Evolution of fluid-like granular ejecta generated by sphere impact

Handle URI:
http://hdl.handle.net/10754/562176
Title:
Evolution of fluid-like granular ejecta generated by sphere impact
Authors:
Marston, Jeremy; Li, Erqiang ( 0000-0002-5003-0756 ) ; Thoroddsen, Sigurdur T. ( 0000-0001-6997-4311 )
Abstract:
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. © 2012 Cambridge University Press.
KAUST Department:
Physical Sciences and Engineering (PSE) Division; Mechanical Engineering Program; High-Speed Fluids Imaging Laboratory
Publisher:
Cambridge University Press (CUP)
Journal:
Journal of Fluid Mechanics
Issue Date:
1-May-2012
DOI:
10.1017/jfm.2012.141
Type:
Article
ISSN:
00221120
Sponsors:
This work was partially supported by an Academic Excellence Alliance grant awarded by the KAUST office of Competitive Research Funds number 7000000028.
Appears in Collections:
Articles; Physical Sciences and Engineering (PSE) Division; Mechanical Engineering Program

Full metadata record

DC FieldValue Language
dc.contributor.authorMarston, Jeremyen
dc.contributor.authorLi, Erqiangen
dc.contributor.authorThoroddsen, Sigurdur T.en
dc.date.accessioned2015-08-03T09:46:33Zen
dc.date.available2015-08-03T09:46:33Zen
dc.date.issued2012-05-01en
dc.identifier.issn00221120en
dc.identifier.doi10.1017/jfm.2012.141en
dc.identifier.urihttp://hdl.handle.net/10754/562176en
dc.description.abstractWe 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. © 2012 Cambridge University Press.en
dc.description.sponsorshipThis work was partially supported by an Academic Excellence Alliance grant awarded by the KAUST office of Competitive Research Funds number 7000000028.en
dc.publisherCambridge University Press (CUP)en
dc.subjectfluidized bedsen
dc.subjectgranular mediaen
dc.subjectjetsen
dc.titleEvolution of fluid-like granular ejecta generated by sphere impacten
dc.typeArticleen
dc.contributor.departmentPhysical Sciences and Engineering (PSE) Divisionen
dc.contributor.departmentMechanical Engineering Programen
dc.contributor.departmentHigh-Speed Fluids Imaging Laboratoryen
dc.identifier.journalJournal of Fluid Mechanicsen
kaust.authorMarston, Jeremyen
kaust.authorLi, Erqiangen
kaust.authorThoroddsen, Sigurdur T.en
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