Jeremy C. Phillips

Entrainment into two-dimensional and axisymmetric turbulent gravity currents

Entrainment of ambient fluid into both two-dimensional and axisymmetric gravity currents is investigated experimentally using a novel neutralization technique. The technique involves the titrative neutralization of an alkaline gravity current which intrudes into and entrains an acidic ambient, and is visualized using a pH indicator solution. Using this technique, we can determine quantitative results for the amount of dilution in the head of the current. The head of the current is able to entrain ambient fluid both by shear instabilities on the current/ambient interface and by over-riding (relatively light) ambient fluid. Guided by our experimental observations, we present two slightly different theoretical models to determine the entrainment into the head of the current as a function of distance from the source for the instantaneous release of a constant volume of fluid in a two-dimensional geometry. By dimensional analysis, we determine from both models that the dimensionless entrainment or dilution ratio, E , defined as the ratio of the volumes of ambient and original fluid in the head, is independent of the initial reduced gravity of the current; and this result is confirmed by our experiments in Boussinesq situations. Our theoretical evaluation of E in terms of the initial cross-sectional area of the current agrees very well with our experimental measurements on the incorporation of an entrainment coefficient α, evaluated experimentally to be 0.063 ± 0.003. We also obtain experimental results for constant-volume gravity currents moving over horizontal surfaces of varying roughness. A particularly surprising result from all the experiments, which is reflected in the theoretical models, is that the head remains essentially unmixed – the entrainment is negligible – in the slumping phase. Thus the heads of gravity currents with identical initial cross-sectional areas but different initial aspect ratios (lock lengths) will begin to be diluted by ambient fluid at different positions and hence propagate at different rates. A range of similar results is determined, both theoretically and experimentally, for the instantaneous release of a fixed volume of (heavy) fluid in an axisymmetric geometry. By contrast, the results of our experiments with gravity currents fed by a constant flux exhibit markedly different entrainment dynamics due to the continual replenishment of the fluid in the head by the constant input of undiluted fluid from the tail.

Sediment-laden gravity currents with reversing buoyancy

There are many natural occurrences of sediment-laden gravity currents in which the density of the interstitial fluid is less than that of the ambient fluid, although the bulk density of the current is greater. Such currents are driven by the excess density of suspended particles. However, after sufficient particles have sedimented, the current will become buoyant, cease its lateral motion and ascend to form a plume. Examples of such currents include brackish underflows in deltas, turbidity currents and pyroclastic flows. Experimental studies are described which show that, due to sedimentation, sediment-laden gravity currents decelerate more rapidly than saline currents of the same density. There is little difference in the experiments between a sediment-laden current with neutrally buoyant interstitial fluid and one with buoyant interstial fluid until sufficient sediment has been lost to cause the latter kind of current to lift-off. A marked deceleration is then observed and a plume is generated, with lift-off occurring along the length of the current. The resulting buoyant plume then generates a gravity current below the upper surface of the fluid in the tank. The deposit from a current with buoyant fluid shows a fairly abrupt decrease in thickness beyond the lift-off distance and has a flatter profile than that from a simple sediment current. A theoretical model is presented, which is based on the two-layer shallow-water equations and incorporates a model of the sedimentation in which particles are assumed to be uniformly suspended by the turbulence of the current. The model shows good agreement with the observed lengths of the experimental currents as a function of time and predicts the lift-off distance reasonably well. These processes have implications for the behaviour of turbidity currents, the interpretation of turbidites, mixing processes in the oceans and the lift-off of pyroclastic flows.

Gum rosin-acetone system as an analogue to the degassing behaviour of hydrated magmas

Solutions of gum rosin and acetone reproduce the volatile- and temperature-dependent viscosity, together with the phase behaviour, of hydrated magmas. A range of experimental exsolution conditions was investigated, including the variation of supersaturation, rate of decompression, solution temperature and volatile content. Degassing processese were controlled by the formation of an exsolution interface above a supersaturated liquid. The end-products ranged from a mildly degassed liquid to a solid foam, which preserved strained vesicles. Solutions of gum rosin and acetone are proposed as a suitable analogue system with which to study magma degassing processes.

Planar collapse of a granular column: experiments and discrete element simulations

The collapse of a granular column is an intriguingly simple table-top experiment which exhibits a host of interesting phenomena. Here, we introduce a planar version in which the collapsing column is only one particle deep perpendicular to the plane of motion to make observations of the internal motion possible. This configuration also particularly lends itself to comparison with discrete element simulations which are performed in tandem. Our experiments confirm that this planar system displays all the same features as collapsing cylinders and rectangular blocks. In particular, the dominant dependence on the initial parameters of the column runout is through a power law of the initial height-to-width aspect ratio. Discrete element simulations, which are found to reproduce the experimental behavior very well, are then used to analyze the velocity field of the collapse process. A predominantly linear velocity profile is found in a moving layer over an evolving static pile. The time-dependent strain rate in t...

A total volatile inventory for Masaya Volcano, Nicaragua

NERC project “Magma dynamics at persistently degassing basaltic volcanoes: A novel approach to linking volcanic gases and magmatic volatiles within a physical model” (NE/F004222/1 and NE/F005342/1).

MeMoVolc report on classification and dynamics of volcanic explosive eruptions

Classifications of volcanic eruptions were first introduced in the early twentieth century mostly based on qualitative observations of eruptive activity, and over time, they have gradually been developed to incorporate more quantitative descriptions of the eruptive products from both deposits and observations of active volcanoes. Progress in physical volcanology, and increased capability in monitoring, measuring and modelling of explosive eruptions, has highlighted shortcomings in the way we classify eruptions and triggered a debate around the need for eruption classification and the advantages and disadvantages of existing classification schemes. Here, we (i) review and assess existing classification schemes, focussing on subaerial eruptions; (ii) summarize the fundamental processes that drive and parameters that characterize explosive volcanism; (iii) identify and prioritize the main research that will improve the understanding, characterization and classification of volcanic eruptions and (iv) provide a roadmap for producing a rational and comprehensive classification scheme. In particular, classification schemes need to be objective-driven and simple enough to permit scientific exchange and promote transfer of knowledge beyond the scientific community. Schemes should be comprehensive and encompass a variety of products, eruptive styles and processes, including for example, lava flows, pyroclastic density currents, gas emissions and cinder cone or caldera formation. Open questions, processes and parameters that need to be addressed and better characterized in order to develop more comprehensive classification schemes and to advance our understanding of volcanic eruptions include conduit processes and dynamics, abrupt transitions in eruption regime, unsteadiness, eruption energy and energy balance.

Experimental observations of pressure oscillations and flow regimes in an analogue volcanic system

Gas-liquid flows, designed to be analogous to those in volcanic conduits, are generated in the laboratory using organic gas-gum rosin mixtures expanding in a vertically mounted tube. The expanding fluid shows a range of both flow and pressure oscillation behaviors. Weakly supersaturated source liquids produce a low Reynolds number flow with foam expanding from the top surface of a liquid that exhibits zero fluid velocity at the tube wall; i.e., the conventional “no-slip” boundary condition. Pressure oscillations, often with strong long-period characteristics and consistent with longitudinal and radial resonant oscillation modes, are detected in these fluids. Strongly supersaturated source liquids generate more energetic flows that display a number of flow regimes. These regimes include a static liquid source, viscous flow, detached flow (comprising gas-pockets-at-wall and foam-in-gas annular flow, therefore demonstrating strong radial heterogeneity), and a fully turbulent transonic fragmented or mist flow. Each of these flow regimes displays characteristic pressure oscillations that can be related to resonance of flow features or wall impact phenomena. The pressure oscillations are produced by the degassing processes without the need of elastic coupling to the confining medium or flow restrictors and valvelike features. The oscillatory behavior of the experimental flows is compared to seismoacoustic data from a range of volcanoes where resonant oscillation of the fluid within the conduit is also often invoked as controlling the observed oscillation frequencies. On the basis of the experimental data we postulate on the nature of seismic signals that may be measured during large-scale explosive activity.

Suppression of large-scale magma mixing by melt–volatile separation

Abstract Many volcanic eruptions are triggered by the injection of hot basic magma into a subsurface reservoir containing cooler and less dense silicic magma. As the basaltic magma cools and crystallises, it may become volatile saturated and exsolve bubbles. Here we present a new quantitative model and supporting laboratory experiments which identify that if a sufficient number of bubbles remain in suspension, then the bulk density of the basalt may fall below that of the silicic magma. However, if the basalt has sufficiently low viscosity, or the cooling rate is sufficiently small, then the bubbles can rise through the basalt, suppressing the large-scale overturn, and forming an intermediate bubbly layer at the interface with the more viscous silicic magma. As the small bubble plumes then rise from this foam and transport vesicular basalt into the upper layer, the chamber now remains density stratified.

The role of gravitational instabilities in deposition of volcanic ash

Volcanic ash is a significant hazard for areas close to volcanoes and for aviation. Gravitational instabilities forming at the bottom of spreading volcanic clouds have been observed in many explosive eruptions. Here we present the first quantitative description of the dynamics of such instabilities, and correlate this with the characteristics of the fall deposit from observations of the 4 May 2010 Eyjafjallajokull (Iceland) eruption. Gravitational instabilities initially took the form of downward-propagating fingers that formed continuously at the base of the cloud, and appeared to be advected passively at the crosswind speed. Measurements of finger propagation are consistent with initial conditions inferred from previous studies of ash cloud dynamics. Dedicated laboratory analogue experiments confirmed that finger downward propagation significantly exceeded the settling speed of individual particles, demonstrating that gravitational instabilities provide a possible mechanism for enhanced sedimentation of fine ash. Our observations challenge the view that aggregation is the primary explanation of proximal fine ash sedimentation, and give direct support for the role of gravitational instabilities in providing regions of high particle concentration that can promote aggregation.

Blocked natural ventilation: the effect of a source mass flux

We analyse the density evolution of fluid within a confined ventilated space resulting from the action of a dense turbulent plume originating at the top of the space with finite source volume flux,