Search

Gravity currents contribute to the transport of heat and mass in atmospheric and aquatic environments. In aquatic systems subject to daily surface cooling, gravity currents propagate through turbulent convective surroundings. Yet, the effects of thermal convection on aquatic gravity currents remain to be quantified. This paper demonstrates how the interaction between penetrative convection and downslope gravity currents impacts the fluid dynamics and transport across littoral aquatic systems. We performed field experiments in a wind‐sheltered lake experiencing differential cooling to resolve the dynamics of thermally driven gravity currents in convective environments. Our in situ observations reveal that convective plumes penetrate gravity currents, generating large vertical fluctuations that foster the erosion of the stratified layer. This enhanced vertical mixing destroys the stratified downslope flow and limits the basin‐scale transport. Our results demonstrate that the interaction between penetrative convection and downslope gravity currents controls the littoral‐pelagic connectivity in aquatic ecosystems. Plain Language Summary: Horizontal differences in fluid density generate flows called gravity currents. These currents transport heat and mass across environmental systems. The dynamics of gravity current and the resulting transport are well understood when the surroundings are quiescent, which is rarely the case in nature. In aquatic systems, turbulent processes such as convection energize the ambient water. Cooling‐driven convection occurs when the surface of aquatic systems loses heat to the atmosphere, which generates sinking thermal plumes. In this paper, we used a lake experiencing differential cooling as an ideal field‐scale laboratory to investigate the effects of convection on the dynamics of gravity currents. Our in situ observations reveal that convective plumes distort the upper interface of gravity currents and limit the flow intensity. These results demonstrate that convection constrains the horizontal transport induced by gravity currents in natural systems. Key Points: Convective plumes penetrate downslope gravity currents and generate vertical interface fluctuationsVertical convective mixing erodes the stratified downslope flow and limits lateral transportThe time of maximal lateral transport is delayed to the relaxation phase once penetrative convection weakens [ABSTRACT FROM AUTHOR]

Copyright of Yellow River is the property of Editorial Board of Yellow River and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Since the operation of Xiaolangdi Reservoir, great social and economic benefits have been brought into play in ensuring the flood control and silt reduction, the safety of living, production, irrigation and ecological water use. Based on the measured data, this paper analyzed the outflow conditions of high water level density flow and the influence factors of sediment discharge of Xiaolangdi Reservoir. The results show that during the high water level operation (above 235 m) of Xiaolangdi Reservoir, two conditions shall be met for the density flow to reach the front of the dam. a) The inflow flood process shall not be less than 3 days and the fine sediment content shall be higher than 40%. b) Inflow discharge Qi shall not be less than 4000 m³/s. And meet the sediment concentration shall not be less than 13.0 kg/m³. Alternatively, if inflow discharge Qi is between 1700 m³/s and 4000 m³/s, and meet the sediment concentration Si shall not be less than 86 minus 0.018Qi. Sediment discharge ratio of reservoir η is inversely proportional to the ratio of the water storage body and the outflow discharge and the ratio of the inflow and outflow discharge. The larger the maximum continuous 3-day inflow sediment transport rate, the stronger the flow power and the better the sediment discharge effect. The research results can provide a certain technical support for the high water level density flow sediment discharge operation of Xiaolangdi Reservoir. [ABSTRACT FROM AUTHOR]

The work presented in this paper is devoted to the study of the phenomena of electron emission that take place at the cathode in an electric arc discharge. This problem remains a subject of careful research conducted by several specialists. Nevertheless, it seems interesting to make a comparison between all the theories currently available. In an electric arc, electrons are emitted from the surface of the cathode. The cathode spot is the place from which the electrons emerge. The current density of the electrons emitted represents 90% of the total current density. The remaining 10% is provided by the positive ions present in front of the cathode. Thus, the determination of the total current density at the cathode in an arc discharge is reduced to the calculation of that of the emitted electrons. The choice of the method for calculating the current density of the electrons emitted at the cathode is crucial for the evaluation of the total current density at the cathode. In our model, the current density of emitted electrons is evaluated using the Murphy-Good formulation which is in the current state, the most general and the most accurate equation. [ABSTRACT FROM AUTHOR]

Copyright of Larhyss Journal is the property of Biskra University, Research Laboratory in Subterranean & Surface Hydraulics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The propagation of density current under different boundary conditions is investigated using high resolution direct numerical simulations (DNS). A revised Kleiser and Schumann influence-matrix method is used to treat the general Robin type velocity boundary conditions and the related 'tau' error corrections in the numerical simulations. Comparison of the simulation results reveals that the boundary conditions change the turbulent flow field and therefore the propagation of the front. This paper mainly focuses on the effects of boundary conditions and initial depth of the dense fluid. The differences in energy dissipation and overall front development in wall-bounded and open channels are examined. Through DNS simulations, it is evident that with the decrease of initial release depth ratio ( $$D/H$$), the effect of the top boundary becomes less important. In wall-bounded channels, there are three distinctive layers in the vertical distribution of energy dissipation corresponding to the contributions from bottom wall, interface, and top wall, respectively. In open channels, there are only two layers with the top one missing due to the shear free nature of the boundary. It is found that the energy dissipation distribution in the bottom layer is similar for cases with the same $$D/H$$ ratio regardless the top boundary condition. The simulation results also reveal that for low Reynolds number cases, the energy change due to concentration diffusion cannot be neglected in the energy budget. To reflect the real dynamics of density current, the dimensionless Froude number and Reynolds number should be defined using the release depth $$D$$ as the length scale. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Civil Engineering is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper describes the results of numerical experiments with a three-dimensional hydrodynamic and mud transport model in which sediment–fluid interaction is taken into account through the effects of hindered settling, buoyancy destruction in the turbulence k–ℇ model and sediment-induced barocline pressure gradients in the momentum equations. The model was applied to a schematic case representing a coastal area with a tidal river, navigation channel and harbour basin, and a real-world case, viz. Rotterdam harbour area in The Netherlands. The results show that the sediment transport into the harbour area, and subsequent siltation rates, increase by a factor 3 to 5 due to the sediment–fluid interaction. It is shown that the larger contribution stems from an increase in vertical gradients in suspended sediment. [ABSTRACT FROM AUTHOR]

On 30 June 2013, 19 Granite Mountain Hotshots firefighters were killed fighting a wildfire near Yarnell in the mountains of Central Arizona. They succumbed when the wildfire, driven by erratic winds, blocked their escape route and overran their location. A previous study is extended to simulate and analyze the downscale organization of convective circulations that redirected the wildfire, which started from the scale of the Rossby Wave Breaking over North America to a convective gust front that redirected the wildfire, trapping the firefighters. Five stages are found: Stage I, the initial deep prolonged gust front; Stage II, a front-to-rear jet and its ascending motions that organized high-based convection; Stage III, high-based dry microburst-induced downdrafts organized initially by ascending flow in Stage II that transported mass and entropy to the surface; Stage IV; multiple meso-γ-scale high centers and confluence zones formed that encompassed the firefighters' location, which established a favorable environment leading to Stage V, canyon-scale circulations formed surrounding the fire. The atmosphere thus transitioned from supporting a deep and long-lived convective density current to elevated dry microbursts with mass and wind outflow into a canyon, redirecting the ongoing wildfire. [ABSTRACT FROM AUTHOR]

Abstract Gravity currents contribute to the transport of heat and mass in atmospheric and aquatic environments. In aquatic systems subject to daily surface cooling, gravity currents propagate through turbulent convective surroundings. Yet, the effects of thermal convection on aquatic gravity currents remain to be quantified. This paper demonstrates how the interaction between penetrative convection and downslope gravity currents impacts the fluid dynamics and transport across littoral aquatic systems. We performed field experiments in a wind‐sheltered lake experiencing differential cooling to resolve the dynamics of thermally driven gravity currents in convective environments. Our in situ observations reveal that convective plumes penetrate gravity currents, generating large vertical fluctuations that foster the erosion of the stratified layer. This enhanced vertical mixing destroys the stratified downslope flow and limits the basin‐scale transport. Our results demonstrate that the interaction between penetrative convection and downslope gravity currents controls the littoral‐pelagic connectivity in aquatic ecosystems.

The paper presents the results of a long-term research program on the improvement of existing shallow circular secondary settling tanks (SSTs) based on the computational fluid dynamics (CFD) investigation of their inner hydrodynamic processes. The results provide insight into the flow and concentration pattern within such SSTs and highlight some important details that largely determine their performance. As a novel detail of the research of SSTs, a direct dependence was found between the inlet height and the length of the radial density jet induced by the entering water–sludge suspension. A hydrodynamically optimized (low positioned) inlet facility decreases the kinetic energy and the length of the density jet, as a result of which it vanishes before hitting outer boundary of the SST. This results in an enhanced performance indicators even of such shallow SSTs, under both dry weather and peak load conditions as well. [ABSTRACT FROM AUTHOR]

We introduce the notion of exotic periodic points of a meromorphic self-map. We then establish the expected asymptotic for the number of isolated or exotic periodic points for holomorphic self-maps with a simple action on the cohomology groups on a compact Kähler manifold. [ABSTRACT FROM AUTHOR]

Here we present observations of a dust storm that occurred on 22 February 2020 in the northwestern Sonoran Desert. In‐situ and remotely sensed measurements and output from numerical simulations suggest that evaporative cooling from cold frontal orographic precipitation spilling over an upwind mountain range generated a density current, with dust uplift occurring as the density current traveled over the emissive desert surface. Because the density current was laden with dust, time series of vertical profiles of aerosol backscatter and extinction from a ceilometer located 25 km downwind of the initial dust emission event show a well‐developed density current structure, including an overturning frontal head with a vertical extent of 1.2 km. Ceilometer measurements and soundings suggest a density current body depth of 400–500 m exhibiting a two‐layer structure that consisted of a positively sheared and dusty lower‐level, and a negatively sheared and pristine upper level. Kelvin‐Helmholtz instability at the top of the density current cold pool generated quasi‐regular oscillations in the height of the dust and pristine‐sky interfacial layer. Ridges and troughs in the height of this interfacial layer were coupled to maxima and minima in surface wind speed and near surface dust concentrations, respectively, with peak dust concentrations located directly under the interfacial layer ridges. These results corroborate several findings from model studies of dust emission and transport by density currents, and suggest that the internal circulation of a density current modifies the timing of dust emission and the patterns of dust concentration within the current body. Plain Language Summary: Here we discuss the case of a unique dust storm that occurred in the western end of the Sonoran Desert. Remote sensing and in‐situ measurements are used to demonstrate that the dust storm was generated by a density current. Lidar backscatter measurements during passage of this density current over a research site in the region are used to trace the path of dust laden air through the density current head and body, and reveal the internal circulation of this feature. These observational results corroborate findings from modeling studies of dust emission and transport by density currents, but also suggest a complex distribution of dust within the density current body. Key Points: Measurements of a dust storm in the western Sonoran Desert that was generated by a density current are presentedThe density current exhibits a 2‐layer internal circulation with a lower dusty layer and an upper pristine layerThese observations corroborate several findings from numerical simulations of dust emission and transport by density currents [ABSTRACT FROM AUTHOR]

The Xiaolangdi Hydro-Project is one of the large projects on the main stem of the Middle Yellow River. It has been operated for more than 10 years, since its impoundment in October, 1999. The reservoir has trapped 2.833×109 m3 of sediment, and caused the total erosion of 1.891×109 t in the Lower Yellow River from October, 1999 through October, 2010. Not only the serious atrophied situation of the Lower Yellow River (LYR) has been resuscitating, but also many new phenomena of sediment transport and behaviors of channel re-establishing are coming into being. They are illustrated and discussed in detail in this paper. [ABSTRACT FROM AUTHOR]

Abstract: The paper presents an experimental study of suspension flow patterns and velocity field inside two types of circular settling tanks with continuous operation. The two tanks differ in inlet- and outlet configurations. Research was focused on the influence of flow field on the sedimentation efficiency of both settling tanks. Experiment was carried out on a settling tank sections made of plexi-glass that represented radial slices of prototype circular settling tanks. Kinematic flow properties inside settling tank sections were determined by computer-aided visualization in order to analyse the whole instantaneous flow field at once. Sedimentation efficiency was assessed by measurements of inlet- and effluent suspension concentrations. Pronounced density current evolved in both tanks, which under certain conditions enhanced sedimentation. Flow field showed qualitative changes during operation and proved to be significantly dependent on average suspension density in the inner chamber of the settling tank. Some important properties of the flow field could also be reconstructed from the spatial distribution of the remaining sludge. Centrally fed settling tank with peripheral effluent generally performed better in terms of sedimentation efficiency mainly due to insufficient height of the effluent baffle in the peripherally fed settling tank. [Copyright &y& Elsevier]

The sustainable use of reservoirs for irrigation, flood protection, water supply and hydropower may be endangered due to unavoidable reservoir sedimentation. Turbidity currents are the main process for the transport and deposit of sediments in reservoirs, especially in the deepest part near the dam where vital structures such as power intakes and bottom outlets are located. Besides other measures such as solid or permeable obstacles, turbidity currents can be influenced by means of an inclined water jet screen. Physical experiments of a turbidity current flowing through a water jet screen were carried out. Velocity profiles, front velocities, and deposit evolutions were determined. The results indicate that in certain configurations, turbidity currents can be partially stopped by the jet screen. Furthermore, the deposits downstream of the screen may be reduced up to a factor of two as compared with deposits of a free-flowing turbidity current. [ABSTRACT FROM AUTHOR]

Cold pool outflows (CPOs) are thought to be the most significant meteorological mechanism of mineral dust emission from the world's largest source in the central and western Sahara in boreal summer. An absence of CPOs from numerical models and reanalyses used to simulate Saharan dust emission leads to considerable error in modeling of dust fluxes from the Sahara. As such, the role of CPOs in the observed variability of dust through the monsoon season remains unclear. To remedy these issues, an improved observational benchmark is needed. In this research, an automated approach to identify and track CPOs in dust imagery from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) is derived. The approach is found to flag 74.2% of events identified manually (26/35). 1,559 events are tracked for June, July and August of 2004–2017. CPOs follow a clear diurnal cycle, peaking at 1700–1900 Universal Time Coordinated. Propagation speeds decay exponentially through their lifetime, but on average speeds are 1.5 ms−1 higher at night. About 22.5% of the observed events exceed a total traveled distance of 300 km, with an overwhelming preference for northwestwards propagation. Common across the southern central and western Sahara, CPO activity shifts north through summer in line with observed dust emission. The exception to this is the development of an intense hotspot of CPO activity in southern Algeria in August, which does not parallel any known late season outbreaks of dust. The results underline the importance of the southernmost Saharan dust sources, activated by frequent CPO occurrence in early summer. Key Points: An objective satellite detection and tracking scheme is developed and used to characterize Saharan cold pool outflows across 14 summersCold pool outflows are widespread across the central and western Sahara, peaking in August close to the Mali‐Niger‐Algeria triple pointPatterns of cold pool outflow activity can help explain an observed decline in late summer dust emission [ABSTRACT FROM AUTHOR]

Copyright of Larhyss Journal is the property of Biskra University, Research Laboratory in Subterranean & Surface Hydraulics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Deep-Sea Tailings Placement (DSTP) systems have been used at several mines to discharge diluted mine tailings at great depths. DSTP often provides considerable environmental and engineering benefits compared to traditional on-land tailings storage. The key element in the viability of a DSTP system is the ability to discharge mine tailings at sufficient depths and at such a location that a density current is formed that carries the tailings along the seabed to the abyssal ocean, and such that adequate dilution can be achieved and environmental impacts minimized. Depending on the composition of the slurry, sub-sea plumes might also form from the density current at different levels of neutral buoyancy, further diluting the effluent. The engineering, environmental and construction aspects of the deep-sea outfall system are illustrated through a case study of the DSTP outfall system on Woodlark Island in Papua New Guinea, located in the tropical south Pacific. [ABSTRACT FROM AUTHOR]

A strong mesoscale convective system affected northeastern Italy on August 8, 2008. Notable damage and two casualties resulted, mainly due to the strong wind gusts. The event is analysed using observations, including surface data from a meso‐network of meteorological stations, radar reflectivity and velocity data from a C‐band Doppler radar, polar satellite images, lightning measurements from a lightning detection network and the ambient thermodynamic conditions derived from local radiosoundings. The role of the cold front is investigated; in particular, the associated strong wind descending from the Alps, which interacted with the preexisting convection in the plain, is analysed to understand the way it affected the storm development. A simple density current model is applied to describe the flow characteristics and to identify the mechanisms that could support the development of such a high wind speed. Lastly, observed wind speeds are compared with the theoretical estimates from the proposed model. The results show that different factors contribute to the very strong wind gusts registered by surface stations. The main contributor to the windstorm is a density current (a) driven by the cold front, (b) maintained by the interaction between the cold air impinging on the Friuli Venezia Giulia plain from the north and the strong regional density gradient and (c) eventually enhanced by the storm cold pool. This study highlights the complex evolution of severe storms in a region on the lee side of the Alpine chain, and emphasizes the role of the orography in the enhancement of storm. [ABSTRACT FROM AUTHOR]

A density current is a (two-phase) flow of a fluid of density which is caused essentially by the influence of a density difference on the gravity. The previous experimental studies have been focused on investigation into properties of gravity current in straight channels. This paper presents a series of experiments in which saline gravity currents flowed through in physical models laboratory of Shahid Chamran University of Ahvaz. Experiments were carried out in 8.5 m length, 20cm width and 70cm height of flume and three radiuses of curvature (40, 80,120 cm). In this study the effect of three radiuses of curvature of channel bends on density flow behavior were studied. Experiments performed by four discharges (0.5,0.7,0.9 and 1.1 L/s) and four concentrations (8, 12, 16, 20 and 40 g/L). An Acoustic Doppler velocimeter (ADV-Vectrino+) was used to record the instantaneous downstream velocity. The Vectrino Velocimeter measures water speed using the Doppler effect. The results show that the elevation of the interface at the outer bank is in every case greater than the one at the inner bank. The results show that due to increasing the relative radius of channel bend decreases elevation of the interface and transverse interfacial slopes. The maximum and minimum superelevation occur in r/b =2 and r/b=6, respectively.

Density currents occur when fluid of onedensity propagates into a fluid of different density. These currents are the main caude forsediment transport in dam reservoirs and estuaries. Thenegative effects of density currents can be minimized using obstacles. Theobstacles can totally or partially block density currents and thussedimentation will occur far from the dams.In this paper, the effect of cylindrical obstacles and their arrangement ondensity current body is experimentally studied. Obstacles with constantdiameter and height were glued on the bed in staggered form. The staggered formwere constant during all the experiments, but transverse longitudinal distancesbetween obstacles were variable. It caused two beds with different arrangementof obstacles. The results indicated that rate of mixing in bed with obstaclesis greater than bed without obstacles. So velocity and height of headin bed with obstacles decrease about 40 present

Abstract: The gravity current dynamics in the presence of regular surface waves is investigated experimentally. The aim is to determine the influence of the wave‐induced velocity field on gravity current kinematic parameters and on the concentration distribution. In the presence of surface waves, the front shows a pulsating behavior while periodic oscillations are observed at the interface between lighter and denser fluids. The dynamics of such processes is related to the wave parameters. Present results confirm that the lock‐exchange generated front velocity is strongly influenced by the oscillatory motion and it may be increased or decreased depending on the Lagrangian mass transport induced by the surface waves. Compared to the propagation in the absence of waves, the dynamics of the concentration field is governed by an enhancement of mixing process due to the orbital motion. [ABSTRACT FROM AUTHOR]

The dynamics of density current over a bottom covered by macro-roughness elements were investigated by laboratory experiments and a computational model using large eddy simulations. The macro-roughness considered had significant size in comparison with the scale of density current. Five different roughness conditions were considered, namely flat bottom (for reference), half spheres, fine gravels, medium gravels, and large gravels. These bottom conditions had variations in roughness element size, shape, angularity, and spatial configuration. The density current was a lock-exchange type with a density difference of 1% between the two fluids initially separated by a gate in the middle. In the computational model, the roughness was captured using two different methods depending on the size of the roughness elements. For the large roughness elements, i.e., the half spheres and the medium and large gravels, an immersed boundary method was used to resolve the surface of each gravel, which was obtained through 3D laser scanning. The realistic and physically correct placement of these scanned objects in the simulation domain was achieved using a computer tool which can detect the collision of rigid bodies and simulate their dynamics. For the fine gravels, a rough wall function was used. The computational model was validated with the data measured in the experiments, including front position and velocity, and point velocity measurement within the current. The results show that density currents over macro-roughness have distinct behavior from those over a smooth boundary. The characteristics (size, angularity, and pavement pattern) of the macro-roughness play a key role in the current development. Macro-roughness significantly retards the front propagation and enhances entrainment. [ABSTRACT FROM AUTHOR]

Gravity or density current is produced due to a density contrast among two different fluids or among two layers of a single fluid. In this research work, the saline gravity current near bed with slope reduction was simulated numerically, using FLUENT software. The experimental results were used in order to calibrate the numerical model. The obtained results showed that density current was simulated accurately, using turbulent model (RNG type) and optimized grid. This result was found especially for the vertical profiles of local velocity and volumetric concentration (for both sub and supercritical sections), the density height, the maximum value of mean velocity and the corresponding height of this velocity. The numerical results showed that both standard and RNG turbulent model was accurate for calculation of saline gravity current height. In this paper, using turbulent model (RNG type), better similarity collapses of velocity profiles were obtained in comparison with concentration profiles. Moreover, due to ambient water height limitation, a circulation flow was found in numerical results. Finally, the numerical model indicated that for given initial conditions, as initial bed slope increases, the location of the density jump is moved farther downstream, and also the amount of ambient water entrained along the supercritical regions increases with the increase of the initial bed slope.

Since the first impoundment of Three Gorges Dam in 2003, algal blooms occur frequently in the near-dam tributaries. It is widely recognized that the impoundment-induced change in hydrodynamic condition with the lower current velocity will make the eutrophication problem even more severe when an excessive amount of nutrients is already loaded into a reservoir and/or its tributaries. Operation tests carried out by Three Gorges Corporation in 2010 point to some feasible reservoir operation schemes that may have positive impacts on reducing the algal bloom level. In our study, an attempt is made to obtain, through a numerical hydrodynamic and water quality modeling and analysis, the reservoir operation rules that would reduce the level of algal blooms in the Xiangxi River (XXR), a near-dam tributary. Water movements and algal blooms in XXR are simulated and analyzed under different scenarios of one-day water discharge fluctuation or two-week water level variation. The model results demonstrate that the reservoir operations can further increase the water exchange between the mainstream of the Three Gorges Reservoir (TGR) and the XXR tributary and thus move a larger amount of algae into the deep water where it will die. Analysis of the model results indicate that the water discharge fluctuation constituted of a lower valley-load flow and a larger flow difference for the short-term operation (within a day), the rise in water level for the medium-term operation (e.g., over weeks), and the combination of the above two for the long-term operation (e.g., over months) can be the feasible reservoir operation rules in the non-flood season for TGR. [ABSTRACT FROM AUTHOR]

We present the results of laboratory experiments on the behaviour of sustained, dense granular flows in a horizontal flume, in which high-gas pore pressure was maintained throughout the flow duration by continuous injection of gas through the flume base. The flows were fed by a sustained (0.5-30 s) supply of fine (75 ± 15 μm) particles from a hopper; the falling particles impacted an impingement surface at concentrations of ~3 to 45 %, where they densified rapidly to generate horizontally moving, dense granular flows. When the gas supplied through the flume base was below the minimum fluidization velocity of the particles (i.e. aerated flow conditions), three flow phases were identified: (i) an initial dilute spray of particles travelling at 1-2 m s−1, followed by (ii) a dense granular flow travelling at 0.5-1 m s−1, then by (iii) sustained aggradation of the deposit by a prolonged succession of thin flow pulses. The maximum runout of the phase 2 flow was linearly dependent on the initial mass flux, and the frontal velocity had a square-root dependence on mass flux. The frontal propagation speed during phase 3 had a linear relationship with mass flux. The total mass of particles released had no significant control on either flow velocity or runout in any of the phases. High-frequency flow unsteadiness during phase 3 generated deposit architectures with progradational and retrogradational packages and multiple internal erosive contacts. When the gas supplied through the flume base was equal to the minimum fluidization velocity of the particles (i.e. fluidized flow conditions), the flows remained within phase 2 for their entire runout, no deposit formed and the particles ran off the end of the flume. Sustained granular flows differ significantly from instantaneous flows generated by lock-exchange mechanisms, in that the sustained flows generate (by prolonged progressive aggradation) deposits that are much thicker than the flowing layer of particles at any given moment. The experiments offer a first attempt to investigate the physics of the sustained pyroclastic flows that generate thick, voluminous ignimbrites. [ABSTRACT FROM AUTHOR]

Direct numerical simulation (DNS) is used to simulate lock-exchange flow to understand density currents over rough surfaces. This work is one of the first DNS to simulate density currents over rough walls. The simulations are performed at a Grashof number of 1.6×107. The non-dimensional height of the roughness elements with respect to the half-height of the channel is 0.12. Roughness reduces the speed of the front. Furthermore, the instabilities are significantly enhanced resulting in secondary instabilities that arise much earlier in time. Roughness introduces an additional vorticity generation mechanism which is comparable to the vorticity generated from Kelvin–Helmholtz and lobe–cleft type of instabilities. Flow exhibits significant differences near the leading edge or the nose of the front of the density currents due to roughness. [ABSTRACT FROM AUTHOR]

The Yarnell Hill Fire, triggered by dry lightning on 28 June 2013, was initiated by hot and dry westerly winds, which rapidly shifted to north-northeast by convective-induced outflows. This sudden wind shift led to the demise of 19 firefighters. This study focuses on the environment and its predictive potential in terms of erratically changing the fire spread. Three numerical sensitivity tests are performed investigating the evolving synoptic-meso-β scale environmental wind flow: (1) deactivating the evaporative cooling, (2) deactivating surface-driven diurnal heating/cooling, and (3) removing the mountain. Results show the strong north-northeasterly wind induced by the density current(s) and the diurnal surface sensible heating played the most significant roles in enhancing the mesoscale environment conducive to the rapid change in the fire spread direction. While the mountain played a less significant role in weakening the magnitude of the airflow affecting the fire, it still had an impact. Additionally, the Hot-Dry-Windy (HDW) index is calculated to determine its predictor role with respect to the atmosphere affecting the fire. The focus is not on feedback from explicit fire heating on the larger environment but rather the role of the environmental physical processes in causing the convectively induced rapid wind shifts. [ABSTRACT FROM AUTHOR]

We present a range of numerical tests comparing the dynamical cores of the operationally used numerical weather prediction (NWP) model COSMO and the university code Dune, focusing on their efficiency and accuracy for solving benchmark test cases for NWP. The dynamical core of COSMO is based on a finite difference method whereas the Dune core is based on a Discontinuous Galerkin method. Both dynamical cores are briefly introduced stating possible advantages and pitfalls of the different approaches. Their efficiency and effectiveness is investigated, based on three numerical test cases, which require solving the compressible viscous and non-viscous Euler equations. The test cases include the density current (Straka et al. in Int J Numer Methods Fluids 17:1-22, ), the inertia gravity (Skamarock and Klemp in Mon Weather Rev 122:2623-2630, ), and the linear hydrostatic mountain waves of (Bonaventura in J Comput Phys 158:186-213, ). [ABSTRACT FROM AUTHOR]

The depositional processes and gas pore pressure in pyroclastic flows are investigated through scaled experiments on transient, initially fluidized granular flows. The flow structure consists of a sliding head whose basal velocity decreases backwards from the front velocity ( U) until onset of deposition occurs, which marks transition to the flow body where the basal deposit grows continuously. The flows propagate in a fluid-inertial regime despite formation of the deposit. Their head generates underpressure proportional to U whereas their body generates overpressure whose values suggest that pore pressure diffuses during emplacement. Complementary experiments on defluidizing static columns prove that the concept of pore pressure diffusion is relevant for gas-particle mixtures and allow characterization of the diffusion timescale ( t) as a function of the material properties. Initial material expansion increases the diffusion time compared with the nonexpanded state, suggesting that pore pressure is self-generated during compaction. Application to pyroclastic flows gives minimum diffusion timescales of seconds to tens of minutes, depending principally on the flow height and permeability. This study also helps to reconcile the concepts of en masse and progressive deposition of pyroclastic flow units or discrete pulses. Onset of deposition, whose causes deserve further investigation, is the most critical parameter for determining the structure of the deposits. Even if sedimentation is fundamentally continuous, it is proposed that late onset of deposition and rapid aggradation in relatively thin flows can generate deposits that are almost snapshots of the flow structure. In this context, deposition can be considered as occurring en masse, though not strictly instantaneously. [ABSTRACT FROM AUTHOR]

Adiabatic theories of ocean circulation and density structure have a long tradition, from the concept of the ventilated thermocline to the notion that deep ocean ventilation is controlled by westerly winds over the Southern Ocean. This study explores these ideas using a recently developed Lagrangian ocean model (LOM), which simulates ocean motions by computing trajectories of water parcels. A unique feature of the LOM is its capacity to model ocean circulations in the adiabatic limit, in which water parcels exactly conserve their densities when they are not in contact with the ocean surface. The authors take advantage of this property of the LOM and consider the circulation and stratification that develop in an ocean with a fully adiabatic interior (with both isopycnal and diapycnal diffusivities set to zero). The ocean basin in the study mimics that of the Atlantic Ocean and includes a circumpolar channel. The model is forced by zonal wind stress and a density restoring at the surface. Despite the idealized geometry, the relatively coarse model resolution, and the lack of atmospheric coupling, the nondiffusive ocean maintains a density structure and meridional overturning that are broadly in line with those observed in the Atlantic Ocean. These are generated by just a handful of key water pathways, including shallow tropical cells described by ventilated thermocline theory; a deep overturning cell in which sinking North Atlantic Deep Water eventually upwells in the Southern Ocean before returning northward as Antarctic Intermediate Water; a Deacon cell that results from a topographically steered and corkscrewing circumpolar current; and weakly overturning Antarctic Bottom Water, which is effectively ventilated only in the Southern Hemisphere. The main conclusion of this study is that the adiabatic limit for the ocean interior provides the leading-order solution for ocean overturning and density structure, with tracer diffusion contributing first-order perturbations. Comparing nondiffusive and diffusive experiments helps to quantify the changes in stratification and circulation that result from adding a moderate amount of tracer diffusion in the ocean model, and these include an increase in the amplitude of the deep meridional overturning cell of several Sverdrups, a 10%%-20%% increase in Northern Hemispheric northward heat transport, a stronger stratification just below the main thermocline, and a more realistic bottom overturning cell. [ABSTRACT FROM AUTHOR]

Particle image velocimetry scalar measurements were carried out on the body of a stably stratified density current with an inlet Reynolds number of 2,300 and bulk Richardson number of 0.1. These measurements allowed the mass and momentum transport between the current and the less dense ambient fluid to be investigated. Reynolds stress, Reynolds flux, and shear production of turbulent kinetic-energy profiles revealed local maxima at the bed, as well as at the interface with the ambient fluid. Profiles of excess density variance and buoyancy production of turbulent kinetic energy revealed only local maxima at the interface with the ambient. These maxima decreased downstream as the stable density gradient reduced the turbulent intensities, until turbulence collapsed. A two-dimensional, unsteady, Reynolds-averaged Navier-Stokes (2DV-URANS) simulation was also performed on this density current. Good agreement was found between the modeled and measured normalized mean flow profiles. A comparison was also made between the measured and modeled outer flow scales of the density current. [ABSTRACT FROM AUTHOR]

The water level of the Three Gorges Reservoir (TGR) rose to 172.5 m in two stages from September to November, 2008. The hydrodynamic parameters and water quality parameters (e.g. flow velocity, temperature, turbidity and nutrient content) had been continuously monitored to reveal the influence of the impounding process of the TGR on the water eutrophication in the Xiangxi Bay (XXB). The maximal daily rise of the water level of the TGR was about 2.38 m. The water exchange between the mainstream (the Changjiang River) and the XXB of the TGR was enhanced by density current, and its main characteristics were a density-stratified flow in different directions. The water exchange reduced the differences of the water quality parameters (for example, temperature, turbidity, nutrient level, pH, etc.) between the mainstream and the XXB during the impounding process. Meanwhile, the water stability index in the mainstream remained small but it was lowered in the XXB. The main causes included the dilution by water of lower concentration of the chlorophyll a ( Chl. a) entering from the mainstream, the decline of the water stability, and the increase of suspended silt, which induced the Chl.a to decrease in the XXB during the impounding process. The grade of the water eutrophication changed from middle eutrophic level to mesotrophic state in the process. However, as available nutrients were imported into the XXB from the mainstream during the impounding process the risk of water eutrophication increases in the XXB in the future. [ABSTRACT FROM AUTHOR]

This study is based on a series of numerical modeling experiments to understand the impact of the Namgang Dam water discharge into the Kangjin Bay (KB). Numerical model performances were tested in terms of various skill analyses, with skill scores for elevation and tidal current over 85 %. The models successfully reproduced the tidal circulation and the response of fresh water input and subsequent dilution of salinity field. The behavior of the fresh water plume in the KB was investigated and analyzed. The different pattern of densitydriven current field was described at tidal phases. The modified tidal ellipse characteristics are estimated as a result of interaction between tidal current and density current. [ABSTRACT FROM AUTHOR]

The Bosphorus Strait accommodates two-way flow between the Aegean and Black Seas. The Aegean (Mediterranean) inflow has speeds of 5 to 15 cm sec−1 in the strait and a salinity contrast of ∼12‰ to 16‰ with the Black Sea surface waters on the shelf. An anastomosed channel network crosses the shelf and in water deeper than 70 m is characterized by first-order channels 5 to 10 m deep, local lateral accretion bedding, muddy in-channel barforms, and a variety of sediment waves both on channel floors and bar crests, crevasse channels entering the overbank area and levée/overbank deposits which are radiocarbon-dated in cores to be younger than ∼7·5 to 8·0 ka. This channel network accommodates the saline density current formed by the Mediterranean inflow. The density contrast between the density underflow and the ambient water mass is ∼0·01 g cm−3, similar to the density contrast ascribed to low-concentration turbidity currents in the deep sea. Channel-floor deposits are sandy to gravelly with local shell concentrations. Low-relief bedforms on the channel floor have relatively straight crests, upflow-dipping cross-stratification, heights 1 to 1·5 m and wavelengths 85 to 155 m. Bankfull flows are subcritical, so these probably are not antidunes. Bar tops are ornamented locally with mudwaves having heights 1 to 2 m and wavelengths ∼20 to 100 m; these are potentially antidunes formed under shallow overbank flows. Towards the shelf edge, the degree of channel bifurcation increases dramatically and bar tops are dissected locally by secondary channels, some of which terminate in hanging valleys. Conical mounds on the shelf (possibly mud volcanoes or sites of fluid seepage) interact with the channel network by promoting accretion of muddy streamlined macroforms in their lee. This channel network may be one of the largest and most accessible natural laboratories on Earth for the study of continuously flowing density currents. Although the driver is salinity contrast, the underflow transports sufficient sediment to form levée wedges and large streamlined barforms, and presumably transports sediment into deep water. [ABSTRACT FROM AUTHOR]

Abstract: Topographic corrugations such as canyons and ridges cross-cutting the path of a dense plume may effectively steer all or part of the plume downslope. Here, topographically steered flows are investigated experimentally, as laminar, dense gravity currents are observed to impinge on and flow along sloping, V-shaped canyons and ridges. Ridges, as well as canyons, were observed to steer the dense water downslope. A dynamical regime, in which the along-slope transport is balanced by a return flow in the Ekman layer to maintain a geostrophically balanced downslope flow along the corrugation, has been proposed. Results from a previously published analytical model describing such flows are compared with the laboratory experiments. The response of the flow to variations in four governing parameters (slope, rotation, volume flux and reduced gravity) is generally described well by the model and results agree qualitatively, although theory slightly underestimates the dense layer thickness. Vertical velocity profiles resolving the Ekman spiral were obtained using a laser Doppler velocimeter and they showed the secondary, transverse circulation superimposed on the primary, downslope flow. A particle flowing down the canyon/along the ridge can be expected to follow a helix-like path, and dye released within the dense layer showed this. The experiments support the analytical model and the dynamical regime proposed for topographically steered flows. The gravity current split in two when the transport capacity of the corrugation was exceeded; one part continued along the slope and the other flowed downslope along the corrugation. [Copyright &y& Elsevier]

Abstract: Large-scale dense bottom currents are geostrophic to leading order, with the main flow direction along the continental slope. Bottom friction makes the water descend to greater depths, but only at a small angle to the horizontal. Here the effect of a submarine ridge that intersects the slope is considered. It is shown that the presence of a submarine ridge greatly enhances the downward transport. By leaning against the ridge it is possible for the dense water to flow downhill, perpendicular to the depth contours, even though the first-order dynamics are geostrophic. The requirement for downward flow next to the ridge is that the frictional transport that it induces is sufficiently large to counteract geostrophic advection along the isobaths and out of the ridge region. The dynamics are similar to those of downward flow in submarine canyons, but ridges appear to be more effective in channeling the dense water downhill, in particular for narrow ridges/canyons with small seaward slope of the ridge/canyon axis. The downward flow is analyzed using a simplified analytical model and the results are compared to data from the Filchner Overflow, which agree qualitatively with the model. [Copyright &y& Elsevier]

A robotic water quality monitoring network is used to resolve the coupled patterns of a natural tracer, specific conductance (SC), and metrics of light scattering and turbidity for Schoharie Creek and downstream Schoharie Reservoir, with particular emphasis on the impacts of runoff events. Strong relationships between these parameters and streamflow, and the propensity for this tributary to plunge in the reservoir in summer and fall based on its lower temperature, are reported. The entry of this stream, the primary tributary, into the reservoir as a turbid density current during runoff events is depicted as distinct and vertically coincident subsurface SC minima and peaks in measures of light scattering. The magnitudes of these signatures imparted to the reservoirs water column are demonstrated to be strongly dependent on the magnitude of the runoff event. The time course of the diminishment of these signatures and longitudinal differences in turbidity within the reservoir are described. The documented patterns of SC and metrics of light scattering provided by the robotic monitoring network offer a rare opportunity to support development and testing of a turbidity model with the necessary attributes of fine temporal and spatial resolution. [ABSTRACT FROM AUTHOR]

A wavelet method is used to estimate kinetic energy and fluxes from data collected under stable conditions during the CASES-99 field campaign. Results in the high frequency range are compared with those obtained by the traditional method used to estimate turbulent moments, which is based on the Reynolds decomposition of variables into a mean and a turbulent part. The fact that the wavelet transform performs much better as a filter than the averaging process accounts for most of the disagreements between results. Since the wavelet method can be applied at very different spectral ranges, it is also used to analyse two different coherent structures: a density current and a train of internal gravity waves. The strong burst of turbulence related to the density current reflects the complexity of the first event. The wavelet method discriminates the different scales of motion, which are present in the perturbation, and is therefore an ideal tool for assessing the interactions between them. A method based on the phase difference between wavelet-transformed time series is then applied to the analysis of the horizontal and vertical structure of the gravity waves, and a three-dimensional image of the oscillations is provided. [ABSTRACT FROM AUTHOR]

By means of flow visualization and quantitative measurement, the diffusion pattern and concentration distribution characteristics of high concentration jets vertically discharged into shallow moving waterbody were experimentally investigated in water channel. The interactions between the high concentration jets and environmental flow conditions were analysed, and the formulae of impinging point coordinate and transverse s[read angle are gained from data analysis. Experimental results indicate that the jets show complicated flow patterns and diffusion characteristics in near region, which are different from common submerged jets, and spread downstream in the manner of density currents. [ABSTRACT FROM AUTHOR]

Because of the difference in morphometry and in the underground heating of the two lobes of Lake Banyoles there is a differential cooling that causes a density current - the denser water of the shallower lobe plunging into the deepest lobe forming a bottom current - which redistributes water between the lobes and replaces that of the northern lobe about every 5 days. This current has been studied during the mixing periods of the years 1989-1991 from temperature and current measurements and it has been found that it increases or slows down depending on the direction of the wind. Furthermore, it is observed that it mainly flows along the west shore of the lake, deflecting towards the right as an effect of the bottom morphometry. However, due to the long time scale of the density current the influence of the earth's rotation should not be neglected in advance. Finally, a simple numerical model has been used to corroborate the magnitude of this current - whose velocity has been directly measured - which is found to be the most important in the lake during the mixing period and so, it dominates the lake-wide circulation [ABSTRACT FROM AUTHOR]