Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session P16: Turbulence: Environmental Flows (3:10pm - 3:55pm CST)Interactive On Demand
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P16.00001: Secondary Flow Structures in Natural Channels Under Open and Ice-coverage Conditions Berkay Koyuncu, Trung Le Secondary flow structure (SFS) under ice-coverage has not been examined so far despite it’s important role in dynamics of meandering streams and rivers. In this work, three-dimensional flow structures are investigated in natural channels from a relatively small streams to large-scale rivers using both field measurement (Acoustic Doppler Current Profiler) and Large Eddy Simulation. Three channels are considered in this work: (a) Buffalo River, Minnesota, United States; (b) Red River of the North, Fargo, North Dakota, United States; and (c) Hong River, Hanoi, Vietnam. Two methods of measurements have been carried out: (a) continuous monitoring; and (b) traverse crossings for these rivers. The measurements show evidence for the existence of SFS across measured cross-sections. To understand the impact of ice-coverage, three-dimensional model of the Red River of the North is setup for Large Eddy Simulation at bank-full condition under both open and covered conditions. Our field data show that ice-coverage condition induces a drastically different 3D structures in comparison to the open-surface condition. Under ice-coverage, a double log-law velocity profile distribution is formed. Due to the presence of SFS, turbulent statistics are also altered across the measured cross-sections. [Preview Abstract] |
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P16.00002: Flocculation of suspended cohesive particles in homogeneous isotropic turbulence Kunpeng Zhao, Florian Pomes, Bernhard Vowinckel, Tian-Jian Hsu, Bofeng Bai, Eckart Meiburg We investigate the dynamics of cohesive particles in homogeneous isotropic turbulence, based on one-way coupled simulations that account for Stokes drag, lubrication, cohesive and direct contact forces. The simulations yield the evolution of floc size and shape due to aggregation and breakage, as function of the governing parameters. Larger turbulent shear and weaker cohesive forces limit the floc size and result in elongated floc shapes. Flocculation proceeds most rapidly when the Stokes number of the primary particles, based on the Kolmogorov length scale and the rms-velocity, is near one. The equilibrium floc size distribution exhibits a preferred size as function of the cohesive number. Consistent with earlier findings, we observe that flocs of size close to the Kolmogorov scale preferentially align themselves with the intermediate strain direction and the vorticity tensor. In contrast, flocs smaller than the Kolmogorov scale tend to align themselves with the extensional strain direction. Based on the numerical data we propose a new flocculation model with a variable fractal dimension, which allows us to predict the temporal evolution of the floc size and shape. Compared to existing models in the literature, the new model has fewer limitations and results in improved accuracy. [Preview Abstract] |
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P16.00003: Oil droplet behavior in a counterrotating vortex pair generated in a Langmuir cell tank Sanjib Gurung, Carlowen Smith, Adila Hoque, Tristen Mee, Shankar Singh, Mumtaz Hassan, Seyedmohammadjavad Zeidi, Andres Tejada-Martinez, David Murphy Langmuir circulation is a form of turbulence occurring in the ocean surface$'$s upper layer due to wind shear and surface waves. The resulting pairs of counterrotating vortices called Langmuir cells affect particle distribution in the water column. Buoyant particles such as oil droplets resulting from entrained oil slicks can be trapped by downwelling in Stommel Retention Zones (SRZ). Using a laboratory facility recreating some aspects of SRZ, we study the fate of such oil droplets, which is not well understood. The experimental facility consists of a $1\times0.2\times0.5$ m$^{3}$ tank in which a shear stress is applied on the side walls using conveyor belts, resulting in a counterrotating vortex pair of variable strength and turbulent kinetic energy. A turbulent oil jet may be repeatably injected downwards into this downwelling region. The resulting droplet size distribution is simultaneously characterized for 5 minutes after oil injection using brightfield imaging at 10 Hz within three fields of view horizontally distributed across the tank. The decrease in median droplet diameter over time for various belt speeds is rationalized using the Trapping Function, which is the ratio of outward to inward forces acting on a droplet at the elevation of the vortex center. [Preview Abstract] |
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P16.00004: Momentum Transport in Heterogeneous Forest Canopies Hawwa Kadum, Ryan Scott, Sarah Smith, Giulia Salmaso, Chad Higgins, Raul Cal, Marc Calaf The effect of spatial heterogeneity on momentum transport in forest canopies is investigated in a series of wind tunnel experiments. Forest models of 1:200 scale were manufactured from 10 PPI reticulated foam with porosity equivalent to a LAI of 5.3. The model forest canopy consists of identical, uniformly distributed trees with spatial heterogeneity introduced via alternating patches and gaps of various sizes. Lacunarity analysis is used to quantify the heterogeneity of these arrangments. The various arrangements are then tested against surface and volume integrated terms of the momentum equation obtained using control volume analysis. Two canopy layers are considered: upper canopy above the canopy height and lower canopy below the canopy height. The lower canopy is dominated by local flow features and its response to heterogeneity is lower than the upper canopy. The upper canopy shows a significant increase in momentum advection and residual terms with increased heterogeneity. The most heterogeneous case shows an advection value that is 14 times the homogeneous case. The stress gradient term also increases with heterogeneity although the difference between upper and lower canopies is less pronounced. The stress gradient term is more influenced by the gap size. [Preview Abstract] |
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P16.00005: Unfolding the Link between Forest Canopy Structure and Flow Morphology Giulia Salmaso, Raúl Bayoán Cal, Chad Higgins, Marc Calaf The focus of this project is on canopy heterogeneity associated with canopy lacunarity with scales \textasciitilde O(100m) associated with a uniform under-canopy roughness and dominated by weak thermal stratification. Such near-canonical cases describe inhomogeneous momentum transport in an otherwise planar homogeneous flow when the canopy is absent. These canonical configurations serve as a logical starting point for the more complex cases. More specifically, in this work, we explore the use of dispersive fluxes as a means not only to quantify the perturbations induced by the canopy heterogeneity on the mean flow but also as an opportunity for developing new parameterizations. For this purpose, Large Eddy Simulations of the atmospheric boundary layer with varying geostrophic forcing will be used. A high-resolution representation of different vegetated canopies, with a changing degree of ``\textit{gapiness''} and heterogeneity, is included. Results will quantify the perturbations induced by the canopy heterogeneities on the mean flow and higher-order statistics. Furthermore, preliminary results of a new scaling relating the contribution of canopy-induced dispersive fluxes as a function of canopy heterogeneity will be presented. [Preview Abstract] |
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P16.00006: Effect of Wood Jams on Flow Structure and Local Sediment Transport Elizabeth Follett, Isabella Schalko, Heidi Nepf Wood jam reintroduction is now considered a key component of river restoration projects, due to ecohydraulic benefits associated with wood presence. The physical attributes and associated impacts on flow structure and sediment transport of wood jams vary depending on relative jam and channel dimensions and bed mobility. Prediction of jam-induced backwater rise and sediment transport is necessary to improve design of restoration projects and model flood hazards. We present recent results demonstrating that an accumulation of wood pieces acts as a porous obstruction, so that flow progressing through the structure experiences frictional losses due to drag on wood pieces in an analogous manner to the drag generated by vegetated canopies. Further, we consider flow diversion and heterogeneity generated by partially spanning jams distributed non-uniformly in depth and for jams formed from increasing numbers of logs, which generate flow diversion in a manner similar to vegetated patches. When jam-induced flow diversion was strong enough to initiate local sediment transport, the jam grew in a self-similar manner, generating a scour hole. However, jams accumulated above immobile beds were restricted by the bed, eventually filling the channel-cross section with increased backwater rise. [Preview Abstract] |
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P16.00007: Response of Evaporating Droplets to Vortices Anu V. S. Nath, Rama Govindarajan, S Ravichandran, Anubhab Roy We study the dynamics of small water droplets in vortical airflow and explore the important differences between the collective dynamics of individual droplets and those of merely a heavier phase of the fluid. We had shown earlier ([1],[2]) that heavy droplets getting centrifuged out of vortices can form caustics (cross each other) within a certain radius, and that this can make a big difference to the collision and coalescence. We now explore how evaporation/ condensation affects this process, given that the Stokes number of each droplet changes as phase change proceeds. We also discuss how the resulting buoyancy injection modifies the flow itself, in model three-dimensional vortical flows as well as in turbulence. References [1] Ravichandran, S., & Govindarajan, R. (2015). Caustics and clustering in the vicinity of a vortex. Physics of Fluids, 27(3), 033305. [2] Govindarajan, R., Ravichandran, S., Ray, S., & Deepu, P. (2016). Caustics and the growth of droplets. APS, 2016, K53-005. [Preview Abstract] |
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P16.00008: A Spatial Heterogeneity Parameter for Canopy Flows Ryan Scott, Hawwa Kadum, Giulia Salmaso, Marc Calaf, Raúl Bayoán Cal Quantifying spatial structure is an integral component of understanding turbulent canopy flows. Generally, canopies are considered homogeneous rough surfaces through Monin-Obukhov similarity theory which assumes the turbulence introduced by spatial heterogeneity within the canopy blends away in the lower ABL. However, the blending height is a function of canopy geometry as well as atmospheric properties necessitating the use of subgrid turbulence models or semi-empirical relationships. In order to parameterize canopy structure, a novel measure of spatial heterogeneity is proposed from lacunarity analysis where the space filling nature of a given canopy reveals relevant length scales. The parameter provides description of spatial heterogeneity which facilitates comparisons of turbulent quantities between canopies. Further details related to the development of the new parameter will be discussed in presentation along with application of the measure to multiple canopy types of interest to the turbulence community. [Preview Abstract] |
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P16.00009: Quantifying the dynamics of transitional clay flows within nearly isotropic turbulence Shaelynn Kaufman, Vaibhav Tipnis, Jim Best, Leonardo Chamorro, Shyuan Cheng, Samya Sen, Randy Ewoldt Water-clay flows are found extensively across Earth's surface in a number of natural and man-made environments. Although progress has been made in the last two decades to characterize their fluid dynamics, the fine-scale fluid mechanics of transitional clay flows (volumetric clay concentrations of 0.01\textless C\textless 0.8) remain poorly understood. This is partially due to the challenges associated with imaging opaque flows. To address this problem, we have developed a novel approach that uses Laponite RD\texttrademark , a synthetic clay capable of producing clear suspensions of clay particles within water, and thus enabling optical quantification of the flow in both Lagrangian and Eulerian frames of reference. In the mixing box, turbulence is generated by a series of eight symmetrical mechanical fans that are capable of generating nearly isotropic turbulence. Time-resolved particle image velocimetry is used to quantify the spatio-temporal dynamics of the flows, their spectral composition, statistics, and energy budget. This paper will provide details of the technique, and results of the experiments performed that show that a correlation exists between the Taylor microscale Reynolds number and Kolmogorov scaling of the temporal spectra as clay concentration increases above C$\ge $0.015, due to turbulence modulation caused by the presence of clay particles. [Preview Abstract] |
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P16.00010: Parameterizing pollutant dispersion and deposition for roadside vegetation Khaled Hashad, Bo Yang, Ke Max Zhang With many communities living close to highways there is a need to mitigate traffic pollution. Roadside vegetation can be used to reduce pollutants by enhancing vertical dispersion and deposition. Most studies conduct either CFD simulations or field measurement to assess roadside vegetation. With growing interest to understand how roadside vegetation impact pollutant concentration, there is a need to develop simpler models that can describe this phenomena. We fit the Gaussian dispersion equations to describe pollutant concentration behind vegetation barriers through understanding the underlying physics driving pollutant dispersion and deposition. Vegetation induces drag on the flow and dissipates turbulence through its small structures like branches and leaves which creates a wake region with low velocity and turbulence. After the wake, the turbulence generated by the shear of the flow above the vegetation and in the wake, reaches the ground and further disperses the pollutants. A multi-region approach was used to describe the concentration behind the barrier. The first region is within the vegetation, followed by the wake region, then the high turbulence region. Data generated from LES simulations, validated through field measurements, was used to generate the model. [Preview Abstract] |
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P16.00011: Effect of size and shape on the transport of particles over the free surface of a natural stream Henri Sanness Salmon, Lucia Jane Baker, Jessica Kozarek, Filippo Coletti Plastics are the most prevalent type of marine debris found in our oceans and great lakes. They come in many shapes and sizes and degrade into very small pieces (microplastics) that irreversibly pollute the environment. Understanding how these objects are transported in natural streams is critical for capturing them before they reach large bodies of water. Here we investigate experimentally the motion of floating particles on the turbulent free surface of the Outdoor Stream Lab, a field-scale meandering stream at the St. Anthony Falls Laboratory. We consider mm-sized tracers as well as cm-sized disks and rods. Particle tracking velocimetry is used to obtain Eulerian and Lagrangian statistics of the particle motion at different flow rates. In particular, we analyze particle velocity, acceleration, spatial and temporal correlations, single-particle and particle-pair dispersion rates. The tracer analysis allows to define the characteristic spatial and temporal scales of the free-surface turbulence and its dispersion properties. The larger particles are directly compared to the tracers, highlighting the influence of shape and size on their velocity and dispersion. [Preview Abstract] |
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