Aeration of a free jet above a spillway Aeration d'un jet libre sur un evacuateur de crues SUMMARY H. CHANSON Lecturer in Hydraulics/Fluid \tlechanrcs. Dept. Ciwl Universiry of Queensland. St. Lucw. QLD 4072. Australia A study of air entrainment above an spillway aerator is presented and discussed w1th a dliTlens1onal analys•s. We conclude that similitude of ajr entramment processes for spillway aerator is not possible between model and prototypt!. New informations on the aeration region are presented and an analytical solution of the upper nappe entrainment is developed. RESUME L 'art tcle presenre une etude de l'entrainement d'air pour un dispositif d'aeration d'ecoulements sur un evacuateur de crues. completee par une etude adimensionnelle. Cette etude conclut que la similitude des processus d'entrainement d'air. pour les aerateurs d\!vacuateur de crues, n'est pas possible entre un modele et un prototype. Cependant de nouveaux resuJtats soot presentee pour la zone d'aeratmn de l'ecoulemem et une solution analytique est developpee pour l'entrainement il travers la nappe supeneure du jet libre. 1 Introduction 1.1 Presenrarion At high now velocities cavitation damage may occur on the spillway surface and leads to a catastrophic failure. With flow velocities greater than 20 to 25 m/sit becomes usual to protect the spillway surface by increasing the compress1biliry of the fluid near the surface through aeration. The entrained airthrough the free surface may protect the spillway floor from cavitation damage if the free surface aeration process provides a sufficient air concentration near the bottom [8. 9]. If there is not enough natural free surface aeration (i.e. downstream of a gate), rbe use of aerauon device (called ae rator ) can provide small quantities of air near the spillway Ooor which prevent lhe cavitation damage [12]. Aerators are located on the spillway floor (Fig. 1) and sometimes on the side walls. Vischer et al. [12] detailed rhe properties of various types of aeration device. Usually a combination of an offset, a ramp and a groove provides the best design. We propose to study the air ent ramment processes of a free jet above a spillway. Firstly a dimen- sional anal ysis is presented . Then the aeration region and the processes occurring in this region are detailed. 1.2 Definuion The local air concentration is defined as the volume of air per unit volume and th is will normally be raken as a time averaged value. Let define t he characteristic depth d as: Rev1sion received January 5, 1991. Open for discussion £111 April 30. 1992. JOURNAL OF HYDRAULIC RESE:\RCH, VOL 29. 1991. '10 5 655