The time evolution of a line puff, a turbulent non-buoyant element with significant momentum, is studied using the renormalization group (RNG) K-∈ model. The numerical results show that the puff motion is characterized by a vortex pair flow; the computed flow details and scalar mixing characteristies can be described by self-similar relations beyond a dimensionless time of around 30. The added mass coefficient of the puff motion is found to be approximately unity, The predicted puff flow and mixing rate are substantially similar to those obtained from the standard K-∈ model and are well supported by experimental data. The computed scalar field reveals significant significant secondary concentration peaks trailing behind in the wake fo the puff. The present results suggestthat the overall mixing rate of a puff is primarily detenmined by the large-scale motion and that stremline curvature probably plays a minor role.