26, 2019 10:31AM - 10:44AM |
S39.00001: Turbulence in Forced Stratified Exchange Flows Katherine M. Smith, John R. Taylor, Jamie Partridge, Adrien Lefauve, Paul Linden, C. P. Caulfield Continuously forced, stratified exchange flows occur in many geophysical systems, such as through channels between ocean basins, between coastal shelves and the deep ocean, and at the mouth of rivers and estuaries. These exchange flows can be susceptible to instabilities that promote the growth of turbulence and increase mixing between the two differing flows. While these mixing processes are assumed to be important to global ocean budgets, they are unresolved within Earth system models and therefore must be fully understood in order to include accurately through sub-grid scale parameterization. In this talk, we present results from three-dimensional direct numerical simulations of stratified exchange flows that are continuously forced by weakly relaxing the buoyancy and streamwise velocity back to their initial mean profiles. We explore a range of large and small values of the bulk Richardson number and, after an initial `spin-up' period, a turbulent steady state is analyzed. Both turbulence and mixing are characterized in each case and comparisons to experimental results are discussed. [Preview Abstract] |
Tuesday, November 26, 2019 10:44AM - 10:57AM |
S39.00002: Large Eddy Simulation of the response of a stratified reservoir with inclined walls to an oscillating surface shear stress Sara Markovic, Vincenzo Armenio We present results of numerical simulations of the response of a reservoir with two-layer stratification to an oscillating surface shear stress. The simulations are carried out at a laboratory scale, using Large Eddy Simulation. We solve the three dimensional Navier-Stokes equations under the Boussinesq approximation for the density field using a model based on \texttt{buoyantBoussinesqPimpleFoam} implemented in the OpenFOAM library. The model was validated by reproducing experimental results for reservoir response to surface shear stress and resonant frequencies of the internal waves. The response of different domain geometries was investigated, obtained varying the inclination angle of the side walls, from zero (vertical walls) to $30^\circ$. When the frequency of the forcing is close to the first mode, resonant internal seiche occurs, in other cases forcing conditions are non-resonant. The results of simulations show that the slope of the side walls dramatically affects the surface mixed layer. Mixing is enhanced and penetrates deeper in the vertical as slope inclination grows. Under resonant forcing, interfacial waves are much more energetic and there is less change in the density profile compared to non-resonant conditions. [Preview Abstract] |
Tuesday, November 26, 2019 10:57AM - 11:10AM |
S39.00003: Interaction between an inclined gravity current and a pycnocline in a two-layer stratification Yukinobu Tanimoto, Nicholas Ouellette, Jeffrey Koseff A series of laboratory experiments were conducted to investigate the characteristics of a dense gravity current flowing down an inclined slope into a quiescent two-layer stratification. The presence of the pycnocline causes the gravity current to split and intrude into the ambient at two distinct levels of neutral buoyancy, as opposed to the classical description of gravity currents in stratified media as being either a pure underflow or interflow. The splitting behavior is observed to be dependent on the Richardson number (Ri) of the gravity current, formulated as the ratio of the excess density and the ambient stratification. For low Ri, underflow is more dominant, while at higher Ri interflow is more dominant. As Ri increases, however, we find that the splitting behavior eventually becomes independent of Ri. Additionally, we have also identified two different types of waves that form on the pycnocline in response to the intrusion of the gravity current. An underflow-dominated regime causes a pycnocline displacement where the speed of the wave crest is locked to the gravity current, whereas an interflow- dominated regime launches an internal wave that moves much faster than the gravity current head or interfacial intrusion. [Preview Abstract] |
Tuesday, November 26, 2019 11:10AM - 11:23AM |
S39.00004: Modelling stratified wall-bounded turbulence using resolvent analysis Muhammad Ahmed, Andrew Thompson, Beverley McKeon The effects an active scalar has on incompressible wall-bounded turbulence are investigated using the resolvent framework (McKeon \& Sharma, 2010, JFM). The state of the flow system is expressed as the result of applying the linear resolvent operator to the nonlinear terms in the governing Navier-Stokes equations with the Boussinesq approximation. To investigate the relationship between velocity and scalar fluctuations, the formulation is extended to include a scalar equation (Dawson et al., 2018, AIAA) and a scalar component acting in the wall-normal direction in the momentum equations. It is found that the Richardson number has a significant effect on the shape and phase of the velocity and scalar modes across the critical layer. In addition, it is shown that active scalar modes have a significant impact on the energy transfer between velocity mode components at varying scales. Furthermore, we visualise mode combinations that are representative of coherent structure observed in the atmosphere and oceans to gain a better understanding of the spatial wavenumber spectrum observed in nature. [Preview Abstract] |
Tuesday, November 26, 2019 11:23AM - 11:36AM |
S39.00005: Large-eddy simulations of stratified turbulence: an anisotropic subgrid-scale closure Sina Khani, Michael Waite The horizontal and vertical grid spacings are generally equal in large-eddy simulations (LES) of stratified turbulence. In simulations of larger-scale motions, however, it is computationally affordable to use different grid spacings in the horizontal and vertical. In this talk, we introduce a new subgrid-scale (SGS) parameterizations based on horizontal filtering of equations of motions in stratified turbulence. The horizontal and vertical SGS dissipations are not disconnected because the vertical derivatives of horizontal SGS fluxes are included in our newly developed closure, unlike the common horizontal SGS closures in atmospheric and oceanic models. Our anisotropic model can successfully reproduce the flow characteristics, moments and parameters, such as time series of kinetic and potential energy, horizontal and vertical wavenumber energy spectra and mixing efficiency, similar to those in direct numerical simulations while the computational cost is considerably reduced in LES. We suggest that our new SGS model can also improve the dissipative performance of the horizontal Smagorinsky closure in current atmospheric and oceanic models without adding any ad hoc energizing terms at smaller scales. [Preview Abstract] |
Tuesday, November 26, 2019 11:36AM - 11:49AM |
S39.00006: Wall shear stress fluctuations induced by shallow-water Langmuir turbulence Bingqing Deng, Anqing Xuan, Lian Shen In shallow water with the existence of surface waves, large-scale full-depth Langmuir circulations can be generated due to the interaction between wind-driven currents and the Stokes drift of shallow-water waves. We perform wall-resolved LES of the Craik-Leibovich equation to study shallow-water Langmuir turbulence. It is found that the full-depth Langmuir circulations directly leave large-scale footprint near the water bottom and hence have significant contributions to the wall shear stress fluctuations. The full-depth Langmuir circulations also alter the distribution of other coherent structures, so that the distribution of the wall shear stress fluctuations contributed by other coherent structures is different from the wall turbulence in the absence of Stokes drift of surface waves. The magnitude of the wall shear stress fluctuations at various locations is predicted by the velocity fluctuations induced by the full-depth Langmuir circulations. [Preview Abstract] |
Tuesday, November 26, 2019 11:49AM - 12:02PM |
S39.00007: DNS of stratified Ekman layers over rough surfaces Sungwon Lee, Iman Gohari, Sutanu Sarkar We investigate the evolution of stratified Ekman boundary layers in the presence of surface roughness. The roughness elements are 2-dimensional bumps and the neutral flow is in the transitionally rough regime. A cooling buoyancy flux which is applied for a finite time period is responsible for the stabilizing stratification. The Reynolds number is moderate so as that a broad parametric study of the influence of roughness height ($h^+$) and cooling flux (normalized Obukhov length, $L^+$) can be performed using direct numerical simulation (DNS). A cooling flux corresponding to $L^+ \approx 700$ is sufficient to cause the initial collapse of turbulence for both smooth and rough surfaces. Buoyancy and the slope of the surface roughness elements act in conjunction to affect the state of boundary-layer turbulence after the initial transient. The final value of the bulk Richardson number ($Ri_b$), which is a function of both $L^+$ and roughness properties, is found to provide guidance on the overall state of the flow, e.g., weak or strongly stable in the sense of Mahrt (1998); continuous, globally intermittent or locally intermittent. [Preview Abstract] |
Tuesday, November 26, 2019 12:02PM - 12:15PM |
S39.00008: Multiphase plumes in a rotating environment Daria Frank, Julien Landel, Stuart Dalziel, Paul Linden The Deepwater Horizon (DwH) blowout in 2010 resulted in a formation of a multiphase plume consisting of oil droplets and gas bubbles. The multiphase component of a plume is commonly characterised by the so-called slip velocity. Additionally, the duration of the DwH spill of several months implies that the plume is likely to have been affected by the Earth's rotation as was conjectured by previous numerical and experimental studies. The complex interplay between the slip velocity of the multiphase effluent and the system rotation as well as their combined effects on the internal plume dynamics and the associated subsurface dispersion of pollutants are still poorly understood. Yet, characterising these effects is important for oil spill mitigation purposes. In this talk, we present results from small-scale laboratory experiments on bubble plumes released into a rotating environment that were conducted for a wide range of Rossby numbers and several bubble slip velocities. By performing a rigorous image analysis, we focus on three particular aspects of the problem: the evolution of the plume structure during its initial rise, the subsurface dispersion of the effluent once the plume has risen through the entire water column and the lateral spreading of the plume on the water surface. [Preview Abstract] |
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