Fig. 5

Aerodynamic properties of falling ash particles obtained using high-speed camera imaging. Black circles, triangles, and crosses show the results of footages HSC1057, HSC1133, and HSC1200, respectively. a Relationship between falling velocity and diameter of ash particles. Curves represent the model of Wilson and Huang (1979), and the corresponding density and form factor (FF = (l_intermediate + l_minor)/(2 × l_major)) values are shown. Solid red curves were obtained based on density variation in the ash deposit measured using a helium pycnometer. Dashed black curves were calculated by the given constant density. Gray square shading indicates the range in particle size and velocity of the aggregates in Sakurajima volcano, as reported in Bagheri et al. (2016). Colorless and gray areas enclosed by gray dash lines indicate the individual and aggregate particles from Eyjafjallajökull volcano, respectively (Taddeucci et al. 2011). b Relationship between Reynolds number (R_e) and drag coefficient (C_d). Dashed curves show the model of Wilson and Huang (1979) with form factors of 0.35 and 0.75. Colorless and gray areas enclosed by gray dash lines indicate the individual and aggregate particles from Eyjafjallajökull volcano, respectively (Taddeucci et al. 2011). c Density and diameter of falling ash particles calculated from Eqs. (1), (2), and (3). The solid red line represents density variation in the ash deposit, as measured using a helium pycnometer