Air entrainment in a particular reservoir outlet

The hydraulic working of reservoir outlets is often affected by the phenomenon of air entrainment connected with the outflow of water stream. As a consequence the evaluation of flow rate of entrained air is in order to verify the working conditions. Both in scientific research and in technical practice a coefficient called "beta" is used for the assessment of the phenomenon: it is the ratio between flow rate of entrained air and water flow rate. The present paper reports the results of a study carried out in a physical model of large dimensions reproducing a reservoir outlet consisting of a vertical shaft supplied by an orifice at the base of a storage tank. The authors used an original experimental technique and a particular elaboration method in order to obtain a formula for determination of coefficient "beta". The results, though achieved in a particular case, contribute to the choice and the specifications of the parameters by which the phenomenon is characterized.