Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session U33: Geophysical Fluid Dynamics: Atmospheric Boundary Layers |
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Chair: Andrew Banko, United States Military Academy Room: 241 |
Tuesday, November 22, 2022 8:00AM - 8:13AM |
U33.00001: Quantifying Three-dimensional Topological Dispersion within Realistic Urban Canopies using Magnetic Resonance Imaging Andrew J Banko, Michael J Benson, Ty Homan, Christopher J Elkins Scalar dispersion within urban canopies is governed by topological dispersion due to the mean flow around buildings and turbulent dispersion due to eddies. Topological dispersion can be important in the near-field of ground level sources, because complex mean flow patterns can produce cross-wind or even up-wind plume spread. However, a 3D understanding of the flow topology in realistic urban canopies is relatively limited. In this work, magnetic resonance imaging is used to measure the 3D mean velocity and concentration fields in a scale model of Oklahoma city based on the JU2003 field tests. Transport boundaries, defined as 2D manifolds with small mean scalar flux across them, are extracted using the Finite Time Lyapunov Exponent (FTLE) field and correlated to the dispersion patterns. Ridges of the backwards FTLE field, which identify hyperbolic manifolds exhibiting exponential convergence of fluid trajectories, act as barriers to scalar transport that redirect the plume. The MRI measurements enable a 3D characterization of transport boundary topology and explain observed cross-wind and vertical scalar transport behavior. Remarkably, although the FTLE field only guarantees small mean scalar flux across a ridge, the structures appear robust to turbulent mixing. |
Tuesday, November 22, 2022 8:13AM - 8:26AM |
U33.00002: Uncrewed Aircraft System Measurements of Atmospheric Surface-Layer Structure during Morning Transition Loiy Al-Ghussain, Sean C Bailey This study applies uncrewed aircraft systems towards the investigation of surface-layer structure during the morning transition. Three uncrewed aircraft systems simultaneously measuring horizontal transects were partnered with a fourth measuring vertical profiles during two consecutive mornings as part of the Collaboration Leading Operational Unmanned Aerial System Development for Meteorology and Atmospheric Physics (CLOUDMAP) measurement campaign near Stillwater, Oklahoma, USA. The results reflect differences in the evolution of spatial statistics, despite very similar synoptic conditions. Conditional averaging was also applied to identify the structure of sweep and ejection motions and results revealed similarities to observations from canonical wall-bounded flow. Finally, it was found that these structures contribute approximately 20% to 30% of overall turbulence kinetic energy, momentum, and sensible heat flux, with an increase observed during the course of the boundary-layer transition. |
Tuesday, November 22, 2022 8:26AM - 8:39AM |
U33.00003: Measurement of seasonal changes in the near-maritime atmospheric surface layer Alex Peralta, Charles Nelson, Cody Brownell Turbulent fluxes of momentum and heat in the Chesapeake Bay region of Maryland, USA are studied over a 12-month period. Measurements from a vertical array of two sonic anemometers and an infrared gas analyzer, reaching 8-m above the water, provide surface layer parameters using the eddy covariance method. Warm waters of the Bay are shown to produce generally unstable atmospheric conditions throughout the summer months, and a more balanced mix of stable and unstable conditions during the late winter. The temperature structure parameter, i.e. the coefficient of the second order structure function for temperature, is calculated and compared to expected scaling per Monin-Obukhov theory. As observed by others, the expected scaling for temperature structure holds well for unstable conditions but is less predictable for neutral and stable conditions. |
Tuesday, November 22, 2022 8:39AM - 8:52AM |
U33.00004: Experimental Observation of Boundary Layer and Movement of Topographically Varying Desert Dunes with Unmanned Systems Kerrick Ray, Victoria A Natalie, Jamey D Jacob Desert dunes offer unique geophysical interactions, and these eolian dominant interactions occur over a significant portion of earth surfaces. There are multiple factors that determine the direction and shape of these dunes, which include sand availability and primary wind modes throughout the year. Trying to replicate these interactions in common water or wind tunnels can be difficult as the size of the dune creates a high Reynolds Number that was proven difficult to replicate with sized down models. This project endeavored to apply unmanned aerial systems (UAS) capabilities to track the sub boundary layer propagation by attaching an anemometer and flying vertical profiles while comparing the data with grounded anemometers. In addition, this research pursued tracking the movement of desert dunes using images capture by unmanned aerial systems (UAS) to create a structure-from-motion to assist when shape modeling. This volatile landscape makes an ideal setup for tracking imagery combined with its effect of atmospheric data. |
Tuesday, November 22, 2022 8:52AM - 9:05AM |
U33.00005: Assessment of UAS based Atmospheric Boundary Layer Sampling of Wind Speed Trevor C Wilson, Brian R Elbing, Jamey D Jacob Uncrewed Aircraft Systems (UAS) have become more involved in weather monitoring and atmospheric sampling. UAS can be equipped with sensors to measure various atmospheric properties such as temperature, velocity, and humidity. However, it is vital to understand the impact of UAS propulsion systems on the sensors and the data they collect. Initial experiments collected wind speed measurements on a quad-copter based UAS and compared it with a nearby fixed mast. A statistical analysis on this data quantified the minimum distance (5.3 rotor diameters) a sensor should be offset from the rotors (above). The current work aims to expand the scope of this previous work while utilizing the recommended sensor location to obtain trusted velocity measurements from a UAS. This includes the utilization of multiple UAS and masts to better characterize the atmospheric boundary layer (ABL), including local shear. Results shown will include plans for optimal ABL sampling as well as preliminary results from these UAS tests with a focus on length and time scales of the atmospheric boundary layer. |
Tuesday, November 22, 2022 9:05AM - 9:18AM |
U33.00006: Development of practical computational fluid dynamics procedures for generating horizontally homogeneous atmospheric boundary layer profiles for wind engineering applications Yunjae Hwang, DongHun Yeo Simulations for wind engineering applications require fully-developed boundary layer approach flow with horizontal homogeneity and zero pressure gradient for accurate characterization of wind fields and the associated aerodynamic pressure on structures. Most numerical approaches using computational fluid dynamics (CFD) have focused on achievement of such approach flows in computational domains whose heights are equal to or less than the atmospheric boundary layer (ABL) height. However, simulations for specific wind engineering applications, such as wind over topographic features or in urban environments, could require the approach flow in domains whose top boundaries are higher than the ABL height because the limited domain height might distort the flow field of interest. Thus, this study proposes a novel approach to create the desired horizontally homogeneous and zero pressure gradient flow characteristics, in a vertically extended computational domain whose height is greater than the ABL depth. The proposed procedure is applied to simulations with various topographic features to investigate the effect of simulation conditions on the flow field of interest over the topography. |
Tuesday, November 22, 2022 9:18AM - 9:31AM |
U33.00007: Unsteady Boundary Layer Flow in Urban Canopies Marco G Giometto, Weiyi Li In atmospheric boundary layer (ABL) flows, non-stationarity is a rule rather than an exception. Under this condition, the flow may experience departures from equilibrium with the underlying surface stress, misalignment of shear and strain rates, and three-dimensionality of turbulence statistics. Existing ABL flow theories are largely established for stationary flow conditions, and cannot predict such behaviors. Here, we analyze the impact of time-varying pressure gradients on mean flow and turbulence over and within urban-like surfaces. To that end, a series of large-eddy simulations of pulsatile flow over cuboid arrays are performed with varying oscillation amplitude and forcing frequency. The flow response is then examined in terms of long-time averaged and phase-dependent flow statistics. The analysis reveals that the aerodynamic roughness length characterizing the surface increases with both frequency and amplitude of the imposed oscillation. In this presentation, we discuss fundamental mechanisms responsible for this behavior and present a phenomenological surface drag model that captures the impact of flow unsteadiness on surface drag. |
Tuesday, November 22, 2022 9:31AM - 9:44AM |
U33.00008: Logarithmic profile of temperature in sheared and unstably stratified atmospheric boundary layers Yu Cheng, Qi Li, Dan Li, Pierre Gentine The impact of buoyancy on the mean velocity, temperature, and scalar concentration profiles in the lower atmosphere is typically investigated within the framework of Monin-Obukhov similarity theory (MOST). MOST is the theoretical foundation for parametrizing surface-atmosphere exchanges in nearly all weather, climate, and hydrological models. According to MOST, the classic logarithmic profiles of mean velocity and temperature break down as the buoyancy effects become important. However, recent studies on turbulent Rayleigh-Bénard convection and natural convection along vertical walls suggest that the mean temperature in the near-surface region still follows a logarithmic profile. Motivated by these new results, we study the mean potential temperature profile in sheared and unstably stratified atmospheric boundary layers using direct numerical simulations and field observations. We find that the mean potential temperature profile remains logarithmic across a wide range of stability parameters, which characterizes the relative importance of buoyancy versus shear effects. Compared to MOST, our results suggest that the buoyancy force does not modify the logarithmic nature of the mean potential temperature profile, but instead modulates its slope, which is no longer universal and differs from 1/κ, where κ is the von Kármán constant. This study provides another perspective on scalar turbulence in the atmospheric boundary layer. |
Tuesday, November 22, 2022 9:44AM - 9:57AM |
U33.00009: Kirigami-inspired wind-steering for ventilation of urban shading structures Lucia Stein-Montalvo, Sigrid Adriaenssens, Elie Bou-Zeid While passive shading solutions—e.g., façades, "streeteries" and cloth canopies protecting street canyons—can regulate temperatures and reduce energy needs in urban settings, existing designs often block air flow, trapping heat and harmful particles. To address the need for simultaneous shading and ventilation, we propose a kirigami-inspired solution. In kirigami, adding cuts to sheets frees sections to buckle and tilt when the sheet is stretched, thus transforming a 2D structure into a porous 3D one. Here, we use Large Eddy Simulation (LES) to examine how such angled features, placed at the top of an otherwise enclosed space, affect ventilation in the presence of oncoming wind. In particular, we study how geometry affects the Air Exchange Rate (ACH) and local air circulation, examining both mean flow (dispersive) and turbulent air exchange. Additionally, our simulation results are complemented by qualitative data from wind tunnel experiments. |
Tuesday, November 22, 2022 9:57AM - 10:10AM |
U33.00010: Contaminant Dispersion In and Around a Sports Stadium - High Fidelity Simulations and Comparisons with Magnetic Resonance Measurements Mark F Owkes, Michael J Benson, Andrew J Banko The intentional or accidental release of a contaminant in an urban environment has the potential to be harmful to a large population. However, predicting the dynamics of the contaminant plume is complicated due to the complex geometry of the urban buildings. Many studies have been performed that provide velocity and concentration data at a few discrete points within the complex flow. This work leverages a rich three-dimensional measurement taken with magnetic resonance imaging (MRI). The experiment was conducted on a scaled model of a sports stadium within a water tunnel and provided contaminant concentration and three components of velocity. The contaminant was released within the stadium to assess the impact of interactions between the internal and external flow fields via several ventilation features, including a roof opening, roof slits, and entrance and egress tunnels. This dataset is compared with LES simulations performed with the same geometry and boundary conditions. The LES simulations further investigate the influence of the wind direction and temporal nature of a contaminate plume in this complex geometry. |
Tuesday, November 22, 2022 10:10AM - 10:23AM |
U33.00011: Unsteady land-sea breeze circulations in the presence of a synoptic pressure forcing Elie R Bou-Zeid, Mohammad H Allouche, Juho Iipponen Land-Sea breezes are some of more complex geophysical turbulent flows: they are unsteady, heterogeneous, and depend on the balance of thermal to mechanical pressure forcing. Studies of these flows have typically focused on time snap-shots and have often ignored the important role of synoptic scale mechanical pressure forcing. In this study, we investigate the diurnal cycles of land-sea breeze (LSB) circulations in the presence of increasingly strong synoptic pressure forcing (expressed as an equivalent geostrophic wind). The relative importance and orientation of the thermal and synoptic forcings are measured through two dimensionless parameters: a bulk inverse Richardson number Ri = W*2/Mg2, where Mg is the geostrophic wind magnitude and W* is a convective buoyant velocity scale, and the angle α between the shore and geostrophic wind. |
Tuesday, November 22, 2022 10:23AM - 10:36AM |
U33.00012: Estimation of near-surface wind speed over complex terrain with machine learning Takenobu Michioka, Keita Kosaka, Hiroshi Takimoto, Ayumu Sato High-resolution wind speeds near the ground surface over complex terrain are reconstructed by the low-resolution wind speeds using a machine-learned super-resolution (SR) model. The hybrid downsampled skip-connection/multi-scale model proposed by Fukami et al (JFM, Vol.870, 2019, pp. 106-120) are applied in the machine-learned SR model. The Reynolds-averaged Navier-Stokes is implemented for flow fields over complex terrains to obtain the high-resolution wind speeds (20 m resolution) at 10 m above the ground. The low-resolution wind speeds (160 m or 320 m resolution) were obtained by applying average pooling to high-resolution wind speeds. The SR model using only the low-resolution wind speeds as its input can not dramatically improve the reproducibility of the high-resolution wind speeds. However, the SR model using both the low-resolution wind speeds and the terrain elevation (the horizontal resolution of 20 m) as its inputs accurately estimates the high-resolution wind speeds. For the dataset at another site which has not been used in training, the SR model using the two inputs approximately reconstructs the high-resolution scalar velocities. |
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