Modelling Settling-Driven Gravitational Instabilities at the Base of Volcanic Clouds Using the Lattice Boltzmann Method

Field observations and laboratory experiments have shown that ash sedimentation can be significantly affected by collective settling mechanisms that promote premature ash deposition, with important implications for dispersal and associated impacts. Among these mechanisms, settling-driven gravitational instabilities result from the formation of a gravitationally-unstable particle boundary layer (PBL) that grows between volcanic ash clouds and the underlying atmosphere. The PBL destabilises once it reaches a critical thickness characterised by a dimensionless Grashof number, triggering the formation of rapid, downward-moving ash fingers that remain poorly characterised. We simulate this process by coupling a Lattice Boltzmann model, which solves the Navier-Stokes equations for the fluid phase, with a Weighted Essentially Non Oscillatory (WENO) finite difference scheme which solves the advection-diffusion-settling equation describing particle transport. Since the physical problem is advection dominated, the use of the WENO scheme reduces numerical diffusivity and ensures accurate tracking of the temporal evolution of the interface between the layers. We have validated the new model by showing that the simulated early-time growth rate of the instability is in very good agreement with that predicted by linear stability analysis, whilst the modelled late-stage behaviour also successfully reproduces quantitative results from published laboratory experiments. The results show that the model is capable of reproducing both the growth of the unstable PBL and the non-linear dependence of the fingers’ vertical velocity on both the initial particle concentration and the particle diameter. Our validated model is used to expand the parameter space explored experimentally and provides key insights into field studies. Our simulations reveal that the critical Grashof number for the instability is about ten times larger than expected by analogy with thermal convection. Moreover, as in the experiments, we found that instabilities do not develop above a given particle threshold. Finally, we quantify the evolution of the mass of particles deposited at the base of the numerical domain and demonstrate that the accumulation rate increases with time, while it is expected to be constant if particles settle individually. This suggests that real-time measurements of sedimentation rate from volcanic clouds may be able to distinguish finger sedimentation from individual particle settling.

The Influence of Particle Concentration on the Formation of Settling-Driven Gravitational Instabilities at the Base of Volcanic Clouds

Settling-driven gravitational instabilities observed at the base of volcanic ash clouds have the potential to play a substantial role in volcanic ash sedimentation. They originate from a narrow, gravitationally unstable region called a Particle Boundary Layer (PBL) that forms at the lower cloud-atmosphere interface and generates downward-moving ash fingers that enhance the ash sedimentation rate. We use scaled laboratory experiments in combination with particle imaging and Planar Laser Induced Fluorescence (PLIF) techniques to investigate the effect of particle concentration on PBL and finger formation. Results show that, as particles settle across an initial density interface and are incorporated within the dense underlying fluid, the PBL grows below the interface as a narrow region of small excess density. This detaches upon reaching a critical thickness, that scales with (ν2/g′)1/3, where ν is the kinematic viscosity and g′ is the reduced gravity of the PBL, leading to the formation of fingers. During this process, the fluid above and below the interface remains poorly mixed, with only small quantities of the upper fluid phase being injected through fingers. In addition, our measurements confirm previous findings over a wider set of initial conditions that show that both the number of fingers and their velocity increase with particle concentration. We also quantify how the vertical particle mass flux below the particle suspension evolves with time and with the particle concentration. Finally, we identify a dimensionless number that depends on the measurable cloud mass-loading and thickness, which can be used to assess the potential for settling-driven gravitational instabilities to form. Our results suggest that fingers from volcanic clouds characterised by high ash concentrations not only are more likely to develop, but they are also expected to form more quickly and propagate at higher velocities than fingers associated with ash-poor clouds.

Giant Molecular Cloud Formation at the Interface of Colliding Supershells in the Large Magellanic Cloud

We investigate the H i envelope of the young, massive GMCs in the star-forming regions N48 and N49, which are located within the high column density H i ridge between two kpc-scale supergiant shells, LMC 4 and LMC 5. New long-baseline H i 21 cm line observations with the Australia Telescope Compact Array (ATCA) were combined with archival shorter baseline data and single dish data from the Parkes telescope, for a final synthesized beam size of 24.75″ by 20.48″, which corresponds to a spatial resolution of ∼ 6 pc in the LMC. It is newly revealed that the H i gas is highly filamentary, and that the molecular clumps are distributed along filamentary H i features. In total 39 filamentary features are identified and their typical width is ∼ 21 (8–49) [pc]. We propose a scenario in which the GMCs were formed via gravitational instabilities in atomic gas which was initially accumulated by the two shells and then further compressed by their collision. This suggests that GMC formation involves the filamentary nature of the atomic medium.

Gravitational instability and the formation of planetary systems of solar-type stars

The fundamental principles of the protoplanetary ring model – the model of formation of planetary systems of stars, which is based on the origin and development of large-scale gravitational instabilities (protoplanetary rings) – are extended to the formation of regular planetary satellites. Based on these principles, a complete model of the formation of planetary systems, including their satellites, (model of gas and dust rings) for solar-type stars is proposed.

Star-Forming Galaxies at Cosmic Noon

Ever deeper and wider look-back surveys have led to a fairly robust outline of the cosmic star-formation history, which culminated around [Formula: see text]; this period is often nicknamed “cosmic noon.” Our knowledge about star-forming galaxies at these epochs has dramatically advanced from increasingly complete population censuses and detailed views of individual galaxies. We highlight some of the key observational insights that influenced our current understanding of galaxy evolution in the equilibrium growth picture: ▪ Scaling relations between galaxy properties are fairly well established among massive galaxies at least out to [Formula: see text], pointing to regulating mechanisms already acting on galaxy growth. ▪ Resolved views reveal that gravitational instabilities and efficient secular processes within the gas- and baryon-rich galaxies at [Formula: see text] play an important role in the early buildup of galactic structure. ▪ Ever more sensitive observations of kinematics at [Formula: see text] are probing the baryon and dark matter budget on galactic scales and the links between star-forming galaxies and their likely descendants. ▪ Toward higher masses, massive bulges, dense cores, and powerful AGNs and AGN-driven outflows are more prevalent and likely play a role in quenching star formation. We outline emerging questions and exciting prospects for the next decade with upcoming instrumentation, including the James Webb Space Telescope and the next generation of extremely large telescopes.

Evaporation-induced Rayleigh–Taylor instabilities in polymer solutions

Understanding the mechanics of detrimental convective instabilities in drying polymer solutions is crucial in many applications such as the production of film coatings. It is well known that solvent evaporation in polymer solutions can lead to Rayleigh-Bénard or Marangoni-type instabilities. Here, we reveal another mechanism, namely that evaporation can cause the interface to display Rayleigh–Taylor instabilities due to the build-up of a dense layer at the air–liquid interface. We study experimentally the onset time ( t p ) of the instability as a function of the macroscopic properties of aqueous polymer solutions, which we tune by varying the polymer concentration ( c 0 ), molecular weight and polymer type. In dilute solutions, t p shows two limiting behaviours depending on the polymer diffusivity. For high diffusivity polymers (low molecular weight), the pluming time scales as c 0 − 2 / 3 . This result agrees with previous studies on gravitational instabilities in miscible systems where diffusion stabilizes the system. On the other hand, in low diffusivity polymers the pluming time scales as c 0 − 1 . The stabilizing effect of an effective interfacial tension, similar to those in immiscible systems, explains this strong concentration dependence. Above a critical concentration, c ^ , viscosity delays the growth of the instability, allowing time for diffusion to act as the dominant stabilizing mechanism. This results in t p scaling as ( ν / c 0 ) 2/3 . This article is part of the theme issue ‘Stokes at 200 (Part 1)’.

Towards instrumental catalogs of gravitational instabilities at local and regional scales by a combined seismology and machine learning approach

<p><span>Seismology allows continuous recording of the activity of gravitational instabilities whatever the context, and is therefore able to provide a tool for the study of the spatio-temporal evolution of the activity of gravity instabilities with a unique resolution. Due to the considerable fall in the costs of the means of acquiring seismological data and the increasing densification of global, regional and local networks observed in recent years, the amount of data to be processed is growing exponentially. Thus access to information is more and more complete but in return the volume of data to be processed becomes considerable. To analyze this volume of data and extract relevant information, it is necessary to develop automatic methods of identification of seismic sources and location to quickly build the most complete seismicity catalogs possible. </span></p><p><span>We present a new machine-learning based method for automatically constructing catalogs of gravitational seismogenic events from continuous seismic data. We have developed a robust and versatile solution, which can be implemented in any context where seismic detection of landslides or other mass movements is relevant. The method is based on spectral detection of seismic signals and the identification of sources with a machine learning algorithm. Spectral detection detects signals with a low signal-to-noise ratio, while the Random Forest algorithm achieves a high rate of positive identification of seismic signals generated by landslides and other seismic sources. The processing chain is implemented to operate in parallel in a high-performance data center, which allows years of continuous seismic data to be explored and a database of events to be rapidly built up. This solution is also deployed for near-real time seismicity catalogs construction in the framework of slow moving landslides monitoring done by the Observatoire Multidisciplinaire des Instabilités de Versants (OMIV). Here we present the preliminary results of the application of this processing chain in different contexts, locally for the monitoring of slow-moving landslides (La Clapière, Super-Sauze, Séchilienne), and at the regional level for the detection of large landslides field (Alaska and Alps).</span></p>

Deformation Regimes in Cratons Caused by Gravitational Instabilities

<p>Most cratonic lithospheres are stable entities that have not been deformed since their formation in the Archean. In contrast, geological and geophysical inferences showed that North China and Wyoming Cratons have been deformed/destroyed under specific geodynamic circumstances (e.g metasomatization, slab dehydration). For instance, lithospheric roots are densified-destabilized and they may eventually sink into the mantle. Here, numerical experiments are used to investigate how high-density anomalies/eclogite in the lower crust that is varying in size, density and geometry may control the lithospheric removal process. Based on a large set of parametric numerical calculations, we first classified the lithospheric removal style (e.g localized, non-localized, high degree, and pierce through). In the case where the eclogite blocks attached to the lower crust, two different conditions develop; localized deformation and non-localized deformation occur due to the small-scale convection. Two new different removal mechanisms are evolved after the eclogite becomes detached from the lower crust; (i) pierce through mechanism subsequent to localized deformation and (ii) high-degree deformation following non-localized deformation. While the width of the eclogite block causes high-degree deformation, it is observed that with increasing thickness it leads to the formation of viscous drips. Experimental results indicate that eclogite block(s) under the cratons may still be there while creating small wavelength MOHO depth variations.</p>