Hanna Hartman, Hurricane Season. hcmf//, 17 November 2019

After occasional previous engagements in the north of England, including a particularly memorable performance at Distractfold's Cut & Splice Festival in 2017, Hanna Hartman was engaged as composer in residence for hcmf's 41st iteration. Hurricane Season, a world premiere, was also an hcmf// commission. It was for objects, electronics, amplification, magnified projection and a rather large-sized magnet (or perhaps more than one), which I imagine most of the commission fee was spent on getting through international customs. The aural component of the piece was fairly unremittingly scratchy and granular, the sort of soundscape one might expect to hear if holding a stethoscope to a dead tree filled with insects. And it was certainly a soundscape – the sounds seemed to result not so much from the motions and frictions of the different materials (though, I assume, most of them actually did come from this) but hover around the space cavernously as a sort of underscoring to the action. Hartman's preoccupation with exploding the finest grains of material was here achieved at once aurally and visually, with the mega-enlarged projection complimenting the various gradations of scrapes and squeaks of the moving objects. I'm coming to this from a New Music perspective, so my frame of reference here is more Mikrophonie than Child of Tree, but one could just as easily go down the route of reading Hurricane Season through the more experimental tradition of allotting objects an aural agency which might be overlooked otherwise.

Evolution of fluid-like granular ejecta generated by sphere impact

We present results from an experimental study of the speed and shape of the ejecta formed when a solid sphere impacts onto a granular bed. We use high-speed imaging at frame rates up to 100 000 f.p.s. to provide direct measurement of individual grain velocities and ejecta angles as well as the overall evolution of the granular ejecta. For larger grain sizes, the emergence velocities of the grains during the ‘early stage flow’, i.e. before the main ejecta ‘curtain’ forms, increase with the kinetic energy of the impacting sphere but are inversely proportional to the time from impact. We also observe that the fastest grains, which can obtain velocities up to five times that of the impacting sphere (${V}_{g} / {V}_{0} = 5$), generally emerge at the earliest times and with the lowest ejection angles. As the grain size is decreased, a more ‘fluid-like’ behaviour is observed whereby the ejected material first emerges as a thin sheet of grains between the sphere and the bed surface, which is also seen when a sphere impacts a liquid pool. In this case, the sheet velocity is approximately double that of the impacting sphere (${V}_{s} / {V}_{0} = 2$) and independent of the bulk packing fraction. For the finest grains we provide evidence of the existence of a vortex ring inside the ejecta curtain where grains following the air flow are entrained through the curtain. In contrast to predictions from previous studies, we find that the temporal evolution of the ejecta neck radius is not initially quadratic but rather approaches a square-root dependence on time, for the finest grains with the highest impact kinetic energy. The evolution therefore approaches that seen for the crown evolution in liquid drop impacts. By using both spherical glass beads and coarse sands, we show that the size and shape distribution are critical in determining the post-impact dynamics whereby the sands exhibit a qualitatively different response to impact, with grains ejected at lower speeds and at later times than for the glass beads.