Transition flow regime on stepped spillways: air–water flow characteristics and step-cavity fluctuations

Abstract

Stepped spillways are man-made hydraulic structures designed to control the release of flow and to achieve a high energy dissipation. The flow pattern for a given stepped chute geometry can be distinguished into different regimes. Herein, the transition flow regime occurs at a range of intermediate discharges and is characterised by strong hydrodynamic fluctuations and intense splashing next to the free-surface. Up to date, only minimal experimental data is available for the transition flow. As this flow regime is likely to occur on stepped spillways designed for skimming flow operation, a knowledge of the transition flow characteristics is important to ensure safe operation. The present article investigates the hydraulics of the transition flow regime on a laboratory spillway, presenting a detailed characterisation of air-water flow properties and an image-based analysis of free-surface fluctuations within successive step-cavities. The results show two different void fraction and turbulence intensity profiles, indicating the existence of an upper and a lower transition flow sub-regime. The image-based analysis suggests the presence of a rapidly and a gradually varied flow region downstream of the inception point for both sub-regimes, whereas full equilibrium flow was not reached in the physical model. Overall, the study contributes towards improving the characterisation of the transition flow by assembling analytical solutions for different two-phase flow parameters, including void fraction, interfacial velocity and step-cavity pool height.

Optical flow estimations in aerated spillway flows: Filtering and discussion on sampling parameters

“…A turbulent shear-layer was forming in the wake of the step edges. Note that a recent study of Kramer and Chanson (2018b) presented detailed two-phase flow measurements at various flow rates (d c /h = 0.5 to 0.8) within both transition flow sub-regimes. The specific discharge of the performed experiments was q = 0.067 m 2 /s (TRA2), which corresponded to a Reynolds number of Re = 4q/ν = 2.7 × 10 5 , where ν is the kinematic viscosity of water.…”

“…Further details of the physical model, e.g. concerning the geometry of the inlet section or on the operation of the weir crest, were published in Zhang and Chanson (2016) and Kramer and Chanson (2018b). A trolley was mounted parallel to the pseudo-bottom (formed by the step edges) of the spillway chute and allowed for the positioning of phase-detection probes at different longitudinal and transverse positions.…”

A review: Aeration efficiency of hydraulic structures in diffusing DO in water

“…The concept of transition flow regime was introduced by Ohtsu and Yasuda [84] . There is a pool of re-circulating water along with a small air cavity, spray water deflection immediately downstream of the stagnant point, and a chaotic appearance associated with the transition flow regime [85] .…”

Numerical evaluation of discharge coefficient and energy dissipation of flow over a stepped morning glory spillway

“…A brief description of the characteristics of flow over stepped chute spillway (Figure 2) would be beneficial, before following the history of the stepped morning glory spillway (Chanson, 2001). The flow regime over stepped spillways might be either nappe flow at low flow rates (Figure 2 (Kramer, 2018). However, there would be no serious problem on low flow rates associated with nappe and transition flow regimes over stepped beds.…”