Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session E23: Geophysical Fluid Dynamics: Stratified Turbulence III |
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Chair: Hieu Pham, Univesrity of California at San Diego Room: 2001 |
Sunday, November 23, 2014 4:45PM - 4:58PM |
E23.00001: Low-Level Jets, Coherent Structures and Turbulence in a Stably Stratified Atmospheric Boundary Layer Iman Gohari, Masoud Jalali B., Scott Baden, Sutanu Sarkar Accurate numerical modeling of stably stratified Atmospheric Boundary Layers (SABL) is known to be a pacing item for progress in numerical weather prediction, atmospheric dispersion and other related applications such as harnessing wind energy. The stabilizing effect of buoyancy not only dampens the strength of turbulence relative to near-neutral and convectively unstable cases but also qualitatively affects turbulence structure by changing anisotropy in velocity, anisotropy in length scale and spatio-temporal intermittency. Low-Level Jets (LLJs) form at O(100) m heights in the rotation-influenced SABL. It is thought that the LLJ leads to two turbulent layers, wherein the turbulence in the top layer originates from shear production and that in the lower one is mainly controlled by surface conditions. Properties of the SABL during the evolution of LLJ are not well understood. Therefore, Large Eddy Simulations of a rotation-influenced SABL with different surface cooling rates have been performed. Fluctuations (coherent structures, turbulence, and internal gravity waves) during LLJ evolution are quantified through second-order moments, high-order derivative statistics, spectra, length scales and eduction of coherent structures. [Preview Abstract] |
Sunday, November 23, 2014 4:58PM - 5:11PM |
E23.00002: On Parameterizing Turbulence in the Stably Stratified Atmospheric Boundary Layer Jordan M. Wilson, Subhas K. Venayagamoorthy Parameterizing turbulent mixing in the stably stratified atmospheric boundary layer remains an active area of research connecting available field measurements with appropriate model parameters. The research presented studies the pertinent mixing lengths for shear- and buoyancy-dominated (or weakly stable and very stable) regimes in the stable atmospheric boundary layer (SABL). Incorporating shear and buoyancy effects, two length scales can be constructed, $L_{kS} = k^{1/2} /S$ and $L_{kN} = k^{1/2}/N$, respectively. Extending the conceptual framework of Mater \& Venayagamoorthy (2014)\footnote{\textit{Phys. Fluids}, 26(3), 036601.}, $L_{kS}$ and $L_{kN}$ are shown to be accurate representations of large-scale motions from which relevant model parameters are developed using observations from three field campaigns. An \textit{a priori} analysis of large-eddy simulation (LES) data evaluates the efficacy of parameterizations applied to the vertical structure of the SABL. The results of this study provide a thorough evaluation of the pertinent mixing lengths in stably stratified turbulence through applications to atmospheric observations and numerical models for the boundary layer extendable to larger-scale weather prediction or global circulation models. [Preview Abstract] |
Sunday, November 23, 2014 5:11PM - 5:24PM |
E23.00003: Stratified Turbulence Measurements in Complex Terrain Using Hot-film Probes and a Collocated Sonic Anemometer C. Hocut, E. Kit, D. Liberzon, H.J.S. Fernando In the fall of 2012 and spring 2013, the Mountain Terrain Atmospheric Modeling and Observations Program (MATERHORN) conducted extensive field experiments at the Granite Mountain Atmospheric Science Testbed (GMAST), US Army Dugway Proving Grounds (DPG), Utah. This provided a unique opportunity to deploy tower mounted three-dimensional hot-film combo probes, consisting of sonic anemometers collocated with hot-film anemometers able to respond to the wind direction. The combo probes follow mean winds using a feedback control loop and use a Neural Network to calibrate the hot-films in-situ. Once calibrated, these probes can handle a vast range of background flow conditions and scales from mesoscale flow down to the Kolmogorov scale. Of particular interest are the observed variation in velocity spectra during the evenings. Sometimes the velocity spectra shows the turbulence is Kolmogorov and is isotropic at small scales while in other spectra there is evidence of turbulence production at finer scales. An explanation on different spectral shapes will be presented as well as the relevant length/time scales of the production events. [Preview Abstract] |
Sunday, November 23, 2014 5:24PM - 5:37PM |
E23.00004: Turbulence and Mixing Near a Sloping Boundary of a Lake Chris Rehmann, Zhimin Li, Hui Hu Fluxes in stratified water bodies such as lakes and oceans are often controlled by turbulence and mixing at sloping boundaries, and determining how the mixed fluid moves from the boundary to the interior is important for estimating basin-wide transport of heat and other scalars. A field experiment in one lake showed that fluid mixed at the boundary can be transported by intrusions that form as the mixed fluid collapses while an experiment in another lake suggested that the transport is caused by advection and dispersion by internal waves. Further work on this problem involves two parallel approaches. An analytical model, based on rapid distortion theory, is used to determine the effect of straining by vertical mode-2 waves on the turbulence and to compute the efficiency of the mixing. This approach is complemented with laboratory measurements of velocity and scalar fields with molecular tagging velocimetry. These measurements allow the scalar fluxes to be quantified as a function of the ratio of the wave frequency and the critical wave frequency. [Preview Abstract] |
Sunday, November 23, 2014 5:37PM - 5:50PM |
E23.00005: Turbulence and dissipation in a computational model of Luzon Strait Masoud Jalali, Sutanu Sarkar Generation sites for topographic internal gravity waves can also be sites of intense turbulence. Bottom-intensified flow at critical slopes leads to convective instability and turbulent overturns [Gayen \& Sarkar (2011)]. A steep ridge with small excursion number, $Ex$, but large super criticality can lead to nonlinear features according to observations [Klymak et al. (2008)] and numerical simulations [Legg \& Klymak (2008)]. The present work uses high resolution 3-D LES to simulate flow over a model with multiscale topography patterned after a cross-section of Luzon Strait, a double-ridge generation site which was the subject of the recent IWISE experiment. A 1:100 scaling of topography was employed and environmental parameters were chosen to match the slope criticality and $Fr$ number in the field. Several turbulent zones were identified including breaking lee waves, critical slope boundary layer, downslope jets, internal wave beams, and vortical valley flows. The multiscale model topography has subridges where a local $Ex$ may be defined. Wave breaking and turbulence at these subridges can be understood if the local value of $Ex$ is employed when using the $Ex$-based regimes identified by Jalali et al. (2014) in their DNS of oscillating flow over a single triangular obstacle. [Preview Abstract] |
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