The time evolution of a line puff --- a turbulent non-buoyant element with significant momentum, is studied numerically via a two-equation turbulence model. The solution reveals a distinct double-vortex flow with significant loss of initial impulse due to pressure interaction, and intense mixing at the front of the puff. The added mass coefficient of the puff motion is found to be approximately 1. The computed flow and puff characteristics strongly support the concept of self-similarity beyond a dimensionless time of 25--30. The predicted puff flow and mixing rate are well-supported by experimental data.