GETTING INSIGHTS INTO THE MECHANISMS OF PARTICLES ACCELERATION FOLLOWING SHOCK WAVES PROPAGATION

The hazard of shock waves generated during volcanic eruptions is underrepresented in risk assessment. The finding of a steel pole impacted by ash particles at velocities up to 980 m/s, in an active volcanic area (Scolamacchia and Schouwenaars, 2009), suggested that shock waves could be able to accelerate the particles due to their efficient coupling with a gas phase.

We are investigating the mechanisms responsible for particles acceleration following shock waves generation.

We used a shock tube consisting of a high-pressure (HP) steel autoclave (450 mm long, 28 mm in diameter), pressurized with argon, and a low-pressure (LP) 140 mm long acrylic glass autoclave, with the same internal diameter of the HP reservoir. Shock waves were generated by the sudden decompression of Ar at atmospheric pressures through the failure of a diaphragm.

Experiments were performed with analogue particles (C-fibers and glass beads) of different sizes (150-500 microns), suspended inside the LP autoclave, or at different distances from the nozzle exit, at P_res/P_amb ratios between 150:1 and 180:1. The condensation front associated with the shock wave propagation was recorded with a high-speed camera (30,000 to 50,000 fps) and piezoelectric sensors. This front attained a maximum velocity of 788 m/s, which is in the range of velocities of ash impacts on the original steel pole, and decreased to 524 m/s at distances of 0.5 ±0.2 cm. Following the condensation front, C-fibers up to 210 microns exhibited large accelerations, with velocities that vary from few tens of m/s up to 479 (±0.5) m/s, at distances of 1.5 (±0.3) cm, in times of 0.1 ms. This drag did not occur for 500 microns glass beads, which were displaced only later by Ar gas exhausting from the system.

These results agree with other experimental and theoretical studies on gas- particle coupling. Nevertheless, other mechanisms for particles acceleration still have to be explored.

Understanding the mechanisms responsible for particles acceleration in the presence of shock waves, as well as the temporal and spatial scale at which they occur, will contribute to improve our current knowledge on the hazard related to these phenomena.