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 G39: Turbulence: Atmospheric Boundary LayerBoundary Layers Turbulence
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Chair: Kalyan Shrestha, University of Texas at Dallas Room: Four Seasons Ballroom 1-3 |
Monday, November 20, 2017 10:35AM - 10:48AM |
G39.00001: Numerical study of turbulent flow over stages of interacting barchan dunes: sediment scour and vorticity dynamics. Chao Wang, William Anderson Large-eddy simulation (LES) results of unidirectional turbulent flow over interacting barchan dunes are presented. A series of interacting barchan dune topographies have been considered wherein a small dune is positioned at locations upflow of a relatively larger dune, and at a slight spanwise offset. The smaller dune is geometrically similar, but one-eighth the volume of the larger dune, thus replicating instantaneous realizations during actual dune interactions. We report that flow channeling in the interdune space induces a mean flow heterogeneity -- termed ``wake veering'' -- in which the location of maximum momentum deficit in the dune wake is spanwise-displaced. The probability density functions of streamwise velocity fluctuation in the interdune space showed wide variability, and were used to select low-frequency, high-magnitude thresholds for conditional sampling. Conditionally- and Reynolds-averaged iso-contours of Q-criterion and differential helicity revealed a persistent roller in interdune space, which strengthened asymmetric sediment erosion via scouring. We assess terms in the Reynolds-averaged streamwise vorticity transport, and show that the roller is primarily sustained by stretching. Finally, we present results of joint time-frequency analysis using wavelet decomposition, which shows that the dune geometry imparts a distinct influence on the local flow. [Preview Abstract] |
Monday, November 20, 2017 10:48AM - 11:01AM |
G39.00002: Large-eddy simulation study of turbulent flow over a complex topography using the terrain-following coordinate transformation method: application to aeolian dunes. Jianzhi Yang, William Anderson Modeling of turbulent flow over complex topography is of great interest in many engineering applications. However, accurate modeling of turbulent flow over complex topography still presents major technical challenges. The immersed boundary method (IBM) has been widely used to deal with the arbitrary domain geometry, due to its advantage of keeping the computation grid and, thus, preservation of the original numerical formulation. But IBM can introduce wall-modeling errors since the terrain and computational mesh points are not collocated. Terrain-following coordinate transformation, however, obviates wall-modeling errors. In this study, an existing large-eddy simulation (LES) code has been generalized for coordination transformation in the wall-normal direction, and applied to flow over a series of topographic configurations (flat surface, as a baseline for comparison, spanwise ridge, and an aeolian dune digital elevation map). Validation of simulation data has, firstly, been performed via comparison with first- and second-order data experimental data. Secondly, for the three-dimensional hill and dune cases, an LES simulation with IBM will be also performed. The implications of these results for physics-based, aeolian morphodynamic modeling will be discussed. [Preview Abstract] |
Monday, November 20, 2017 11:01AM - 11:14AM |
G39.00003: Cross-plane Stereo-PIV measurements in a refractive index matched flume of to elucidate the turbulent flow structure over 3D bedforms Nathaniel Bristow, Gianluca Blois, James Best, Kenneth Christensen Barchan dunes are three-dimensional, crescent-shaped bedforms found in regions of unidirectional flow and limited sediment supply, and while most commonly associated with aeolian environments, recent observations have shown them to exist in river beds, along continental shelves, and on the surfaces of Mars and Titan. As barchans migrate in the direction of the flow, they interact with their neighbors, typically by way of a collision. The morphodynamics of such collision processes are complex, where the role of the turbulent flow structure is strongly coupled to that of the sediment transport and morphological change. Here we study the flow structure in a decoupled manner through measurements of the turbulent flow over fixed-bed models of barchan dunes in various configurations involved in a barchan collision process. Particle image velocimetry is used to measure the flow in a refractive-index matched flume environment that enables uninhibited access to the whole flow field around these geometrically complex bedforms. Presented herein are results from stereo PIV measurements showing, for the first time experimentally, the turbulent flow structure in the cross-plane behind the horns of a barchan undergoing a collision with a larger, downstream barchan. [Preview Abstract] |
Monday, November 20, 2017 11:14AM - 11:27AM |
G39.00004: Time-resolved PIV measurements of the atmospheric boundary layer over wind-driven surface waves Corey Markfort, Matt Stegmeir Complex interactions at the air-water interface result in two-way coupling between wind-driven surface waves and the atmospheric boundary layer (ABL). Turbulence generated at the surface plays an important role in aquatic ecology and biogeochemistry, exchange of gases such as oxygen and carbon dioxide, and it is important for the transfer of energy and controlling evaporation. Energy transferred from the ABL promotes the generation and maintenance of waves. A fraction of the energy is transferred to the surface mixed layer through the generation of turbulence. Energy is also transferred back to the ABL by waves. There is a need to quantify the details of the coupled boundary layers of the air-water system to better understand how turbulence plays a role in the interactions. We employ time-resolved PIV to measure the detailed structure of the air and water boundary layers under varying wind and wave conditions in the newly developed IIHR Boundary-Layer Wind-Wave Tunnel. The facility combines a 30-m long recirculating water channel with an open-return boundary layer wind tunnel. A thick turbulent boundary layer is developed in the 1 m high air channel, over the water surface, allowing for the study of boundary layer turbulence interacting with a wind-driven wave field. [Preview Abstract] |
Monday, November 20, 2017 11:27AM - 11:40AM |
G39.00005: A New Similarity theory for Strongly Unstable Atmospheric Surface Layer Yong Ji, Zhen-Su She We apply the structural ensemble dynamics (SED) theory to analyze mean velocity and streamwise turbulence intensity distribution in unstable atmospheric surface layer (ASL). The turbulent kinetic energy balance equation in ASL asserts that above a critical height $z_{L} $, the buoyancy production cannot be neglected. The SED theory predicts that a stress length function displays a generalized scaling law from $z$ to $z^{4/3}$. The $z_{L} $ derived from observational data show a two-regime form with Obukhov length $L$ , including a linear dependence for moderate heat flux and a constant regime for large heat flux, extending the Monin-Obukhov similarity theory which is only valid for large $\left| L \right|$. This two-regime description is further extended to model turbulent intensity, with a new similarity coordinate $L_{z} $ such that the observational data collapse for all $L$. Finally, we propose a phase diagram for characterizing different ASL flow regimes, and the corresponding flow structures are discussed. In summary, a new similarity theory for unstable atmosphere is constructed, and validated by observational data of the mean velocity and streamwise turbulence intensity distribution for all heat flux regimes. [Preview Abstract] |
Monday, November 20, 2017 11:40AM - 11:53AM |
G39.00006: LES-based generation of high-frequency fluctuation in wind turbulence obtained by meteorological model Tetsuro Tamura, Masaharu Kawaguchi, Hidenori Kawai, Tao Tao The connection between a meso-scale model and a micro-scale large eddy simulation (LES) is significant to simulate the micro-scale meteorological problem such as strong convective events due to the typhoon or the tornado using LES. In these problems the mean velocity profiles and the mean wind directions change with time according to the movement of the typhoons or tornadoes. Although, a fine grid micro-scale LES could not be connected to a coarse grid meso-scale WRF directly. In LES when the grid is suddenly refined at the interface of nested grids which is normal to the mean advection the resolved shear stresses decrease due to the interpolation errors and the delay of the generation of smaller scale turbulence that can be resolved on the finer mesh. For the estimation of wind gust disaster the peak wind acting on buildings and structures has to be correctly predicted. In the case of meteorological model the velocity fluctuations have a tendency of diffusive variation without the high frequency component due to the numerically filtering effects. In order to predict the peak value of wind velocity with good accuracy, this paper proposes a LES-based method for generating the higher frequency components of velocity and temperature fields obtained by meteorological model. [Preview Abstract] |
Monday, November 20, 2017 11:53AM - 12:06PM |
G39.00007: Characterization of wind-shear effects on the entrainment zone in convective boundary layers Armin Haghshenas, Juan Pedro Mellado Direct numerical simulations and dimensional analysis are used to investigate wind-shear effects on a convective boundary layer (CBL) that grows into a linearly stratified atmosphere. Consistent with the literature, we observe that strong wind shear enhances entrainment and thickens the entrainment zone. In addition, we show that under a weak wind-shear condition, the entrainment zone is characterized by a convective-layer thickness, $(\Delta z_i)_c$, which is a constant fraction of the CBL depth. But as the wind shear increases, shear production starts to dominate turbulent transport across the entrainment zone, and as a result, the characteristic length scale in that region changes to a shear-layer thickness, $(\Delta z_i)_s$. We associate this length scale to the asymptotic depth of a stably stratified shear layer. We find that the dependence of wind-shear effects on surface buoyancy flux, buoyancy stratification, wind velocity, and the CBL depth can be expressed just by one dimensionless variable, $(\Delta z_i)_s/(\Delta z_i)_c$. Shear effects are observed only if this variable, which can be easily measured in the atmosphere, is larger than one. We also show that the observed dependence varies linearly when it is expressed in terms of the inverse of a bulk Richardson number. [Preview Abstract] |
Monday, November 20, 2017 12:06PM - 12:19PM |
G39.00008: Large-Scale Effects near the Surface and in the Entrainment Zone of Convective Boundary Layers Katherine Fodor, Juan-Pedro Mellado Free convection in the atmospheric boundary layer organises on large-scales into a distinct cellular pattern, similar to that found in Rayleigh-B\'{e}nard convection. How these large-scales affect small-scale properties near the surface and in the entrainment zone remains poorly understood, partly due to a lack of accurate data. We address this issue by comparing direct numerical simulations of convective boundary layers with Rayleigh-B\'{e}nard convection, using filter operators to define the large-scales. We find that spatial filtering according to the decorrelation length, $L_d$, of the vertical velocity, and not the depth of the convective region, leads to comparable kinetic energy partition between large- and small-scales across cases. Hence we use this spatial filter to study small-scale statistical properties conditioned separately in large-scale updraught and downdraught regions. We also find that temporal filtering according to the decorrelation time, $T_d$, yields similar results, both qualitatively and quantitatively, but spatial filtering is preferable as $T_d$ is highly variable and unpredictable, whereas $L_d$ scales with the boundary layer depth. [Preview Abstract] |
Monday, November 20, 2017 12:19PM - 12:32PM |
G39.00009: Roughness model for LES of turbulent flow over multiscale fractal urban-like topography. Xiaowei Zhu, William Anderson Urban-like topographies are composed of a wide spectrum of topographic elements, which results in a multiscale, fractal-like surface height distribution. This presents unique challenges for large-eddy simulation (LES), since the corresponding low-pass filtered details of the topography are spatially resolved, while affects associated with the high-pass filtered topography must be modeled with some closure. For truly fractal landscapes, we show that the affects of descendent topographic generations of terrain can be captured through application of the equilibrium logarithmic law. In the present study, LES has been used to investigate flow over fractal-like topographies, where the number of topographic generations and fractal dimension were systematically varied. For any fractal dimension, the roughness length increases dramatically for the first few generations, before converging to a constant value. To leverage the self-similar nature of the fractal-like topography, we demonstrate that a fixed constant of proportionality can be used a priori to relate roughness length and root-mean-square topographic height, topography skewness, and fractal dimension. We validate the model by accurately predicting mean streamwise velocity profiles. [Preview Abstract] |
Monday, November 20, 2017 12:32PM - 12:45PM |
G39.00010: ABSTRACT WITHDRAWN |
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