Particle-laden Gravity Currents Research Articles

Investigation of particle laden gravity currents using the light attenuation technique

An extensional use of the light attenuation technique is described to study the dynamics of particle laden flows. The method is validated against an analytical absorption/scattering model. It is then applied to particle laden gravity currents (PLGCs) and provides access to quantitative measurements of the particle mass fraction field. The influence of particle concentration on the turbulent dynamics of the flow and the front velocity are evaluated and discussed. In all experiments heterogeneities in the particle distribution are measured, with the maximum particle concentration in the PLGC front and intermediate particle concentration at the turbulent interface. The results highlight two regimes. In the dilute case, particles have no measurable impact on the turbulent structures observed in the flow nor on the bulk velocity. Particle sedimentation occurs due to mixing between the carrier and surrounding fluids. In the dense case, turbulent structures strongly weaken and the bulk velocity of the PLGC quantitatively decreases as the effective flow viscosity grows.

Turbidity currents propagating down an inclined slope: particle auto-suspension

The turbidity current (TC), a ubiquitous fluid–particle coupled phenomenon in the natural environment and engineering, can transport over long distances on an inclined terrain due to the suspension mechanism. A large-eddy simulation and discrete element method coupled model is employed to simulate the particle-laden gravity currents over the inclined slope in order to investigate the auto-suspension mechanism from a Lagrangian perspective. The particle Reynolds number in our TC simulation is$0.01\sim 0.1$and the slope angle is$1/20 \sim 1/5$. The influences of initial particle concentration and terrain slope on the particle flow regimes, particle movement patterns, fluid–particle interactions, energy budget and auto-suspension index are explored. The results indicate that the auto-suspension particles predominantly appear near the current head and their number increases and then decreases during the current evolution, which is positively correlated with the coherent structures around the head. When the turbidity current propagates downstream, the average particle Reynolds number of the auto-suspension particles remains basically unchanged, and is higher than that of other transported particles. The average particle Reynolds number of the transported particles exhibits a negative correlation with the Reynolds number of the current. Furthermore, the increase in particle concentration will enhance the particle velocity, which allows the turbidity current to advance faster and improves the perpendicular support, thereby increasing the turbidity current auto-suspension capacity. Increasing slope angle will result in a slightly larger front velocity, while the effect of that on the total force is insignificant.

Characteristics and controls of the runout behaviour of non-Boussinesq particle-laden gravity currents – A large-scale experimental investigation of dilute pyroclastic density currents

One of the most dangerous aspects of explosive volcanism is the occurrence of dilute pyroclastic density currents that move at high velocities of tens to about a hundred of metres per second outwards from volcanic vents. Predicting the runout behaviour of these turbulent flows of hot particles and air is complicated by strong changes in the flow density resulting from entrainment of ambient air, sedimentation of particles, as well as heating and expansion of the gas phase. Current hazard models that are based on the behaviour of aqueous gravity currents cannot capture all aspects of the flow dynamics, and thus pyroclastic density current dynamics remain comparatively poorly understood. Here we interrogate the runout behaviour of dilute pyroclastic density currents in large-scale experiments using hot volcanic material and gas. We demonstrate that the flows transition through four dynamic regimes with distinct density and force characteristics. The first, inertial regime is characterized by strong deceleration under high density differences between the flow and ambient air where suspended particles carry a main proportion of the flows' momentum. When internal gravity waves start to propagate from the flow body into the advancing flow front, the currents transition into a second, inertia-buoyancy regime while flow density continues to decline. In this regime, subsequent arrivals of fast-moving internal gravity waves into the front replenish momentum and lead to sudden short-lived front accelerations. In the third regime, when the density ratio between flow and ambient air decreases closer to a value of unity, buoyancy forces become negligible, but pressure drag forces are large and constitute the main flow retarding force. In this inertia-pressure drag regime, internal gravity waves cease to reach the front. Finally, and with the density ratio decreasing below 1, the current transitions into a buoyantly rising thermal in regime 4.Unlike for aqueous gravity currents, the Froude number is not constant and viscous forces are negligible in these gas-particle gravity currents. We show that, in this situation, existing Boussinesq and non-Boussinesq gravity current models strongly underpredict the front velocity for most of the flow runout for at least half of the flow propagation. These results are not only important for hazard mitigation of pyroclastic density currents but are also relevant for other turbulent gas-particle gravity currents, such as powder snow avalanches and dust storms.

Shallow-water equations and box model simulations of turbidity currents from a moving source

This paper presents simplified simulations, using the shallow-water equations (top panels) and a box model (bottom panels) of the particle-laden gravity currents generated by a vehicle moving along the seafloor. It analyzes the resulting deposits and their dependence on the two dominant parameters in the system: the ratio of the vehicle to spreading current speed (vehicle Froude number $F\phantom{\rule{0}{0ex}}r$) and the ratio of particle settling to vehicle speed. Estimates of the maximum extent of these deposits as a function of those two parameters are also computed, in both the supercritical regime, $F\phantom{\rule{0}{0ex}}r>\sqrt{2}$, and the subcritical regime, $F\phantom{\rule{0}{0ex}}r<\sqrt{2}$.

Particle-laden gravity currents interacting with stratified ambient water using direct numerical simulations

Particle-laden gravity currents interacting with stratified ambient water using direct numerical simulations

Separation of particle-laden gravity currents down a slope in linearly stratified environments

Particle-laden gravity currents down a slope in stratified fluid are important processes in lake, estuary, and ocean environments. By conducting direct numerical simulations, this study investigates the detailed dynamic features of lock-exchange particle-laden gravity currents down a slope in linearly stratified environments. The front velocity, separation depth, water entrainment ratio, and energy budget are quantitatively analyzed. This evolutionary process can be divided into three stages, i.e., the acceleration stage, deceleration stage, and separation stage, if the relative stratification parameter is larger than unity. At the acceleration stage, as the collapse of the dense fluid leads to fast entrainment of ambient water into the current, the entrainment ratios have large values, while the settling velocity and the ambient stratification are shown to have less impact on both the entrainment ratios and the front velocity. At the deceleration stage, a larger slope angle, a weaker ambient stratification, and a smaller settling velocity bring a greater front velocity. At the separation stage, the head of the current leaves the slope and intrudes into the environment; meanwhile, the dense fluid at the body of the current also intrudes into the ambient water because the density contrast has largely been reduced due to water entrainment, particle settling, and the density increase in the ambient fluid. A predictive model is developed to determine the separation depth by considering the presence of particles. The fingerlike horizontal intrusions enhance the entrainment effect between the current and the ambient water. A stronger ambient stratification suppresses the conversion of the potential energy to the kinetic energy, while a larger settling velocity accelerates the conversion of the kinetic energy to the dissipated energy.

Residual-based variational multi-scale modeling for particle-laden gravity currents over flat and triangular wavy terrains

In this paper, a numerical formulation for simulating dilute particle-laden gravity currents at large Reynolds number (Re) is developed using a residual-based variational multi-scale formulation (RBVMS), in which the coupling between the velocity fine scales and density concentration equation residual is represented. The proposed formulation is utilized to simulate the lock-exchange particle-laden gravity currents at Re=10,000. Firstly, we validated the proposed formulation by simulating the gravity currents over flat terrain. Flow statistics are thoroughly compared against high-resolution simulation results such as DNS or experimental results in the literature. It shows the RBVMS formulation presented here can reproduce quite accurate results with much lower mesh resolution. Then, the particle-laden gravity currents over triangular wavy terrains with changing wave heights are simulated. The effect of the wave height on the flow statistics is investigated.

Identification of particle-laden flow features from wavelet decomposition

A wavelet decomposition based technique is applied to air pressure data obtained from laboratory-scale powder snow avalanches. This technique is shown to be a powerful tool for identifying both repeatable and chaotic features at any frequency within the signal. Additionally, this technique is demonstrated to be a robust method for the removal of noise from the signal as well as being capable of removing other contaminants from the signal. Whilst powder snow avalanches are the focus of the experiments analysed here, the features identified can provide insight to other particle-laden gravity currents and the technique described is applicable to a wide variety of experimental signals.

Reynolds number and settling velocity influence for finite-release particle-laden gravity currents in a basin

Three-dimensional highly resolved Direct Numerical Simulations (DNS) of particle-laden gravity currents are presented for the lock-exchange problem in an original basin configuration, similar to delta formation in lakes. For this numerical study, we focus on gravity currents over a flat bed for which density differences are small enough for the Boussinesq approximation to be valid. The concentration of particles is described in an Eulerian fashion by using a transport equation combined with the incompressible Navier-Stokes equations, with the possibility of particles deposition but no erosion nor re-suspension. The focus of this study is on the influence of the Reynolds number and settling velocity on the development of the current which can freely evolve in the streamwise and spanwise direction. It is shown that the settling velocity has a strong influence on the spatial extent of the current, the sedimentation rate, the suspended mass and the shape of the lobe-and-cleft structures while the Reynolds number is mainly affecting the size and number of vortical structures at the front of the current, and the energy budget.

Direct numerical simulation of bi-disperse particle-laden gravity currents in the channel configuration

• Direct numerical simulation of bi-disperse particle-laden gravity currents. • 2D and 3D simulations in the lock-exchange configuration. • Full computation of kinetic, potential and dissipative terms of the energy balance. • Influence of the different initial proportion of fine and coarse particles. • Computation of important quantities of interest for geophysics. We present a numerical investigation of bi-disperse particle-laden gravity currents in the lock-exchange configuration. Previous results, based on numerical simulation and laboratory experiments, are used to establish comparisons. Our discussion focuses on explaining how the presence of more than one particle diameter influences the main features of the flow, such as deposit profile, the evolution of the front location and suspended mass. We develop the complete energy budget equation for bi-disperse flows. A set of two and three-dimensional direct numerical simulations (DNS), with different initial compositions of coarse and fine particles, are carried out for Reynolds number equal to 4000. Such simulations show that the energy terms are strongly affected by varying the initial particle fractions. The addition of a small amount of fine particles into a current predominantly composed of coarse particles increases its run-out distance. In particular, it is shown that higher amounts of coarse particles have a dumping effect on the current development. Comparisons show that the two-dimensional simulation does not reproduce the intense turbulence generated in 3D cases accurately, which results in a significant difference in the suspended mass, front position as well as the dissipation term due to the advective motion.

Direct numerical simulation of cylindrical particle-laden gravity currents

We present results from direct numerical simulations (DNS) of cylindrical particle-laden gravity currents. We consider the case of a full depth release with monodisperse particles at a dilute concentration where particle–particle interactions may be neglected. The disperse phase is treated as a continuum and a two-fluid formulation is adopted. We present results from two simulations at Reynolds numbers of 3450 and 10,000. Our results are in good agreement with previously reported experiments and theoretical models. At early times in the simulations, we observe a set of rolled up vortices that advance at varying speeds. These Kelvin–Helmholtz (K–H) vortex tubes are generated at the surface and exhibit a counter-clockwise rotation. In addition to the K–H vortices, another set of clockwise rotating vortex tubes initiate at the bottom surface and play a major role in the near wall dynamics. These vortex structures have a strong influence on wall shear-stress and deposition pattern. Their relations are explored as well.

Two- and three-dimensional Direct Numerical Simulation of particle-laden gravity currents

In this numerical study, we are interested in the prediction of a mono-disperse dilute suspension particle-laden flow in the typical lock-exchange configuration. The main originality of this work is that the deposition of particles is taken into account for high Reynolds numbers up to 10000, similar to the experimental ones. Unprecedented two- and three-dimensional Direct Numerical Simulations (DNS) are undertaken with the objective to investigate the main features of the flow such as the temporal evolution of the front location, the sedimentation rate, the resulting streamwise deposit profiles, the wall shear velocity as well as the complete energy budget calculated without any approximations for the first time. It is found that the Reynolds number can influence the development of the current front. Comparisons between the 2D and 3D simulations for various Reynolds numbers allow us to assess which quantities of interest for the geoscientist could be evaluated quickly with a 2D simulation. We find that a 2D simulation is not able to predict accurately the previously enumerated features obtained in a 3D simulation, with maybe the exception of the sedimentation rate for which a qualitative agreement can be found.

Scaling for lobe and cleft patterns in particle-laden gravity currents

Abstract. Lobe and cleft patterns are frequently observed at the leading edge of gravity currents, including non-Boussinesq particle-laden currents such as powder snow avalanches. Despite the importance of the instability in driving air entrainment, little is known about its origin or the mechanisms behind its development. In this paper we seek to gain a better understanding of these mechanisms from a laboratory scale model of powder snow avalanches using lightweight granular material. The instability mechanisms in these flows appear to be a combination of those found in both homogeneous Boussinesq gravity currents and unsuspended granular flows, with the size of the granular particles playing a central role in determining the wavelength of the lobe and cleft pattern. When scaled by particle diameter a relationship between the Froude number and the wavelength of the lobe and cleft pattern is found, where the wavelength increases monotonically with the Froude number.

A comparison of the shear stress distribution in the bottom boundary layer of experimental density and turbidity currents

The internal stress distribution within weakly depositional turbidity currents has often been assumed to be similar to saline gravity currents. This assumption is investigated by analyzing a series of experiments to quantify and compare the shear stress distribution in the bottom boundary layer (BBL) of saline and particle-laden gravity currents. Vertical profiles of Reynolds stresses, viscous stresses and turbulent kinetic energy (TKE) were obtained from the mean downstream velocity profiles and turbulent velocity fluctuations, and were broadly similar in both flow types, suggesting that saline gravity currents are a good analogue to turbidity currents. Maximum positive Reynolds stresses occur where the velocity gradient is largest in the BBL but below this maximum, the Reynolds stresses decrease significantly and are balanced by an increase of viscous stresses. The bulk drag coefficients CD is defined for both flows using three methods, (i) a log-fit method based on the law of the wall, (ii) the observed maximum total stress and (iii) direct measurements of turbulent velocities. The CD values of both flow types were broadly similar but each method led to CD values of different orders of magnitude. The log-fit method yielded the largest drag coefficients of O(10−2) whereas measurements of turbulent velocities gave relatively small values of O(10−4). The best correlation with drag coefficients observed in field measurements of O(10−3) was obtained by using the maximum total stresses next to the wall. The variation of CD is discussed in relation to parameterization methods in experimental and numerical modeling.

Suspended load and bed-load transport of particle-laden gravity currents: the role of particle–bed interaction

The development of particle-enriched regions (bed-load) at the base of particle-laden gravity currents has been widely observed, yet the controls and relative partitioning of material into the bed-load is poorly understood. We examine particle-laden gravity currents whose initial mixture (particle and fluid) density is greater than the ambient fluid, but whose interstitial fluid density is less than the ambient fluid (such as occurs in pyroclastic flows produced during volcanic eruptions or when sediment-enriched river discharge enters the ocean, generating hyperpycnal turbidity currents). A multifluid numerical approach is employed to assess suspended load and bed-load transport in particle-laden gravity currents under varying boundary conditions. Particle-laden flows that traverse denser fluid (such as pyroclastic flows crossing water) have leaky boundaries that provide the conceptual framework to study suspended load in isolation from bed-load transport. We develop leaky and saltation boundary conditions to study the influence of flow substrate on the development of bed-load. Flows with saltating boundaries develop particle–enriched basal layers (bed-load) where momentum transfer is primarily a result of particle–particle collisions. The grain size distribution is more homogeneous in the bed-load and the saltation boundaries increase the run-out distance and residence time of particles in the flow by as much as 25% over leaky boundary conditions. Transport over a leaky substrate removes particles that reach the bottom boundary and only the suspended load remains. Particle transport to the boundary is proportional to the settling velocity of particles, and flow dilution results in shear and buoyancy instabilities at the upper interface of these flows. These instabilities entrain ambient fluid, and the continued dilution ultimately results in these currents becoming less dense than the ambient fluid. A unifying concept is energy dissipation due to particle–boundary interaction: leaky boundaries dissipate energy more efficiently at the boundary than their saltating counterparts and have smaller run-out distance.

The Significance of Grain-Size Breaks in Turbidites and Pyroclastic Density Current Deposits

Turbidite beds and pyroclastic density current deposits commonly show abrupt internal changes in grain size with stepwise upward fining. Grain-size breaks divide a turbidite bed into a lower sandy zone, a middle silty zone, and an upper muddy zone. Zone boundaries need not correspond with Bouma division boundaries, but the sandy zone usually consists of A, B, and occasionally C divisions, the silty zone is composed of one or more of the B, C, and D divisions, and the muddy zone correlates with division E. Pyroclastic density current deposits likewise display a tripartite zonation with prominent grain-size breaks. Divergence of particle-laden gravity currents into a lower dense body and an upper dilute turbulent wake can explain the formation of the lower grain-size break. Fine particles are elutriated into the wake region by ambient entrainment through the current head, while coarse grains are transported in the body. Particle size and concentration stratification is sustained by the turbulence and velocity structure of gravity currents, with a velocity maximum and turbulence minimum also dividing currents into two regions. Two stages of deposition result. The upper grain-size break may be produced by buoyant lofting, with fine particles convected into the water column or atmosphere as the flow wanes.

A front-capture scheme for the simulation of homogeneous and particle-laden gravity currents

This paper presents a new finite volume scheme to efficiently simulate gravity currents flowing over complex surfaces. The two-dimensional shallow-water equations, with terms to account for friction and particle transport, are solved using a non-oscillatory technique. By applying a form drag at the front or head of the dense current, the scheme also implicitly captures the correct Froude number condition at the moving front as it intrudes into the less dense ambient fluid. The Froude number of the head region predicted by the numerical simulation is in good agreement with experimental results for a homogeneous current over a horizontal surface if a realistic profile drag coefficient is chosen. This new scheme avoids the development complexities of a general front-tracking scheme (e.g., handling merging fronts and multiple currents) and the computational cost of solving the full three-dimensional Euler equations while providing a fast, accurate simulation of gravity currents.

On the application of box models to particle-driven gravity currents

New laboratory experiments on different types of lock-exchange particle-driven gravity currents advancing into a flume of fresh water are presented. These include purely saline currents, monodisperse particle-laden gravity currents with both fresh and saline interstitial fluid, and bidisperse particle-laden currents. For each case a simple box model is developed. These agree well with the experimental data. We find that particulate gravity currents with saline interstitial fluid flowing into ambient fresh fluid are best described using a Froude number of 0.52 in the box model (cf. Huppert & Simpson 1980). However, particulate gravity currents with fresh interstitial fluid are best described using a higher Froude number of 0.67. The change in Froude number reflects the different shape and structure associated with the different density of interstitial fluid. For all experiments, box models provide accurate predictions for up to twenty lock-lengths.