Interactions between cavity flow and main stream skimming flows: an experimental study

Abstract

In the last two decades, research on the hydraulics of skimming flows down stepped chutes was driven by needs for better design guidelines. Skimming flows are characterized by significant momentum transfer from the main stream to the recirculation zones. Investigations are difficult because of the complex nature of the flow, the strong flow aeration, and the interactions between entrained air and turbulence. This study provides a comprehensive database on main stream and cavity flow interactions in skimming flows down a stepped chute. Measurements were conducted on a large facility (α = 15.9°, h = 0.05 and 0.1 m, W = 1 m) with precise instrumentation based upon a Froude similitude. Air–water velocity and turbulence measurements demonstrated a well-defined mixing layer developing downstream of each step edge in which the velocity profiles had the same shape as classical monophase flow results. A comparative analysis of air–water flow properties for Froude similar flow conditions showed a good agreement between the two step heights in terms of dimensionless distributions of air content, velocity, and turbulence intensity, but dimensionless bubble count rates, turbulence levels, and bubble chord sizes were improperly scaled.Key words: stepped chute, skimming flow, flow recirculation, momentum exchange, physical modelling.

Step cavity and gabion aeration on a gabion stepped spillway

“…The present results were consistent with the visual observations (see above) and it was thought that the interaction between the mainstream flow and seepage through the gabion faces contributed to the drastic change in step cavity void fraction profiles. Above the step cavity between step edges, a greater aeration was consistently observed in the lower mainstream flow region (0 < y/h < 0.2-0.3) than at step edges, as reported by Matos et al (2001) and Gonzalez and Chanson (2004). Some typical distributions of void fraction at several longitudinal locations between the step edges 7 and 9 are presented in Figure 3.…”

“…On the gabion stepped chute and in the step cavities, the aeration process differed from conventional spillway structures (e.g. Matos 2001, Gonzalez andChanson 2004). The amount of entrained air was a function of discharge, gabion size and shape, flow regime, cavity shape and location.…”

“…Most studies focused on the air-water properties in the skimming flow above step edges. Only a handful documented the flow properties in the steps cavities (Gonzalez and Chanson 2004, Felder and Chanson 2011, Felder 2013. Little data is available on gabion stepped spillway flows, besides Peyras et al (1992) and Wuthrich and Chanson (2014).…”

Physical Modelling, Scale Effects, and Self-Similarity of Stepped Spillway Flows

“…Several studies yielded Equation (7) (e.g. Matos 2000, Boes 2000, Chanson and Toombes 2002, Gonzalez and Chanson 2004, but a few documented the velocity distribution in the upper spray region (Chanson and Carosi 2007). Equations (7) and (8) are compared with experimental data in Figure 4.…”

Stepped spillways for embankment dams. Review, progress and development in overflow hydraulics

“…Data for y < 0 were recorded in the step cavity. Velocity measurements between step edges highlighted further the developing mixing layer downstream of each step edge (GONZALEZ and CHANSON 2004). Above vanes, the mainstream turbulence levels were usually lower than between vanes and than in absence of vanes.…”