Hydraulics of the developing flow region of stepped spillways. I: Physical modeling and boundary layer development

On a stepped spillway, the steps act as macroroughness elements, contributing to enhanced energy dissipation and significant aeration. In a skimming flow, the upstream flow motion is nonaerated, and the free surface appears smooth and glossy up to the inception point of free-surface aeration. In this developing flow region, a turbulent boundary layer grows until the outer edge of the boundary layer interacts with the free surface and air entrainment takes place. The flow properties in the developing flow region were documented carefully in a large stepped spillway model (1V:1H; h = 0.10 m). The upstream flow was controlled by a broad-crested weir and critical flow conditions were observed along most of the weir crest, although the pressure distributions were not hydrostatic at the upstream and downstream ends. Downstream of the broad crest and upstream of the inception point, the free surface was smooth, although some significant free-surface curvature was observed for all discharges. The boundary layer growth was faster than on a smooth chute for identical flow conditions. The inception point of free-surface aeration was observed when the boundary layer thickness reached 80% of the flow depth: δ=d ≈ 0.8. The location of the inception point of free-surface aeration and the flow depth at inception were compared successfully to previous laboratory and prototype results

Air-water flow properties in stepped chutes with modified step and cavity geometries

Skimming air-water flow properties were investigated in a stepped chute configured with triangular steps, chamfered steps, and partially blocked step cavities. The turbulent interactions between air and water were examined using a synchronised system consisting of a dual-tip phase-detection probe and a pressure transducer mounted side-by-side. In comparison to uniform triangular steps, the chamfered steps were found to cause a reduction in air entrainment and an increase in mean velocity gradient next to the pseudo-bottom. Partial cavity blockages appeared to have little effect on air entrainment, but were linked to an increased presence of large-scale structures in the overflow, which likely resulted from a reduction in mutual sheltering between adjacent step elements. The results indicated that modifications of step and cavity geometries might have significant implications on stepped chute design

Gabion stepped spillway: interactions between free-surface, cavity, and seepage flows

On a gabion stepped chute, the steps contribute to the dissipation of turbulent kinetic energy, free-surface aeration may be intense, and there are complex interactions between the free-surface flow and seepage motion. Detailed measurements were conducted in a relatively large gabion stepped spillway model. Using a combination of high-speed movies and phase-detection probe measurements, the air–water flow properties in the step cavities and in the gabions were documented. Strong air–water exchanges between seepage and stepped cavity flows were observed. The data showed a complex bubbly seepage motion in the gabions associated with a high level of interactions between seepage and free-surface flows, leading to a modification of the step cavity recirculation and lesser flow resistance

Interaction between free-surface aeration and total pressure on a stepped chute

Stepped chutes have been used as flood release facilities for several centuries. Key features are the intense free-surface aeration of both prototype and laboratory systems and the macro-roughness caused by the stepped cavities. Herein the air bubble entrainment and turbulence were investigated in a stepped spillway model, to characterise the interplay between air bubble entrainment and turbulence, and the complicated interactions between mainstream flow and cavity recirculation motion. New experiments were conducted in a large steep stepped chute (θ=45°, h=0.10m, W=0.985m). Detailed two-phase flow measurements were conducted for a range of discharges corresponding to Reynolds numbers between 2×10 and 9×10. The total pressure, air-water flow and turbulence properties were documented systematically in the mainstream and cavity flows. Energy calculations showed an overall energy dissipation of about 50% regardless of the discharge. Overall the data indicated that the bottom roughness (i.e. stepped profile) was a determining factor on the energy dissipation performance of the stepped structure, as well as on the longitudinal changes in air-water flow properties. Comparative results showed that the cavity aspect ratio, hence the slope, has a marked effect on the residual energy

Air entrainment and turbulence in hydraulic jumps: Free-surface fluctuations and integral turbulent scales

An open channel flow can change from a supercritical to subcritical flow with a strong dissipative process: a hydraulic jump. Herein some new measurements of free-surface fluctuations next to the jump toe and integral turbulent scales in the roller are presented with a focus on turbulent hydraulic jumps with a marked roller. The results highlighted the fluctuating nature of the impingement perimeter in terms of both longitudinal and transverse locations. The air-water flow measurements highlighted the intense flow aeration. The turbulent velocity distributions presented a shape similar to a wall jet solution, and the integral turbulent length scale distributions exhibited a monotonic increase with increasing vertical elevation within 0.2 < Lz/d1 < 0.8 in the shear layer, where Lz is the integral turbulent length scale and d1 the inflow depth

On void fraction and flow fragmentation in two-phase gas-liquid free-surface flows

In free-surface flows, the interactions between high-velocity liquid and atmosphere may lead to strong gas-liquid mixing and complex multiphase flow interactions. In this study, the void fraction power spectrum density (PSD) is used to provide an alternative view of the air-water flow composition. The high frequency range of the PSD curve reflects contributions of small chord times, while the low frequency range contains contributions of both small and large chord times. Likewise, the interactions between the smallest bubbles contribute approximately uniformly to the entire frequency range of the spectrum, while any interaction involving large bubbles/drops will be modulated by 1/f . It is shown that the void fraction spectra are a powerful tool in providing an alternative view of the air-water flow composition

Effects of step and cavity shapes on aeration and energy dissipation performances of stepped chutes

The effects of step edge and cavity shapes on skimming flow properties were investigated in a large-size 45° stepped chute model configured with uniform triangular steps, partially blocked cavities, and chamfers. The focus of this experimental study was the air–water flow regime and the energy dissipation performances. Visually, the partial cavity blockage and chamfers were respectively associated with an increase and a decrease in flow stability, while causing no substantial change in the general flow regimes. Comparisons of characteristic air–water properties indicated better aeration performance for the sharp edges than for the chamfers. A substantial reduction in friction factor was observed with the chamfers, while partial cavity blockages appeared to slightly improve flow resistance. A strongly negative correlation between total air entrainment and flow resistance was identified, which was more observable for the sharp edges. A comparative study revealed that sparsely spaced sharp edges at slopes between 30 and 45° might be optimal in terms of aeration and energy dissipation performances

Turbulence and aeration in hydraulic jumps: free-surface fluctuation and integral turbulent scale measurements

In an open channel, a change from a supercritical to subcritical flow is a strong dissipative process called a hydraulic jump. Herein some new measurements of free-surface fluctuations of the impingement perimeter and integral turbulent time and length scales in the roller are presented with a focus on turbulence in hydraulic jumps with a marked roller. The observations highlighted the fluctuating nature of the impingement perimeter in terms of both longitudinal and transverse locations. The results showed further the close link between the production and detachment of large eddies in jump shear layer, and the longitudinal fluctuations of the jump toe. They highlighted the importance of the impingement perimeter as the origin of the developing shear layer and a source of vorticity. The air–water flow measurements emphasised the intense flow aeration. The turbulent velocity distributions presented a shape similar to a wall jet solution with a marked shear layer downstream of the impingement point. The integral turbulent length scale distributions exhibited a monotonic increase with increasing vertical elevation within 0.2 < Lz/d1 < 0.8 in the shear layer, where Lz is the integral turbulent length scale and d1 the inflow depth, while the integral turbulent time scales were about two orders of magnitude smaller than the period of impingement position longitudinal oscillations