Guillaume Carazzo

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 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.

On the rise of turbulent plumes : Quantitative effects of variable entrainment for submarine hydrothermal vents, terrestrial and extra terrestrial explosive volcanism

[1]xa0The maximum height reached by a turbulent plume rising in a stratified environment is a key tool to estimate the flux released at its source, particularly for large-scale flows because flux can often be very hard to measure directly. This height is known to be mainly controlled by the stratification of the ambient fluid, source buoyancy flux, and the efficiency of turbulent mixing between the plume and the external fluid. The latter effect has been only superficially explored in spite of its fundamental control on the dynamics. Here we show that commonly used one-dimensional models incorporating a constant entrainment coefficient do not provide satisfying predictions. We propose a new model allowing for variable entrainment which gives excellent predictions of maximum heights reached by laboratory plumes in stratified environments. We then apply our formalism to natural plumes produced by explosive volcanic eruptions under terrestrial, paleo-Martian, and Venusian conditions and by submarine hydrothermal activity at mid-ocean ridges. Source mass discharge rates deduced from maximum volcanic column heights for terrestrial eruptions are found to be greater than those estimated with the commonly used constant entrainment parameterization by a factor of 2. In the paleo-Martian atmosphere, existing models overestimate plume heights by 14–27%. In the current atmosphere of Venus, the maximum height reached by a volcanic plume is also found to be smaller than previously estimated for large eruption rates. The source heat flux released by the TAG field (Atlantic Ocean) deduced from several submarine hydrothermal plumes is found greater by a factor 3 with our model.

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.

Dynamics of the major plinian eruption of Samalas in 1257 A.D. (Lombok, Indonesia)

The 1257xa0A.D. caldera-forming eruption of Samalas (Lombok, Indonesia) was recently associated with the largest sulphate spike of the last 2xa0ky recorded in polar ice cores. It is suspected to have impacted climate both locally and at a global scale. Extensive fieldwork coupled with sedimentological, geochemical and physical analyses of eruptive products enabled us to provide new constraints on the stratigraphy and eruptive dynamics. This four-phase continuous eruption produced a total of 33–40xa0km3 dense rock equivalent (DRE) of deposits, consisting of (i) 7–9xa0km3 DRE of pumiceous plinian fall products, (ii) 16xa0km3 DRE of pyroclastic density current deposits (PDC) and (iii) 8–9xa0km3 DRE of co-PDC ash that settled over the surrounding islands and was identified as far as 660xa0km from the source on the flanks of Merapi volcano (Central Java). Widespread accretionary lapilli-rich deposits provide evidence of the occurrence of a violent phreatomagmatic phase during the eruption. With a peak mass eruption rate of 4.6u2009×u2009108u2009kg/s, a maximum plume height of 43xa0km and a dispersal index of 110,500xa0km2, the 1257xa0A.D. eruption stands as the most powerful eruption of the last millennium. Eruption dynamics are consistent with an efficient dispersal of sulphur-rich aerosols across the globe. Remarkable reproducibility of trace element analysis on a few milligrammes of pumiceous tephra provides unequivocal evidence for the geochemical correlation of 1257xa0A.D. proximal reference products with distal tephra identified on surrounding islands. Hence, we identify and characterise a new prominent inter-regional chronostratigraphic tephra marker.

The recent Plinian explosive activity of Mt. Pelée volcano (Lesser Antilles): The P1 AD 1300 eruption

Plinian explosive eruptions represent a major volcanic hazard in the Lesser Antilles Arc that must be carefully assessed based on reconstructions of past activity. The present study focusses on a detailed time evolution of the P1 eruption (AD 1300) at Mt Pelée volcano (Martinique). After an initial dome-forming stage, a Plinian phase commenced. The P1 Plinian-style sequence is mostly a pumice fall deposit with an inversely graded base, interbedded with a surge deposit, and overlain by final flow/surge deposit. Field data on deposit dispersal, thickness, and grain-size distribution are used together with physical models to reconstruct the dynamical evolution of this eruption. We find that the mass eruption rate increased from 2×107 to 9×107xa0kgxa0su2009−u20091, producing a 19–22-km-high Plinian plume, initially stable but which ultimately collapsed to form a ~1.3-km-high fountain. Empirical models of deposit thinning suggest that the minimum volume of pyroclastic deposits is 0.15xa0km3, about 25xa0% that previously estimated. Published data on magmatic water contents in glass inclusions are used together with mass discharge rates to elucidate the mechanisms leading to column collapse. Conditions at the base of the column were close to the plume/fountain transition soon after the Pelean/Plinian-style transition due to the competing effects of increase in both gas content and mass discharge rate. After a short stage of partial collapse, the column underwent a total collapse due to an increasing discharge rate.

Turbulent Entrainment Into Volcanic Plumes: New Constraints From Laboratory Experiments on Buoyant Jets Rising in a Stratified Crossflow

Predictions for the heights and downwind trajectories of volcanic plumes using integral models are critical for the assessment of risks and climate impacts of explosive eruptions, but are strongly influenced by parameterizations for turbulent entrainment. We compare four popular parameterizations using small scale laboratory experiments spanning the large range of dynamical regimes in which volcanic eruptions occur. We reduce uncertainties on the wind entrainment coefficient β which quantifies the contribution of wind-driven radial velocity shear to entrainment and is a major source of uncertainty for predicting plume height. We show that models better predict plume trajectories if: i) β is constant or increases with the plume buoyancy to momentum flux ratio; ii) the superposition of the axial and radial velocity shear contributions to the turbulent entrainment is quadratic rather than linear. Our results have important implications for predicting the heights and likelihood of collapse of volcanic columns.