Importance of convection in the compaction mechanisms of anisotropic granular media.

We report the experimental observation of vortex patterns in a vertically tapped granular media. Depending on the tapping acceleration, two behaviors are observed. For high acceleration a convection vortex appears in the whole media, whereas for low acceleration two unstable vortices appear in the upper part of the media and slowly compact the lower part. We explain the formation of the vortices and relate them to granular convection. Our results demonstrate the importance of compression wave propagation on granular compaction.