Abstract: The multi-stage orifice spillway tunnel is a type of internal energy dissipator that realizes energy dissipation through strong shear mixing of water flow, and its cavitation resisting performance is a key index affecting the safety of its operation. This paper presents a systematic analysis of the cavitation characteristics of the flows in a multi-stage orifice plate spillway tunnel based on the prototype hydraulic observation data of the spillway tunnel No.1 of the Xiaolangdi Dam, and examines the influence of scale effect on the incipient cavitation number by comparing with the measurements on our 1:40 vacuum tank model. The results show that the flow cavitation number is insensitive to changes in the reservoir water stage, while the incipient cavitation number increases with a rising stage. Consequently, a higher reservoir stage will lead to a smaller gate opening threshold of incipient cavitation. For a multi-stage orifice spillway tunnel, its incipient cavitation number is significantly influenced by scale effect; thus, the value obtained from a scale model must be modified. And its structure size and flow velocity are the main factors affecting the incipient cavitation number. Based on this, we work out a new method for correcting the incipient cavitation number of a vacuum tank model, achieving an error less than 2% against the prototype data and thus deepening our understanding of multi-stage orifice spillway tunnel hydraulics.

Key words: orifice spillway tunnel, scale effect, incipient cavitation number, prototype hydraulic observation, vacuum tank tests