Role of air in granular jet formation.
A steel ball impacting on a bed of very loose, fine sand results in a surprisingly vigorous jet which shoots up from the surface of the sand [D. Lohse, Phys. Rev. Lett. 93, 198003 (2004)10.1103/PhysRevLett.93.198003]. When the ambient pressure p is reduced, the jet is found to be less vigorous [R. Royer, Nature Phys. 1, 164 (2005)10.1038/nphys175]. In this Letter we show that p also affects the rate of penetration of the ball: Higher pressure increases the rate of penetration, which makes the cavity created by the ball close deeper into the sand bed, where the hydrostatic pressure is stronger, thereby producing a more energetic collapse and jetting. The origin of the deeper penetration under normal ambient pressure is found to lie in the extra sand fluidization caused by the air flow induced by the falling ball.
Caballero
Gabriel
G
Faculty of Science and J.M. Burgers Centre for Fluid Dynamics, University of Twente, Enschede, The Netherlands.
Bergmann
Raymond
R
van der Meer
Devaraj
D
Prosperetti
Andrea
A
Lohse
Detlef
D
D000388:Air; D011312:Pressure; D012822:Silicon Dioxide; D013232:Steel; D013997:Time Factors
D013232:Steel; D012822:Silicon Dioxide