Report on the Program “Fluid-mediated particle transport in geophysical flows” at the Kavli Institute for Theoretical Physics, UC Santa Barbara, September 23 to December 12, 2013

International audienceThe KITP program held at UC Santa Barbara in the fall of 2013 addressed the dynamics of dispersed particulate flows in the environment. By focusing on the prototypes of Aeolian transport and turbidity currents, it aimed to establish the current state of our understanding of such two-phase flows, to identify key open questions, and to develop collaborative research strategies for addressing these questions. Here we provide a brief summary of the program outcome. Introduction Flows of a continuous fluid phase containing dispersed particles represent a ubiquitous phenomenon, with numerous applications in nature and technology. They can give rise to a great variety of qualitatively distinct flow regimes governed by different balances of inertial, viscous, gravitational and interparticle forces, depending on such aspects as the density ratio between particles and fluid, the nature of the particle-particle interactions, on whether the flows are dilute or concentrated, conservative or nonconservative, and Newtonian or non-Newtonian in nature, to name just a few. Even the narrower field of geophysical particle-laden flows covers a wide variety of phenomena, ranging from Aeolian transport, dust storms and powder snow avalanches to volcanic ash plumes, sediment transport in rivers, estuaries and oceans, and dense pyroclastic and debris flows. While all of the above flows have distinctly different features, they nevertheless share a number of common aspects as well. To advance our capabilities to describe flows of this nature, the community will have to draw heavily on such fundamental research areas as the physics of suspensions and granular flows. The KITP program aimed to review the current state of our understanding of such flows, to identify the key open questions that remain, and to develop collaborative research strategies for addressing these questions via a combination of laboratory experiments, computational investigations and field observations

Propagation and deposition of non-circular finite release particle-laden currents

The dynamics of non-axisymmetric turbidity currents is considered here for a range of Reynolds numbers of O(10^4) when based on the initial height of the release. The study comprises a series of experiments and highly resolved simulations for which a finite volume of particle-laden solution is released into fresh water. A mixture of water and polystyrene particles of mean diameter dp=300 μm and mixture density ρc=1012 kg/m^3 is initially confined in a hollow cylinder at the centre of a large tank filled with fresh water. Cylinders with two different cross-sectional shapes, but equal cross-sectional areas, are examined: a circle and a rounded rectangle in which the sharp corners are smoothened. The time evolution of the front is recorded as well as the spatial distribution of the thickness of the final deposit via the use of a laser triangulation technique. The dynamics of the front and final deposit are significantly influenced by the initial geometry, displaying substantial azimuthal variation especially for the rectangular case where the current extends farther and deposits more particles along the initial minor axis of the rectangular cross-section. Several parameters are varied to assess the dependence on the settling velocity, initial height aspect ratio and volume fraction. Even though resuspension is not taken into account in our simulations, good agreement with experiments indicates that it does not play an important role in the front dynamics, in terms of velocity and extent of the current. However, wall shear stress measurements show that incipient motion of particles and particle transport along the bed are likely to occur in the body of the current and should be accounted to properly capture the final deposition profile of particles

Propagation and deposition of non-circular finite release particle-laden currents

The dynamics of non-axisymmetric turbidity currents is considered here for a range of Reynolds numbers of O (104) when based on the initial height of the release. The study comprises a series of experiments and highly resolved simulations for which a finite volume of particle-laden solution is released into fresh water. A mixture of water and polystyrene particles of mean diameter ̃=300m and mixture density ̃=1012kg/m3 is initially confined in a hollow cylinder at the centre of a large tank filled with fresh water. Cylinders with two different cross-sectional shapes, but equal cross-sectional areas, are examined: a circle and a rounded rectangle in which the sharp corners are smoothened. The time evolution of the front is recorded as well as the spatial distribution of the thickness of the final deposit via the use of a laser triangulation technique. The dynamics of the front and final deposits are significantly influenced by the initial geometry, displaying substantial azimuthal variation especially for the rectangular case where the current extends farther and deposits more particles along the initial minor axis of the rectangular cross section. Several parameters are varied to assess the dependence on the settling velocity, initial height aspect ratio, and volume fraction. Even though resuspension is not taken into account in our simulations, good agreement with experiments indicates that it does not play an important role in the front dynamics, in terms of velocity and extent of the current. However, wall shear stress measurements show that incipient motion of particles and particle transport along the bed are likely to occur in the body of the current and should be accounted to properly capture the final deposition profile of particles

New developments in the analysis of column-collapse pyroclastic density currents through numerical simulations of multiphase flows

Abstract. A granular multiphase model has been used to evaluate the action of differently sized particles on the dynamics of fountains and associated pyroclastic density currents. The model takes into account the overall disequilibrium conditions between a gas phase and several solid phases, each characterized by its own physical properties. The dynamics of the granular flows (fountains and pyroclastic density currents) has been simulated by adopting a Reynolds-averaged Navier-Stokes model for describing the turbulence effects. Numerical simulations have been carried out by using different values for the eruptive column temperature at the vent, solid particle frictional concentration, turbulent kinetic energy, and dissipation. The results obtained provide evidence of the multiphase nature of the model and describe several disequilibrium effects. The low concentration (≤5 × 10−4) zones lie in the upper part of the granular flow, above the fountain, and above the tail and body of pyroclastic density current as thermal plumes. The high concentration zones, on the contrary, lie in the fountain and at the base of the current. Hence, pyroclastic density currents are assimilated to granular flows constituted by a low concentration suspension flowing above a high concentration basal layer (boundary layer), from the proximal regions to the distal ones. Interactions among the solid particles in the boundary layer of the granular flow are controlled by collisions between particles, whereas the dispersal of particles in the suspension is determined by the dragging of the gas phase. The simulations describe well the dynamics of a tractive boundary layer leading to the formation of stratified facies during Strombolian to Plinian eruptions

Estimation of Sediment Resuspension and Deposition in Coastal Waters

The Louisiana Gulf Coast is a dynamic system of heavy influence on the cultures that live and prosper around it. Land in this area is in jeopardy of being lost. In 2017, the Coastal Protection and Restoration agency will issue a new State Master and this thesis provides a more intricate way of numerically predicting the behaviors of associated sediments. A model for the estimation of resuspension and deposition is proposed and prepared for integration into the existing model. The silt and clay fractions of the bed sediment and the sediment inflow were modeled by the widely used hydrodynamic models of Delft3D and ECOMSED, using the Young and Verhagen wave properties to obtain orbital velocities and bed shear stress. The critical shear stress for erosion was based on empirical formulas developed by van Rijn

Suspension-Driven gravity surges on horizontal surfaces: Effect of the initial shape

We present results from highly resolved direct numerical simulations of canonical (axisymmetric and planar) and non-canonical (rectangular) configurations of horizontal suspension-driven gravity surges. We show that the dynamics along the initial minor and major axis of a rectangular release are roughly similar to that of a planar and axisymmetric current, respectively. However, contrary to expectation, we observe under certain conditions the final extent of the deposit from finite releases to surpass that from an equivalent planar current. This is attributed to a converging flow of the particle-laden mixture toward the initial minor axis, a behaviour that was previously reported for scalar-driven currents on uniform slopes [31]. This flow is observed to be correlated with the travelling of a perturbation wave generated at the extremity of the longest side that reaches the front of the shortest side in a finite time. A semi-empirical explicit expression (based on established relations for planar and axisymmetric currents) is proposed to predict the extent of the deposit in the entire x-y plane. Finally, we observe that for the same initial volume of a suspension-driven gravity surge, a release of larger initial horizontal aspect-ratio is able to retain particles in suspension for longer periods of time

A Combined Field and Numerical Approach to Understanding Dilute Pyroclastic Density Current Dynamics and Hazard Potential: Auckland Volcanic Field, New Zealand

The most dangerous and deadly hazards associated with phreatomagmatic eruption in the Auckland Volcanic Field (AVF; Auckland, New Zealand) are those related to volcanic base surges - dilute, ground-hugging, particle laden currents with dynamic pressures capable of severe to complete structural damage. We use the well-exposed base surge deposits of the Maungataketake tuff ring, (Manukau coast, Auckland) to reconstruct flow dynamics and destructive potential of base surges produced during the eruption. The initial base surge(s) snapped trees up to 0.5 m in diameter near their base as far as 0.7-0.9 km from the vent. Beyond this distance the trees were encapsulated and buried by the surge in growth position. Using the tree diameter and yield strength of the wood we calculate that dynamic pressures (Pdyn) in excess of 12–35 kPa are necessary to cause the observed damage. Next we develop a quantitative model for flow of and sedimentation from a radiallyspreading, dilute pyroclastic density currents (PDCs) to determine the damage potential of the base surges produced during the early phases of the eruption and explore the implications of this potential on future eruptions in the region. We find that initial conditions with velocities on the order of 65 m s- 1, bulk density of 38 kg m-3 and initial, near-vent current thicknesses of 60 m reproduce the fieldbased Pdyn estimates and runout distances. A sensitivity analysis revealed that lower initial bulk densities result in shorter run-out distances, more rapid deceleration of the current and lower dynamic pressures. Initial velocity does not have a strong influence on run-out distance, although higher initial velocity and slope slightly decrease runout distance due to higher rates of atmospheric entrainment. Using this model we determine that for base surges with runout distances of up to 4 km, complete destruction can be expected within 0.5 km from the vent, moderate destruction can be expected up to 2 km, but much less damage is expected up to the final runout distance of 4 km. For larger eruptions (base surge runout distance 4–6 km), Pdyn of \u3e 35 kPa can be expected up to 2.5 km from source, ensuring complete destruction within this area. Moderate damage to reinforced structures and damage to weaker structures can be expected up to 6 km from source. In both cases hot ash may still cause damage due to igniting flammable materials in the distal-most regions of a base surge. This work illustrates our ability to combine field observations and numerical models to explore the depositional mechanisms, macroscale current dynamics, and potential impact of dilute PDCs. Thus, this approach may serve as a tool to understand the damage potential and extent of previous and potential future eruptions in the AVF

Efeitos causados por obstáculos na hidrodinâmica de correntes de turbidez: uma abordagem experimental

This research aims to evaluate the effects of the presence of obstacles on turbidity currents hydrodynamics. Nine physical simulations of a poorly sorted mixture of water and coal (Cvol = 5%; D50 = 47 μm) were run in a laboratory test channel with three flow discharges (5, 10 and 15 L.min-1) in three different topographic configurations: runs without obstacles; runs with three 3 cm-high obstacles and runs with three 6 cm-high obstacles. The results showed that greater height of obstacles leads to greater blockage of the flow, causing changes on vertical profiles of velocity shape, flow regime (supercritical to subcritical), geometry and flow circulation in the zone between obstacles. Obstacle height reduction by 50% led to similar behavior of the turbidity current as the no-obstacles condition. After passing over the sequence of the obstacle, the turbidity current tended to regenerate a hydrodynamic structure comparable to the no-obstacles conditions.Esta pesquisa visa avaliar os efeitos da presença de obstáculos na hidrodinâmica das correntes de turbidez. Nove simulações físicas de uma mistura mal selecionada de água e carvão (Cvol = 5%; D50 = 47 μm) foram executadas em um canal de acrílico medindo 4.00 x 0.49 x 0.24 m. Foram utilizadas três vazões de injeção (5, 10 e 25 L.min-1) em três configurações topográficas diferentes: canal sem obstáculos, canal com três obstáculos de 3 cm de altura e canal com três obstáculos de 6 cm de altura. Os resultados mostraram que quanto maior a altura dos obstáculos, maior o bloqueio do escoamento, causando alterações nos perfis verticais da forma, da velocidade, do regime de escoamento (supercrítico para subcrítico), da geometria e da zona de recirculação entre os obstáculos. A redução da altura em 50% (3 cm) indica que a corrente de turbidez desenvolveu um comportamento semelhante à condição sem obstáculos. Após ultrapassar a sequência de obstáculos, o escoamento tende a regenerar sua própria estrutura hidrodinâmica da mesma forma que a condição imediatamente anterior aos obstáculos

Characterisation of reverse grading in ignimbrites through image analysis and experimental granular currents

Pyroclastic density currents (PDCs) are hot, density-driven fast-moving flows of gas, rock and ash produced by volcanic events such as explosive eruptions, the fallback of eruption columns or the collapse of lava domes. They are deadly geological hazards which have caused >90 000 deaths since 1600 AD. We must improve our understanding of PDCs and their deposits to improve our ability to prepare for future events. PDCs are rarely observed up close due to their hazardous nature and as such real time analysis is difficult. Through the use of models and the interpretations of deposits, known as ignimbrites, we can improve our understanding of the flow dynamics of PDCs. The deposits of PDCs can provide important information about how these deadly volcanic hazards behave in time and space. Reverse grading of clasts is often observed in these deposits and can be interpreted in different ways such as growing eruption intensity where larger clasts are introduced over time. Alternatively, it could record kinematic sorting (the ‘muesli effect’) where small grains percolate downwards and large grains rise. The link between current dynamics and reverse grading is previously untested in aerated granular currents.This study used aerated granular currents created in an analogue flume to investigate how reverse grading may be related to kinematic sorting. These experiments are complemented by sedimentological characterisation of ignimbrites through image analysis along with static tests of kinematic sorting. Our results show that aerated currents are stratified through kinematic sorting whereby larger grains are carried towards the top of a current and smaller grains are closer to the base. Stratification of the current controls the composition of the flow boundary zone and therefore the clasts which are able to deposit. Through quantitative analysis, we show that kinematic sorting during flow is directly linked to creating reversely graded deposits