Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session D34: Geophysical Fluid Dynamics: Atmospheric II |
Hide Abstracts |
Chair: Chenning Tong, Clemson University Room: Georgia World Congress Center B406 |
Sunday, November 18, 2018 2:30PM - 2:43PM |
D34.00001: Multi-point Monin-Obukhov similarity in the atmospheric surface layer Chenning Tong, Khuong Xuan Nguyen The Monin-Obukhov similarity hypothesis (MOST) is the theoretical foundation for understanding the atmospheric surface layer. However, it has long been recognized that some important statistics do not follow MOST, indicating incomplete similarity. We propose a generalized MOST, termed multi-point Monin-Obukhov similarity (MMO), hypothesizing that (1) the surface layer has complete similarity, which however can only be represented by multi-point statistics, requiring a horizontal characteristic length scale, which is absent in MOST; (2) The Obukhov length, $L$ is this length scale; (3) All non-dimensional surface-layer multi-point statistics, depend only on the non-dimensional height and separations between the points. The similarity properties (or a lack thereof) for one-point statistics (MOST) can be derived from those of multi-point statistics. A key aspect of the MMO is that at heights much smaller than $-L$ in the convective surface layer, both shear and buoyancy can be important. When applied to the two-dimensional horizontal turbulence spectra, MMO predicts a two-layer structure with three scaling ranges. MMO provides a new framework for analyzing the turbulence statistics and for understanding the dynamics in the atmospheric surface layer. |
Sunday, November 18, 2018 2:43PM - 2:56PM |
D34.00002: Lagrangian Measurements of Canopy Flow Statistics Ron Shnapp, Yardena Bohbot-Raviv, David Peri, Eyal Fattal, Alexander Liberzon Air pollution in the environment is a major concern given the rise in population, and increasing trends of urban population density. Atmospheric turbulence causes dilution of pollutants in the environment. Thus, studying the fluid dynamics in the rough surface layer canopy flows is important for air pollution modeling, both for predictive and real-time emergency response applications. We study the Lagrangian description of turbulence, measuring the velocity of moving flow tracers. Lagrangian statistics are key for most turbulent dispersion models, yet in the canopy flow they are lacking. Our goal is to provide direct Lagrangian measurements in a wind tunnel canopy flow model. We preform Lagrangian measurements of the canopy flow modeled in a wind tunnel laboratory through the 3D-PTV method. To do so we use a unique real-time-image-processing system. A one of its kind dataset of Lagrangian measurements was gathered in this way, both inside and above the canopy layer. We use the dataset to investigate Lagrangian flow characteristics in the canopy model, deriving Lagrangian statistics directly for the first time. These results will aid to construct and validate dispersion models in the atmospheric surface layer. |
Sunday, November 18, 2018 2:56PM - 3:09PM |
D34.00003: Multi-point Monin-Obukhov similarity in the convective atmospheric surface layer using matched asymptotic expansions Mengjie Ding, Chenning Tong The Multi-point Monin-Obukhov similarity (MMO) was proposed to address the incomplete similarity in the original Monin-Obukhov similarity theory (MOST). However, similar to MOST, MMO was also proposed as hypotheses based on phenomenology. Here we derive analytically MMO for the case of the horizontal Fourier transforms of the velocity and potential temperature fluctuations using the spectral forms of the Navier-Stokes and potential temperature equations. We show that for large-scale motions (wavenumber $k<1/z$) in a convective surface layer, the solution is uniformly valid with respect to $z$ (i.e., as $z$ decreases from $z>-L$ to $z<-L$), where $z$ is the height from the surface and $L$ is the Obukhov length. However, for $z<-L$ the solution is not uniformly valid with respective to $k$ as it increases from $k < -1/L$ to $k > -1/L$, resulting in a singular perturbation problem, which we analyze using the method of matched asymptotic expansions. The scales and scaling ranges identified in MMO are derived. We also derive the corrections to the spectra due to finite ratios of the length scales. The analytical derivations for the case of two-point horizontal separations provide strong support to MMO for general multi-point velocity and temperature differences. |
Sunday, November 18, 2018 3:09PM - 3:22PM |
D34.00004: Large-eddy simulation of turbulent flow over spanwise-offset barchan dunes: interdune roller sustained by vortex stretching. Chao Wang, William Anderson The feedback between fluid-flow processes and barchan dune geometry is a key determinant in the evolution of dune fields. Large-eddy simulation was used to model turbulent flow over a series of static barchan dunes. The dune configurations captured realistic stages of a so-called "offset interaction", wherein a relatively small dune is placed upflow of a relatively larger dune, thereby guaranteeing interaction since the former migrates faster than the latter. We conditionally sampled the flow based on events low in probability but high in magnitude; this is relevant to erosion and sediment transport since models indicate a nonlinear scaling between aero-/hydro-dynamic surface stress and sediment flux. Conditional sampling revealed a persistent roller in the space between the upflow and downflow dune. We show that the rotational sign of the roller drives scour from the large dune, and facilitates dune interaction. Terms in the Reynolds-averaged streamwise vorticity transport showed that the roller is primarily sustained by stretching in the interdune space. With this, we show isocontours of local (differential) helicity, which confirms that the roller is persistently scouring sediment from the downflow dune. |
Sunday, November 18, 2018 3:22PM - 3:35PM |
D34.00005: Large-Eddy Simulation of Stably-Stratified Atmospheric Boundary Layers over Flat and Sloped Terrains Cheng-Nian Xiao, Inanc Senocak Numerical studies of turbulent stably-stratified atmospheric boundary layer are known to pose significant challenges as a result of the smaller near-surface turbulence length scale caused by negative buoyancy effects when compared to neutral boundary layers. Due to advances in computing power, large-eddy simulation (LES) with fine spatial resolutions has emerged as a powerful tool to study the complex flow features in stably-stratified boundary layers; however, the quality of the predictions is strongly dependent on the subgrid-scale models and the near-surface parameterization. Since, in addition to the buoyancy destruction of turbulence, gravity effects due to a nonzero terrain slope angle can also alter the near-surface flow structure, there is a major challenge for LES models to accurately account for unresolved flow-turbulence interactions in stably stratified boundary layers close to the surface. In the present work, a number of different models are applied to carry out LES of stably-stratified boundary layers over flat and sloped terrains to investigate the effects of terrain slope angle on the predictions and assess the performance of subgrid-scale models.
|
Sunday, November 18, 2018 3:35PM - 3:48PM |
D34.00006: A study of city–scale atmospheric circulations, and their transition from plume to bubble Hamidreza Omidvar, Elie Bou-Zeid, Qi Li, Juan Pedro Mellado, Petra Klein The Atmospheric Boundary Layer (ABL) structure and turbulence over urban and rural areas are mainly modulated by the wind speed and the Urban Heat Island intensity. The balance between the mean wind advection and thermal upward convection is believed to determine the large-scale flow pattern over the city. When the advection of the wind is the dominant factor, a plume of air rises from the city, and is advected away to the downwind areas. In contrast, when the ABL is dominated by the convection from the hotter city surface, the urban plume is recirculated into the city, forming a bubble shape circulation. Here, we use large eddy simulations to probe these different possible flow regimes. We reduce the problem using dimensional analysis, and investigate how the circulation regime is influenced by two non-dimensional parameters: (i) the ratio of convective velocities that contrasts urban and rural buoyancy fluxes, and (ii) the ratio of the surface friction velocity and the thermal convection velocity over the city. Finally, we examine how the turbulence characteristics shift when the circulation regime transitions from plume to bubble. |
Sunday, November 18, 2018 3:48PM - 4:01PM |
D34.00007: Roughness effects in a stratified atmospheric boundary layer (ABL) Sungwon Lee, Iman Gohari, Sutanu Sarkar We examine roughness effects in the ABL by conducting Direct Numerical Simulation (DNS) of a canonical problem, the Ekman boundary layer (EBL), at moderate Reynolds number (Re*=700). The roughness takes the form of periodic two-dimensional bumps whose nondimensional amplitude and aspect ratio are changed among cases. The EBL is in a transitionally rough regime and, without stratification, the effect of roughness is found to be mild in contrast to the stratified case. Stratification is imposed through a stabilizing cooling flux that is applied for approximately an inertial period. The flat-bottom case exhibits initial collapse of turbulence which eventually recovers, albeit with large-amplitude oscillations in turbulent kinetic energy. There is a strong thermal inversion and a low-level jet characteristic of a very stable (Mahrt 1998) regime. In contrast, the 4 bump EBL belongs to the weakly stable regime. Relative to the flat surface, the 4 bump case has a weaker thermal inversion, lower bulk Richardson number, and stronger near-surface turbulence. The role of coherent structures will be discussed through visualizations and statistics obtained by triple decomposition. |
Sunday, November 18, 2018 4:01PM - 4:14PM |
D34.00008: Experimental Observations of the Boundary Layer in Desert Dunes with Unmanned Aircraft Victoria Alexander Natalie, Jamey D Jacob To observe and relate the effects of terrain on lower atmospheric phenomena, empirical data is obtained through atmospheric measurements and photogrammetric terrain modeling. The results of which are analyzed to determine correlations between topography and boundary layer characteristics. Lower atmospheric boundary layer conditions are modeled by wind speed, temperature, pressure, and humidity sensors flown onboard small Unmanned Aerial Vehicles (UAVs). These sensors include commercially available sonic anemometers and hotwires, as well as 3D printed multi hole probes that are designed, printed, and calibrated for this application. High resolution modeling of varying terrain and types of Aeolian dune structures are created using cameras onboard small UAVs. The observed dune structures have been developed under different conditions and are hallmarks of rapid geomorphology. Understanding the relationship that the atmosphere has with these structures can provide insight on less rapidly formed Aeolian landforms as well. |
Sunday, November 18, 2018 4:14PM - 4:27PM |
D34.00009: Spatially Sampled Turbulence Measurements Acquired Using Unmanned Aerial Vehicles Caleb Canter, Sean C.C. Bailey To address the challenge of spatially interrogating the turbulent structure of the atmospheric boundary layer over a wide range of length scales we use highly instrumented, autonomous unmanned aerial vehicles (UAVs). This approach has the advantage of being able to spatially sample the flow field over a large range of spatial scales while providing a reduced reliance on Taylor’s frozen flow hypothesis regarding assumptions of temporal evolution of the turbulence. This talk will provide an overview of the systems which have been developed and are being used as part of this work, as well as present atmospheric data acquired by these systems as part of the ISARRA LAPSE-RATE measurement campaign in Colorado's San Luis valley. |
Sunday, November 18, 2018 4:27PM - 4:40PM |
D34.00010: Large-eddy simulation of complex terrain flows: an application to the Perdigao site Inanc Senocak, Cheng-Nian Xiao Detailed investigations and an enhanced understanding of atmospheric flows over complex terrain are necessary for many civil and engineering applications such as air pollution studies, weather prediction, wind energy harvesting, agriculture and aviation. Due to advances in high-performance computing with accelerators, large-eddy simulations (LES) of complex terrain flows with spatial resolutions on the order of ten meters have become feasible. However, technical challenges such as accurate near-surface subgrid-scale parameterizations for complex terrains need to be overcome in order to obtain reliable results from LES. In the current work, results from LES of flow around the Perdigao terrain under neutrally-stratified conditions are presented. A Cartesian immersed boundary method with a logarithmic reconstruction scheme and a near-surface subgrid-scale model are applied to simulate separated flows over the double-humped terrain of Perdigao . Simulation results are compared against the field measurements, and the effect of turbulent inflow conditions on the simulations are investigated. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700