Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session Z33: Geophysical Fluid Dynamics: Stratified Flows II |
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Chair: Alexis Kaminski, UC Berkeley Room: 241 |
Tuesday, November 22, 2022 12:50PM - 1:03PM |
Z33.00001: Stratified shear instabilities in estuarine fluid muds Alexis K Kaminski, Junbiao Tu, William D Smyth Stratified shear instabilities are an important physical process in a variety of environmental flows, and understanding their dynamics is key to modelling turbulent fluxes. In estuaries, the underlying stratification may arise not only from heat and salt, but also from suspended sediment. Here, we compare echosounder observations of shear instabilities growing on a lutocline in a tidally-driven estuarine flow with direct numerical simulations of Kelvin-Helmholtz instability. We explore the relationship between characteristic lengthscales describing to the height of the primary billow (measured from the echosounder data) and the corresponding initial Richardson number and turbulent dissipation rate. While we show that useful parameterizations can be found for both quantities, our estimates suggest that sediment stratification leads to altered scalings which remain to be fully explained. |
Tuesday, November 22, 2022 1:03PM - 1:16PM |
Z33.00002: Direct numerical simulations of particles settling in stratified fluids Abdullah Abdal, Lyes Kahouadji, Seungwon Shin, Jalel Chergui, Damir Juric, Omar K Matar Settling particles moving through sharp interfaces or continuously stratified layers are found in a variety of engineering and geophysical applications. Generally, engineering applications include settling particles in a column of immiscible fluids, where viscosity and surface tension play a key role in the settling dynamics. The non-dimensional parameterisation for a column of immiscible fluids consists of the Bond number, the Archimedes number, and the viscosity ratio. For geophysical applications, the fluid comprises sharp density layers due to temperature or salinity gradients, and the parameterisation consists of the non-dimensional Froude number as well as the particle Reynolds number. We perform three-dimensional direct numerical simulations to validate our numerical framework, which uses a hybrid front-tracking level-set interface capturing algorithm, with existing experimental and computational studies of settling particles in both immiscible and miscible fluid columns. The results show that stratification has an important effect on the particle settling velocities caused by the particle dragging a column of fluid with different physical parameters to the neighbouring fluid. |
Tuesday, November 22, 2022 1:16PM - 1:29PM |
Z33.00003: Critical density triplets for the arrestment of a sphere falling in a sharply stratified two-layer fluid Roberto Camassa, Lingyun Ding, Richard M McLaughlin, Ashuwin Vaidya We study the motion of a rigid sphere falling in a two-layer stratified fluid under the action of gravity in the potential flow regime. Experiments at a moderate Reynolds number of approximately 20 to 450 indicate that a sphere with the precise critical density, higher than the bottom layer density, can display behaviors such as bounce or arrestment after crossing the interface. We experimentally demonstrate that such a critical sphere density increases linearly as the bottom fluid density increases with a fixed top fluid density. Additionally, the critical density approaches the bottom layer fluid density as the thickness of density transition layer increases. We propose an estimation of the critical density based on the potential energy. With assuming the zero layer thickness, the estimation constitutes an upper bound of the critical density with less than 0.043 relative difference within the experimental density regime 0.997 $g/cm^{3}$ $\sim $ 1.11 $g/cm^{3}$ under the zero layer thickness assumption. By matching the experimental layer thickness, we obtain a critical density estimation with less than 0.01 relative difference within the same parameter regime. TIme permitting, we will discuss additional experimental observations including depth of arrestment, and theoretical and computational approaches. |
Tuesday, November 22, 2022 1:29PM - 1:42PM |
Z33.00004: Internal wave families in stratified flow past a bluff body Divyanshu Gola, Sheel Nidhan, Sutanu Sarkar The wave field of a horizontally moving circular disk in a stratified fluid is studied using large eddy simulations. Linear as well as nonlinearly stratified backgrounds are investigated. The diameter based Reynolds number is 5000 and the minimum background Froude number is 1 for all cases. In linear stratification, steady lee waves and unsteady wake generated waves are found as in previous work. In the case of nonlinear stratification, an additional third family, namely, steady Kelvin wake waves, which resemble the Kelvin ship wave pattern, is also found. The qualitative difference between the steady lee wave and the steady Kelvin wake wave is shown by visualization on horizontal planes. Linear theory provides insight into the amplitude and structure of these two different types of steady waves. The wavenumber-frequency properties of the family of unsteady wake generated waves are obtained. Trapping of these unsteady waves while traversing a background with nonlinear stratification is also discussed. |
Tuesday, November 22, 2022 1:42PM - 1:55PM |
Z33.00005: Quantifying the Effect of Stable Stratification on Flow past Horizontal Cylinder Holland Kartchner, Som Dutta Stably stratified flows are encountered in different natural and industrial settings. The current research quantifies the effect stable stratification has on the fundamental phenomena of flow past a cylinder. Specifically, when the stratification is orthogonal to the direction of the flow, and the wake has a two-dimensional characteristics. These flows are simulated using open source spectral-element based high-order navier-stokes solver Nek5000. These simulations aim to capture and categorize the different flow structures induced by a cylinder in a stably-stratified flow. Simulations are first conducted for a wide range of Densimetric Froude numbers (Fr) and Reynolds numbers (Re), where Re is calculated using diameter as the length scale. Simulated Fr ranges between 0.01 to 1000 and simulated Re ranges between 10 to 180. The drag, lift, and Strouhal number are quantified. This allowed for the effect of increasing stratification on the vortex shedding to be quantified, including how the stratification changed the vortex shedding downstream of the cylinder. The structures formed are also classified using Dynamic Mode Decomposition (DMD). |
Tuesday, November 22, 2022 1:55PM - 2:08PM |
Z33.00006: Spatiotemporal analysis of coherent structures in the wake of an underwater hill Jinyuan Liu, Pranav Suresh Puthan Naduvakkate, Sutanu Sarkar Oceanic stirring and mixing at bottom topography in the deep ocean crucially impacts the interior ocean state. In this study, we conduct large eddy simulations of a canonical problem: flow past a conical hill that is submerged in a rotating, stratified fluid. The Reynolds number is Re = UD/ν = 25000 and the Froude number is Fr = U/Nh = 0.15. Cases with various levels of rotation strength ranging from none to strong rotation (low value of Rossby number Ro = U/fD) are examined. |
Tuesday, November 22, 2022 2:08PM - 2:21PM |
Z33.00007: On spatiotemporal intermittency of infrasound fields in stable atmospheres Christophe Millet, Francois Lott, Gil Averbuch Ducting of infrasound in the stratosphere is sensitive to upward-propagating gravity waves (GWs). GWs are generated by tropospheric sources such as convection or jet/front systems. The effects of GWs on the resolved scales are parameterized in Global Climate Models (GCMs). GW fields can be computed as a combination of a large number of individual wave packets, whose horizontal wavenumber, direction and phase velocity are chosen randomly. This representation allows the phenomenon of intermittency, which is known to produce positive outcomes regarding the long-standing cold-pole bias of GCMs. Intermittency redistributes the GW drag in the stratosphere, in a way which is likely impossible to reach using a spectral parameterization of GWs. In this work, it is shown that the intermittency of GWs can be observed in infrasound data, because of the nonlinearity between the forcing of GW sources and the statistical effects on acoustic waveguides higher up. It is also demonstrated that a minor shift in the phase velocity of GWs at the launching altitude may lead to significant changes in the acoustic waveguides. This sensitivity suggests that infrasound signals, that are recorded worlwide, may provide relevant additional constraints to tune stochastic parameterizations of GWs in GCMs. |
Tuesday, November 22, 2022 2:21PM - 2:34PM |
Z33.00008: Wakes of inclined prolate spheroid in the stratified environment - body forces and vortex dynamics Sheel Nidhan, Jose L Ortiz-Tarin, Sutanu Sarkar Despite their widespread presence in underwater applications, stratified wakes of slender bodies placed at an angle of incidence have received significantly less attention compared to their unstratified counterpart. To this end, we perform large eddy simulations (LES) to study the flow past a 6:1 prolate spheroid placed at an angle of incidence of $\alpha = 10^{\circ}$. The diameter-based Reynolds number ($Re= U_{\infty}D/\nu$) is set to a value of $5\times10^{3}$ and four values of diameter-based Froude numbers ($Fr = U_\infty/ND$) are analyzed: $Fr= \infty, 6, 1.9,$ and $1$. Visualizations of coefficient of pressure ($C_p$) and friction ($C_f$) contours reveal asymmetry in the $Fr = \infty$ and $6$ flows while, at $Fr = 1$ and $Fr= 1.9$, the flow over the body does not have any visible asymmetry. We will discuss the effect of this asymmetry (and its absence in strongly stratified cases) on the streamwise vortex pair shed from the body and, thereby, on mean and turbulent fields. |
Tuesday, November 22, 2022 2:34PM - 2:47PM |
Z33.00009: Asymmetric stratified wakes Chan-Ye Ohh, Madeleine Oliver, Geoffrey R Spedding Previous studies have suggested that all stratified wakes arrive at some universal state where the information about the wake generator is lost. However, there is scant evidence on whether wakes seeded with some specific asymmetric initial conditions can retain information about their origin through late stages of their evolution. In particular, there is interest in the wake of an inclined spheroid, as the asymmetry from the combination of drag wake and separation vortices presents a special opportunity for unique pattern generation. Here, a 6:1 prolate spheroid is towed in a refractive-index-matched salt-stratified fluid with varying incidence angle, θ = {0,10,20}°, Reynolds number, ReD = {0.5, 1, 2} × 104 and Froude number, Fr = {16, 32, 64, ∞}. Both stereo-PIV (2D3C) of two orthogonal planes and tomo-PIV (3D3C) are used to quantify the development of 3D flow structures in the near-wake. |
Tuesday, November 22, 2022 2:47PM - 3:00PM |
Z33.00010: Triadic resonances in the internal wave modes with background shear Patibandla B Ramana, Manikandan Mathur, Anubhab Roy We consider a monochromatic primary wave-field of discrete internal wave modes in a finite-depth, two dimensional uniformly stratified shear flow. Using a weakly-nonlinear theory, we study the resonances consisting of the superharmonic (2ω) part of the secondary wave field. In the absence of shear, it is known that a wave triad should satisfy a frequency condition, a horizontal wavenumber condition and a mode number condition for resonance. In the presence of weak-shear, using an asymptotic theory, we show that the mode number condition is not necessary. This results in the activation of several new resonances leading to the possibility of self-interaction and resonances close to ω = 0, even with arbitrarily weak shear. A similar asymptotic theory can be extended to other inhomogeneities (non-uniform stratification, non-cartesian geometry, etc.) as well. For an exponential background shear flow, we track the location of these resonances in the parameter space of Richardson number (Ri) and ω for a range of modal interactions and present their behaviour. |
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