Buoyant Layer Research Articles

Downstream stratification of the buoyant contaminants produced by a highly lazy plume in a longitudinally ventilated tunnel

In an underground tunnel with longitudinal ventilation, the stratification of the buoyant contaminants is crucial for the safety of the workers and passengers. A series of experiments are conducted in a water tank to investigate the density stratification in the downstream of the buoyancy source. Highly lazy plumes are produced by releasing coloured brine solution into a longitudinally ventilated tunnel which is placed in a large reservoir filled with fresh water. The transient distribution of the reduced gravity is measured using a light attenuation technique. The results suggest that the vertical distribution of the reduced gravity in the buoyant layer is approximately linear. The gradient Richardson number demonstrates that the downstream stratification can be very unstable under the effect of forced longitudinal ventilation. The difference between the reduced gravity at the ceiling and that at the floor Δg′, the average reduced gravity g′‾ and the thickness of the buoyant layer h are chosen to quantify the stratification. With the increase in the longitudinal ventilation velocity, both Δg′ and g′‾ decrease but h tends to increase. As the source buoyancy flux increases, all the three parameters become larger. Based on the experimental data and dimensional analyses, quantitative models are proposed to estimate the above three parameters. It is hoped that the models may provide useful references for the downstream distribution of the buoyant contaminants produced by highly lazy plumes in a longitudinally ventilated tunnel.

A New Thermal Categorization of Ice‐Covered Lakes

AbstractLakes are traditionally classified based on their thermal regime and trophic status. While this classification adequately captures many lakes, it is not sufficient to understand seasonally ice‐covered lakes, the most common lake type on Earth. We describe the inverse thermal stratification in 19 highly varying lakes and derive a model that predicts the temperature profile as a function of wind stress, area, and depth. The results suggest an additional subdivision of seasonally ice‐covered lakes to differentiate underice stratification. When ice forms in smaller and deeper lakes, inverse stratification will form with a thin buoyant layer of cold water (near 0°C) below the ice, which remains above a deeper 4°C layer. In contrast, the entire water column can cool to ∼0°C in larger and shallower lakes. We suggest these alternative conditions for dimictic lakes be termed “cryostratified” and “cryomictic.”

Predicting Lift-Off Time When Deep-Frying Potato Dough Snacks

When frying potato snacks, it is typically observed that the dough, which is submerged in hot oil, after some critical time increases its buoyancy and floats to the surface. The lift-off time is a useful metric in ensuring that the snacks are properly cooked. Here we propose a multiphase mathematical model for the frying of potato snacks, where water inside the dough is evaporated from both the top and bottom surfaces of the snack at two receding evaporation fronts. The vapor created at the top of the snack bubbles away to the surface, whereas the vapor released from the bottom surface forms a buoyant blanket layer. By asymptotic analysis, we show that the model simplifies to solving a one-dimensional Stefan problem in the snack coupled to a thin-film equation in the vapor blanket through a nonlinear boundary condition. Using our mathematical model, we predict the change in the snack density as a function of time and investigate how lift-off time depends on the different parameters of the problem.

Seasonal and interannual variability of landfast sea ice in Atka Bay, Weddell Sea, Antarctica

Abstract. Landfast sea ice (fast ice) attached to Antarctic (near-)coastal elements is a critical component of the local physical and ecological systems. Through its direct coupling with the atmosphere and ocean, fast-ice properties are also a potential indicator of processes related to a changing climate. However, in situ fast-ice observations in Antarctica are extremely sparse because of logistical challenges and harsh environmental conditions. Since 2010, a monitoring program observing the seasonal evolution of fast ice in Atka Bay has been conducted as part of the Antarctic Fast Ice Network (AFIN). The bay is located on the northeastern edge of Ekström Ice Shelf in the eastern Weddell Sea, close to the German wintering station Neumayer III. A number of sampling sites have been regularly revisited each year between annual ice formation and breakup to obtain a continuous record of sea-ice and sub-ice platelet-layer thickness, as well as snow depth and freeboard across the bay. Here, we present the time series of these measurements over the last 9 years. Combining them with observations from the nearby Neumayer III meteorological observatory as well as auxiliary satellite images enables us to relate the seasonal and interannual fast-ice cycle to the factors that influence their evolution. On average, the annual consolidated fast-ice thickness at the end of the growth season is about 2 m, with a loose platelet layer of 4 m thickness beneath and 0.70 m thick snow on top. Results highlight the predominately seasonal character of the fast-ice regime in Atka Bay without a significant interannual trend in any of the observed variables over the 9-year observation period. Also, no changes are evident when comparing with sporadic measurements in the 1980s and 1990s. It is shown that strong easterly winds in the area govern the year-round snow distribution and also trigger the breakup of fast ice in the bay during summer months. Due to the substantial snow accumulation on the fast ice, a characteristic feature is frequent negative freeboard, associated flooding of the snow–ice interface, and a likely subsequent snow ice formation. The buoyant platelet layer beneath negates the snow weight to some extent, but snow thermodynamics is identified as the main driver of the energy and mass budgets for the fast-ice cover in Atka Bay. The new knowledge of the seasonal and interannual variability of fast-ice properties from the present study helps to improve our understanding of interactions between atmosphere, fast ice, ocean, and ice shelves in one of the key regions of Antarctica and calls for intensified multidisciplinary studies in this region.

DESIGN AND DEVELOPMENT OF FLOATING PULSATILE DRUG DELIVERY OF LOSARTAN POTASSIUM

Objective: The objective of the present investigation was to the development of floating pulsatile drug delivery system of Losartan potassium (LP) tablets for obtaining no drug release during floating followed by pulsed, rapid drug release to achieve chronotherapeutic release. In hypertension, the risk of getting heart attacks early in the morning is high and therefore, there was need to develop drug delivery, which will release drugs at morning hours and provide efficacious therapy. LP is a short biological half-life (1.5-2.5h) and readily absorbed from the stomach and upper gastrointestinal tract.
 Methods: Tablet formulation was prepared by press coating of rapid release core tablets and core tablets were further top coated with a buoyant layer of HPMC K4M and sodium bicarbonate. Various grades of HPMC polymer (E5/E15/E50) were used for the pulsatile coating layer. The developed formulations were characterized for physical characteristics, floating lag time, floating time, release lag time, drug content, swelling index, in vitro dissolution studies, DSC and XRD.
 Results: The FTIR and DSC studies predicted that there was no chemical interaction between drug and excipients. The core tablet coated with HPMC E50 showed a high swelling index and release the drug 97.60±1.2% at 6h. Buoyant layer with 80 mg HPMC K4M and 25 mg sodium bicarbonate gave satisfactory floating lag time.
 Conclusion: The system showed an excellent lag phase followed by burst release in the distal small intestine, which gives site and time-specific delivery of LP acting as per chronotherapy for treatment of hypertension.

Development of laid-in knitted fabric for buoyant swimwear

As the use of buoyant swimwear by children in residential swimming becomes more common, the investigation of the effects of buoyant textiles is crucial for the optimisation of functional buoyant clothing design. This study investigated buoyant knitted fabrics developed using an inlaid knitting technique for buoyant swimwear application. The impact of the inner diameter, outer diameter, and the linear density of the inlaid tube on the buoyant ability of the buoyant fabrics was analyzed using multiple linear regression. The result demonstrated that the fabric’s net buoyant force was significantly affected by all three parameters, with inner diameter having the greatest effect, followed by the outer diameter and linear density of the inlaid tube. The increase in the net buoyant force of the fabric can be predicted by the increase in the inner diameter of its inlaid tube and linear density and decrease in the outer diameter. The divergence of the results for specimens inlaid with Silicon tube and from the result predicted by multiple linear regression indicates that the net buoyant force is also affected by the linear density and the wall thickness of the inlaid tube. The new knowledge of this study can contribute to the production of buoyant layers inside the buoyant swimwear in replace of air chambers to improve the limitation of non-distributed buoyancy, accidental punctuation and bulkiness in conventional buoyant swimwear.

Flow and heat transfer performance of supercritical pressure carbon dioxide in pipes with discrete double inclined ribs

Heated flow of supercritical pressure carbon dioxide in pipes with discrete double inclined ribs (DDIR) were numerically studied in this work. The turbulent flow and heat transfer was solved by a modified Shear Stress Transport k-ω model, where a variable turbulent Prandtl number formulation was incorporated. The model accuracy was verified by experimental data covering wide working conditions. Numerical runs were performed at 7.58 MPa, mass flux of 200 ~ 800 kg /(m2•s), and heat flux of 56.7 kW/m2. Results show that the strong buoyancy effect existing in smooth pipes was heavily inhibited by DDIR and no serious deterioration occurred. Sensitivity analysis on geometrical parameters suggest that the rib spacing the most sensitive one to the occurrence of heat transfer deterioration. The influence of inclination angle was much weaker than the rib height and spacing. The overall thermal performance of the DDIR pipe was the best when Bo* was around 4.5 × 10−6, with a PEC of 3~4. In the regions with very strong or negligible buoyancy, however, PEC was a much lower value between 1~2. The major role of discrete ribs is to interrupt the continuous development of near-wall buoyant layer and subsequently eliminate the negative effect of buoyancy on heat transfer. Ribs cannot effectively increase the positive effect of buoyancy on heat transfer. Finally, Bo* was found to be appropriate for the performance evaluation and optimization of enhancement techniques for supercritical heat transfer.

Modeling Arctic Boundary Layer Cloud Streets at Grey-zone Resolutions

To better understand how model resolution affects the formation of Arctic boundary layer clouds, we investigated the influence of grid spacing on simulating cloud streets that occurred near Utqiagvik (formerly Barrow), Alaska, on 2 May 2013 and were observed by MODIS (the Moderate Resolution Imaging Spectroradiometer). The Weather Research and Forecasting model was used to simulate the clouds using nested domains with increasingly fine resolution ranging from a horizontal grid spacing of 27 km in the boundary-layer-parameterized mesoscale domain to a grid spacing of 0.111 km in the large-eddy-permitting domain. We investigated the model-simulated mesoscale environment, horizontal and vertical cloud structures, boundary layer stability, and cloud properties, all of which were subsequently used to interpret the observed roll-cloud case. Increasing model resolution led to a transition from a more buoyant boundary layer to a more shear-driven turbulent boundary layer. The clouds were stratiform-like in the mesoscale domain, but as the model resolution increased, roll-like structures, aligned along the wind field, appeared with ever smaller wavelengths. A stronger vertical water vapor gradient occurred above the cloud layers with decreasing grid spacing. With fixed model grid spacing at 0.333 km, changing the model configuration from a boundary layer parameterization to a large-eddy-permitting scheme produced a more shear-driven and less unstable environment, a stronger vertical water vapor gradient below the cloud layers, and the wavelengths of the rolls decreased slightly. In this study, only the large-eddy-permitting simulation with gird spacing of 0.111 km was sufficient to model the observed roll clouds.

Numerical modeling of hydrodynamic processes of deep-sea hydrothermal plumes: A case study on Daxi hydrothermal field, Carlsberg Ridge

A RANS-based computational fluid dynamic model is developed to simulate the propagation and turbulent mixing of hydrothermal plumes in a stratified deep-sea environment and it is employed to study the Daxi hydrothermal field (DHF), Carlsberg Ridge in the Indian Ocean. Based on the results of in-situ measurements of the hydrothermal plume in the DHF, the parameter sensitivity is analyzed to evaluate the effects of the volume flux released from the vent orifice and the ambient buoyancy frequency on the plume dynamics. The results reveal the three-layer velocity field and obtain several important scalings, including the maximum plume rise height and neutrally buoyant plume height. Thus, it can be concluded that the classical entrainment assumption is applicable only to the plume stem below 0.6 times of the maximum plume rise height, and above that level, the structure of the flow field tends to be complicated due to the lateral spreading in the neutrally buoyant layer and circulation in the plume cap region. In addition, the simulation results show that both turbulent kinetic energy and turbulence dissipation rate attain their maximum values near the vent. However, the maximum turbulent viscosity is observed in the plume cap region, indicating strong turbulent mixing in that region. A quantitative analysis of the hydrothermal plumes in the DHF is performed by the proposed model and verified by the in-situ measurements, and the results suggest that the discrete heat flux of the DHF is about 39.38 MW. The proposed model can provide useful information for tracing the submarine hydrothermal vents, studying the matter and energy cycling in the ocean basin, and the exploration of submarine polymetallic sulfides resources and environmental assessment.

A physical conjecture for the dipolar–multipolar dynamo transition

In numerical simulations of planetary dynamos there is an abrupt transition in the dynamics of both the velocity and magnetic fields at a ‘local’ Rossby number of 0.1. For smaller Rossby numbers there are helical columnar structures aligned with the rotation axis, which efficiently maintain a dipolar field. However, when the thermal forcing is increased, these columns break down and the field becomes multi-polar. Similarly, in rotating turbulence experiments and simulations there is a sharp transition at a Rossby number of ${\sim}0.4$. Again, helical axial columnar structures are found for lower Rossby numbers, and there is strong evidence that these columns are created by inertial waves, at least on short time scales. We perform direct numerical simulations of the flow induced by a layer of buoyant anomalies subject to strong rotation, inspired by the equatorially biased heat flux in convective planetary dynamos. We assess the role of inertial waves in generating columnar structures. At high rotation rates (or weak forcing) we find columnar flow structures that segregate helicity either side of the buoyant layer, whose axial length scale increases linearly, as predicted by the theory of low-frequency inertial waves. As the rotation rate is weakened and the magnitude of the buoyant perturbations is increased, we identify a portion of the flow which is more strongly three-dimensional. We show that the flow in this region is turbulent, and has a Rossby number above a critical value $Ro^{crit}\sim 0.4$, consistent with previous findings in rotating turbulence. We suggest that the discrepancy between the transition value found here (and in rotating turbulence experiments), and that seen in the numerical dynamos ($Ro^{crit}\sim 0.1$), is a result of a different choice of the length scale used to define the local $Ro$. We show that when a proxy for the flow length scale perpendicular to the rotation axis is used in this definition, the numerical dynamo transition lies at $Ro^{crit}\sim 0.5$. Based on this we hypothesise that inertial waves, continually launched by buoyant anomalies, sustain the columnar structures in dynamo simulations, and that the transition documented in these simulations is due to the inability of inertial waves to propagate for $Ro>Ro^{crit}$.

Motion of buoyant, flexible aquatic vegetation under waves: Simple theoretical models and parameterization of wave dissipation

When submerged, flexible vegetation bends back and forth under waves, and stems are tilted only a small angle from vertical, simple models for stem motion and wave dissipation can be derived. Here, previous simple models for the wave-induced bending of elastic vegetation are extended to account for buoyancy. Buoyancy results in stem tension which, together with fluid drag, is incorporated in the Euler-Bernoulli problem, in which each stem is modeled as a cantilevered elastic beam. Solutions are governed by a new ‘dimensionless buoyancy’ β, in addition to the ‘dimensionless stiffness’ S identified by previous researchers. If β≪S1/2, buoyancy is negligible and previous results for elastic stems are recovered. Specifically, stems are nearly immobile for S≫1, but for S≪1 stems move with surrounding water except in a thin ‘elastic boundary layer’ extending a distance S1/4l* above the bed, where l*= stem length. Conversely, if β≫S1/2, then elasticity is negligible along most of the length of the stem and new behaviour is found. Specifically, stems are nearly immobile for β≫1, but for β≪1 stems move with surrounding water except in a thin ‘buoyant boundary layer’ extending a distance β1/2l* above the bed. For essentially inflexible cases (S≫1 or β≫1), simulated depth-integrated wave dissipation roughly equals the value Dr predicted for rigid stems. For highly flexible cases (i.e. for S and β both ≪1), dissipation is limited to elastic or buoyant boundary layers, and therefore scales with the maximum of S1/4Dr and β1/2Dr. For the simple stems considered here, which have constant diameter and density, simulated dissipation for all S and β was approximated by the expression [(S+β2/4)/(4+S+β2/4)]1/4Dr. This simple formula may require modification for vegetation with complex geometry. Nevertheless, this analysis identifies β as a key parameter for inclusion in dissipation formulations, together with parameters such as S identified by previous authors.

Improving diurnal rainfall phase over the Southern Great Plains in warm seasons by using a convective triggering design

AbstractThis study reports the improvements in the diurnal rainfall phase over the Southern Great Plains (SGP) with the Community Atmosphere Model by incorporating two convective initiation designs into the Zhang–Mcfarlane scheme. To capture the convection initiation of the nocturnal rainfall regime, the triggers emphasized the importance of suppression of convective inhibition and identification of elevated buoyancy for initiating convection. Consistent with the single‐column study of Wang et al. (2015), the model rainfall bias reduced with an increase in the heavy nocturnal rainfall and with the suppression of the spurious daytime convection in global domain experiments. We found that, with the climate model resolution of 1°, the nocturnal convection over the SGP can be well represented when the destabilization mechanisms of potentially buoyant layer at the low‐level troposphere is appropriately captured. Such destabilization is caused by the moisture convergence between the Rocky Mountains–SGP circulations and the uplifted low‐level jets from the Gulf of Mexico. Similar improvements are also found in other regions on the lee side of high terrains, such as the Tibetan Plateau.

Numerical simulation of two coalescing turbulent forced plumes in linearly stratified fluids

A computational fluid dynamic model that can solve the Reynolds-averaged Navier-Stokes equations and the species transport equation is developed to simulate two coalescing turbulent forced plumes, which are released with initial momentum and buoyancy flux into a linearly stable stratified environment. The velocity fields, turbulence structures, and entrainment of two plumes with different source separations and source buoyancy fluxes are analyzed quantitatively, in comparison with a series of physical experiments. An empirical parameterization is proposed to predict the amplification of the maximum rise height of two coalescing forced plumes caused by superposition and mutual entrainment. The maximum values of both turbulent kinetic energy and turbulence dissipation rate decrease monotonically with the increase in source separation of the two turbulent plumes. However, the trajectory of the maximum turbulent viscosity attained in the plume cap region presents two notable enhancements. This variation may be attributed to the turbulence transported from the touching region and the strong mixing around the neutrally buoyant layer between two plumes, while the mixing is caused by the lateral convection and the rebound after overshooting. The plume entrainment coefficient in near vent stems has a positive relationship with the source Richardson number. A transition of flow regimes to plume-like flows would occur when the contribution of initial momentum is important. The entrainment coefficient will decrease in the touching region of two plumes due to mutual entrainment, while the superposition of plumes can lead to distortion of the boundary of plume sectors.

Response of a Coastal Plume Formed by Tidally Modulated Estuarine Outflow to Light Upwelling-Favorable Wind

AbstractThis study presents observations of a buoyant plume off Winyah Bay, South Carolina, which was formed under conditions of high freshwater discharge and upwelling-favorable wind forcing. Analysis of observations demonstrates that the response of the anticyclonic bulge formed by tidally modulated estuarine outflow to the light upwelling-favorable wind is more complex than the previously studied far-field response. The latter can be described by a slab-like model with mixing concentrating at the offshore edge of a buoyant layer. The observed plume depth increased from ~3 m near the mouth to 6 m at the offshore edge, with plume depth changing in a steplike fashion rather than continuously. CTD profiles near these steps revealed overturning indicative of vigorous mixing. Estimates of the gradient Richardson number confirmed the likelihood of mixing/entrainment not only at the offshore edge of the plume but also in the proximity of the observed steps. We hypothesize that these steps represent tidal fronts that undergo geostrophic adjustment and are advected offshore by the superimposed Ekman drift. Scaling analysis suggests that mixing and entrainment at the observed interior fronts can be enhanced by superposition of geostrophic and wind-induced shear.

Coastal outflow currents into a buoyant layer of arbitrary depth

The long-wave, reduced-gravity, shallow-water equations (the semi-geostrophic equations) are used to study the outflow of a river into the ocean. While previous models have studied dynamics driven by gradients in density, the focus here is on the effects of potential vorticity anomaly (PVa). The river water is taken to have the same density as a finite-depth upper layer of oceanic fluid, but the two fluids have different, uniform, potential vorticities. Under these assumptions, the governing equations reduce to two first-order, nonlinear partial differential equations which are integrated numerically for a prescribed efflux of river water and PVa. Results are found to depend strongly on the sign of the PVa, with all fluid turning downstream (in the direction of Kelvin-wave propagation) when the river water has positive PVa and anticyclonic flow upstream of the river mouth when the PVa is negative. In all cases, a nonlinear Kelvin wave propagates at finite speed ahead of the river water. Away from the river mouth, the uniformity of one set of Riemann invariants allows for similarity solutions that describe the shape of the outflow, as well as a theory that predicts properties of the Kelvin wave. A range of behaviours is observed, including flows that develop shocks and flows that continue to expand offshore. The qualitative behaviour of the outflow is strongly correlated with the value of a single dimensionless parameter that expresses the ratio of the speed of the flow driven by the Kelvin wave to that driven by image vorticity.

Effects of neutral buoyancy outer boundary condition in models of deep convection in giant planets

Cloud motions have revealed strong east–west jet streams on Jupiter and Saturn. Two different paradigms have emerged for the origin of these jets. In the shallow dynamics paradigm zonal flow is driven largely by solar radiation and weather dynamics in the troposphere. In the deep convection paradigm forcing is thought to originate primarily via heat flow from the deep interior. Most previous anelastic models that focus on the development of deep zonal flows implement a constant entropy difference from the inner to outer boundary with no internal heat sources. For models with strong density gradients these constant entropy boundary conditions yield strong convective forcing at the outer boundary. However, observations indicate that giant planet tropospheres are stably stratified or perhaps near neutrally buoyant. Based on these observations we present here a deep convection parameter study with a zero entropy gradient (neutral stability) outer boundary condition and a uniform internal heat sink. We find that near convective onset, a neutrally buoyant shallow layer modifies the flow strongly, pushing active convection to occur near the bottom boundary. Furthermore, deeply seated jets develop efficiently, even with relatively weak forcing. Thus, for near critical and weak forcing the zero entropy gradient top boundary condition yields flows that resemble Boussinesq results, even for strong density gradients. For strongly convecting flows, the jet structure is more similar to cases with constant entropy difference. However, with strong convective mixing, the zero entropy gradient outer boundary condition results in relatively extensive regions of dynamical stability (reversed entropy gradient). Under these conditions, surface flows can develop with larger scale non-zonal features and vorticity structures that have planetary characteristics.

Anatomy of a glacial meltwater discharge event in an Antarctic cove.

Glacial meltwater discharge from Antarctica is a key influence on the marine environment, impacting ocean circulation, sea level and productivity of the pelagic and benthic ecosystems. The responses elicited depend strongly on the characteristics of the meltwater releases, including timing, spatial structure and geochemical composition. Here we use isotopic tracers to reveal the time-varying pattern of meltwater during a discharge event from the Fourcade Glacier into Potter Cove, northern Antarctic Peninsula. The discharge is strongly dependent on local air temperature, and accumulates into an extremely thin, buoyant layer at the surface. This layer showed evidence of elevated turbidity, and responded rapidly to changes in atmospherically driven circulation to generate a strongly pulsed outflow from the cove to the broader ocean. These characteristics contrast with those further south along the Peninsula, where strong glacial frontal ablation is driven oceanographically by intrusions of warm deep waters from offshore. The Fourcade Glacier switched very recently to being land-terminating; if retreat rates elsewhere along the Peninsula remain high and glacier termini progress strongly landward, the structure and impact of the freshwater discharges are likely to increasingly resemble the patterns elucidated here.This article is part of the theme issue ‘The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change’.

Formation of cratonic lithosphere during the initiation of plate tectonics

Earth’s oldest near-surface material, the cratonic crust, is typically underlain by thick lithosphere (>200 km) of Archean age. This cratonic lithosphere likely thickened in a high-compressional-stress environment, potentially linked to the onset of crustal shortening in the Neoarchean. Mantle convection in the hotter Archean Earth would have imparted relatively low stresses on the lithosphere, whether or not plate tectonics was operating, so a high stress signal from the early Earth is paradoxical. We propose that a rapid transition from heat pipe–mode convection to the onset of plate tectonics generated the high stresses required to thicken the cratonic lithosphere. Numerical calculations are used to demonstrate that an existing buoyant and strong layer, representing depleted continental lithosphere, can thicken and stabilize during a lid-breaking event. The peak compressional stress experienced by the lithosphere is 3×–4× higher than for the stagnant-lid or mobile-lid regimes immediately before and after. It is plausible that the cratonic lithosphere has not been subjected to this high stress state since, explaining its long-term stability. The lid-breaking thickening event reproduces features observed in typical Neoarchean cratons, such as lithospheric seismological reflectors and the formation of thrust faults. Paleoarchean “pre-tectonic” structures can also survive the lid-breaking event, acting as strong rafts that are assembled during the compressive event. Together, the results indicate that the signature of a catastrophic switch from a stagnant-lid Earth to the initiation of plate tectonics has been captured and preserved in the characteristics of cratonic crust and lithosphere.

Role of dispersed particles on the dynamics of an umbrella cloud of a forced plume in a linearly stratified environment

The vertical release of a lighter buoyant fluid, commonly referred to as a forced plume, into dense environment is a common occurrence in ocean and atmosphere. Such releases may have heterogeneity in them in form of particles with varied size, shape, and volume fraction. Normally in field conditions, the particle size ranges from a few microns to millimetres, and the volume fraction ranges from $$\phi _{v}=0.1{-}10$$ %. In this study, the effects of low values of $$\phi _{v}$$ (corresponding to a two-way coupled system) on the dynamics and structure of a plume umbrella cloud formed in a linearly stratified ambient were examined. Spherical particles with mean diameter $$d_{p}=100\, \upmu \hbox {m}$$ , density, $$\rho _{p}=2500\, \hbox {kg}\,\hbox {m}^{-3}$$ , and $$\phi _{v}=0{-}0.7\%$$ were injected along with the lighter plume fluid. Due to the phenomena of “particle fall-out” and “particle re-entrainment”, it was observed that a plume trough characterized by radius, $$R_{c}$$ , and depth, $$L_p$$ , forms below the neutral buoyant layer of an umbrella cloud. The plume trough formation is linked to the draw-down of the fluid from the neutral buoyant layer by the sedimenting particles. This trough either sustains or collapses depending on the plume conditions at the source, namely, the diameter $$d_{0}$$ , fluid buoyancy, $$g^{\prime }_{0}$$ , vertical velocity, $$W_{0}$$ , and $$\phi _{v}$$ . In all the experiments, $$g^{\prime }_{0}$$ and $$d_{0}$$ were kept constant while $$\phi _{v}=0{-}0.7\%$$ and $$W_{0}=0.2{-}0.65 \,\hbox {m}\,\hbox {s}^{-1}$$ were varied. The experiments revealed that the sustaining and collapsing trough regimes could be qualitatively demarcated based on a source effective Richardson number, $$Ri^{*}_{0}$$ , that accounts for the combined effect of $$\phi _{v}$$ and $$W_{0}$$ . It was found that when $$Ri^{*}_{0}<0.018$$ , the plume trough collapses, otherwise it is sustained. However, $$Ri^{*}_{0}$$ failed to predict the variations in $$R_{c}$$ and $$L_{p}$$ and hence was unsuitable for quantifying the plume trough. Therefore, it was established that the characteristics of a plume trough (i.e. $$R_{c}$$ and $$L_{p}$$ ) depend independently on the particle volume fraction $$(\phi _{v})$$ and source Richardson number $$(Ri_{0})$$ , while the demarcation of the trough regimes could be done using $$Ri^{*}_{0}$$ . For a constant $$Ri_{0}= \frac{-g^{\prime }_{0}d_{0}}{W_{0}^{2}}$$ , with an increase in $$\phi _{v}$$ the trough radius, $$R_{c}$$ , was found to decrease. In contrast, $$L_p$$ increases for a sustaining trough and decreases for a collapsing trough. For a constant $$\phi _{v}$$ , a decrease in $$Ri_{0}$$ caused both $$R_{c}$$ and $$L_{p}$$ to increase irrespective of a sustaining or collapsing plume trough. Using equations of motion for sedimenting particles, an analytical expression for the trough radius, $$R_{c}$$ , was formulated that accurately explains the experimental results. A relation for trough radius, $$R_{c}$$ , that elucidates the dynamics of particle re-entrainment was also obtained. If a particle is within the value of $$R_{c}$$ it is re-entrained into the plume, otherwise settles at the bottom. These results provide useful information needed for modeling particle-laden forced plumes.

When plumes tickle continents

Continental stability may be linked to a shallow, buoyant mantle layer, and the deepest craton roots can be destabilized and removed by mantle plumes.