Inclined gravity currents filling basins: the impact of peeling detrainment on transport and vertical structure

2017 ◽  
Vol 820 ◽  
pp. 400-423 ◽  
Author(s):  
Charlie A. R. Hogg ◽  
Stuart B. Dalziel ◽  
Herbert E. Huppert ◽  
Jörg Imberger
Keyword(s):  
Gravity Current ◽  
Uniform Density ◽  
Ambient Fluid ◽  
Transport Pathway ◽  
Density Profiles ◽  
Model Match ◽  
Dense Fluid ◽  
Vertical Density ◽  
Multiprocess Models ◽  
The Impact

Transport of dense fluid by an inclined gravity current can control the vertical density structure of the receiving basin in many natural and industrial settings. A case familiar to many is a lake fed by river water that is dense relative to the lake water. In laboratory experiments, we pulsed dye into the basin inflow to visualise the transport pathway of the inflow fluid through the basin. We also measured the evolving density profile as the basin filled. The experiments confirmed previous observations that when the turbulent gravity current travelled through ambient fluid of uniform density, only entrainment into the dense current occurred. When the gravity current travelled through the stratified part of the ambient fluid, however, the outer layers of the gravity current outflowed from the current by a peeling detrainment mechanism and moved directly into the ambient fluid over a large range of depths. The prevailing model of a filling box flow assumes that a persistently entraining gravity current entrains fluid from the basin as the current descends to the deepest point in the basin. This model, however, is inconsistent with the transport pathway observed in visualisations and poorly matches the stratifications measured in basin experiments. The main contribution of the present work is to extend the prevailing filling box model by incorporating the observed peeling detrainment. The analytical expressions given by the peeling detrainment model match the experimental observations of the density profiles more closely than the persistently entraining model. Incorporating peeling detrainment into multiprocess models of geophysical systems, such as lakes, will lead to models that better describe inflow behaviour.

scholarly journals An Analysis of Two-Dimensional Stratified Gravity Current Flow using Open FOAM

2018 ◽  
Vol 7 (4.35) ◽  
pp. 589
Author(s):  
W.K Lam ◽  
L. Chan ◽  
H. Hasini ◽  
A. Ooi
Keyword(s):  
Current Flow ◽  
Gravity Current ◽  
Propagation Speed ◽  
Ambient Air ◽  
Front Velocity ◽  
Test Case ◽  
Two Dimensional ◽  
Ambient Fluid ◽  
Dense Fluid ◽  
Reduced Density

Direct numerical simulations (DNSs) of two-dimensional stratified gravity-current are simulated using OpenFOAM. Three different aspect ratio, h0/l0 (where h0 is the height of the dense fluid and l0 is the length of the dense fluid) are simulated with stratification ranging from 0 (homogenous ambient) to 0.2 with a constant Reynolds number (Re) of 4000. The stratificationof the ambient air is determined by the density difference between the bottom and the top walls of the channel (ρb- ρ0, where ρb is the density at the bottom of the domain and ρ0 is the density at the top). The magnitude of the stratification (S=ԑb/ԑ) can be determined by calculating the reduced density differences of the bottom fluid with the ambient fluid (ԑb = (ρb- ρ0)/ ρ0) and the dense fluid with the ambient fluid (ԑ = (ρc-ρ0)/ ρ0, where ρc represents the density of the dense fluid). The configuration of the simulation is validated with a test case from Birman, Meiburg & Ungraish and the contour and front velocity (propagation speed) were in good agreement. The gravity current flow in the stratified ambient is analyzed qualitatively and compared with the gravity current in the homogenous ambient. Gravity current in homogenous ambient (S=0) and weak stratification (S=0.2) are supercritical flow where the flow is turbulent and Kelvin-Helmholtz (K-H) billow formed behind the gravity current head. The front location of the gravity is reduced as the stratification increase and denotes that the front velocity of the gravity current is reduced by the stratification.

Vortex shedding by a circular cylinder in lock-exchange density current 

2021 ◽  
Author(s):  
Ana M Ricardo ◽  
Giovanni Di Lollo ◽  
Moisés Brito ◽  
Claudia Adduce ◽  
Rui M.L. Ferreira
Keyword(s):  
Circular Cylinder ◽  
Fresh Water ◽  
Vortex Shedding ◽  
Gravity Current ◽  
Shear Instability ◽  
Density Current ◽  
Bluff Bodies ◽  
Density Currents ◽  
Ambient Fluid ◽  
Dense Fluid

<p>Flow around bluff bodies have been attracting the interest of the research community for more than a century. The physical mechanisms associated with the vortex shedding in the wake of bluff bodies is still of fundamental research interest. However, flow-structure interaction in density currents has not received enough attention. The transient nature of the interaction between the density driven flow and a stationary object constitutes the motivation for the present laboratory study aiming at investigating the vortex generation and fate on the wake of a circular cylinder in a density current.</p><p>The experiments were conducted in a horizontal and rectangular cross-section channel with 3.0 m long, 0.175 m wide and 0.4 m deep. The gravity current was generated using the classic lock-exchange configuration. A sliding stainless-steel gate with 1 mm thickness, sealed by PVC board glued in the sidewall, was positioned at 0.3 m from the left hand side of the channel. The experiment starts when the gate is suddenly removed, leaving the dense fluid to flow along the bottom of the channel, while the ambient fluid moves above in the opposite direction. The dense fluid consists in a mixture of fresh water and salt while the ambient fluid is a solution fresh water and ethanol (96%). The amount of salt and alcohol added in each mixture was determined in order to obtain a given density difference and to ensure the same refractive index in both fluids. Two different currents were tested with reduced gravity equal to 0.06 ms<sup>-2</sup> and 0.24 ms<sup>-2</sup>. For each test ten repetitions were carried out. Instantaneous velocity maps were acquired with a Particle Image Velocimetry system at 15 Hz. Polyamide seeding particles of density equal to 1.03 were added in both dense and ambient fluids.</p><p> The Reynolds number varied between 1500 and 4000. The results show that vortex shedding varies as the current reaches and overtakes the cylinder. Boundary layer detachment and shear instability is initiated shortly before the snout reaches the cylinder. A pattern of well-defined symmetrical vortexes is formed as a result of the initial shear instability. As the head of the current engulfs the cylinder, stronger turbulence diffusion contributes to reduce vortex coherence. Vortexes are smaller and detach sooner, while is not clear if shedding is alternate or simply random. The formation length is smaller than that of a steady flow with the same Re. When the back of the current passes, the formation length is increased and vortex shedding becomes periodical again. A striking feature is that the Von Kármán street is frequently symmetrical rather than exhibiting a pattern of alternate vortices.</p><p>This research was funded by national funds through Portuguese Foundation for Science and Technology (FCT) project PTDC/CTA-OHR/30561/2017 (WinTherface).</p>

Critical regime of gravity currents flowing in non-rectangular channels with density stratification

2018 ◽  
Vol 840 ◽  
pp. 579-612
Author(s):  
L. Chiapponi ◽  
M. Ungarish ◽  
S. Longo ◽  
V. Di Federico ◽  
F. Addona
Keyword(s):  
Cross Section ◽  
Power Law ◽  
Rectangular Cross Section ◽  
Gravity Current ◽  
Gravity Currents ◽  
Critical Conditions ◽  
Uniform Density ◽  
Ambient Fluid ◽  
Real Numbers ◽  
Positive Real

We present theoretical and experimental analyses of the critical condition where the inertial–buoyancy or viscous–buoyancy regime is preserved in a uniform-density gravity current (which propagates over a horizontal plane) of time-variable volume ${\mathcal{V}}=qt^{\unicode[STIX]{x1D6FF}}$ in a power-law cross-section (with width described by $f(z)=bz^{\unicode[STIX]{x1D6FC}}$, where $z$ is the vertical coordinate, $b$ and $q$ are positive real numbers, and $\unicode[STIX]{x1D6FC}$ and $\unicode[STIX]{x1D6FF}$ are non-negative real numbers) occupied by homogeneous or linearly stratified ambient fluid. The magnitude of the ambient stratification is represented by the parameter $S$, with $S=0$ and $S=1$ describing the homogeneous and maximum stratification cases respectively. Earlier theoretical and experimental results valid for a rectangular cross-section ($\unicode[STIX]{x1D6FC}=0$) and uniform ambient fluid are generalized here to a power-law cross-section and stratified ambient. Novel time scalings, obtained for inertial and viscous regimes, allow a derivation of the critical flow parameter $\unicode[STIX]{x1D6FF}_{c}$ and the corresponding propagation rate as $Kt^{\unicode[STIX]{x1D6FD}_{c}}$ as a function of the problem parameters. Estimates of the transition length between the inertial and viscous regimes are also derived. A series of experiments conducted in a semicircular cross-section ($\unicode[STIX]{x1D6FC}=1/2$) validate the critical values $\unicode[STIX]{x1D6FF}_{c}=2$ and $\unicode[STIX]{x1D6FF}_{c}=9/4$ for the two cases $S=0$ and $1$. The ratio between the inertial and viscous forces is determined by an effective Reynolds number proportional to $q$ at some power. The threshold value of this number, which enables a determination of the regime of the current (inertial–buoyancy or viscous–buoyancy) in critical conditions, is determined experimentally for both $S=0$ and $S=1$. We conclude that a very significant generalization of the insights and results from two-dimensional (rectangular cross-section channel) gravity currents to power-law cross-sections is available.

A flow-front instability in viscous gravity currents

1998 ◽  
Vol 369 ◽  
pp. 1-21 ◽  
Author(s):  
DON SNYDER ◽  
STEPHEN TAIT
Keyword(s):  
Gravitational Instability ◽  
Gravity Current ◽  
High Viscosity ◽  
Gravity Currents ◽  
Ambient Fluid ◽  
Cell Width ◽  
Dense Fluid ◽  
Unstable Layer ◽  
Gravitational Mechanism ◽  
Hele Shaw Cell

We describe an instability that appears at the front of laminar gravity currents as they intrude into a viscous, miscible ambient fluid. The instability causes a current to segment into fingers aligned with its direction of flow. In the case of currents flowing along a rigid floor into a less dense fluid, the case of primary interest here, two mechanisms can produce this instability. The first is gravitational and arises because the nose of the gravity current is elevated above the floor and overrides a buoyantly unstable layer of ambient liquid. The second is a form of viscous fingering analogous to a Saffman–Taylor instability in a Hele-Shaw cell. Whereas the ambient fluid must be more viscous than the current in order for the latter instability to occur, the gravitational instability can occur even if the ambient fluid is less viscous, as long as it is viscous enough to elevate the nose of the current and trap a layer of ambient fluid. For the gravitational mechanism, which is most important when the current and ambient fluids have comparable viscosities, the wavelength when the instability first appears is proportional to a length scale constructed with the viscosity, the flux and the buoyancy. The Saffman–Taylor-type mechanism is most important when the ambient liquid is much more viscous than the current. We have carried out experiments with miscible fluids in a Hele-Shaw cell that show that, at the onset of instability, the ratio of the finger wavelength to the cell width is a constant approximately equal to 2. This result is explained by using the principle that the flow tends to minimize the dissipation associated with the finger perturbation. For the gravity currents with high viscosity ratios, the ratio of the wavelength to the current thickness is also a constant of about 2, apparently consistent with the same mechanism. But, further analysis of this instability mechanism is required in order to assess its role in wavelength selection for gravity currents.

Gravity induced vertical motion of dense fluids into saturated granular beds

2021 ◽  
Author(s):  
Rui M L Ferreira ◽  
Gabriel Solis ◽  
Claudia Adduce ◽  
Ana Margarida Ricardo
Keyword(s):  
Porous Layer ◽  
High Speed ◽  
Rectangular Cross Section ◽  
Gravity Current ◽  
Gravity Currents ◽  
Frame Rate ◽  
Ambient Fluid ◽  
Porous Bed ◽  
Loose Packing ◽  
Dense Fluid

<p>Gravity currents propagating over and within porous layers occurs in natural environments and in industrial processes. The particular modes by which the dense fluid flows into the porous layer is a subject that is not sufficiently understood. To overcome this research gap, we conducted laboratory experiments aimed at describing experimentally the dynamics of the drainage flow.</p><p>The experiments were conducted in a horizontal channel with a rectangular cross-section. The channel is 3.0 m long, 0.05 m wide. The porous bottom was composed of 5 cm and 10 cm layers of 3 mm borosilicate spheres – unimodal bed – and of a mixture of 3 mm (50% in weight) and 5 mm spheres (50%) – bi-modal bed. The porosity of the unimodal bed ranged between 0.60 and 0.64 (compatible with loose packing). The porosity of the bi-modal bed ranged between 0.61 and 0.65. All gravity currents were generated by releasing suddenly denser fluid locked by a thin vertical barrier placed at 0.2 m from the channel end. The dense fluid consists in a mixture of freshwater and salt (coloured with Rhodamine) while the ambient fluid is a solution of freshwater and ethanol. The density difference between the ambient fluid and the current, and the need to maintain the same refractive index, determine the amount of salt and alcohol added in each mixture. Here we report the findings of currents with a reduced gravity of 0.06 ms<sup>-2</sup>.</p><p>Each experiment was recorded by an high-speed camera with a frame-rate of 386 Hz and a resolution of 2320 x 1726 pxxpx. Measurements were based on light absorption techniques: a LED light panel 0.3 m high and 0.61 m long was used as back illumination. All images were calibrated to ascribe, pixel by pixel, a concentration value from a 8 bit gray level. Different calibrations were performed for the porous layer and for the surface current.</p><p>Results show that, in the slumping phase, the gravity current flows with velocities compatible with those over rough beds. As the current progresses further attenuation of momentum is noticed owing to mass loss to the porous bed.</p><p>The flow in the porous bed reveals plume instability akin to a Saffman-Taylor instability. The growth of the plumes seems independent from the initial fluid height in both types of porous beds. The wavelength and the growth rate of the plumes depends on the bed material. Plumes grow faster in the case of the bi-modal bed and the wavelength of the bi-modal bed is about 1.5 as that of the unimodal bed. It is hypothesised that the gravity-induced porous flow is best parameterized by a Péclet number defined as a ratio of dispersive (mechanical diffusion) and advective modes of transport. Smaller wavelengths and slower growths are attained for stronger dispersion, characterisitic of the unimodal bed. For bimodal beds, permeability is larger, and thus also advection. This causes the flow to concentrate in faster growing but farther apart plumes.</p><p> </p><p>This research was funded by national funds through Portuguese Foundation for Science and Technology (FCT) project PTDC/CTA-OHR/30561/2017 (WinTherface).</p>

Experiments on the dynamics of a gravity current head

1978 ◽  
Vol 88 (2) ◽  
pp. 223-240 ◽  
Author(s):  
R. E. Britter ◽  
J. E. Simpson
Keyword(s):  
Gravity Current ◽  
Uniform Flow ◽  
Experimental Results ◽  
Momentum Balance ◽  
Two Dimensional ◽  
Ambient Fluid ◽  
Dimensional Flow ◽  
Dense Fluid ◽  
Two Dimensional Flow ◽  
The Cross

Some of the dense fluid at the front of an advancing gravity current is observed to be mixed with the ambient fluid. This process continues when the cross-stream non-uniformities at the head of the current are suppressed by advancing the floor beneath the head. In the resulting two-dimensional flow regular billows are visible. This paper considers experimentally and analytically the inviscid gravity current head and specifically includes the observed mixing at the head. Experimental results were obtained with an apparatus in which the head of the gravity current was brought to rest by an opposing uniform flow. The mixing appears to occur through Kelvin-Helmholtz billows generated on the front of the head and controls the dynamics of the head. A momentum balance is used to analyse the flow and the problem is closed by quantitatively introducing the billow structure.

scholarly journals Modelling H2 and its effects on star formation using a joint implementation of gadget-3 and KROME

2021 ◽  
Vol 504 (2) ◽  
pp. 2325-2345
Author(s):  
Emanuel Sillero ◽  
Patricia B Tissera ◽  
Diego G Lambas ◽  
Stefano Bovino ◽  
Dominik R Schleicher ◽  
...  
Keyword(s):  
Star Formation ◽  
Formation Rate ◽  
Physical Parameters ◽  
Density Profiles ◽  
Star Forming ◽  
Chemical Enrichment ◽  
Numerical Resolution ◽  
Stellar Radiation ◽  
Wide Range ◽  
The Impact

ABSTRACT We present p-gadget3-k, an updated version of gadget-3, that incorporates the chemistry package krome. p-gadget3-k follows the hydrodynamical and chemical evolution of cosmic structures, incorporating the chemistry and cooling of H2 and metal cooling in non-equilibrium. We performed different runs of the same ICs to assess the impact of various physical parameters and prescriptions, namely gas metallicity, molecular hydrogen formation on dust, star formation recipes including or not H2 dependence, and the effects of numerical resolution. We find that the characteristics of the simulated systems, both globally and at kpc-scales, are in good agreement with several observable properties of molecular gas in star-forming galaxies. The surface density profiles of star formation rate (SFR) and H2 are found to vary with the clumping factor and resolution. In agreement with previous results, the chemical enrichment of the gas component is found to be a key ingredient to model the formation and distribution of H2 as a function of gas density and temperature. A star formation algorithm that takes into account the H2 fraction together with a treatment for the local stellar radiation field improves the agreement with observed H2 abundances over a wide range of gas densities and with the molecular Kennicutt–Schmidt law, implying a more realistic modelling of the star formation process.

Modelling of the Development of the Vertical Density Profile of MDF during Hot Pressing

2007 ◽  
Vol 2 (2) ◽  
Author(s):  
Arun Gupta ◽  
Patrick Jordan ◽  
Shusheng Pang
Keyword(s):  
Moisture Content ◽  
Density Profile ◽  
Hot Pressing ◽  
Major Influence ◽  
Transfer Model ◽  
Uniform Density ◽  
Moisture Movement ◽  
Vertical Density Profile ◽  
Vertical Density ◽  
Pressing Operation

The hot pressing operation is one of the most important operations in medium density fibreboard (MDF) manufacture. Complicated dynamic interactions occur during pressing, including heat transfer, moisture movement, development of gas pressure, internal stress development and relaxation, wood consolidation, resin curing, bonding between particles and eventual development of a non-uniform density distribution through the panel thickness. Consequently the mat experiences continuously changing internal conditions (temperature and moisture content) as the pressing operation proceeds. The vertical density profile (VDP) has a major influence on the MDF strength and physical properties. This influence of the VDP on the board properties is generally recognised, but the formation of the density profile and their specific effects on the board performance have proved difficult to quantify. A mathematical model based on theoretical analysis and experimental information is being developed. In the model, the mat is divided into a number of thin parallel layers. The deformation of each layer is a function of stress, temperature and moisture content of the layer. The model incorporates the variation of the mat mechanical and rheological properties with moisture content and temperature. The changes in temperature and moisture content are provided by a separate heat and mass transfer model. The present model can predict stress, strain, layer deformation and density across the thickness during pressing. The performance of the model was validated by experiments conducted in a pilot-scale press. Twelve MDF boards were made with different pressing parameters, and the VDP were measured and compared with the simulation results from the model. The model could predict the density profile with an acceptable accuracy for the main variables that control the manufacturing of MDF boards.

scholarly journals Regional characteristics of spring Asian dust and its impact on aerosol chemistry over northern China

2006 ◽  
Vol 6 (6) ◽  
pp. 12825-12864 ◽  
Author(s):  
Y. L. Sun ◽  
G. S. Zhuang ◽  
Z. F. Wang ◽  
Y. Wang ◽  
W. J. Zhang ◽  
...  
Keyword(s):  
Dust Storm ◽  
Fine Particles ◽  
Coastal Region ◽  
Northern China ◽  
Asian Dust ◽  
Aerosol Chemistry ◽  
Transport Pathway ◽  
Regional Characteristics ◽  
Source Regions ◽  
The Impact

Abstract. TSP and PM2.5 aerosol samples were synchronously collected at six sites along the transport pathway of dust storm from desert regions to coastal areas in the spring of 2004. The aerosol concentration and composition were measured to investigate the regional characteristics of spring Asian dust and its impact on aerosol chemistry over northern China. Based on the daily PM10 concentrations in 13 cities, the northern China could be divided into five regions, i.e., Northern Dust Region, Northeastern Dust Region, Western Dust Region, Inland Passing Region, and Coastal Region. Northern Dust Region was characterized by high content of Ca and Northeastern Dust Region was characterized by low one instead. Northeastern Dust Region was a relatively clean area with the lowest concentrations of pollutants and secondary ions among all sites. Inland Passing Region and Coastal Region showed high concentrations of pollutants, of which As and Pb in Inland Passing Region, and Na+, SO42− and NO3− in Coastal Region were the highest, respectively. The impact of dust on air quality was the greatest in the cities near source regions, and this impact decreased in the order of Yulin/Duolun > Beijing > Qingdao/Shanghai as the increase of transport distance. The spring Asian dust was inclined to affect the chemical components in coarse particles near source regions and those in fine particles in the cities far from source regions. Dust storm could mix significant quantities of pollutants on the pathway and carry them to the downwind cities or dilute the pollutants in the cities over northern China. Each dust episode corresponded to a low ratio of NO3−/SO42− with the lowest value appearing after the peak of dust storm. Asian dust played an important role in buffering and neutralizing the acidity of atmosphere in the cities over northern China, which could lead to the pH in the aerosols increase ~1 in spring.

scholarly journals Architectural Diversity of Submarine Lobate Deposits

2021 ◽  
Vol 9 ◽  
Author(s):  
Tim R. McHargue ◽  
David M. Hodgson ◽  
Eitan Shelef
Keyword(s):  
Turbulent Flows ◽  
Deep Water ◽  
Relative Size ◽  
Transport Pathway ◽  
Branching Points ◽  
Channel Patterns ◽  
The Impact ◽  
Source Materials ◽  
Permeability Structure ◽  
Distributary Channel

Lobate deposits in deep-water settings are diverse in their depositional architecture but this diversity is under-represented in the literature. Diverse architectures result from multiple factors including source material, basin margin physiography, transport pathway, and depositional setting. In this contribution, we emphasize the impact of differing source materials related to differing delivery mechanisms and their influence on architecture, which is an important consideration in source-to-sink studies. Three well imaged subsurface lobate deposits are described that display three markedly different morphologies. All three lobate examples, two from intraslope settings offshore Nigeria and one from a basin-floor setting offshore Indonesia, are buried by less than 150 m of muddy sediment and are imaged with high resolution 3D reflection seismic data of similar quality and resolution. Distinctively different distributary channel patterns are present in two of the examples, and no comparable distributaries are imaged in a third example. Distributary channels are emphasized because they are objectively recognized and because they often represent elements of elevated fluid content within buried lobate deposits and thus influence permeability structure. We speculate that the different distributary channel patterns documented here resulted from different processes linked to source materials: 1) a lobate deposit that is pervasively channelized by many distributaries that have branched at numerous points is interpreted to result from comparatively mud-rich, stratified, turbulent flows; 2) an absence of distributaries in a lobate deposit is interpreted to result from collapse of mud-poor, turbulent flows remobilized from littoral drift; and 3) a lobate deposit with only a few, long, straight distributaries with few branching points is interpreted to be dominated by highly viscous flows (i.e., debris flows). We propose a conceptual model that illustrates the relationship between the proportion of mud in contributing flows and the relative size and runout distance of lobate deposits. We conclude that reconciling 3D seismic morphologies with outcrop observations of channels, scours, and amalgamation zones, and simple application of hierarchical schemes, is problematic. Furthermore, when characterizing unconfined deep-water deposits in the subsurface, multiple models with significant differences in predicted permeability structure should be considered.

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