Summary: Modeling the highly inelastic interactions inside hair, and between the hair and the character body, is a determinant feature for achieving realistic hair animation. However, the extremely large number of hair strands has so far prevented animators from handling these interactions. It is also unclear if they should be treated at the level of individual hair strands. This paper proposes an approach to handle these interactions, based on a new clustering paradigm. The high friction forces between neighboring hair strands, yielding a high motion coherence, is simulated by clustering these strands into an anisotropic viscous volume representing a hair wisp. All other interactions between hair strands are approximated at the wisp level, thanks to an anisotropic behavior that allows pairs of wisps to interpenetrate or to collide depending on their relative orientations. The wisps also offer the necessary structure to efficiently simulate the collisions with the character body. The model results in visually realistic animations, where complex wisp interactions capture both continuities and discontinuities easily observed in thick, long hair