Buoyant Plume Dynamics Research Articles

Evaluating crown scorch predictions from a computational fluid dynamics wildland fire simulator

BackgroundCrown scorch—the heating of live leaves, needles, and buds in the vegetative canopy to lethal temperatures without widespread combustion—is one of the most common fire effects shaping post-fire canopies. Despite the ability of computational fluid dynamic models to finely resolve fire activity and buoyant plume dynamics including heterogenous 3D distributions of forest canopy heating, these models have had only limited use in simulating fire effects and have not been used to evaluate crown scorch. Here, we demonstrate a method of evaluating crown scorch using a computational fluid dynamics model, FIRETEC, and validate this approach by simulating the experiments that were used to develop Van Wagner’s 1973 crown scorch model.ResultsThe average scorch height prediction from FIRETEC compares well with the empirical model derived by Van Wagner, which is the most widely used empirical model for crown scorch. We further find that the 3D buoyant plume dynamics from a steady and homogeneous idealized heat source on the ground results in a spatially heterogenous crown scorch pattern reflecting complex heating dynamics that are best represented by percent scorch rather than height of scorch.ConclusionsThe ability of the computational fluid dynamics model to capture variation in crown scorch due to 3D buoyant plume dynamics provides direct links between forest structure, fire behavior, and fire effects that can be used by forest managers and researchers to better understand how fires result in crown damage under various environmental and management scenarios.

Export of Ice Sheet Meltwater from Upernavik Fjord, West Greenland

Abstract Meltwater from Greenland is an important freshwater source for the North Atlantic Ocean, released into the ocean at the head of fjords in the form of runoff, submarine melt, and icebergs. The meltwater release gives rise to complex in-fjord transformations that result in its dilution through mixing with other water masses. The transformed waters, which contain the meltwater, are exported from the fjords as a new water mass Glacially Modified Water (GMW). Here we use summer hydrographic data collected from 2013 to 2019 in Upernavik, a major glacial fjord in northwest Greenland, to describe the water masses that flow into the fjord from the shelf and the exported GMWs. Using an optimum multi-parameter technique across multiple years we then show that GMW is composed of 57.8% ± 8.1% Atlantic Water (AW), 41.0% ± 8.3% Polar Water (PW), 1.0% ± 0.1% subglacial discharge, and 0.2% ± 0.2% submarine meltwater. We show that the GMW fractional composition cannot be described by buoyant plume theory alone since it includes lateral mixing within the upper layers of the fjord not accounted for by buoyant plume dynamics. Consistent with its composition, we find that changes in GMW properties reflect changes in the AW and PW source waters. Using the obtained dilution ratios, this study suggests that the exchange across the fjord mouth during summer is on the order of 50 mSv (1 Sv ≡ 106 m3 s−1) (compared to a freshwater input of 0.5 mSv). This study provides a first-order parameterization for the exchange at the mouth of glacial fjords for large-scale ocean models.

Effect of rotation on buoyant plume dynamics

Effect of rotation on buoyant plume dynamics

Hydrophysical Structure and Current Dynamics of the Kodor River Plume

This work is focused on the river plume generated by the Kodor River which is the largest river of Abkhazia and the northeastern coast of the Black Sea. Based on in situ hydrological data, quadcopter aerial images and satellite observations, hydrophysical structure and circulation in the Kodor plume was studied at different river discharge conditions. Discharge of the Kodor River, which inflows to sea from several deltaic branches forms a relatively shallow plume with large area, as compared to the Mzymta River, the largest estuarine river of the study region, at similar hydrological, meteorological, and oceanographic conditions. Due to its small vertical size and large salinity gradient at the border with the subjacent saline sea, the Kodor plume is characterized by quick response on wind forcing variability. The Kodor plume also has large spatial inhomogeneity, caused by its formation conditions (several closely located freshwater sources prone to abrupt discharge variability), as well as regional bathymetry (interaction between the plume and shoals). During low discharge period, the inflowing Kodor River has relatively low kinetic energy, which causes abrupt deceleration of the jet in the sea area adjacent to the river mouth. As a result, the inertia of the inflowing jet abruptly decreases and large velocity and salinity gradients are formed in vicinity of river estuary, which hinder formation of anticyclonic circulation at the near-field part of the plume. This feature induces transport of freshwater discharge off the river mouth and its accumulation in the far-field part of the small plume under moderate wind forcing conditions that is not typical for large river plumes. The obtained results are important for understanding general aspects of dynamics of buoyant plumes formed by rivers with small discharge rates.

Collapse of particle-laden buoyant plumes

Particle loading affects the dynamics of buoyant plumes, since the difference between particle and fluid densities is much greater than that in the fluid alone. In stratified environments, plume rise is density limited; after initial overshoot, the plume reaches a terminal level and spreads radially. Particles dropping from this horizontal intrusion may be re-entrained. This recycling of dense matter reduces plume buoyancy and intrusion height and, for sufficient load, can lead to plume collapse. Entrainment-based formulae yield a steady-state plume rise. We identify a new conserved quantity for such plumes. Integrating paths of particles dropping from the intrusion yields the fraction re-entrained. A simple mathematical model predicts from buoyancy ratio at source ($P=$ negative particle buoyancy divided by positive fluid buoyancy) whether a particle-laden plume will collapse. Under this model, for small settling velocity, a particle-laden plume will not collapse if $P<0.368$. Above this, collapse depends also on the amount of particle-free ambient fluid entrained in the overshoot region. For pure plumes, experimental evidence suggests that this is small. For forced plumes, more substantial overshoot and entrainment is shown to increase the critical ratio. An extension, based on successive recycling, estimates time to collapse. To investigate further we develop a simple computational model, coupling a ‘top-hat’ plume model, an analytical formula for radially decaying concentrations in the intrusion and an axisymmetric finite-volume solution for time-dependent settling and entrainment. The model can predict the impact of particle load on final rise, as well as the occurrence and time scales of plume collapse.

Radium isotopes as tracers of hydrothermal inputs and neutrally buoyant plume dynamics in the deep ocean

Abstract Radium (Ra) isotopes are enriched in hydrothermal fluids, have a wide range of half-lives and are minimally impacted by scavenging removal processes; therefore, they have the ability to provide key information on the fate of trace elements in both near- and far- field hydrothermal plumes. To expand our understanding of Ra isotopes and their usefulness as tracers of hydrothermal activity, we measured Ra isotopes in both high and low temperature vent fluids, and in neutrally buoyant plumes. The time scales of plume transport were derived from Ra isotopes in neutrally buoyant plumes emanating from 15°S on the East Pacific Rise (EPR) and the TAG vent field on the Mid Atlantic Ridge (MAR) relative to their source ratio in high-temperature vent fluids. In near-field ( 223 Ra: t 1/2 = 11.4 d, 224 Ra: t 1/2 = 3.66 d) suggest plume ages of ~ 1 month and ~ 5 d over the EPR and MAR, respectively. Farther afield (> 100 km from vent), the distribution of the longer-lived 228 Ra (t 1/2 = 5.75 y) is indicative of hydrothermal plumes with different points of origin along the EPR. Radium-228 derived plume ages were used to calculate the residence time of hydrothermal dissolved Fe, which was in the range of 9–20 y (with an upper limit of 50 y depending on model assumptions). These are shorter than typical deep ocean Fe residence times, a result of scavenging removal of Fe from the neutrally buoyant plume during transport. This study highlights the utility of Ra isotopes in providing time-scales of hydrothermally derived trace element cycling in the deep ocean.

Large-eddy simulation and parameterization of buoyant plume dynamics in stratified flow

Characteristics of laboratory-scale bubble-driven buoyant plumes in a stably stratified quiescent fluid are studied using large-eddy simulation (LES). As a bubble plume entrains stratified ambient water, its net buoyancy decreases due to the increasing density difference between the entrained and ambient fluids. A large fraction of the entrained fluid eventually detrains and falls along an annular outer plume from a height of maximum rise (peel height) to a neutral buoyancy level (trap height), during which less buoyant scalars (e.g. small droplets) are trapped and dispersed horizontally, forming quasi-horizontal intrusion layers. The inner/outer double-plume structure and the peel/intrusion process are found to be more distinct for cases with small bubble rise velocity, while weak and unstable when the slip velocity is large. LES results are averaged to generate distributions of mean velocity and turbulent fluxes. These distributions provide data for assessing the performance of previously developed closures used in one-dimensional integral plume models. In particular, the various LES cases considered in this study yield consistent behaviour for the entrainment coefficients for various plume cases. Furthermore, a new continuous peeling model is derived based on the insights obtained from LES results. Comparing to previous peeling models, the new model behaves in a more self-consistent manner, and it is expected to provide more reliable performance when applied in integral plume models.

Aerosol source plume physical characteristics from space‐based multiangle imaging

Models that assess aerosol effects on regional air quality and global climate parameterize aerosol sources in terms of amount, type, and injection height. The multiangle imaging spectroradiometer (MISR) aboard NASA’s Terra satellite retrieves total column aerosol optical thickness (AOT), and aerosol type over cloud‐free land and water. A stereo‐matching algorithm automatically retrieves reflecting‐layer altitude wherever clouds or aerosol plumes have discernable spatial contrast, with about 500‐m accuracy, at 1.1‐km horizontal resolution. Near‐source biomass burning smoke, volcanic effluent, and desert dust plumes are observed routinely, providing information about aerosol amount, particle type, and injection height useful for modeling applications. Compared to background aerosols, the plumes sampled have higher AOT, contain particles having expected differences in Angstrom exponent, size, single‐scattering albedo, and for volcanic plume and dust cloud cases, particle shape. As basic thermodynamics predicts, thin aerosol plumes lifted only by regional winds or less intense heat sources are confined to the boundary layer. However, when sources have sufficient buoyancy, the representative plumes studied tend to concentrate within discrete, high‐elevation layers of local stability; the aerosol is not uniformly distributed up to a peak altitude, as is sometimes assumed in modeling. MISR‐derived plume heights, along with meteorological profile data from other sources, make it possible to relate radiant energy flux observed by the moderate resolution imaging spectroradiometer (MODIS), also aboard the Terra spacecraft, to convective heat flux that plays a major role in buoyant plume dynamics. A MISR climatology of plume behavior based on these results is being developed.

Predicting the ignition of crown fuels above a spreading surface fire. Part I: model idealization

A model was developed to predict the ignition of forest crown fuels above a surface fire based on heat transfer theory. The crown fuel ignition model (hereafter referred to as CFIM) is based on first principles, integrating: (i) the characteristics of the energy source as defined by surface fire flame front properties; (ii) buoyant plume dynamics; (iii) heat sink as described by the crown fuel particle characteristics; and (iv) energy transfer (gain and losses) to the crown fuels. Fuel particle temperature increase is determined through an energy balance relating heat absorption to fuel particle temperature. The final model output is the temperature of the crown fuel particles, which upon reaching ignition temperature are assumed to ignite. CFIM predicts the ignition of crown fuels but does not determine the onset of crown fire spread per se. The coupling of the CFIM with models determining the rate of propagation of crown fires allows for the prediction of the potential for sustained crowning. CFIM has the potential to be implemented in fire management decision support systems.

Coherent vortical structures in numerical simulations of buoyant plumes from wildland fires

The structure and dynamics of buoyant plumes arising from surface-based heat sources in a vertically sheared ambient atmospheric flow are examined via simulations of a three-dimensional, compressible numerical model. Simple circular heat sources and asymmetric elliptical ring heat sources that are representative of wildland fires of moderate intensity are considered. Several different coherent vortical structures that dominate the plume structure and evolution are evident in the simulations, and these structures correspond well with those observed in plumes from wildland fires. For the circular source, these structures include: (i) a counter-rotating vortex pair aligned with the plume trajectory that is associated with a bifurcation of the plume, (ii) transverse shear-layer vortices on the upstream face of the plume, and (iii) vertically oriented wake vortices that form periodically with alternating sign on either side of the downstream edge of the plume base. For the elliptical ring source, a streamwise counter-rotating vortex pair is apparent on each flank, and a transverse horizontal vortex is observed above the head of the source. In all simulations the plume cross section is represented poorly by a self-similar Gaussian distribution.

Comment on “Tracking fronts in solutions of the shallow‐water equations” by A. F. Bennett and P. F. Cummins

Journal of Geophysical Research: OceansVolume 93, Issue C12 p. 15749-15750 Regular SectionFree Access Comment on “Tracking fronts in solutions of the shallow-water equations” by A. F. Bennett and P. F. Cummins James O'Donnell, James O'DonnellSearch for more papers by this authorRichard W. Garvine, Richard W. GarvineSearch for more papers by this author James O'Donnell, James O'DonnellSearch for more papers by this authorRichard W. Garvine, Richard W. GarvineSearch for more papers by this author First published: 15 December 1988 https://doi.org/10.1029/JC093iC12p15749AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. References Bennett, A. F., P. F. Cummins, Tracking fronts in solutions to the shallow-water equations, J. Geophys. Res., 93, 1293– 1301, 1988. Chern, I. L., J. Glimm, O. McBryan, B. Plohr, S. Yaniv, Front tracking for gas dynamics, J. Comput. Phys., 62, 83– 110, 1986. Garvine, R. W., Frontal jump conditions for models of shallow, buoyant surface layer hydrodynamics, Tellus, 33, 301– 312, 1981. Garvine, R. W., A steady state model of buoyant surface plume hydrodynamics in coastal waters, Tellus, 34, 293– 306, 1982. Garvine, R. W., Radial spreading of buoyant, surface plumes in coastal waters, J. Geophys. Res., 89, 1989– 1996, 1984. Garvine, R. W., Estuary plumes and fronts in shelf waters: A layer model, J. Phys. Oceanogr., 17, 1877– 1896, 1987. Houghton, D. D., The effects of rotation on the formation of hydraulic jumps, J. Geophys. Res., 74, 1351– 1360, 1969. Lax, P. D., B. Wendroff, Systems of conservation laws, Commun. Pure Appl. Math., 13, 217– 237, 1960. O'Donnell, J., Buoyant plumes: A numerical model, M.S. thesis,Coll. of Mar. Stud., Univ. of Del.,Newark,1981. O'Donnell, J., A numerical model of the dynamics of buoyant discharges, Ph.D. dissertation,Coll. of Mar. Stud., Univ. of Del.,Newark,1986. O'Donnell, J., A numerical technique to incorporate frontal boundaries in two-dimensional layer models of ocean dynamics, J. Phys. Oceanogr., 1988. O'Donnell, J., R. W. Garvine, A time dependent, two-layer frontal model of buoyant plume dynamics, Tellus, Ser. A, 35, 73– 80, 1983. Parrett, C. A., M. J. P. Cullen, Simulation of hydraulic jumps in the presence of rotation and mountains, Q. J. R. Meteorol. Soc., 110, 147– 165, 1984. Pratt, L. J., On inertial flow over topography, I, Semigeostrophic adjustment to an obstacle, J. Fluid Mech., 131, 195– 218, 1983. Richtmyer, R. D., A survey of difference methods for non-steady fluid dynamicsTech. Note 63-2Natl. Center for Atmos. Res., Boulder, Colo., 1963. Roe, P. L., Characteristic based schemes for the Euler equations, Annu. Rev. Fluid Mech., 18, 337– 365, 1986. Simons, T. J., Generation and propagation of downwelling fronts, J. Phys. Oceanogr., 8, 571– 581, 1978. Volume93, IssueC1215 December 1988Pages 15749-15750 ReferencesRelatedInformation

A time dependent, two-layer frontal model of buoyant plume dynamics

The dynamics of buoyant plumes, such as those observed at the mouths of the Mississippi and Connecticut Rivers is investigated by considering the one dimensional, time dependent behavior of a two-layer, frontal model. The equations governing the flow in the main body of the plume are the long wave equations. The flow at the frontal boundary is modeled using jump conditions which incorporate the important dissipative processes. These equations are solved numerically using a shock patching technique that is accurate to second order in the long wave region and to first order at the front. The solutions demonstrate the controlling influence of the inlet on the flow field and the importance of time-dependent processes for the dynamics of buoyant plumes. DOI: 10.1111/j.1600-0870.1983.tb00186.x

A time dependent, two-layer frontal model of buoyant plume dynamics

A time dependent, two-layer frontal model of buoyant plume dynamics