---

In the present study, free surface vortex dynamic was experimentally investigated in a horizontal intake. Air entrainment rate into the intake due to the air-core vortices was also discussed. The results presented are the relationship between the vortex type and the intake hydraulic parameters, general pattern of surface displacement of the vortex core and its relation with the vortex type, the required time duration to fully development of the vortex core, and finally evaluation of the vortex induced air entrainment rate. In this research by defining intake number as intake Froude number over the intake relative submergence, a relationship was established between the vortex type and the intake number. Moreover, it was shown that while the intake number increases, surface instability of the vortex core decreases, in which, for the intake numbers greater than one, surface movement of the vortex core is limited to an area of the twice of the intake diameter. Then, another relationship was also established between the time requirement of the vortex air-core formation and the intake number, and it was shown that there will be an exponentially decrease in the mentioned time scale, while the intake number increases. In this context, a relationship was suggested and compared with one of previous works. In the last section, the dependency between air entrainment rate due to the air-core vortices and the intake number was considered, and another relationship was also suggested and compared with previous works.