Edouard Kaminski

Turbulent entrainment in jets with arbitrary buoyancy

Explosive volcanic jets present an unusual dynamic situation of reversing buoyancy. Their initially negative buoyancy with respect to ambient fluid first opposes the motion, but can change sign to drive a convective plume if a sufficient amount of entrainment occurs. The key unknown is the entrainment behaviour for the initial flow regime in which buoyancy acts against the momentum jet. To describe and constrain this regime, we present an experimental study of entrainment into turbulent jets of negative and reversing buoyancy. Using an original technique based on the influence of the injection radius on the threshold between buoyant convection and partial collapse, we show that entrainment is significantly reduced by negative buoyancy. We develop a new theoretical parameterization of entrainment as a function of the local (negative) Richardson number that (i) predicts the observed reduction of entrainment and (ii) introduces a similarity drift in the velocity and buoyancy profiles as a function of distance from source. This similarity drift allows us to reconcile the different estimates found in the literature for entrainment in plumes.

The size distribution of pyroclasts and the fragmentation sequence in explosive volcanic eruptions

In an explosive eruption, the atmospheric column dynamics depend strongly on the mass fraction of gas in the erupting mixture, which is fixed by fragmentation in the volcanic conduit. At fragmentation, gas present in vesicular magmatic liquid gets partitioned between a continuous phase separating magma clasts and a dispersed phase in individual bubbles within the clasts. As regards flow behavior, it is the former, continuous, gas phase which matters, and we show that its amount is determined by the fragment size. Analysis of 25 fall deposits and 37 flow deposits demonstrates that ash and pumice populations follow a power law size distribution such that N, the number of fragments with radii larger than r, is given by N r -D. D values range from 2.9 to 3.9 and are always larger than 3.0 in fall deposits. D values for pyroclastic flow deposits are systematically smaller than those of fall deposits. We show that at fragmentation the amount of continuous gas phase is an increasing function of the D value. Large D values cannot be attributed to a single fragmentation event and are due to secondary fragmentation processes. Laboratory experiments on bubbly magma and on solid pumice samples demonstrate that primary breakup leads to D values of 2.5±0.1 and that repeated fragment collisions act to increase the D value. A model for size-dependent refragmentation accounts for the observations. We propose that in a volcanic conduit, fragmentation proceeds as a sequence of events. Primary breakup releases a small amount of gas and is followed by fragment collisions. Due to refragmentation and decompression, both the mass and volume fractions of continuous gas increase. The final D value, and hence the mass fraction of continuous gas at the vent, depends on the time spent between primary fragmentation and eruption out of the vent.

The route to self-similarity in turbulent jets and plumes

The description of entrainment in turbulent free jets is at the heart of physical models of some major flows in environmental science, from volcanic plumes to the dispersal of pollutant wastes. The classical approach relies on the assumption of complete self-similarity in the flows, which allows a simple parameterization of the dynamical variables in terms of constant scaling factors, but this hypothesis remains under debate. We use in this paper an original parameterization of entrainment and an extensive review of published experimental data to interpret the discrepancy between laboratory results in terms of the systematic evolution of the dynamic similarity of the flow as a function of downstream distance from the source. We show that both jets and plumes show a variety of local states of partial self-similarity in accordance with the theoretical analysis of George (1989), but that their global evolution tends to complete self-similarity via a universal route. Plumes reach this asymptotic regime faster than jets which suggests that buoyancy plays a role in more efficiently exciting large-scale modes of turbulence.

Laminar starting plumes in high-Prandtl-number fluids

Experimental studies of laminar axisymmetric starting plumes are performed to investigate the dependence of the flow on the Prandtl number, focusing on large Prandtl numbers. Thermal plumes are generated by a small electric heater in a glass tank filled with viscous oils. Prandtl numbers in the range of 7-10 4 were investigated. Experimental conditions are such that viscosity variations due to temperature differences are negligible. Plumes ascend in two different regimes as a function of distance to source. At short distances, the plumes accelerate owing to the development of the viscous boundary layer. At distances larger than about five times the heater size, the ascent velocity is constant and increases as a function of the Prandtl number, as predicted by theory for steady plumes. This velocity is, within experimental error, proportional to the steady plume centreline velocity.

Expansion and quenching of vesicular magma fragments in Plinian eruptions

The conditions of pumice generation in Plinian eruptions are studied. A physical model describes the behavior of gas bubbles in a magma fragment which is carried upward in a volcanic conduit and an atmospheric eruption column. The effects of pressure release and cooling are calculated for a range of eruption conditions. The magma fragment expands in the conduit and stops expanding soon after leaving the vent, when a thin viscous rind forms against the cold mixture of magmatic gas and air. This rind prevents further volume changes. Pumice vesicularity is a function of the decompression rate in the conduit, which depends on ascent velocity and fragmentation depth. It is also sensitive to the cooling rate in the atmospheric column, which depends on vent radius and mass discharge rate. Different fragments follow different trajectories in the column and are subjected to different cooling rates. This generates a range of vesicularities which reflects the eruptive conditions. All else being equal, pumice vesicularity increases as magma viscosity decreases. These predictions are consistent with observations. Pumices provide quantitative constraints on conduit flow conditions and mass discharge rate. These concepts are applied to two Plinian eruptions. Vesicularity values for the Bishop Tuff, Long Valley caldera, require a mass discharge rate between 108 and 109 kg s−1. Vesicularity variations during Plinian phase 1 of the Minoan eruption, Santorini, are explained by the conduit radius increasing from about 30 m to 120 m. Both cases require large decompression rates in the eruption conduit, suggesting that flow pressures were close to lithostatic values.

Lithosphere structure beneath the Phanerozoic intracratonic basins of North America

Four intracratonic basins of North America, the Hudson Bay, Michigan, Illinois and Williston basins, have similar ages and are close to one another. Yet, they exhibit different subsidence histories characterised by different time-scales and sediment thicknesses. They can be explained by local lithosphere thinning and by the cooling of the induced thermal anomaly. Within the framework of 1D thermal models for vertical heat transport, each basin requires a different lithosphere thickness or a different boundary condition at the base of the lithosphere. Heat flow and seismic studies show that, beneath the North American craton, the lithosphere is too thick for the assumption of purely vertical heat transfer to be valid. Thermal models are developed to account for finite thermal anomaly width and for two types of basal boundary conditions, fixed temperature or fixed heat flux. Different subsidence histories are explained by deep lithospheric anomalies of different sizes. The stability of thick continental roots requires the mantle part of the lithosphere to be compositionally buoyant with respect to ‘normal’ convecting mantle. Localised lithospheric thinning, due for example to plume penetration, results in the emplacement of compositionally denser mantle into the lithosphere. This represents a load which drives permanent flexure. The cooling time and the characteristics of flexure allow constraints on the dimensions of these deep lithospheric anomalies. There are no solutions for lithosphere thicknesses less than 170 km. The Williston and Illinois basins are associated with wide (V200 km) and thin anomalies (V100 km), whereas the Michigan and Hudson Bay are located on top of narrow (V100 km) and tall (V200 km) anomalies. fl 2000 Elsevier Science B.V. All rights reserved.

An experimental study of the surface thermal signature of hot subaerial isoviscous gravity currents: Implications for thermal monitoring of lava flows and domes

Received 22 July 2011; revised 23 November 2011; accepted 23 November 2011; published 7 February 2012. [1] Management of eruptions requires a knowledge of lava effusion rates, for which a safe thermal proxy is often used. However, this thermal proxy does not take into account the flow dynamics and is basically time-independent. In order to establish a more robust framework that can link eruption rates and surface thermal signals of lavas measured remotely, we investigate the spreading of a hot, isoviscous, axisymmetric subaerial gravity current injected at constant rate from a point source onto a horizontal substrate. We performed laboratory experiments and found that the surface thermal structure became steady after an initial transient. We develop a theoretical model for a spreading fluid cooled by radiation and convection at its surface that also predicts a steady thermal regime. We show that, despite the model’s simplicity relative to lava flows, it yields the correct order of magnitude for the effusion rate required to produce the radiant flux measured on natural lava flows. For typical thermal lava properties and an effusion rate between 0.1 and 10 m 3 s � 1 , the model predicts a steady radiated heat flux ranging from 10 8 to 10 10 W. The assessed effusion rate varies quasi-linearly with the steady heat flux, with much weaker dependence on the flow viscosity. This relationship is valid only after a transient time which scales as the diffusive time, ranging from a few days for small basaltic flows to several years for lava domes. The thermal proxy appears thus less reliable to follow sharp variations of the effusion rate during an eruption.

Marginal stability of atmospheric eruption columns and pyroclastic flow generation

Explosive volcanic eruptions frequently generate fall and flow deposits simultaneously, which can be attributed to a marginally stable atmospheric column in transitional conditions between the buoyant and collapse regimes. This behavior is reproduced by laboratory experiments and numerical simulations. Ten well-documented eruptions are used to test theoretical models of explosive eruptions. Three types of deposits, fall, flow, and composite deposits made of intercalated flow and fall units, are observed in these eruptions. Estimates of mass discharge rate and initial volatile concentration in the magma are available for each eruptive phase. Using the simple assumptions that (1) the mass fraction of gas in the mixture is equal to the initial volatile content of magma and (2) jet expansion outside the vent is unconstrained by crater dimensions, theoretical predictions are not consistent with the data. Agreement between data and theory may be achieved by appealing to imperfect degassing of pyroclasts, which lowers the gas content of the erupted mixture. The effective amount of continuous gas phase carrying pyroclasts in suspension depends on the size distribution of pyroclasts. In coarse pyroclast populations a large amount of magmatic gas remains trapped in bubbles within the pyroclasts and is not involved in the bulk volcanic flow. A new regime diagram based on estimates of the effective gas content in the erupted mixture allows good agreement with the observations. For given mass flux and initial dissolved volatile content, changes of the size distribution of pyroclasts may have a strong effect on atmospheric column behavior.

Modeling olivine CPO evolution with complex deformation histories—Implications for the interpretation of seismic anisotropy in the mantle

Relating seismic anisotropy to mantle flow requires detailed understanding of the development and evolution of olivine crystallographic preferred orientation (CPO). Recent experimental and field studies have shown that olivine CPO evolution depends strongly on the integrated deformation history, which may lead to differences in how the corresponding seismic anisotropy should be interpreted. In this study, two widely used numerical models for CPO evolution—D-Rex and VPSC—are evaluated to further examine the effect of deformation history on olivine texture and seismic anisotropy. Building on previous experimental work, models are initiated with several different CPOs to simulate unique deformation histories. Significantly, models initiated with a preexisting CPO evolve differently than the CPOs generated without preexisting texture. Moreover, the CPO in each model evolves differently as a function of strain. Numerical simulations are compared to laboratory experiments by Boneh and Skemer (2014). In general, the D-Rex and VPSC models are able to reproduce the experimentally observed CPOs, although the models significantly over-estimate the strength of the CPO and in some instances produce different CPO from what is observed experimentally. Based on comparison with experiments, recommended parameters for D-Rex are: M* = 10, λ* = 5, and χ = 0.3, and for VPSC: α = 10–100. Numerical modeling confirms that CPO evolution in olivine is highly sensitive to the details of the initial CPO, even at strains greater than 2. These observations imply that there is a long transient interval of CPO realignment which must be considered carefully in the modeling or interpretation of seismic anisotropy in complex tectonic settings.

Laboratory experiments of forced plumes in a density‐stratified crossflow and implications for volcanic plumes

The mass eruption rate feeding a volcanic plume is commonly estimated from its maximum height. Winds are known to affect the column dynamics causing bending and hence reducing the maximum plume height for a given mass eruption rate. However, the quantitative predictions including wind effects on mass eruption rate estimates are not well constrained. To fill this gap, we present a series of new laboratory experiments on forced plumes rising in a density-stratified crossflow. We identify three dynamical regimes corresponding to increasing effect of wind on the plume rise. The transition from one regime to another is governed by two dimensionless velocity scales defined as a function of source and environmental parameters. The results are found consistent with the conditions of historical eruptions and provide new empirical relationships to estimate mass eruption rate from plume height in windy conditions, leading to valuable tools for eruption risk assessment.