Author(s): Simone Pagliara; Stefan Felder; Benjamin Hohermuth; Robert Boes

Linked Author(s): Stefan Felder, Robert Boes

Keywords: Aerated flows; Bottom outlet; High-head gates; High-speed flows; Physical modelling; Shockwaves

Abstract: Reservoir dams are decisive for energy supply and water resources management, thus playing a crucial role in modern society and economy. Low-level outlets (LLOs) are important safety structures of dams, aiming at regulating the water level in the reservoir, and at allowing a fast drawdown in case of maintenance or emergency situations. A typical LLO consists of a pressurized inflow controlled by a gate and a free-surface outlet tunnel. The transition from pressurized to supercritical free-surface flow is responsible for the generation of a high-speed water jet, resulting in significant turbulence levels, air entrainment, and air transport along the tunnel. Sub-atmospheric pressures typically develop downstream of the gate, potentially triggering significant problems such as gate vibration and cavitation. A sufficient flow aeration is crucial to mitigate these issues, and an appropriate air supply system should be designed downstream of the gate. Several empirical design equations have been developed in literature to predict the relative air demand (i.e., the ratio of air discharge through the vent and water discharge in the outlet tunnel) of LLOs, accounting for the effects of flow patterns, air vent loss coefficient, tunnel slope, length, and roughness. Nevertheless, the comparison against prototype data showed a significantly larger air demand compared to the model-based equations mainly due to geometrical differences, resulting in non-negligible effects of tunnel roughness, profile transitions, and scale. In this regard, there has been no systematic study investigating the effect of tunnel profile transition on air demand and flow patterns. To narrow this gap, large-scale physical model tests were carried out to investigate the effects of an abrupt and a gradual linear tunnel profile transition on the LLO performance, for various combinations of gate opening, energy head at the gate and air vent properties. The two tunnel profile transitions resemble those commonly found in real-world prototypes. Preliminary observations showed that the profile transitions significantly affect the flow patterns compared to tunnels featuring no transitions, resulting in more complex and larger shockwave formation downstream of the gate. This also leads to an overall increase in the air demand for similar inflow conditions. This study provides preliminary recommendations for a safe design of LLO tunnels featuring gradual and abrupt profile transitions, contributing to a safer design of such structures.

DOI: https://doi.org/10.3929/ethz-b-000675921

Year: 2024