Abstract
Four image-based techniques—i.e., shadowgraphic image method (SIM), high-speed particle image velocimetry (HSPIV), bubble tracking method (BTM), and bubble image velocimetry (BIV)—are employed to investigate an aerator flow on a chute with a 17° inclination angle. The study focuses on their applications to the following issues: (1) to explore the characteristic positions of three water–air interfaces; (2) to interpret the evolution process of air bubbles shed from the wedged tip of the air cavity; (3) to identify the probabilistic means for characteristic positions near the fluctuating free surface; (4) to explore the probability distribution of intermittent appearance of air bubbles in the flow; (5) to obtain the mean streamwise and transverse velocity distributions of the water stream; (6) to acquire velocity fields, both instantaneous and mean, of air bubbles; (7) to construct a two-phase mean velocity field of both water flow and air-bubbles; and (8) to correlate the relationship among the probability distribution of air bubbles, the mean streamwise and transverse velocity profiles of air bubbles, and water stream. The combination of these techniques contributes to a better understanding of two-phase flow characteristics of the chute aerator.
Highlights
The sliding jet from the free jet is generated downstream the wedged tip of air cavity, where shedding of air bubbles takes place (Figure 4)
The sliding jet, accompanied by entrained air bubbles, moves farther downstream and impinges upon the chute surface at the instantaneous impingement point
Using the mean velocity field measured by high-speed particle image velocimetry (HSPIV) and the counterpart obtained by bubble image velocimetry (BIV) for
Summary
Cavitation damage in concrete surfaces of many spillways and chutes has been reported at e.g., Keban Dam in Turkey and Karum Dam in Iran [1,2]. The aerator lifts the approach flow from the boundary to build up an air cavity and a free jet with an upper and a lower surface—air is entrained into the water flow from both surfaces. Rutschmann and Hager [13] adopted two approaches that related the air entrainment coefficient to parameters of chute slope, deflector angle, deflector/offset height, approach flow depth, and velocity and air cavity sub-pressure. With a series of laboratory tests, Gaskin et al [14] expressed the air entrainment coefficient in terms of chute slope, deflector angle, and a Froude number of the approach flow. The key parameters that governed the air transport downstream of an aerator included approach-flow Froude number, chute slope, and deflector angle [19]
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