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 L32: Geophysical Fluid Dynamics: Special Topics IGeophysical
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Chair: Andres Tejada-Martinez, University of South Florida Room: 104 |
Monday, November 20, 2017 4:05PM - 4:18PM |
L32.00001: How trees uptake carbon, release water and cool themselves in air: a marriage between biophysics and turbulent fluid dynamics Tirtha Banerjee, Rodman Linn Resolving the role of the biosphere as a terrestrial carbon sink and the nature of nonlinear couplings between carbon and water cycles across a very wide range of spatiotemporal scales constitute the scope of this work. To achieve this goal, plant physiology models are coupled with atmospheric turbulence simulations. The plant biophysics code is based on the following principles: (1) a model for photosynthesis; (2) a mass transfer model through the laminar boundary layer on leaves; (3) an optimal leaf water use strategy regulated by stomatal aperture variation; (4) a leaf-level energy balance to accommodate evaporative cooling. Leaf-level outputs are upscaled to plant, canopy and landscape scales using HIGRAD/FIRETEC, a high fidelity large eddy simulation (LES) framework developed at LANL. The coupled biophysics-CFD code can take inputs such as wind speed, light availability, ambient CO2 concentration, air temperature, site characteristics etc. and can deliver predictions for leaf temperature, transpiration, carbon assimilation, sensible and latent heat flux, which is used to illustrate the complex the complex interaction between trees and their surrounding environments. These simulation capabilities are being used to study climate feedbacks of forests and agroecosystems. [Preview Abstract] |
Monday, November 20, 2017 4:18PM - 4:31PM |
L32.00002: A simple model of the effect of ocean ventilation on ocean heat uptake Balu Nadiga, Nathan Urban Transport of water from the surface mixed layer into the ocean interior is achieved, in large part, by the process of ventilation---a process associated with outcropping isopycnals. Starting from such a configuration of outcropping isopycnals, we derive a simple model of the effect of ventilation on ocean uptake of anomalous radiative forcing. This model can be seen as an improvement of the popular anomaly-diffusing class of energy balance models (AD-EBM) that are routinely employed to analyze and emulate the warming response of both observed and simulated Earth system. We demonstrate that neither multi-layer, nor continuous-diffusion AD-EBM variants can properly represent both surface-warming and the vertical distribution of ocean heat uptake. The new model overcomes this deficiency. The simplicity of the models notwithstanding, the analysis presented and the necessity of the modification is indicative of the role played by processes related to the down-welling branch of global ocean circulation in shaping the vertical distribution of ocean heat uptake. [Preview Abstract] |
Monday, November 20, 2017 4:31PM - 4:44PM |
L32.00003: Lagrangian transport by breaking surface waves Luc Deike, N.E. Pizzo, W.K. Melville The Lagrangian transport due to non-breaking and breaking focusing wave packets is examined. We present direct numerical simulations of the two-phase air-water Navier-Stokes equations describing focusing wave packets, investigating the Lagrangian drift by tracking tracer particles in the water before, during, and after the breaking event. The net horizontal transport for non-breaking focusing packets is well described by the classical Stokes drift, both at the surface and in the bulk of the fluid, where the e-folding scale of the evanescent vertical profile is given by the characteristic wavenumber. For focusing wave packets that lead to breaking, we observe an added drift that can be ten times larger than the classical Stokes drift for a non-breaking packet at the surface, while the initial depth of the broken fluid scales with the wave height at breaking. We find that the breaking induced Lagrangian transport scales with the breaking strength. A simple scaling argument is proposed to describe this added drift and is found to be consistent with the direct numerical simulations. Applications to upper ocean processes are discussed. [Preview Abstract] |
Monday, November 20, 2017 4:44PM - 4:57PM |
L32.00004: The statistical characteristics of rain-generated stalks on water surface Xinan Liu, Ren Liu, James H Duncan Laboratory measurements of the stalks generated by the impact of raindrops are performed in a 1.22-m-by-1.22-m water pool with a water depth of 0.3 m. Simulated raindrops are generated by an array of 22-gauge hypodermic needles that are attached to the bottom of an open-surface rain tank. The raindrop diameter is about 2.6 mm and the height of the rain tank above the water surface of the pool is varied from 1~m to 4.5~m to provide different impact velocities. A number of parameters, including the diameter, height and initial upward velocity of the center jets (stalks) are measured with a cinematic laser-induced- fluorescence technique. It is found that the maximum potential energy of the stalk and the joint distribution of stalk height and diameter are strongly correlated to the impact velocities of raindrops. Comparisons between the rain experiments and single drop impacts on a quiescent water surface are also shown. [Preview Abstract] |
Monday, November 20, 2017 4:57PM - 5:10PM |
L32.00005: LES-based study of water wave effects on stably-stratified atmospheric turbulent flows. Tao Cao, Lian Shen Turbulent air flow over ocean waves is a key physical process in the marine atmospheric boundary layer. In this study, we use large-eddy simulation to investigate the effect of various water wave parameters on stably stratified air turbulence. By decomposing a physical quantity into three parts, namely the plane average, wave fluctuation, and turbulence fluctuation, we isolate wave-coherent fluctuation in the air flow from the turbulence fluctuation to quantify the interaction between wave-coherent fluctuation and turbulence. It is found that the effect of the wave-induced fluctuation on the turbulence depends on the wave age and wave steepness. The smaller the wave age and the larger the wave steepness, the stronger the coupling between the surface waves and the turbulence above, and the larger modification of the turbulence by the surface waves. [Preview Abstract] |
Monday, November 20, 2017 5:10PM - 5:23PM |
L32.00006: Numerical study of wind over breaking waves and generation of spume droplets. Zixuan Yang, Shuai Tang, Yu-Hong Dong, Lian Shen We present direct numerical simulation (DNS) results on wind over breaking waves. The air and water are simulated as a coherent system. The air-water interface is captured using a coupled level-set and volume-of-fluid method. The initial condition for the simulation is fully-developed wind turbulence over strongly-forced steep waves. Because wave breaking is an unsteady process, we use ensemble averaging of a large number of runs to obtain turbulence statistics. The generation and transport of spume droplets during wave breaking is also simulated. The trajectories of sea spray droplets are tracked using a Lagrangian particle tracking method. The generation of droplets is captured using a kinematic criterion based on the relative velocity of fluid particles of water with respect to the wave phase speed. From the simulation, we observe that the wave plunging generates a large vortex in air, which makes an important contribution to the suspension of sea spray droplets. [Preview Abstract] |
Monday, November 20, 2017 5:23PM - 5:36PM |
L32.00007: The smallest jet drops produced by bursting bubbles Frederik Brasz, Casey Bartlett, Peter Walls, Elena Flynn, James Bird Aerosol droplets are produced from the breakup of jets formed when small air bubbles burst at an air-liquid interface. These jet drops transfer sea salt and organic matter from the oceans to the atmosphere, where they act as cloud condensation nuclei and can spread pathogens. The smallest aerosols persist the longest in the air and advect the furthest from their source, but because they are too small to be observed directly, little is known about what size ocean bubbles create them or how their formation depends on seawater properties. We show, both experimentally and numerically, that the minimum size of primary jet drops is set by the interplay between viscous and inertial-capillary forces and is significantly smaller than previous estimates. We find that viscous stresses modify both the shape of the collapsing bubble and the breakup of the resulting jet, leading to a non-monotonic size relationship between the bubble and primary jet drop. [Preview Abstract] |
Monday, November 20, 2017 5:36PM - 5:49PM |
L32.00008: Aerosolization of crude oil and dispersant slicks due to bubble bursting Kaushik Sampath, Nima Afshar-Mohajer, Won-Seok Heo, Joshua Gilbert, David Austin, Kirsten Koehler, Joseph Katz Bubble bursting in oceanic whitecaps is a well-known mechanism of marine aerosol generation. When crude oil spills occur, these aerosols may be oil-laden, leading to public health concerns. The introduction of dispersants aimed at accelerating the breakup of slicks, which greatly reduce the oil-water interfacial tension, potentially alters the aerosol number and size distributions. To characterize these effects, controlled bubble plumes are injected into a vertical seawater column ($\phi $0.6m, 1.8m high) with an oil-contaminated surface. The aerosol concentrations are measured in the micron (0.5-20\textmu m) and nano (10-370nm) size ranges. Tests are performed at the same air injection rate for varying bubble diameters (614, 263 and 89$\mu $m), slick thicknesses (50 and 500$\mu $m), and oil-water interfacial tensions. The latter are achieved by using crude oil, crude oil premixed with the dispersant Corexit 9500A at a ratio of 1:25, and pure dispersant. The results confirm that bubble bursting causes aerosolization of oil in the micron range, which increases with the introduction of dispersant. An order of magnitude increase in the concentration of nano-aerosols occurs for the largest bubbles, but not the smaller ones, and only for slicks containing pure dispersant or 500 $\mu $m thick crude oil-dispersant mixtures. [Preview Abstract] |
Monday, November 20, 2017 5:49PM - 6:02PM |
L32.00009: A numerical study of the wave shoaling effect on wind-wave momentum flux. Xuanting Hao, Lian Shen Momentum transfer between wind and waves is crucial to many physical processes in air-sea interactions. For decades, there has been a number of observational evidence that the surface roughness in the nearshore region is notably higher than in the open sea. In order to explain the mechanism behind this important phenomenon, in particular the wave shoaling effect on surface roughness, we conduct a series of numerical experiments using the wind-wave module of WOW (Wave-Ocean-Wind), a high-fidelity computational framework developed in house. We use prescribed monochromatic waves with linear shoaling effect incorporated, while the wind field is simulated using wall-resolved large-eddy simulation. A comparison between a shallow water wave case and deep water wave cases shows remarkably stronger wave effects on the wind for the former. Detailed analyses show that the increased surface roughness is closely associated with the increased form drag that is mainly due to the reduced wave age in wave shoaling. [Preview Abstract] |
Monday, November 20, 2017 6:02PM - 6:15PM |
L32.00010: Surfing surface gravity waves Nick Pizzo A simple criterion for water particles to surf an underlying surface gravity wave is presented. It is found that particles travelling near the phase speed of the wave, in a geometrically confined region on the forward face of the crest, increase in speed. The criterion is derived using the equation of John (Commun. Pure Appl. Maths, vol. 6, 1953, pp. 497--503) for the motion of a zero-stress free surface under the action of gravity. As an example, a breaking water wave is theoretically and numerically examined. Implications for upper-ocean processes, for both shallow- and deep-water waves, are discussed. [Preview Abstract] |
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