Spreading angle of gas and particle jets from shock-tube experiments

Shock-tube experiments can be used to investigate the relationship between ejection dynamics and shallow plumbing system characteristics in volcanic scenario. Gas and-particle jets were generated using shock-tube rapid decompression experiments and their gas and particle spreading angles have been quantified using high-speed video image analysis. The spreading angle is defined as the angular deviation from a vertical trajectory. The experimental variables investigated include different: 1) vent geometry, 2) tube length, 3) particle load, 4) particle size distribution, 5) temperature and always 15 MPa initial overpressure in the shock-tube reservoir.

Shock-tube system
The shock-tube used to generate the gas and particle is made of Nimonic 105 alloy. For the collection of the data presented here, the shock-tube has been deployed at ambient temperature and heated with an external furnace at 500°C. for an initial overpressure 15 MPa. The shock-tube reservoir has an internal diameter of 28 mm and is 240 mm long. The original length of the tube, 240 mm, is reduced to 60 mm fixing at the bottom of the tube a full cylinder of Teflon 180 mm long. The reservoir is filled with particles and sealed from ambient conditions via three tailored metal disk and incrementally pressurized with argon gas. When the disk(s) bursts at the desired overpressure, four triangle-shaped segments bent upwards generating some internal diameter variation (between 26 and 28 mm). A pressure sensor, at the metal disk level, records the pressure drop in the reservoir and the onset of the ejection. Four vent geometries have been deployed: a nozzle with converging walls, a cylindrical and two funnel-shaped vents with 15° and 30° flaring walls, respectively. The sample used in all the experiments consists in a mechanically crushed basaltic lava, Schaumlava, original of the Eifel region, Germany. The sample is used in three distinct grain size distributions: 1-2 mm, 0.5-1 mm and 0.125-0.250 mm. The particles present natural roughness and remain solid at all stages of the experiments. Each set of experimental conditions has been repeated at least three times.

Spreading angle measurements
Gas and particle spreading angle were manually measured using the ImageJ plugin MTrackJ, both are freely available. To measure the gas spreading angle, we drew a short tangent along the visible gas right at the lip of the vent. The measurements were taken from the moment the gas exit the lip of the vent until the gas is discernible at the sides of the particle-laden jet. For the particle spreading angle, we drew a tangent starting at the lip of the vent and considering the entire vertical field of view (ca. 20 cm). The measurement was taken on both sides and averaged. We performed the first measurement of particle spreading angle where the jet filled the field of view. The particle spreading angle was measured ignoring outliers, i.e. those particles (n<10) that showed abnormal trajectories and did not allow conclusions about the source. To retrieve the actual angle with respect to the reference origin point of ImageJ, the measured angles need to be corrected by 90 and 180 if the video was analysed vertical or horizontal, respectively. The information about the correction to be used is provided in ‘2020-030_Cigala-et-al_List_of_experiments.xlsx’.