Abstract: The dynamics of turbulent fountain cap are complex due to the flow reversion and fluctuation, which suggests the studies on quasi-steady fountain cap cannot fully describe the flow features of the cap, e.g. the fountain-cap entrainment. From our direct numerical simulation, we observe and evidence the instantaneous dynamics of fountain cap are related to a series of coherent fluid patches. We then analyse the correlation between the dynamics of coherent fluid patches and the fountain-cap volume, weight and mass center. Finally, we conclude that there is more entrainment into the fountain cap when the volume of the cap is growing, and vice versa.