Buoyant Layer Research Articles

A newly recognised mafic sill-hosted Ni-sulfide deposit emplaced during the 2.4 Ga Widgiemooltha dike swarm event, Eastern Goldfields, Western Australia

AbstractThe Cathedrals Ni-Cu prospect, located at the western margin of the Eastern Goldfields of the Yilgarn Craton, is hosted within a mafic intrusion interpreted as a sill complex. U-Pb dating of apatite from the sill yielded a crystallisation age of 2336 ± 64 Ma, inferring an association of sill emplacement and Ni mineralisation related to emplacement of the c. 2400 Ma Widgiemooltha dike swarm. The sill is typically differentiated into a lower olivine orthocumulate layer overlain by a dolerite unit containing xenoliths of partially assimilated granitoids in its upper portion. The latter is interpreted to be the result of stoping and melting of the granitic hanging wall, thereby creating a gravitationally stable buoyant melt layer beneath the top contact. Ni-Cu-Fe sulfides are increasingly abundant towards the base of the sill, ranging from globular disseminated sulfides to net-textured and massive sulfides at the basal contact. The presence and orientation of sulfide globule-bubble pairs indicates a primary near-horizontal orientation. Massive sulfides commonly exhibit a loop texture with pyrrhotite grains surrounded by pentlandite and chalcopyrite. Despite the variety of sulfide textures, sulfur isotopes have a homogeneous mantle-like signature without significant mass independent fractionation. Mineral chemistries that indicate sulfide prospectivity in larger intrusions do not work as effectively in this small sill, therefore new indicators may need to be developed to explore for similar deposits. To date, there are no other known magmatic deposits of this age in Australia. Sills of this age may be more prospective than previously recognised.

Brine-water experimental study on the natural ventilation flow rate induced by a localized buoyancy source in an inclined narrow space

Natural ventilation is an energy-saving strategy for diluting indoor contaminants. A series of brine-water experiments are conducted to investigate the mechanisms influencing the ventilation flow rate induced by stratified buoyancy-driven flows in inclined narrow spaces, such as tunnels, corridors and pathways. The results indicate that the flow stratification is determined by source location, tunnel inclination and source buoyancy flux. While Bernoulli’s theorem still can be used to estimate the ventilation flow rate, flow stratification and the buoyant layer thickness should be taken into consideration in such an inclined configuration. The flow is divided into three flow regimes in terms of the mechanisms controlling the flow rate q. A dimensionless parameter η is proposed to categorize the flow regimes. When η < 0.5, the air inflow is mainly induced by plume entrainment and q is approximately equal to that in a horizontal tunnel. When 0.5 < η < 2, q is influenced by the stratification and increases with η. When η > 2, the flow is approximately well-mixed and a constant dimensionless ventilation flow rate is obtained. Models are proposed to estimate q in different flow regimes. The study can be a reference for natural ventilation design in such configurations.

Experimental study of buoyancy-driven ventilation with a single opening between two adjacent building zones

In adjacent building zones connected by a single opening, a thermal plume produces a buoyant layer accumulating in the upper space of the experimental zone. The pressure difference between inside and outside the chamber will lead to air flowing in from outside and buoyant fluid flowing out from inside. This study aims to examine the effect of the vertical opening combination on flow regimes in adjacent building zones. The findings indicate that there may be three different flow regimes depending on the relative positions of the opening in the shared wall and the neutral level. The steady flow in two regimes with double stratification formed in the two zones is modeled separately and the theoretical prediction is compared with the experimental results. The flow regime of natural ventilation in the adjacent zones is only related to the height of the outlet, heat source height and effective opening area of the ventilation zones, but not to the buoyancy flux. The interface changes linearly with the height of the heat source at first until the nozzle is immersed in the buoyancy layer, and then the interface height remains constant. The relative size of the openings will also affect the volume flow rate through them, and keeping the areas of the openings along the flow passage equal will maximize the ventilation rate. These research results can provide layout guidelines for building doors and windows and will help designer improve the natural ventilation effect of adjacent building zones.

Formation of the Naxos nested domes and crustal differentiation by convection and diapirism

The Naxos dome, in the middle of the Aegean domain, exposes the former root of the Alpine orogenic belt and represents a key natural example to investigate the development of gravitational instabilities during orogenic evolution and their impact on crustal differentiation. The Naxos dome is cored by migmatites with structures depicting second order domes with a diameter of 1–2 km nested in the first order deca-kilometer scale dome that formed at the onset of orogenic collapse. Zircon grains from the migmatites record a succession of crystallization-dissolution cycles with a period of 1–2 Myr. These features have been attributed to the development of convective and diapiric gravitational instabilities, related to thermally induced and compositional buoyancy. In this paper, we test the pertinence of this model with a thermal-mechanical numerical experiment performed with a volume of fluid method (VOF) known to preserve material phase interfaces during large deformation of viscous layers. Partial melting of the crust is modeled by strain-rate and temperature dependent viscosity and temperature dependent density. Moreover, horizontal layers with density, viscosity and heat production variations mimic more felsic or more mafic lithologies in a crust of intermediate composition. With basal heating, gravitational instabilities initiate with local segregation of the buoyant versus heavier layers, followed by diapiric upwelling of buoyant pockets of aggregated less dense material. Convection starts after 5 Myr, approximately when half of the crust has a viscosity lower than 1019 Pa s. The size of the convection cells increases as the temperature rises in the crust and reaches ∼25 km in diameter after ca. 20 Myr, which defines the size of first order domes. Some of the heterogeneous material is entrained in the convection cells with a revolution period of 1 to 3 Myr. However, most of the denser material accumulates in the lower crust, while the buoyant material segregates at the top of the convection cells and forms diapirs that correspond to second order domes, of several kilometers in diameter and nested within the first order domes. This model, which reproduces the first order characteristic dimensions of the Naxos nested domes and the periodicity of their zircon geochronological record, demonstrates the efficiency of gravitational instabilities in the formation of migmatite domes and, more generally, in the multi-scale dynamics of crustal differentiation leading to a felsic upper crust, an intermediate middle crust and a mafic lower crust.

Dampening effect of global flows on Rayleigh–Taylor instabilities: implications for deep-mantle plumes vis-à-vis hotspot distributions

SUMMARY It is a well-accepted hypothesis that deep-mantle primary plumes originate from a buoyant source layer at the core–mantle boundary (CMB), where Rayleigh–Taylor instabilities (RTIs) play a key role in the plume initiation process. Previous studies have characterized their growth rates mainly in terms of the density, viscosity and layer-thickness ratios between the denser overburden and the source layer. The RTIs, however, develop in the presence of global flows in the overlying mantle, which can act as an additional factor in the plume mechanics. Combining 2-D computational fluid dynamic (CFD) model simulations and a linear stability analysis, this paper explores the influence of a horizontal global mantle flow in the instability dynamics. Both the CFD simulation results and analytical solutions reveal that the global flow is a dampening factor in reducing the instability growth rate. At a threshold value of the normalized global flow velocity, short- as well as long-wavelength instabilities are completely suppressed, allowing the entire system to advect in the horizontal direction. Using a series of real-scale numerical simulations, this paper also investigates the growth rate as a function of the density contrast, expressed in Atwood number ${A}_T = ({{{\rho }_1 - {\rho }_2}})/({{{\rho }_1 + {\rho }_2}})$, and the viscosity ratio $\ {\mu }^* = \ {\mu }_1/{\mu }_2$, where ${\rho }_1,\ {\mu }_{1\ }$ and ${\rho }_{2,}\ {\mu }_{2\ }$are densities and viscosities of the overburden mantle and source layer, respectively. It is found that increase in either ${A}_T$ or ${\mu }^*$ promotes the growth rate of a plume. In addition, the stability analysis predicts a nonlinearly increasing RTI wavelength with increasing global flow velocity, implying that the resulting plumes widen their spacing preferentially in the flow direction of kinematically active mantle regions. The theory accounts for additional physical parameters: source-layer viscosity and thickness in the analysis of the dominant wavelengths and their corresponding growth rates. The paper finally discusses the problem of unusually large inter-hotspot spacing, providing a new conceptual framework for the origin of sporadically distributed hotspots of deep-mantle sources.

Timing and Abundance of Clathrate Formation Control Ocean Evolution in Outer Solar System Bodies: Challenges of Maintaining a Thick Ocean within Pluto

Clathrate hydrates may represent a sizable fraction of material within the icy shells of Kuiper Belt objects and icy moons. They influence the chemical and thermal evolution of subsurface oceans by locking volatiles into the ice shell and by providing more thermal insulation than pure water ice. We model the formation of these crystalline compounds in conditions relevant to outer solar system objects, using Pluto as an example. Although Pluto may have hosted a thick ocean in its early history, Pluto’s overall heat budget is probably insufficient to preserve liquid today if its outer shell is pure water ice. One previously proposed reconciliation is that Pluto’s ocean has a winter jacket: an insulating layer of methane clathrate hydrates. Unfortunately, assessments of the timing, quantity, and type of clathrate hydrates forming within planetary bodies are lacking. Our work quantifies the abundance of clathrate-forming gases present in Pluto’s ocean from accreted ices and volatiles released during thermal metamorphism throughout Pluto’s history. We find that if Pluto formed with the same relative abundances of ices found in comets, then a buoyant layer of mixed methane and carbon dioxide clathrate hydrates may form above Pluto’s ocean, though we find it insufficient to preserve a thick ocean today. In general, our study provides methodology for predicting clathrate formation in ocean worlds, which is necessary to predict the evolution of the ocean’s composition and whether a liquid layer remains at present.

Modulation of the Marine Environment in the Natal Bight

Modulation of the marine environment in the Natal Bight (~29.1°S, 31.6°E) was studied using daily high-resolution climate reanalysis products and monthly satellite green- and red-band reflectance in the period 2002–2022. The KwaZulu-Natal shelf edge is characterized by a narrow band of upwelling next to the warm Agulhas Current. Strong, reversing longshore winds ~7 m/s and meandering poleward flow ~1 m/s pulse the system, but along the leeward coast that forms the Natal Bight, environmental conditions are buffered by a weak cyclonic gyre. Wind and current shear create a shadow zone that aggregates plankton, recycles nutrients, and sustains marine resources. The seasonal cycle is of high amplitude: the surface heat balance reaches +70 W/m2 in December, followed by river discharges ~3 M m3/yr of fresh nutrient-rich water that peak in February. This induces a buoyant surface layer that inhibits wind wave turbulence during summer. By contrast, winter (June–August) cooling −95 W/m2 and frequent cyclonic storminess deepen the mixed layer from 25 to 65 m, enabling wind wave turbulence to reach the seafloor (Tugela Bank). Red-band reflectance increases 3-fold from summer to winter and is significantly correlated with net heat balance −0.54, daily wave heights &gt; 2.5 m +0.51, mixed layer depth +0.47, sea surface temp −0.41, and wind vorticity −0.39. Daily longshore winds from the northeast and southwest were, unexpectedly, most amplified in spring (August–October). The seasonality exhibits sequential effects that supports year-round marine nutrification in the Natal Bight. Intra-seasonal fluctuations were related to meandering of the Agulhas Current and changes in longshore winds and shelf waves that impart significant pulsing of near-shore currents at 4–9-day periods. Although the cyclonic gyre in the Natal Bight spins up and down, SST variance was found to be relatively low in its center, where external influences are buffered. Considering linear trends for winds and runoff and surface temperature over the period 1950–2021, we found that northeasterlies increased, runoff decreased, and inshore sea surface temperatures have warmed slowly relative to the adjacent land surface temperature. New insights derive from the use of monthly satellite red-band reflectance and daily 10 km climate reanalysis fields to understand how air–land–sea fluxes modulate the marine environment in the Natal Bight.

Re-evaluation of the role of volatiles in the rupture of magma chambers and the triggering of crystal-rich eruptions

The role of volatile exsolution in triggering volcanic eruption is re-evaluated using numerical simulations of a growing and solidifying magma chamber. The modelled magma chamber is silicic, emplaced at about 2 Kbar, and H2O-saturated. The rate of volume additions to the chamber by the release of volatiles is controlled by cooling and solidification and is significantly lower than magma infilling rates. Thus, the increase in volume due to volatile exsolution is unlikely to be the primary cause of magma chamber failure unless the magma chamber is already on the brink of failure. However, the formation of buoyant H2O rich layers within the mush that surrounds the magma chamber makes the system gravitationally unstable. If the host rock is not permeable and volatiles accumulate, their buoyancy might overcome the strength of the wall resulting in dyke injection and possibly an eruption. Eruptions that are triggered or facilitated by volatiles exsolution are associated to the presence of mush in the chamber and are expected to be crystal-rich. Triggering of eruption by volatiles buoyancy explains the common occurrence of large ranges of crystals ages and protracted crystal residence times at low temperature.

Scaling analysis of buoyant boundary layer flow induced by flux heating with a spatial gradient at Pr ≪ 1, Pr ∼ 1 and Pr ≫ 1

Buoyancy induced convective boundary layer flow is investigated with a scale law analysis in this study. Thermal forcing at the heated wall adopts the second-type boundary condition, and a spatial gradient is specified for the heat flux profile. A full range of Prandtl number is considered and separately analyzed, i.e. Pr << 1, Pr ∼ 1 and Pr >> 1, since the convective flow fundamentally differentiates for the three Pr ranges. Important scale laws quantifying different states of the transient flow are determined by the scaling analysis and they are validated against numerical calculations. The comparison suggests that the proposed scale laws could satisfactorily quantify the boundary layer flow and the corresponding regression constants R2 are around 0.999. The results indicate that scale laws for the three Pr ranges differ only in the Pr term and the Rayleigh number, streamwise location and heating parameter dependencies are identical. It is also demonstrated that, in the start-up state, the boundary layer growth is always two-dimensional at non-zero spatial gradient conditions and the heated wall temperature increases temporally according to τ1/2, which are profoundly different from the extensively investigated isothermal problems. The present work also identifies a special flow condition. That is, when the heat flux gradient equals 2, the heated wall temperature and characteristic velocity of the flow are reduced to vary linearly with streamwise location and the thermal boundary layer thickness becomes independent of streamwise location.

Mantle thermochemical variations beneath the continental United States through petrologic interpretation of seismic tomography

The continental lithospheric mantle plays an essential role in stabilizing continents over long geological time scales. Quantifying spatial variations in thermal and compositional properties of the mantle lithosphere is crucial to understanding its formation and its impact on continental stability; however, our understanding of these variations remains limited. Here we apply the Whole-rock Interpretive Seismic Toolbox For Ultramafic Lithologies (WISTFUL) to estimate thermal, compositional, and density variations in the continental mantle beneath the contiguous United States from MITPS_20, a joint body and surface wave tomographic inversion for Vp and Vs with high resolution in the shallow mantle (60–100 km). Our analysis shows lateral variations in temperature beneath the continental United States of up to 800–900°C at 60, 80, and 100 km depth. East of the Rocky Mountains, the mantle lithosphere is generally cold (350–850°C at 60 km), with higher temperatures (up to 1000°C at 60 km) along the Atlantic coastal margin. By contrast, the mantle lithosphere west of the Rocky Mountains is hot (typically >1000°C at 60 km, >1200°C at 80–100 km), with the highest temperatures beneath Holocene volcanoes. In agreement with previous work, we find that the chemical depletion predicted by WISTFUL does not fully offset the density difference due to temperature. Extending our results using Rayleigh-Taylor instability analysis, implies the lithosphere below the United States could be undergoing oscillatory convection, in which cooling, densification, and sinking of a chemically buoyant layer alternates with reheating and rising of that layer.

On the dynamics of buoyant jets in a linearly stratified ambient

We report mean flow and turbulence characteristics of a buoyant jet evolving in a linearly stratified ambient with stratification strength N=0.4 s−1. The velocity and density fields are captured experimentally using simultaneous particle image velocimetry and planar laser-induced fluorescence technique. We report our findings by strategically choosing four axial locations such that it covers different flow regimes; namely, momentum-dominated region, buoyancy-dominated region, neutral buoyant layer, and plume cap region. The results at these axial locations are presented as a function of the radial co-ordinate to provide a whole field picture of the flow dynamics. From the mean axial velocity and density fields, it is seen that the velocity and the scalar (density) widths are of the same magnitude in the momentum-dominated region but show significant difference in the buoyancy-dominated region and beyond. It is also seen that the axial velocity for the buoyant jet is consistently higher than pure jet at different axial locations due to buoyancy-aided momentum. With the help of turbulent kinetic energy (TKE) budget analysis, it is seen that the shear production (P) and TKE dissipation (ϵ) for a buoyant jet are higher compared to the case of pure jet at different axial locations, cementing the role of buoyancy and stratification on the flow dynamics. Further, it is observed that the buoyancy flux (B) aids and destroys TKE intermittently in the radial direction, and it is at least O(102) smaller than P, ϵ, and the mean flow buoyancy flux (F). Finally, the relative strength of the turbulent transport of momentum to that of scalar in the radial direction is quantified using the turbulent Prandtl number, Prt. It is seen that Prt ≈1 upto the neutral buoyant layer and ≈ 0.6 in the plume cap region. The current set of results obtained from experiments are first of its kind and elucidates various aspects of the flow which hitherto remained unknown and will also prove to be useful in testing numerical simulations for buoyancy-driven flows.

Ocean Surface Salinity Response to Atmospheric River Precipitation in the California Current System

Abstract Atmospheric rivers (ARs) result in precipitation over land and ocean. Rainfall on the ocean can generate a buoyant layer of freshwater that impacts exchanges between the surface and the mixed layer. These “fresh lenses” are important for weather and climate because they may impact the ocean stratification at all time scales. Here we use in situ ocean data, collocated with AR events, and a one-dimensional configuration of a general circulation model, to investigate the impact of AR precipitation on surface ocean salinity in the California Current System (CCS) on seasonal and event-based time scales. We find that at coastal and onshore locations the CCS freshens through the rainy season due to AR events, and years with higher AR activity are associated with a stronger freshening signal. On shorter time scales, model simulations suggest that events characteristic of CCS ARs can produce salinity changes that are detectable by ocean instruments (≥0.01 psu). Here, the surface salinity change depends linearly on rain rate and inversely on wind speed. Higher wind speeds (U &gt; 8 m s−1) induce mixing, distributing freshwater inputs to depths greater than 20 m. Lower wind speeds (U ≤ 8 m s−1) allow freshwater lenses to remain at the surface. Results suggest that local precipitation is important in setting the freshwater seasonal cycle of the CCS and that the formation of freshwater lenses should be considered for identifying impacts of atmospheric variability on the upper ocean in the CCS on weather event time scales. Significance Statement Atmospheric rivers produce large amounts of rainfall. The purpose of this study is to understand how this rain impacts the surface ocean in the California Current System on seasonal and event time scales. Our results show that a greater precipitation over the rainy season leads to a larger decrease in salinity over time. On shorter time scales, these atmospheric river precipitation events commonly produce a surface salinity response that is detectable by ocean instruments. This salinity response depends on the amount of rainfall and the wind speed. In general, higher wind speeds will cause the freshwater input from rain to mix deeper, while lower wind speeds will have reduced mixing, allowing a layer of freshwater to persist at the surface.

Compositions of dissolved organic matter in the ice-covered waters above the Aurora hydrothermal vent system, Gakkel Ridge, Arctic Ocean

Abstract. Hydrothermal vents modify and displace subsurface dissolved organic matter (DOM) into the ocean. Once in the ocean, this DOM is transported together with elements, particles, dissolved gases and biomass along with the neutrally buoyant plume layer. Considering the number and extent of actively venting hydrothermal sites in the oceans, their contribution to the oceanic DOM pool may be substantial. Here, we investigate the dynamics of DOM in relation to hydrothermal venting and related processes at the as yet unexplored Aurora hydrothermal vent field within the ultraslow-spreading Gakkel Ridge in the Arctic Ocean at 82.9∘ N. We examined the vertical distribution of DOM composition from sea ice to deep waters at six hydrocast stations distal to the active vent and its neutrally buoyant plume layer. In comparison to background seawater, we found that the DOM in waters directly affected by the hydrothermal plume was molecularly less diverse and 5 %–10 % lower in number of molecular formulas associated with the molecular categories related to lipid and protein-like compounds. On the other hand, samples that were not directly affected by the plume were chemically more diverse and had a higher percentage of chemical formulas associated with the carbohydrate-like category. Our results suggest that hydrothermal processes at Aurora may influence the DOM distribution in the bathypelagic ocean by spreading more thermally and/or chemically induced compositions, while DOM compositions in epipelagic and mesopelagic layers are mainly governed by the microbial carbon pump dynamics and surface-ocean–sea-ice interactions.

Large scale patches of Calanus finmarchicus and associated hydrographic conditions off the Lofoten archipelago

Large swarms of individuals at different spatiotemporal scales characterise the distributions of many animal species. In the ocean several mesozooplankton taxa aggregate in large patches or swarms driven by active behavioural responses to hydrographic structures, although intrinsic biotic characteristics of species´ life cycles not related with the environment can also affect spatial distributions. To understand the mechanisms of aggregation and disentangle environmental and pure spatial effects contributing to patch formation, we extensively sampled a large Calanus finmarchicus patch off the Lofoten islands, northern Norway, in spring 2017 by means of Laser Optical Plankton Counter (LOPC) cross shelf transects and biophysical samplings at fixed stations. We observed a clear association between the buoyant layer of the fresh, cold Norwegian Coastal Current (NCC) and high surface copepod abundances. Off shelf lateral displacements of the NCC along its path may retain copepods from the offshore basins. At deeper layers, copepod abundances increased markedly at regions with low sea level anomalies and reduced vertical flows. Our results also suggest that copepods performed short range diel vertical migration within the patch. Potential future changes in the seasonality and structure of the NCC and their impact on swarm occurrence and formation are discussed.

Lithospheric Structure of Venusian Crustal Plateaus

AbstractCrustal plateaus are Venusian highlands characterized by tectonized terrains. It is commonly interpreted that their topography is isostatically supported and that they represent fossils of an extinct tectonic regime. Using gravity and topography we perform a comprehensive investigation of the lithospheric structure of six crustal plateaus. We computed the admittance (gravity to topography wavelength‐dependent ratio) for each region and compared them to modeled admittances. Three compensation scenarios were tested: Airy isostasy, a surface‐loading flexural model, and a flexural model with surface and subsurface loads. Our results show that the topography of most plateaus is supported by crustal thickening and that the addition of a mantle support component is not necessary at the investigated wavelengths. The elastic thickness was constrained to be less than 35 km with a best‐fitting average of 15 km, confirming that these regions are consistent with an isostatic regime. The average crustal thickness of the plateaus ranges from 15 to 34 km, and if they are in Airy isostasy, this implies that the global average crustal thickness of Venus is about 20 km. Phoebe Regio is the sole exception of our analysis in that crustal thicknesses that are compatible with the other plateaus are obtained only when a buoyant layer is included. Heat flow estimations computed from the elastic thickness indicate that the plateaus formed under higher heat flow conditions compared to the current global average and could have caused localized melting. Present‐day heat flow predictions suggest that eclogitization could occur where the crust is thickest.

Offshore Spreading of a Supercritical Plume Under Upwelling Wind Forcing: A Case Study of the Winyah Bay Outflow

In this study, we present observations of the Winyah Bay (WB) plume (SC, United States) formed by high freshwater discharge and a moderate upwelling-favorable wind acting continuously for ∼1.5 days prior to the shipboard survey. If a similar wind forcing persists over a longer period, the plume turns upstream (against its natural propagation) and curves offshore forming a “filament” with minimal transverse spreading, as seen in numerous satellite images. The observed plume comprises a train of tidal sub-plumes undergoing rotational adjustment and being transported offshore by Ekman dynamics. The WB outflow is supercritical in terms of the interior Froude number. Moderate wind extends this supercritical regime farther offshore. The plume is characterized by interior fronts associated with consecutive tidal pulses. Age of the buoyant water can be distinguished by the buoyant layer mixing (evident in the layer’s thickness and salinity anomaly) along with the transformation of its TS properties. However, relatively little transverse (lateral) spreading of buoyant water occurs: the equivalent freshwater layer thickness remains surprisingly consistent, approximately 0.8 m, over more than 20 km in the direction of the bulge extension. It is hypothesized that the supercritical regime constrains the transverse spreading of a plume. Microstructure measurements reveal higher dissipation rates below the base of the older (offshore) part of the plume. This is attributed to internal wave radiation from a newly discharged tidal pulse into an older plume, with the buoyant layer acting as a waveguide. Theoretical estimations of the internal wave properties show that the interior front is highly supercritical, while the observed dissipation maximum agrees with the theoretical wave structure.

Scaling laws for stagnant-lid convection with a buoyant crust

SUMMARY Stagnant-lid convection, where subduction and surface plate motion is absent, is common among the rocky planets and moons in our solar system, and likely among rocky exoplanets as well. How stagnant-lid planets thermally evolve is an important issue, dictating not just their interior evolution but also the evolution of their atmospheres via volcanic degassing. On stagnant-lid planets, the crust is not recycled by subduction and can potentially grow thick enough to significantly impact convection beneath the stagnant lid. We perform numerical models of stagnant-lid convection to determine new scaling laws for convective heat flux that specifically account for the presence of a buoyant crustal layer. We systematically vary the crustal layer thickness, crustal layer density, Rayleigh number and Frank–Kamenetskii parameter for viscosity to map out system behaviour and determine the new scaling laws. We find two end-member regimes of behaviour: a ‘thin crust limit’, where convection is largely unaffected by the presence of the crust, and the thickness of the lithosphere is approximately the same as it would be if the crust were absent; and a ‘thick crust limit’, where the crustal thickness itself determines the lithospheric thickness and heat flux. Scaling laws for both limits are developed and fit the numerical model results well. Applying these scaling laws to rocky stagnant-lid planets, we find that the crustal thickness needed for convection to enter the thick crust limit decreases with increasing mantle temperature and decreasing mantle reference viscosity. Moreover, if crustal thickness is limited by the formation of dense eclogite, and foundering of this dense lower crust, then smaller planets are more likely to enter the thick crust limit because their crusts can grow thicker before reaching the pressure where eclogite forms. When convection is in the thick crust limit, mantle heat flux is suppressed. As a result, mantle temperatures can be elevated by 100 s of degrees K for up to a few Gyr in comparison to a planet with a thin crust. Whether convection enters the thick crust limit during a planet’s thermal evolution also depends on the initial mantle temperature, so a thick, buoyant crust additionally acts to preserve the influence of initial conditions on stagnant-lid planets for far longer than previous thermal evolution models, which ignore the effects of a thick crust, have found.

Emptying-filling boxes with non-Boussinesq plumes and fountains

When a light fluid is continuously released from the top (as a fountain) or from the bottom (as a plume) into a box with a top opening, a buoyant layer of constant thickness and density forms under the ceiling at steady state. Both configurations have been investigated using theoretical approaches and small scale air-helium experiments. As a practical application, for a fixed buoyant flux, the fountain and the plume configurations have been compared with respect to their mixing efficiency.

Effect of sub‐atmospheric pressures on buoyancy‐dominated hydrocarbon fuel fires: An experimental analysis on thermal flickering frequency

SummaryThermal flickering of a fire is caused by the buoyancy‐induced Kelvin‐Helmholtz‐type instability in the buoyant shear layer between burned gases and ambient air. The study of flickering frequency and inherent buoyancy‐induced instability are of great importance for fire essential parameters and their applications like fire detection, burner, and furnace designs. The flickering frequency of hydrocarbon fuel fires involved methane, ethylene, and propane was studied at low pressures ranging from 0.03 to 0.1 MPa. Three different fuel flow rates were configured for each fuel type of diffusion flame. Generally, the flickering behaviors were observed falling into three regimes: tip flickering, intermittent flickering, and continuous flickering. The continuous flickering appeared when the fuel flow rate or pressure increased above a particular level. The observed thermal flickering frequency varied from about 8 Hz at 0.03 MPa to 12 Hz at 0.1 MPa and was found insensitive to fuel type and flow rate. Thermal flickering frequency increased with the increasing of pressure with a power of 0.27. Buoyancy force dominates the flame flicking behavior under a wide range of sub‐atmospheric pressures and fuel exit velocities. In addition, multiple frequencies phenomenon was observed in the tests.

Evolution of a wall-attached buoyant plume in confined boxes: Direct numerical simulations, entrainment coefficient and an integral model

The evolution of a wall-attached plume in a confined box is studied here with the aid of three dimensional direct numerical simulations (DNS). The plume originates from a local line heat source of length, L, placed at the bottom left corner of the box. The Reynolds number of the wall plume, based on box height and buoyant velocity scale, is ReH=14530 and boxes of two different aspect ratios (ratio of box width to height) for a particular value of L are simulated. We observe that the plume develops along the vertical sidewall while remaining attached to it before spreading across the top wall to form a buoyant fluid layer and eventually moving downwards and filling the whole box. The original filling box model of Baines and Turner (1969) is modified to incorporate the wall shear stress, and the results from the DNS are compared against the new model. In modelling plumes, we find that the entrainment coefficient (α) for wall-attached plumes is reduced to approximately half of that in the free plume, and the main reason is a diminished contribution of turbulence production to α resulting from a restricted ability of the large-scale eddies to transport momentum. Also, unlike the free plume where away from the source inertial forces balances buoyancy forces, here in our simulations of wall-attached plumes this balance is marginally off, likely due to wall friction. A reasonable agreement is observed between our model and DNS data for the volume and momentum fluxes in the quiescent uniform environment and also for the time-dependent buoyancy profile calculated far away from the plume.