Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session G34: Geophysical Fluid Dynamics: Stratified Flows IIIGeophysical
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Chair: Sina Khani, Princeton University Room: 102 |
Monday, November 20, 2017 10:35AM - 10:48AM |
G34.00001: Sensitivity of mixing efficiency to resolution of the buoyancy scale in large-eddy simulations of stratified turbulence Sina Khani Mixing efficiency is studied in large-eddy simulations (LES) of stratified turbulence when the grid spacing $\Delta$ varies from being $< \sim L_b$ to $\gg L_b$, where $L_b$ is the buoyancy scale. It is shown that the irreversible mixing efficiency $\gamma$ is fairly close to that resulted from direct numerical simulations (DNS) if the buoyancy scale $L_b$ is well resolved in LES. Also, when the buoyancy scale is resolved, the vertical length scale $\mathcal{L}_v \sim L_b$, and we can scale $\gamma$ as a function of the vertical Froude number $Fr_v$ and turbulent Prandtl number $Pr_t$. If we assume $Pr_t \approx 1$, in the regime of stratified turbulence where the horizontal Froude number $Fr_h \ll 1$ and $Fr_v \sim 1$, $\gamma$ goes to a constant value $1/3$. This value of $\gamma$ has been recently reported as an upper bound of mixing efficiency in the deep ocean stratified regions near topographies. Overall, our work suggests that similar results to those from DNS approach can be yielded in LES of stratified turbulence, while the computational costs are significantly decreased in LES in comparison with expensive DNS runs. [Preview Abstract] |
Monday, November 20, 2017 10:48AM - 11:01AM |
G34.00002: Evaluation of small scale anisotropy in stably stratified turbulence Amrapalli Garanaik, Subhas Karan Venayagamoorthy Direct numerical simulations (DNS) of stably stratified turbulent flows are performed to evaluate Kolmogorov's hypothesis of small scale isotropy. True estimation of small scale turbulent quantities such as the rate of dissipation of turbulent kinetic energy ($\epsilon$) and rate of dissipation of temperature variance ($\chi$) requires nine mean square velocity gradients and three mean square scalar gradients respectively. It is common practice in oceanography to measure dissipation quantities from just one or two gradient components for both $\epsilon$ and $\chi$ by invoking Kolmogorov’s small scale isotropy hypothesis. Though the use of the isotropic assumption at small scales in oceanic flows is both tempting and necessary, such an assumption is likely to result in an over- or under estimation of small scale turbulent quantities for two main reasons. First, the Reynolds number for oceanic flows is not always sufficiently high enough to justify isotropy at small scales. Second, oceanic flows are strongly influenced by stable vertical stratification resulting in large scale anisotropy. In this study we have revisited the small scale isotropy assumption in stably stratified flows and provide an estimation of departure from isotropy as a function of turbulent Froude number. [Preview Abstract] |
Monday, November 20, 2017 11:01AM - 11:14AM |
G34.00003: Study of convective overturns and mixing in a near-bottom, oscillating turbulent flow Pranav Puthan, Masoud Jalali, Vamsi Chalamalla, Sutanu Sarkar Mixing, due to convective overturns, in a stratified fluid column is studied using a large-eddy simulation (LES) numerical technique. An oscillating pressure gradient on a slope sets up an oscillating flow that acts on the background stratification to generate a density overturn as the flow reverses from down to upslope through zero velocity. In the present problem, the pressure gradient for later time is set to zero at this zero-velocity point, and the pathway to turbulence from the density overturn is quantified. The results are compared to those of a case where the oscillating pressure gradient is maintained throughout the cycle. Implications are drawn to the applicability of Thorpe-scale estimation of the turbulent dissipation rate from density overturns in the case of wave breaking by convective instability. [Preview Abstract] |
Monday, November 20, 2017 11:14AM - 11:27AM |
G34.00004: Subsampled Numerical Experiments as a Guide for Field Deployment of Thermistor Chains Justin Shaw, Marek Stastna Thermistor chains are a standard tool for recording temperature profiles in geophysical flows. Density values can be inferred from readings and the resulting density field analyzed for the passage of internal waves, Kelvin-Helmholtz billows, and other dynamic events. The number and spacing of the thermistors, both on and between chains, determines which events can be identified in the dataset. We examine the effect of changing these variables by subsampling a set of numerical experiments to simulate thermistor chain locations. A pseudo spectral method was used to solve the incompressible Navier-Stokes equations under the Boussinesq approximation. The resulting flows are a set of high resolution seiches where the depth was held constant across experiments, and the length was varied. Sampling a known, commonly occurring flow with relatively simple geometry allows for a clear analysis of the effects of thermistor placement in the capture of dynamic events. We will discuss three dimensional deployment strategies, as well as EOF and DMD analyses if there is time. [Preview Abstract] |
Monday, November 20, 2017 11:27AM - 11:40AM |
G34.00005: Stratified flow past a conical obstacle at low Froude number Masoud Jalali, Sutanu Sarkar, Geno Pawlak The structure of flow in the lee of topographic features such as islands and headlands is affected by stratification, in particular, vertical motion is suppressed at low Froude number, $Fr = U/Nh < < O(1)$. High-resolution, turbulence-resolving simulations using an immersed boundary method (IBM) have been performed to study the flow in a stratified background over a 3D conical obstacle. The blocked flow moves around the object where subsequent flow separation and vortex shedding leads to an unsteady wake. The body also generates lee waves. We will discuss simulation results regarding the vortex dynamics and internal wave properties. [Preview Abstract] |
Monday, November 20, 2017 11:40AM - 11:53AM |
G34.00006: The nature of transition between downslope and lateral splitting flows in low Froude number stratified flows approaching long ridges Arjun Jagannathan, Kraig Winters, Laurence Armi We investigate the transition between downslope and laterally splitting flow states for low Froude number stratified flows approaching long but finite ridges. In these flows, upstream influence is established through two distinct and competing mechanisms. The streamwise upstream propagating internal wave modes are responsible for blocking and establishment of an accelerated, hydraulically controlled overflow whereas the oblique modes triggered by the finite extent of the obstacle promote the transition to a horizontal splitting flow. Scaling arguments reveal the time scales of the processes leading to this flow adjustment. We demonstrate through numerical experiments that for sufficiently long ridges, asymmetric downslope flows can persist over time scales that are important from a meteorological perspective. We also examine the case of a ridge with variable height and find that, remarkably, even as the flow over the tall section transitions to a splitting flow, a significantly enhanced downslope flow is established over the shorter section. We show that this flow enhancement can be explained using stratified hydraulic theory and flux conservation principles. [Preview Abstract] |
Monday, November 20, 2017 11:53AM - 12:06PM |
G34.00007: Direct numerical simulation of stably stratified Ekman flow over a wavy surface Sungwon Lee, S. M. Iman Gohari, Sutanu Sarkar The stratified Ekman layer over a wavy surface is examined using DNS. The neutral Ekman layer is subjected to a stabilizing cooling flux for approximately an inertial period. The nondimensional amplitude and aspect ratio of the waviness are changed. For sufficiently strong cooling flux, the flat-bottom case exhibits initial collapse of turbulence, emergence of a low-level jet and eventual turbulence recovery. The wavy-surface results are compared with the flat-bottom case to understand how surface roughness influences buoyancy effects in the atmospheric boundary layer. [Preview Abstract] |
Monday, November 20, 2017 12:06PM - 12:19PM |
G34.00008: The response of a basin scale seiche due to variation of the aspect ratio of the density surface Andrew Grace, Marek Stastna, Francis Poulin Experimental analysis of the relaxation of a tilted density interface by Horn et.al. showed how mechanisms such as Kelvin--Helmholtz Billows and the formation of solitons affect the degeneration of a basin scale seiche. The emergence of the different degenerative mechanisms were found by varying the thermocline depth and seiche amplitude/layer thickness ratio. The study found that the parameter space can be divided into five regimes where different mechanisms dominate the motion and the subsequent degeneration of the seiche. However, this study neglected the dependence of the degeneration mechanisms on the amplitude to length ratio (aspect ratio) of the seiche. Through the use of high resolution pseudospectral simulations of the stratified, incompressible Navier--Stokes Equations under the Boussinesq approximation we investigate how the primary degenerative mechanism of the seiche changes when the aspect ratio of the initial condition is varied. We have found that there is clear difference in the dominant degeneration mechanism of the seiche when the aspect ratio becomes small enough. We found with large aspect ratios, the dominant degeneration mechanism appears to be due to shear while at smaller aspect ratios the dominant mechanism is due to the formation of dispersive waves. [Preview Abstract] |
Monday, November 20, 2017 12:19PM - 12:32PM |
G34.00009: Free Falling in Stratified Fluids Try Lam, Lionel Vincent, Eva Kanso Leaves falling in air and discs falling in water are examples of unsteady descents due to complex interaction between gravitational and aerodynamic forces. Understanding these descent modes is relevant to many branches of engineering and science such as estimating the behavior of re-entry space vehicles to studying biomechanics of seed dispersion. For regularly shaped objects falling in homogenous fluids, the motion is relatively well understood. However, less is known about how density stratification of the fluid medium affects the falling behavior. Here, we experimentally investigate the descent of discs in both pure water and in stable linearly stratified fluids for Froude numbers Fr $\approx$ 1 and Reynolds numbers Re between 1000$\--$2000. We found that stable stratification (1) enhances the radial dispersion of the disc at landing, (2) increases the descent time, (3) decreases the inclination (or nutation) angle, and (4) decreases the fluttering amplitude while falling. We conclude by commenting on how the corresponding information can be used as a predictive model for objects free falling in stratified fluids. [Preview Abstract] |
Monday, November 20, 2017 12:32PM - 12:45PM |
G34.00010: Characterizing the performance of an affordable, multichannel conductivity probe for density measurements in stratified flows Balaji Subramanian, Marco Carminati, Paolo Luzzatto-Fegiz In stratified flows, conductivity (combined with temperature) is often used to measure density. The conductivity probes typically used can resolve very fine spatial scales, but on the downside they are fragile, expensive, sensitive to environmental noise and have only single channel capability. Recently a low-cost, robust, arduino-based probe called Conduino was developed, which can be valuable in a wide range of applications where resolving extremely small spatial scales is not needed. This probe uses micro-USB connectors as actual conductivity sensors with a custom designed electronic board for simultaneous acquisition from multiple probes, with conductivity resolution comparable to commercially available PME conductivity probe. A detailed assessment of performance of this Conduino probe is described here. To establish time response and sensitivity as a function of electrode geometry, we build a variety of shapes for different kinds of applications, with tip spacing ranging from 0.5-2.5 mm, and with electrode length ranging from 2.3-6 mm. We set up a two-layer density profile and traverse it rapidly, yielding a time response comparable to PME. The Conduino's multi-channel capability is used to operate probe arrays, which helps to construct density fields in stratified flows. [Preview Abstract] |
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