Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session L13: Geophysical Fluid Dynamics: Turbulence under Stable Stratification |
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Sponsoring Units: DFD GPC Chair: Luca Brandt, KTH Royal Institute of Technology Room: C124 |
Monday, November 21, 2016 4:30PM - 4:43PM |
L13.00001: ABSTRACT WITHDRAWN |
Monday, November 21, 2016 4:43PM - 4:56PM |
L13.00002: On the Orientation of Turbulent Structures in Stably Stratified Shear Flows Frank Jacobitz, Adam Moreau, Joylene Aguirre The orientation of turbulent structures in stably stratified shear flows are investigated using the results of a series of direct numerical simulations. The Richardson number is varied from $Ri=0$, corresponding to unstratified shear flow, to $Ri=1$, corresponding to strongly stratified shear flow. The evolution of the turbulent kinetic energy changes from growth for small Richardson numbers to decay for strong stratification. The orientation of turbulent structures in the flows is determined by the three-dimensional two-point autocorrelation coefficient of velocity magnitude, vorticity magnitude, and fluctuating density. An ellipsoid is fitted to the surface given by a constant autocorrelation coefficient value and the major and minor axes are used to determine the inclination angle of turbulent structures in the plane of shear. The inclination angle is observed to be fairly unaffected by the choice of the autocorrelation coefficient value. In was found that the inclination angle decreases with increasing Richardson number. The structure of the turbulent motion, as characterized by the inclination angle, is therefore directly related to the eventual evolution of the turbulence, as described by the growth or decay rate of the turbulent kinetic energy. [Preview Abstract] |
Monday, November 21, 2016 4:56PM - 5:09PM |
L13.00003: ABSRACT WITHDRAWN |
Monday, November 21, 2016 5:09PM - 5:22PM |
L13.00004: ABSTRACT WITHDRAWN |
Monday, November 21, 2016 5:22PM - 5:35PM |
L13.00005: Mixing efficiency of turbulent patches in stably stratified flows Amrapalli Garanaik, Subhas Karan Venayagamoorthy A key quantity that is essential for estimating the turbulent diapycnal (irreversible) mixing in stably stratified flow is the mixing efficiency $R_f^*$, which is a measure of the amount of turbulent kinetic energy that is irreversibly converted into background potential energy. In particular, there is an ongoing debate in the oceanographic mixing community regarding the utility of the buoyancy Reynolds number $(Re_b)$, particularly with regard to how mixing efficiency and diapycnal diffusivity vary with $Re_b$. Specifically, is there a universal relationship between the intensity of turbulence and the strength of the stratification that supports an unambiguous description of mixing efficiency based on $Re_b$? The focus of the present study is to investigate the variability of $R_f^*$ by considering oceanic turbulence data obtained from microstructure profiles in conjunction with data from laboratory experiments and DNS. Field data analysis has done by identifying turbulent patches using Thorpe sorting method for potential density. The analysis clearly shows that high mixing efficiencies can persist at high buoyancy Reynolds numbers. This is contradiction to previous studies which predict that mixing efficiency should decrease universally for $Re_b$ greater than $O(100)$. [Preview Abstract] |
Monday, November 21, 2016 5:35PM - 5:48PM |
L13.00006: Multiscale equations for strongly stratified turbulent flows Greg Chini, Cesar Rocha, Keith Julien, Colm-cille Caulfield Strongly stratified turbulent shear flows are of fundamental importance owing to their widespread occurrence and their impact on diabatic mixing, yet direct numerical simulations of such flows remain challenging. Here, a reduced, multiscale description of turbulent shear flows in the presence of strong stable density stratification is derived via asymptotic analysis of the governing Boussinesq equations. The analysis explicitly recognizes the occurrence of dynamics on disparate spatiotemoporal scales, and yields simplified partial differential equations governing the coupled evolution of slowly-evolving small aspect-ratio (`pancake') modes and isotropic, strongly non-hydrostatic stratified-shear (e.g. Kelvin--Helmholtz) instability modes. The reduced model is formally valid in the physically-relevant regime in which the aspect-ratio of the pancake structures tends to zero in direct proportion to the horizontal Froude number. Relative to the full Boussinesq equations, the model offers both computational and conceptual advantages. [Preview Abstract] |
Monday, November 21, 2016 5:48PM - 6:01PM |
L13.00007: Interaction between a vertical turbulent buoyant jet and a thermocline Ekaterina Ezhova, Luca Brandt, Claudia Cenedese We study the behaviour of an axisymmetric vertical turbulent jet in an unconfined stratified environment by means of well-resolved large eddy simulations (LES). The stratification is two layers separated by a thermocline and the thermocline thickness considered is smaller and on the order of the jet diameter at the thermocline entrance. We quantify mean jet penetration, stratified turbulent entrainment and study the generation of internal waves. The mean jet penetration is predicted based on the conservation of the source energy in the thermocline. The entrainment coefficient for the thin thermocline agrees with the theoretical model for a two-layer stratification with a sharp interface. A secondary flow towards the jet top appears in the upper part of the thick thermocline. The jet generates internal waves at frequencies in agreement with similar experiments. We shall also report the results of LES of a turbulent plume in a stratified fluid modelling subglacial discharge from a submarine glacier in stratifications typical of Greenland fjords. We consider a free plume from a round source of various diameters with double the total discharge estimated from the field data. We quantify plume dynamics and compare the results for plumes and jets. [Preview Abstract] |
Monday, November 21, 2016 6:01PM - 6:14PM |
L13.00008: A numerical investigation of the interaction between a horizontal density gradient and an oscillating turbulent flow Steven Kaptein, Matias Duran-Matute, Vincenzo Armenio, Federico Roman, Herman Clercx In coastal areas, river outflow provides a large buoyancy input that leads to strong horizontal density gradients. These density gradients are associated to complex hydrodynamics such as, penetration of fresh water currents in the ocean, coastal currents or strain-induced periodic stratification. One key governing mechanism is the interaction between stirring by the tides and horizontal density gradients which influences mixing . In order to investigate this mechanism and gain new insight into the mixing process, wall-resolving large eddy simulation (LES) are performed. The tide is simulated using a horizontal oscillating pressure gradient that acts perpendicular to a horizontal (unstable) linear density gradient. A decomposition of the density allows to apply periodic boundary conditions in the streamwise and spanwise directions, for both the velocity and the density. As the Reynolds number is limited by the computational time required for LES, simulations are performed for different values of the Richardson number. [Preview Abstract] |
Monday, November 21, 2016 6:14PM - 6:27PM |
L13.00009: ABSTRACT WITHDRAWN |
Monday, November 21, 2016 6:27PM - 6:40PM |
L13.00010: Accurate Calculation of the Linear Response Function of General Circulation Models Pedram Hassanzadeh, Zhiming Kuang A linear response function (LRF), $M$, relates the response, $x$, of a nonlinear system, such as the atmosphere, to weak external forcings, $f$, and tendencies, $\dot{x}$, via $\dot{x}=Mx+f$. Knowing the LRF of general circulation models (GCMs) helps with better understanding their internal and forced variability. But even for simple GCMs, $M$ cannot be calculated from first principles due to the lack of a complete theory for eddy-mean flow feedbacks. We present a new framework to accurately calculate the LRFs of GCMs using Green's function: by applying a sufficiently large set of localized forcings, one at a time, to the GCM, then calculating the time-mean responses, and finally finding the LRF via matrix inversion. We discuss the accuracy and properties of the LRF of an idealized GCM that has been calculated using this approach. An eddy flux closure matrix that determines the turbulent eddy flux responses to mean-flow changes is also calculated. Some results on using this LRF to quantify the eddy feedbacks and probe causality in the midlatitude large-scale circulation will be discussed. It will also be shown that the poor performance of another common approach to calculating the LRFs, the Fluctuation-Dissipation Theorem, is linked to the non-normality of the LRFs of GCMs. [Preview Abstract] |
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