Weak shock waves in the eruption column

Livshits, L. D.; Bolkhovitinov, L. G.

doi:10.1038/267420a0

Cited by 11 publications

(6 citation statements)

References 2 publications

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“…The internal structure of volcanic jets, particularly the steady shock and rarefaction waves that arise when the jet is underexpanded (as described in section 4.3), is difficult to document, both due to the paucity of field observations and because of the opacity of volcanic fluids. Reported internal waves (at Tolbatchik, USSR [Livshits and Bolkhovitinov, 1977]), were not the stationary waves described here, but rather were traveling surge waves attributed to oscillations of exit flow velocities above and below sonic. Actually, conclusive evidence for supersonic flow and internal waves would be the simple observation of substantial jet pluming at the conduit exit, due to Prandtl-Meyer expansion and subsequent inward curvature of the jet boundaries' (sections 2.3 and 2.4).…”

Section: Steady State Jet Structurementioning

confidence: 66%

Laboratory studies of volcanic jets

Kieffer¹,

Sturtevant²

1984

J. Geophys. Res.

140

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The study of the fluid dynamics of violent volcanic eruptions by laboratory experiment is described, and the important fluid‐dynamic processes that can be examined in laboratory models are discussed in detail. In preliminary experiments, pure gases are erupted from small reservoirs. The gases used are Freon 12 and Freon 22, two gases of high molecular weight and high density that are good analogs of heavy and particulate‐laden volcanic gases; nitrogen, a moderate molecular weight, moderate density gas for which the thermodynamic properties are well known; and helium, a low molecular weight, lowdensity gas that is used as a basis for comparison with the behavior of the heavier gases and as an analog of steam, the gas that dominates many volcanic eruptions. Transient jets erupt from the reservoir into the laboratory upon rupture of a thin diaphragm at the exit of a convergent nozzle. The gas accelerates from rest in the reservoir to high velocity in the jet. Reservoir pressures and geometries are such that the fluid velocity in the jets is initially supersonic and later decays to subsonic. The measured reservoir pressure decreases as the fluid expands through repetitively reflecting rarefaction waves, but for the conditions of these experiments, a simple steady‐discharge model is sufficient to explain the pressure decay and to predict the duration of the flow. Density variations in the flow field have been visualized with schlieren and shadowgraph photography. The observed structure of the jet is correlated with the measured pressure history. The starting vortex generated when the diaphragm ruptures becomes the head of the jet. Though the exit velocity is sonic, the flow head in the helium jet decelerates to about one‐third of sonic velocity in the first few nozzle diameters, the nitrogen head decelerates to about three‐fourths of sonic velocity, while Freon maintains nearly sonic velocity. The impulsive acceleration of reservoir fluid into the surrounding atmosphere produces a compression wave. The strength of this wave depends primarily on the sound speed of the fluid in the reservoir but also, secondarily with opposite effect, on the density: helium produces a relatively strong atmospheric shock while the Freons do not produce any optically observable wave front. Well‐formed N waves are detected with a microphone far from the reservoir. Barrel shocks, Mach disks, and other familiar features of steady underexpanded supersonic jets form inside the jet almost immediately after passage of the flow head. These features are maintained until the pressure in the reservoir decays to sonic conditions. At low pressures the jets are relatively structureless. Gas‐particle jets from volcanic eruptions may behave as pseudogases if particle concentrations and mass and momentum exchange between the components are sufficiently small. The sound speed of volcanic pseudogases can be as large as 1000 m s−1 or as small as a few tens of meters per second depending on the mass loading and initial temperature. Fluids of high sound speed ...

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Section: Steady State Jet Structurementioning

confidence: 66%

Laboratory studies of volcanic jets

Kieffer¹,

Sturtevant²

1984

J. Geophys. Res.

140

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“…A similar phenomenon observed during the 1975 Ngauruhoe volcano eruption has been explained in terms of condensation cloud induced by the propagation of shock waves in the atmosphere causing the sudden change of the water phase, similarly to what observed during nuclear‐test explosions [ Nairn , ]. Sometimes in eruptive clouds, weak shock waves can be also generated as a result of the local change of the ash‐rich plume density, as during the 1975 Tolbatchik eruption [ Livshits and Bolkhovitinov , ].…”

Section: Flashing Arcs: How Sound Becomes Visiblementioning

confidence: 99%

Acoustic wavefield and Mach wave radiation of flashing arcs in strombolian explosion measured by image luminance

Genco

Ripepe

Marchetti

et al. 2014

Geophysical Research Letters

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Explosive activity often generates visible flashing arcs in the volcanic plume considered as the evidence of the shock-front propagation induced by supersonic dynamics. High-speed image processing is used to visualize the pressure wavefield associated with flashing arcs observed in strombolian explosions. Image luminance is converted in virtual acoustic signal compatible with the signal recorded by pressure transducer. Luminance variations are moving with a spherical front at a 344.7 m/s velocity. Flashing arcs travel at the sound speed already 14 m above the vent and are not necessarily the evidence of a supersonic explosive dynamics. However, seconds later, the velocity of small fragments increases, and the spherical acousto-luminance wavefront becomes planar recalling the Mach wave radiation generated by large scale turbulence in high-speed jet. This planar wavefront forms a Mach angle of 55°with the explosive jet axis, suggesting an explosive dynamics moving at M o = 1.22 Mach number.

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“…Thus shocks can be initiated by catastrophic vent widening at the beginning or during eruptive activity. Furthermore, atmospheric shock waves related to volcanic activity have been described at Hekla [Thorarinsson and Sigvaldason, 1972] and Mount Pe16e [Perret, 1935] as flashing arcs and as condensation fronts at Ngauruhoe [Nairn, 1976] and at Tolbatchik, Kamchatka [Livshits and Bolkhoritinov, 1977]. The shock tube interpretation is especially useful for describing the effects of individual explosions occurring during eruptive sequences (e.g., Krakatoa), a phenomenon that is difficult to assess from interpretation of stratigraphy.…”

Section: Discussionmentioning

confidence: 99%

Hydrodynamic Aspects of Caldera‐Forming Eruptions' Numerical Models

Wohletzt¹,

McGetchin²,

SandforID³

et al. 1984

Collected Reprint Series

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Comparison of results from a two-dimensional numerical eruption simulation (KACHINA) to calculations based upon a shock tube analog supports the conclusion that the hydrodynamics during the initial minutes of large caldera-forming ash flow eruptions may be dominated by blast wave phenomena. Field evidence for this phenomenology is pyroclastic surge deposits commonly occurring both directly below caldera-related ash flow sheets, on top of a preceding Plinian fall deposit (ground surge), and separating individual ash flow units. We model the eruption of the Tshirege member of the Bandelier Tuff (1.1 Ma B.P.) from the Vailes caldera, New Mexico. In the model a magma chamber at 100 MPa (1 kbar) and 800øC is volatile rich, with an average H20 abundance above saturation greater than 8.7 wt % increasing to nearly 100 wt % near the very top of the chamber. Using a shock tube analogy, decompression of the chamber through a wide-open dikelike vent 0.1 km wide and 1 to 5 km long forms a shock wave of 3 MPa (= 30 atm) with a velocity greater than 1.0 km s-•. Steady flow of material erupted from the vent begins after 20 to 100 s based upon a 7-km depth from the ground surface to a reflective (density) boundary in the chamber and a rarefaction wave velocity of 100 to 600 m s-x. The velocity of the ash front behind the shock wave is 300 to 500 m s-x. The shock tube model serves as a basis to evaluate the consistency of the KACHINA code results which are similar to a one-dimensional problem along the symmetry axis. The results of the KACHINA simulation show in some detail the effect of multiple reservoir rarefaction reflections and possibly Prandtl-Meyer expansion in generating compressive wave fronts following the initial shock. The rarefaction resonance not only prolongs unsteady flow in the vent but tends to promote surging flow of ash behind the leading shock. Furthermore, these results are consistent with a blast wave characterized as a shock front followed by one or more pulses of entrained ash. The blast wave shocks ambient air to higher pressures and temperatures, the magnitudes of which depend strongly on the initial chamber overpressure, distance, and direction from the vent. In consideration of volcanic hazards our numerical model shows that a shock wave compressed the atmosphere to pressures of •-0.2 to 0.7 MPa (2-7 atm) and temperatures of •-200 ø to 300øC for distances to 10 km from the Bandelier vent(s).

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Weak shock waves in the eruption column

Cited by 11 publications

References 2 publications

Laboratory studies of volcanic jets

Laboratory studies of volcanic jets

Acoustic wavefield and Mach wave radiation of flashing arcs in strombolian explosion measured by image luminance

Hydrodynamic Aspects of Caldera‐Forming Eruptions' Numerical Models

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