Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session E30: Geophysical Fluid Dynamics: Stratified Flows I |
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Chair: Alberto Scotti, University of North Carolina at Chapel Hill Room: North 229 B |
Sunday, November 21, 2021 2:45PM - 2:58PM |
E30.00001: Towards Simulation of Stratified Turbulent Wakes at Very High Reynolds numbers NIDIA CRISTINA, Peter Diamessis, Kristopher L Rowe, Greg Thomsen Analyzing the dynamics of stratified turbulent wakes through numerical simulation poses large computational and numerical challenges. We will present a high-order flow solver using a Fourier spectral method in the horizontal and a modal spectral element method (SEM) discretization in the wall-normal direction. The use of a modal SEM retains the flexibility of localized flow resolution of the wake core and, in combination with static condensation, results in a large number of small tridiagonal systems that allows us to have an algorithm that is as inexpensive as second-order finite difference schemes. Particular attention has been paid to aspects of the code design/implementation aimed to obtain simulations of wakes at a body-based Reynolds number $Re \sim O(10^6)$. We will discuss code performance, and present results from implicit Large Eddy Simulations of stratified sphere wakes at internal Froude number, $Fr=4$. The preliminary analysis will focus on studying turbulent fine structures during the intermediate-late stage of the wake evolution when the flow is considered to be under strongly stratified conditions. The duration of potentially different regimes at high $Re$ would then be quantified on appropriate regime diagrams. |
Sunday, November 21, 2021 2:58PM - 3:11PM |
E30.00002: Propagation of Internal Waves Generated by Localized Convection Daniel Lecoanet, Matteo Cantiello, Eliot Quataert, Evan H Anders, Louis-Alexandre Couston, Mathieu Bouffard, Benjamin Favier, Michael Le Bars When a stably-stratified fluid lays adjacent to a convective region, turbulent convection can excite internal gravity waves. Here we study the propagation of convectively excited waves to determine their amplitude and effects far from the generation region. We present a series of simplified analytical calculations that suggest how to use an eigenvector decomposition to efficiently model the wave propagation. We compare the results of this eigenvector decomposition to 2D numerical simulations. The eigenvector decomposition correctly describes the wave propagation, except for strongly damped waves, which can be modeled with a WKB approach. Put together, our calculations give robust predictions for the propagation of convectively-generated internal waves. |
Sunday, November 21, 2021 3:11PM - 3:24PM |
E30.00003: Role of independent stratification mechanisms on stable open-channel flows Inanc Senocak, Cheng-nian Xiao Long-lived or static stable stratification of the atmosphere is a common feature in high-latitude regions due to the absence of a neutral residual layer that serves as a memory of the daytime convective mixing. To study this problem, we perform a series of direct numerical simulations of turbulent open-channel flows, serving as a simple model for the atmospheric surface layer over a flat terrain. In this model problem, a neutrally stratfied turbulent flow is stabilized simultaneously by a constant negative surface buoyancy flux as well as an independent ambient stratification characterized by its own Brunt-Väisälä frequency N. When both types of stratification mechanisms are active, marked departures from MOST profiles are possible despite the overall weakly stable state of the bulk flow. We show that the degree of deviation from the neutral dimensionless shear as a function of the vertical coordinate can be used as a qualitative measure of the strength of stable stratification. Furthermore, an extended version of MOST, originally formulated by Zilintinkevich and Calanca in 2000, shows promise in capturing the dimensionless shear, but the extended similarity theory is less accurate in predicting the dimensionless gradients of scalar quantities. |
Sunday, November 21, 2021 3:24PM - 3:37PM |
E30.00004: Regime identification for stratified wakes from limited measurements using a library-based sparse regression formulation Mitul Luhar, Vamsi Krishna Chinta, Chan-Ye Ohh, Geoffrey R Spedding Bluff body wakes in stratified fluids are known to exhibit a rich variety of dynamic behavior that can be categorized into different dynamic regimes based on Reynolds number (Re) and Froude number (Fr). In this work, we attempt to identify the dynamic regime from limited measurement data in a stratified wake with (nominally) unknown Re and Fr. A large database of candidate basis functions is compiled by pooling the DMD modes obtained in prior DNS. A sparse model is built using the Forward Regression with Orthogonal Least Squares (FROLS) algorithm, which sequentially identifies DMD modes that best represent the data and calibrates their amplitude and phase. After calibration, the velocity field can be reconstructed using a weighted combination of the dominant DMD modes. The dynamic regime for the measurements is estimated via a projection-weighted average of Re and Fr corresponding to the identified modes. Regime identification is carried out from a limited number of 2D velocity snapshots from numerical and experimental datasets, as well as 3 point measurements in the wake of the body. A metric to assess confidence is introduced based on the observed predictive capability. This approach holds promise for the implementation of data-driven fluid pattern classifiers. |
Sunday, November 21, 2021 3:37PM - 3:50PM |
E30.00005: Coherence in turbulent stratified wakes through the lens of modal analysis Sheel Nidhan, Oliver T. T Schmidt, Sutanu Sarkar Coherent structures are extracted in stratified disk wakes and analyzed using spectral proper orthogonal decomposition (SPOD) at moderately high $Re = 5 \times 10^{4}$ and $Fr = 2, 10$. The obtained SPOD modes vary with modal index ($n$), frequency ($St$), and streamwise distance ($x/D$). Both wakes exhibit a low-rank behavior and the relative contribution of the low-rank modes to the turbulent kinetic energy increases with $x/D$. The vortex shedding (VS) mechanism, which corresponds to $St \approx 0.11-0.13$ in both wakes, is dominant throughout the domain, unlike their unstratified counterpart. The energy around the VS frequency appears in the outer region of the wake in the form of internal gravity waves (IGWs) beyond $Nt = Nx/U = 6 - 8$. Overall, we find that the coherence of wake turbulence, initiated by the VS mode at the body, is prolonged to the far downstream owing to the buoyancy. Also, this coherence is spatially modified by buoyancy into horizontal layers and IGWs. Low-order truncations of SPOD modes efficiently reconstruct important second-order statistics. |
Sunday, November 21, 2021 3:50PM - 4:03PM |
E30.00006: Coherent structures in the stratified near-wake of an inclined 6:1 prolate spheroid Chan-Ye Ohh, Geoffrey R Spedding Stratified wakes of various axisymmetric bodies have been well-studied where the wakes can be scaled based on the effective drag coefficient. However, non-axisymmetric wakes from a body at high incidence (e.g. inclined spheroid), complex surfaces (e.g. wingtip, fins), or a large maneuver introduces special flow structures such as strong trailing vortices that may generate patterns that can persist into the late wake. The current project seeks to investigate how these coherent structures interact with the drag wake in the presence of background stratification. An experiment is conducted on the near-wake of a towed 6:1 prolate spheroid with incidence angles θ = {0, 10, 20}° at Reynolds number based on the diameter Re_{D} = {0.5, 1, 2} × 10^{4} and Froude number Fr = {8, 16, 32, ∞}. To capture the complex 3D flow structure, tomographic PIV is used where a measurement depth of more than half wake width (~ 0.85D) is achieved. The averaged wake decay rate and the vortex pair propagation are compared with existing literature and described as functions of Re, Fr, and θ to find scalings that can be used to test extrapolation to higher Re, and also to provide information for potential pattern detection. |
Sunday, November 21, 2021 4:03PM - 4:16PM |
E30.00007: Dynamical Analysis On The Geometrical Properties Of Turbulent Coherent Structures Abhishek Paraswarar Harikrishnan, Rupert Klein, Nikki Vercauteren We study Ekman flow simulations which describe the Atmospheric Boundary Layer (ABL) in terms of its coherent structures. We classify coherent structures into six categories, all of which can be identified by using scalar criteria. An optimum threshold τ_{p} is defined for each scalar field by identifying the region of percolation transition. At τ_{p}, we extract the structures and follow them in time with in-house codes. We study their geometrical evolution with three parameters: Shape Index, Curvedness and Stretching. A point on a feature space with these parameters identifies whether a structure is rod-like, blob-like or sheet-like. If we follow the evolution of a rod-like structure, every transient state ζ is indicated by a point in the feature space. We can now think of this feature space as a phase space which hosts all possible geometrical states of coherent structures in the system. We use two instantaneous metrics to determine the dynamical properties of these geometrical states: the local dimension of the phase space d(ζ) and its persistence in time θ(ζ), where ζ denotes a location in the geometrical phase space. We compute these quantities for numerous samples of rod-like, blob-like and sheet-like structures in order to gain insight into the preferred shape of a structure. |
Sunday, November 21, 2021 4:16PM - 4:29PM |
E30.00008: Quantifying and predicting intermittency in stably stratified plane Poiseuille flow Haoyang Cen, Artem Korobenko, Qi Zhou Under sufficiently strong stratification, fully developed wall-bounded turbulent flows could transition to intermittent flows in which laminar and turbulent patches coexist. Using direct numerical simulations, we explore the boundary at which such transitions occur in the parameter space for stably stratified plane Poiseuille flow. A range of friction Reynolds and Richardson numbers, parameters that are observed to control the dynamics, are covered by the numerical simulations, which enable us to examine the applicability of various dimensionless parameters for predicting the occurrence of intermittency. We quantify the volume fraction of turbulent patches in each simulation and use these statistics to predict the occurrence of intermittency. Preliminary results suggest that the intermittency criterion documented for stratified plane Couette flow (Deusebio et al., J. Fluid Mech., volume 781, 2015) may not apply for the plane Poiseuille flow case. Alternative criteria for predicting intermittency are examined. |
Sunday, November 21, 2021 4:29PM - 4:42PM |
E30.00009: Experimental and theoretical study of a stratified boundary layer above a tilted undulated plate Sarah Christin, Patrice Meunier, Stephane Le Dizes Stratification plays a key role in the dynamics of geophysical flows. Stratified fluids support waves, called internal gravity waves (IGW), which can lead by their interactions to instabilities, turbulence, and ultimately mixing. Understanding these processes is a main concern for geophysicists who try to describe how energy is distributed and transported in these flows. |
Sunday, November 21, 2021 4:42PM - 4:55PM |
E30.00010: Experimental properties of continuously-forced, shear-driven, stratified turbulence Adrien Lefauve, Paul F Linden We study the experimental properties of exchange flows in a stratified inclined duct (SID), which are simultaneously turbulent, strongly stratified by a mean vertical density gradient, driven by a mean vertical shear, and continuously forced by gravity. We focus on the ‘core’ shear layer away from the duct walls, where these flows are excellent experimentally-realisable approximations of canonical hyperbolic-tangent stratified shear layers, whose forcing allows mean and turbulent properties to reach quasi-steady states. |
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