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 D35: Stratified Turbulence and Convection |
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Chair: Philip Marcus, University of California, Berkeley Room: Oregon Ballroom 204 |
Sunday, November 20, 2016 2:57PM - 3:10PM |
D35.00001: Localized turbulent spots in a stratified shear flow John Taylor Despite the large Reynolds numbers involved, turbulence in geophysical flows is often highly intermittent in space and time as the stabilizing effects of density stratification inhibit vertical motions. Direct numerical simulations of stratified turbulence exhibit highly localized `bursting' events. The transient nature of these bursts makes them difficult to study systematically. Here, we use a new control technique to study localized patches of turbulence in stratified shear flows. The Richardson number, controlling the `heaviness' of dense fluid is adjusted in time to maintain a fixed level of turbulent kinetic energy. This process allows us to maintain localized turbulent spots and study their properties. [Preview Abstract] |
Sunday, November 20, 2016 3:10PM - 3:23PM |
D35.00002: Instabilities, exact coherent structures and layer formation in horizontally shearing body-forced stably-stratified flow Dan Lucas, C.P. Caulfield, Rich Kerswell We consider turbulence driven by a large scale horizontal shear by way of the Kolmogorov flow (sinusoidal body forcing) and a background linear stable stratification imposed in the third direction. This provides a tractable arena to investigate the formation of coherent structures, which in this case organise the flow into horizontal layers by inclining the background shear as the strength of the stratification is increased. The coherent structures can be traced back to new instabilities of the base flow which have a vertical wavelength depending on Richardson number. We investigate how the vertical lengthscales observed in the turbulence are related to the exact solutions and compare to the other well studied examples of instability driving layer formation. We also expose the chaotic motions of the stratified turbulence by locating unstable periodic orbits embedded therein. [Preview Abstract] |
Sunday, November 20, 2016 3:23PM - 3:36PM |
D35.00003: Zombie Turbulence and More in Stratified Couette Flow Philip Marcus, Joe Barranco, Suyang Pei, Chung-Hsiang Jiang Zombie turbulence occurs in rotating, shearing vertically-stratified flows such as stratified Couette flows. The turbulence is triggered by a neutrally-stable eigenmode with a critical layer receptive to finite-amplitude perturbations. Once excited, the critical layer becomes a vortex layer pair that rolls up into discrete vortices. Those vortices excite new critical layers, and the process repeats ad infinitum. When the vortex amplitudes become sufficiently large, the flow becomes turbulent. Although possessing a mid-range energy spectrum with $E(k) \propto k^{-5/3}$, the turbulence is non-Kolmogorov, highly anisotropic, and with large turbulent, but coherent, structures that retain the length scales of the spacing between the critical layers. The motivation for this study is protoplanetary disks (PPDs) where new stars form. In the PPD the Brunt-Vaisala frequency $N$ increases as a function of distance from the midplane where it is zero. We cannot trigger the initial finite amplitude instability where $N$ is small (close to the midplane). However, computations in PPDs and Couette flows show that zombie turbulence forms where $N$ is large, and then a new type of turbulence, that is neither zombie nor Kolmogorov turbulence, fills in the remainder of the domain even where $N=0. [Preview Abstract] |
Sunday, November 20, 2016 3:36PM - 3:49PM |
D35.00004: Conditional Analysis of Dynamically Distinct Regions in Stratified Turbulence Gavin Portwood, Stephen de Bruyn Kops, John Taylor, Hasem Salehipour, Colm-cille Caulfield Stratified flows have been shown to exhibit broadly intermittent flow dynamics at large scales. In DNS of forced homogeneous stratified turbulence, we employ a conditional averaging technique to distinguish compositional flow regions which define the entire flow domain. Here, we condition on the vertical density gradient at inertial and buoyancy length scales to subdivide homogeneous stratified turbulence into three distinct regions that may be characterised by $\mathrm{Gn} \equiv \epsilon/ \nu N^2$. We show that flows across the Fr-Re parameter space exhibit regions of (a) moderately `quiescent' flow with few three-dimensional overturnings, (b) `layered' turbulent regions which have constrained vertical length scales, and (c) three dimensional `patches' of turbulence and that these regions may be characterised by $\mathrm{Gn} \sim O(1)$, $\mathrm{Gn} \sim O(10)$, and $\mathrm{Gn} \sim O(100)$, respectively. We conjecture that treating stratified turbulence as an instantaneous assemblage of these different regions in varying proportions may explain some of the apparently highly scattered flow dynamics and statistics previously reported in the literature. [Preview Abstract] |
Sunday, November 20, 2016 3:49PM - 4:02PM |
D35.00005: Formation of temperature front in stably stratified turbulence Yoshifumi Kimura, Peter Sullivan, Jackson Herring An important feature of stably stratified turbulence is the significant influence of internal gravity waves which makes stably stratified turbulence unique compared to homogeneous isotropic turbulence. In this paper, we investigate the genesis of temperature fronts--a crucial subject both practically and fundamentally--in stably stratified turbulence using Direct Numerical Simulations (DNS) of the Navier-Stokes equation under the Boussinesq approximation with $1024^3$ grid points. Vertical profiles of temperature fluctuations show almost vertically periodic sawtooth wavy structures with negative and positive layers stacked together with clear boundaries implying a sharp temperature fronts. The sawtooth waves consist of gradual decreasing temperature fluctuations with rapid recovery to a positive value as the frontal boundary is crossed vertically. This asymmetry of gradients comes from the structure that warm temperature region lies on top of cool temperature region, and can be verified in the skewed probability density function (PDF) of vertical temperature gradient. We try to extract the flow structures and mechanism for the formation and maintenance of the strong temperature front numerically. [Preview Abstract] |
Sunday, November 20, 2016 4:02PM - 4:15PM |
D35.00006: Structure and Mixing of a Turbulent Meandering Plume Part 1: Concentration and Velocity Structure D.L. Young, A.I. Larsson, D.R. Webster While much is known about the dynamics and mixing of straight non-buoyant plumes of chemical tracer, comparatively little is understood about the dynamics of meandering plumes, where meander is defined as large scale movement of the plume centerline. Meandering chemical plumes occur in atmospheric and other environmental flows, such as the flow past natural obstacles. In this study, we present simultaneous PIV, PTV, and LIF measurements of a phase-locked meandering plume, the motion of which is forced by the periodic oscillation of a diverting plate. The plume evolves in a turbulent boundary layer in a moderate-\textit{Re} open channel flow. Similar measurements are made for a straight plume for comparison. Analysis of the LIF data reveals that, for the meandering plume compared to the straight plume, the centerline phase-averaged concentration decreases more rapidly with distance downstream and the plume width increases more rapidly with distance downstream. This indicates a more rapid dilution of tracer. Furthermore, the concentration profiles, along transects perpendicular to the plume centerline, are not symmetric about the meandering plume centerline. Analysis of the velocity data indicates that the large-scale alternating-sign vortices induced by the diverting plate are the dominant feature of the flow. The vortices force the plume to meander and govern the spatial distribution of the phase-averaged concentration, phase-averaged vorticity, Reynolds stress, and TKE. [Preview Abstract] |
Sunday, November 20, 2016 4:15PM - 4:28PM |
D35.00007: Structure and Mixing of a Turbulent Meandering Plume Part 2: Turbulent Mixing and Eddy-Diffusivity D.R. Webster, D.L. Young, A.I. Larsson Turbulent mixing in a meandering non-buoyant chemical plume is far less understood than in a straight plume -- partially due to the difficulty separating the plume meander fluctuations from the turbulent fluctuations. In this study we present high resolution measurements of the covariance of the turbulent fluctuations of velocity and concentration in a phase-locked meandering plume, acquired by combining simultaneous PTV velocity and LIF concentration measurements. The effectiveness of the eddy-diffusivity model for predicting the turbulent flux is assessed. Analysis of the data reveals that the spatial distribution of the turbulent flux is governed by the large-scale alternating-sign vortices that induce the plume meander. Further, regions of high turbulent flux are co-located with areas of large phase-averaged concentration gradients. As a result, the eddy-diffusivity framework models the turbulent flux effectively. As expected from turbulent mixing theory, the eddy-diffusivity coefficient plateaus at a constant value once the plume width reaches the size of the largest eddies (i.e., the scale of the water depth in this open channel flow). However, when the plume width is less than the water depth the eddy-diffusivity coefficient scales with the plume width to the 3/4 power. This differs from the theoretical 4/3 scaling that results from the assumption of an inertial subrange. The extent of the inertial subrange is extremely limited in the current moderate-\textit{Re} open channel flow. [Preview Abstract] |
Sunday, November 20, 2016 4:28PM - 4:41PM |
D35.00008: A comparison of the turbulent entrainment process in line plumes and wall plumes David Parker, Henry Burridge, Jamie Partridge, Paul Linden Flows driven by sources of buoyancy appear in a large number of geophysical and industrial applications. The process of turbulent entrainment in these flows is key to understanding how they evolve and how one might model them. It has been observed that the entrainment is reduced when a line source of buoyancy is positioned immediately adjacent to a wall. To gain insight into the effect of the wall on the entrainment process we perform simultaneous PIV and LIF on both line plumes, in the absence of any boundary, and when the source is adjacent to a vertical boundary forming a wall plume. The experiments are designed to isolate the effect of the wall by using the same experimental setup and parameters for both flows with the addition of the wall and half the buoyancy flux used in the wall plume case. Of particular interest is the effect the large scale eddies, forming at the edge of the plume and engulfing ambient fluid, have on the entrainment process. By using velocity statistics in a coordinate system based on the instantaneous scalar edge of the plume, a technique we have recently used to analyse similar effects in an axisymmetric plume, the significance of this large scale engulfment will be quantified. [Preview Abstract] |
Sunday, November 20, 2016 4:41PM - 4:54PM |
D35.00009: Statistical parameters of thermally driven turbulent anabatic flow Roni Hilel, Dan Liberzon Field measurements of thermally driven turbulent anabatic flow over a moderate slope are reported. A collocated hot-films-sonic anemometer (Combo) obtained the finer scales of the flow by implementing a Neural Networks based \textit{in-situ} calibration technique. Eight days of continuous measurements of the wind and temperature fluctuations reviled a diurnal pattern of unstable stratification that forced development of highly turbulent unidirectional up slope flow. Empirical fits of important turbulence statistics were obtained from velocity fluctuations' time series alongside fully resolved spectra of velocity field components and characteristic length scales. TKE and TI showed linear dependence on Re, while velocity derivative skewness and dissipation rates indicated the anisotropic nature of the flow. Empirical fits of normalized velocity fluctuations power density spectra were derived as spectral shapes exhibited high level of similarity. Bursting phenomenon was detected at 15{\%} of the total time. Frequency of occurrence, spectral characteristics and possible generation mechanism are discussed. [Preview Abstract] |
Sunday, November 20, 2016 4:54PM - 5:07PM |
D35.00010: Phenomenology of turbulent convection Mahendra Verma, Anando Chatterjee, Abhishek Kumar, Ravi Samtaney We simulate Rayleigh-B\'{e}nard convection (RBC) in which a fluid is confined between two thermally conducting plates. We report results from direct numerical simulation (DNS) of RBC turbulence on $4096^{3}$ grid, the highest resolution hitherto reported, on 65536 cores of Cray XC40, Shaheen II, at KAUST. The non-dimensional parameters of our simulation are: the Rayleigh number $\mathrm{Ra} = 1.1\times 10^{11}$ (the highest ever for a pseudo-spectral simulation) and Prandtl number of unity. We present energy flux diagnostics of shell-to-shell (in wave number space) transfer. Furthermore, noting that convective flows are anisotropic due to buoyancy, we quantify anisotropy by subdividing each wavenumber shell into rings and quantify ring energy spectrum. An outstanding question in convective turbulence is the wavenumber scaling of the energy spectrum. Our pseudo-spectral simulations of turbulent thermal convection coupled with novel energy transfer diagnostics have provided a definitive answer to this question. We conclude that convective turbulence exhibits behavior similar to fluid turbulence, that is, Kolmogorov's $k^{-5/3}$ spectrum with forward and local energy transfers, along with a nearly isotropic energy distribution. [Preview Abstract] |
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