Air–water flows down stepped chutes: turbulence and flow structure observations

Abstract

: Interactions between turbulent waters and atmosphere may lead to strong air-water mixing.This experimental study is focused on the flow down a staircase channel characterised by very strong flow aeration and turbulence. Interfacial aeration is characterised by strong air-water mixing extending down to the invert. The size of entrained bubbles and droplets extends over several orders of magnitude, and a significant number of bubble/droplet clusters was observed. Velocity and turbulence intensity measurements suggest high levels of turbulence across the entire air-water flow. The increase in turbulence levels, compared to single-phase flow situations, is proportional to the number of entrained particles.

Turbulence, dynamic similarity and scale effects in high-velocity free-surface flows above a stepped chute

“…Equation (5) is valid for both step heights h = 0.05 and 0.10 m on the 21.8°stepped chute only. Overall the present results were very close to the earlier observations of Chanson and Toombes (2002a), Gonzalez (2005) and Relvas and Pinheiro (2008) with the same chute slope.…”

“…4). Such a pattern was observed before for a range of slopes and step heights (Boes 2000a;Chanson and Toombes 2002a;Yasuda and Chanson 2003). It is believed to result from strong interferences between vortex shedding in the shear layers behind each step edge and the free-surface.…”

“…The air-water flow is characterised a complicated twophase turbulent motion with void fractions ranging from some small, often non-zero values close to the invert to 100% above a pseudo ''free-surface'' that is usually defined as the location where the void fraction equals 90% (Cain and Wood 1981;Wood 1991). A key feature is the intense interactions between entrained air and turbulence (Brocchini and Peregrine 2002;Chanson and Toombes 2002a). Presently, limited information is available on the two-phase flow properties of high-velocity free-surface flows, despite some milestone contributions (Straub and Anderson 1958;Cain and Wood 1981), in particular in terms of the turbulent properties.…”

“…The experimental channel was previously used by Chanson and Toombes (2002a) and Gonzalez (2005). The test section consisted of a 1 m wide, 0.6 m long broad-crested weir followed by a steep stepped section (step height h, step length l).…”

“…where C is the void fraction, y 0 = y/Y 90 , y is distance measured normal to the pseudo-invert, Y 90 is the characteristic distance where C = 90% (Chanson and Toombes 2002a). K 0 is an integration constant and D o is a function of the depth-averaged void fraction C mean only:…”