Impact in a granular medium
Seminar Room 1, Newton Institute
AbstractThe penetration of a projectile impacting a granular medium is studied experimentally in two configurations: a quasi two-dimensional (2D) case where a cylindrical projectile impacts a packing of spherical grains contained in a Hele-Shaw cell, and a three-dimensional (3D) case where a spherical projectile impacts the grains contained in a cylindrical vessel. In the 2D situation, the projectile trajectory can be extracted by using image analysis and compared to a simple model including a friction law between the projectile and the grains, a viscous dissipation in the bed, and a force from the collisions between the projectile and the granular medium. For the 3D case, a radial confinement is observed to influence strongly the penetration depth of the projectile impacting the grains contained in a finite vessel. The presence of close lateral walls reduces the penetration depth, and the characteristic distance for these wall effects has been found to be of the order of one projectile diameter . In order to have a better understanding of the projectile dynamics during the impact, Particle Image Velocimetry (PIV) analysis is used in the 2D experiment to extract the velocity field of the grains in the bulk as the projectile penetrates the granular medium. Velocity profiles for the grains in the bulk will be discussed both for an unbounded sample and for confined granular medium.  A. Seguin, Y. Bertho, and P. Gondret, Phys. Rev. E 78 (2008), 010301(R)
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