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 L25: Turbulence: Planetary Boundary Layers 
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Chair: Catherine Gorle, Stanford University Room: 2005 
Monday, November 24, 2014 3:35PM  3:48PM 
L25.00001: Coherency and Large Scale Motions in Turbulent Ekman Flow Cedrick Ansorge, Juan Pedro Mellado We study turbulence in the planetary boundary layer using direct numerical simulations of neutrally and stably stratified Ekman flow. The Reynolds number is varied in the range $500<\delta^+<1500$ where $\delta^+$ is the boundary layer thickness $\delta$ expressed in wall units. We vary the stratification, expressed in terms of a bulk Richardson number from very weak stability, where turbulence acts as a passive scalar, to very strong stability, where the flow relaminarizes partly. When the aspect ratio is sufficiently large, i.e. at a horizontal extent of about $(20\delta)^2$, largescale modes are present in the flow. These largescale modes govern the spatiotemporal structure of external and global intermittency in the flow. We use a dual approach to investigate the largescale motions and coherency in the flow: The analysis of spatially resolved fields of the turbulent flow is complemented by temporally fully resolved data at vertical intersections through the domain. From this data we quantify the expected error of local measurements carried out over a finite period of time with respect to the ensemble average. Moreover, we analyze the coherency in this complex flow, in particular of the largescale structures occurring when the flow is exposed to a stable stratification. [Preview Abstract] 
Monday, November 24, 2014 3:48PM  4:01PM 
L25.00002: Griddependent Convection in WRFLES Jason Simon, Bowen Zhou, Fotini Chow Traditional numerical weather prediction (NWP) models parameterize the boundary layer with planetary boundary layer (PBL) schemes, which assume a coarse resolution so that energycontaining eddies are nearly exclusively subgrid scale (SGS). Newer NWP models can also be used as largeeddy simulation (LES) models, which use a grid resolution that is sufficiently fine to resolve energycontaining eddies. For atmospheric flows the energycontaining eddies are typically on the scale of the PBL depth [O(1 km)]. The range of resolutions between the maximum appropriate resolution for LES and the minimum for PBL schemes is the turbulent gray zone, or terra incognita. The resolution limit for atmospheric LES is largely unexamined despite its dynamical significance. Here we examine the Weather Research and Forecasting model in LES mode (WRFLES). We attempt to identify the symptoms of the turbulent gray zone with WRFLES under primarily convective conditions using the Wangara Day 33 case. Griddependence, a signal of the gray zone, is evaluated by considering the stability profile, resolved convection, higherorder statistical profiles, and turbulence spectra. Also considered are the effects of isotropic mixing lengthscales, domain extent and spatially heterogeneous surface fluxes. [Preview Abstract] 
Monday, November 24, 2014 4:01PM  4:14PM 
L25.00003: Coherent vorticity extraction in turbulent channel flow using anisotropic wavelets Katsunori Yoshimatsu, Teluo Sakurai, Kai Schneider, Marie Farge, Koji Morishita, Takashi Ishihara We examine the role of coherent vorticity in a turbulent channel flow. DNS data computed at frictionvelocity based Reynolds number 320 is analyzed. The vorticity is decomposed using threedimensional anisotropic orthogonal wavelets. Thresholding of the wavelet coefficients allows to extract the coherent vorticity, corresponding to few strong wavelet coefficients. It retains the vortex tubes of the turbulent flow. Turbulent statistics, e.g., energy, enstrophy and energy spectra, are close to those of the total flow. The nonlinear energy budgets are also found to be well preserved. The remaining incoherent part, represented by the large majority of the weak coefficients, corresponds to a structureless, i.e., a noiselike background flow. [Preview Abstract] 

L25.00004: ABSTRACT WITHDRAWN 
Monday, November 24, 2014 4:27PM  4:40PM 
L25.00005: LES of turbulent boundary layer flow over urbanlike roughness elements Tetsuro Tamura, Makoto Tsubokura, Tsuyoshi Nozu, Keiji Onishi LES of turbulent boundary layer flow over urbanlike roughness elements has been performed. Final goal of this paper is to elucidate the availability of LES on the wind flow within the canopy among buildings in cities. Firstly rectangular blocks, definitely larger than those on conventional rough wall such as grain or sand, are homogeneously arrayed and aboveregion equilibrium profiles of mean velocity and turbulent statistics are investigated. Also, in order to predict the fluctuating velocity characteristics of urban boundary layer, actual complicatedshaped buildings are used for reproducing the surface shape in cities. For numerical modeling, this study employs the unstructuredgrid system where grid lines correctly fit to the building shape and BCM (Building Cube Method) which is formulated on very fine Cartesian mesh system. Based on the GIS data, BCM employs the external forcing technique named IBM (Immersed Boundary Method). Also, in BCM, computational process is so simple that the parallel algorithm and the memory access obtain the perfect efficiency. Using both the LES results, turbulence structures in the urban canopy are discussed. Appropriate 3D vortical structures can be recognized at inflow, along the street and among a pack of tall buildings. [Preview Abstract] 
Monday, November 24, 2014 4:40PM  4:53PM 
L25.00006: Turbulent boundary layer flow over distributions of cubes and evaluation of transient dynamics William Anderson, Qi Li, Elie BouZeid We have used largeeddy simulation with an immersed boundary method to study turbulent flows over a distribution of uniform height, staggered cubes. The computational domain was designed such that both the roughness sublayer and a region of the aloft inertial layer was resolved. With this, we record vertical profiles of time series of fluctuations of streamwise velocity and vertical velocity (where fluctuation is computed as a quantity's deviation from its timeaveraged value during a time period over which the simulation exhibits statistical stationarity). Contour images of fluctuating velocity component shown relative to vertical position and time reveals an advectivelag between the passage of a high or lowmomentum region in the aloft inertial layer and excitation or relaxation of cubescale coherent vortices in the sublayer. We quantify this advective lag and demonstrate how these events precede elevated Reynolds stresses associated with turbulent sweeps at the cube height. We propose that coherent, low and high momentum regions in the inertial layer are responsible for the reported advective lag. Vortex identification techniques are used to illustrate the presence of hairpin packets encapsulating low momentum regions, thereby supporting our hypothesis. Based on this, a simple, semiempirical model for prediction of advective lag with height is developed. In spite of its simplicity, the model manages to capture the advective lag profiles reasonably well. [Preview Abstract] 
Monday, November 24, 2014 4:53PM  5:06PM 
L25.00007: Identifying turbulent coherent structures during LLJ events Velayudhan Praju Kiliyanpilakkil, Guillermo Araya, Sukanta Basu, Arquimedes RuizColumbi\'e, Walter Gutierrez, Luciano Castillo Turbulent structures in the unstable atmospheric boundary layer have been extensively studied in the past. However our recent research show that the stateofthe art Weather Research \& Forecasting model (WRF) model needs improvement in the simulation of nocturnal low level jet (LLJ) characteristics. Under these scenarios, the nocturnal stable boundary layer offers some gray areas to explore, particularly when conditions of high stability and strong vertical wind shear occur. Furthermore, the interactions of nighttime intermittent turbulence (high frequency) with coherent structures play an essential role in transport processes. In the present study, using wavelet analysis techniques, the WRF largeeddy simulation data are evaluated for coherent structure features during LLJ occurrences over the West Texas region. Those structural attributes will be compared to those observed by the high frequency (50 Hz) of the 200meter meteorological tower (Reese, West Texas Mesonet). Additionally, the meteorological tower data are used to evaluate the influence of data acquisition frequency on small turbulent scale detection. [Preview Abstract] 
Monday, November 24, 2014 5:06PM  5:19PM 
L25.00008: Numerical prediction of pollutant dispersion and transport in an atmospheric boundary layer St\'ephanie Zeoli, Laurent Bricteux The ability to accurately predict concentration levels of air pollutant released from point sources is required in order to determine their environmental impact. A wall modeled largeeddy simulation (WMLES) of the ABL is performed using the OpenFoam based solver SOWFA (Churchfield and Lee, NREL). It uses Boussinesq approximation for buoyancy effects and takes into account Coriolis forces. A synthetic eddy method is proposed to properly model turbulence inlet velocity boundary conditions. This method will be compared with the standard pressure gradient forcing. WMLES are usually performed using a standard Smagorinsky model or its dynamic version. It is proposed here to investigate a subgrid scale (SGS) model with a better spectral behavior. To this end, a regularized variational multiscale (RVMs) model (Jeanmart and Winckelmans, 2007) is implemented together with standard wall function in order to preserve the dynamics of the large scales within the Ekman layer. The influence of the improved SGS model on the wind simulation and scalar transport will be discussed based on turbulence diagnostics. [Preview Abstract] 
Monday, November 24, 2014 5:19PM  5:32PM 
L25.00009: The vertical structure of eddy diffusivity in pure slope flows over smooth surfaces Marco Giometto, Jiannong Fang, Marc B. Parlange Thermally driven slope flows are ubiquitous in nature and play a major role in regulating local microclimates in valleys, glaciers and icesheets. They control in large part surface momentum, heat and moisture fluxes, and their effects must be accounted for in weather prediction and climate models. Due to the interplay between shear and buoyancy in generating and destroying turbulence, the thermal and hydrodynamic boundary layers that characterize slope flows are very shallow, with turbulent motions of reduced size, when compared to those populating neutral and convective boundary layers. This poses serious difficulties in terms of computational resolution and modeling requirements for thermally driven slope flows. Not surprisingly, therefore, considerable effort has been devoted in designing parameterizations, for use in larger scale models. In this presentation we explore new parameterizations guided by direct numerical simulations (DNS) to determine the vertical profiles of momentum and buoyancy eddy diffusivities for pure slope flows over smooth surfaces. Mean flow profiles from onedimensional models are then compared against DNS and results will be discussed. [Preview Abstract] 
Monday, November 24, 2014 5:32PM  5:45PM 
L25.00010: The evolution of large scale dense gas clouds at Jack Rabbit Pablo Huq, Tom Spicer Typically ammonia and chlorine are stored or transported as pressurized liquefied gas. There have been many accidents involving storage tanks and also accidents during transport. There is a need for accurate evaluation of the hazards associated with accidental releases of ammonia and chlorine which typically result in denser than air clouds which are toxic. The dense gas cloud slumps under the action of gravity into a thin layer with stable density gradients which suppress ambient atmospheric turbulence, and so complicating the physics of mixing. We present similarity analyses of one and two ton experimental releases of ammonia and chlorine at Jack Rabbit. Similarity analysis discriminates inertiabuoyancy and viscousbuoyancy regimes. Sequences of visualizations are used to determine propagation speeds of dense clouds. There is good agreement between observed speeds and the predictions of similarity analysis of the propagation of radial, dense gas clouds. Finally, comparison of one ton with two ton releases for both ammonia and chlorine lead to insights on scaling which are likely to be useful in the design of even larger scale experiments on dense gas clouds arising from similar configurations. [Preview Abstract] 
Monday, November 24, 2014 5:45PM  5:58PM 
L25.00011: Nonequilibrium Behavior of the Daytime Atmospheric Boundary Layer, from LES Balaji Jayaraman, James Brasseur, Tyler McCandless, Sue Haupt LES of the daytime atmospheric boundary layer (ABL) over flat topography is universally developed as an equilibrium ABL with steady surface heat flux Q$_{\mathrm{0}}$ and steady unidirectional ``geostrophic'' wind vector \textbf{V}$_{\mathrm{g}}$ above a capping inversion, where \textbf{V}$_{\mathrm{g}}$ also defines a spatially uniform transverse mean pressure gradient. The LES approaches a quasiequilibrium state characterized statistically by the ratio of boundary layer depth to Obukhov length scale. In contrast, the true daytime ABL is driven by surface heat flux increases to peak midday and drops in the afternoon, and by mesoscale wind vectors \textbf{U}$_{\mathrm{g}}$ that change in magnitude and direction during the day. We study the consequences of mesoscale weather on ABL dynamics by forcing ABL LES with a WRF simulation of the Midwest during 3 days of frontal passage over Kansas. Assuming horizontal homogeneity, we derive the relationship between \textbf{U}$_{\mathrm{g}}$ and \textbf{V}$_{\mathrm{g}}$ and study ABL response with systematic variation in Q$_{\mathrm{0}}$ and the magnitude and direction of \textbf{U}$_{\mathrm{g}}$. Interesting results include: (1) asymmetry nonequilibrium diurnal response of the ABL; (2) directional changes in surface layer winds relevant to wind turbine function; and (3) changes in ABL stability state arising solely from changes in the direction of \textbf{U}$_{\mathrm{g}}$. [Preview Abstract] 
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