Abstract
The air bubble entrainment and self–aeration phenomena in free-surface water flows reveal a rich interplay of fundamental science and engineering, and the size distribution of the entrained bubbles enhances the air–water gas flux, improves the gas transfer, and influences the cavitation erosion protection in high–speed flows. In the present study, we investigate the bubble–formation mechanism of free–surface air entrainment and the related microscopic bubble scale in the laboratory. This paper provides a quantitative description of bubble entrainment. The entrapment deformation of the local free surface over a period follows a power–law scaling and entrains a bubble when the entrapped surface becomes enclosed in the unstable movement period. Both the size scale and shape character of the entrapped free surface determine the size and skewness of the distribution of the air bubble. The entrapment deformation process confirms that the instability behaviour of the local air–water interface results in the onset of bubble entrainment. Further research is necessary to elucidate the instability criterion dominated by the interface instability and promote a new understanding of multiphase flow generation and development.
Highlights
Free–surface air entrainment in a high–speed water flow creates dense plumes of bubbles that can diffuse across a full cross-section; it has air quantity or void fractions exceeding 60–70%
When the free–surface turbulent intensity exceeded a critical fluctuation velocity, the theory of the “bubble being created by a water droplet” posited that bubble entrainment resulted from the water droplet falling back and impacting the free surface[8,9] and that self-aerated flows contained a “water droplet layer” and an “air bubble layer”[10,11]
When the free–surface turbulent intensity in open-channel flows exceeds a critical fluctuation velocity to overcome gravity and the surface tension restraint, the lifetime of a surface–deformation– generated bubble falls into two periods
Summary
Free–surface air entrainment in a high–speed water flow creates dense plumes of bubbles that can diffuse across a full cross-section; it has air quantity or void fractions exceeding 60–70%. When the free–surface turbulent intensity exceeded a critical fluctuation velocity, the theory of the “bubble being created by a water droplet” posited that bubble entrainment resulted from the water droplet falling back and impacting the free surface[8,9] and that self-aerated flows contained a “water droplet layer” and an “air bubble layer”[10,11] Based on this mechanism, a series of critical hydraulic conditions for the inception of self-aeration were deduced[12,13,14], which were related to the flow depth, channel slope and wall roughness. We use optical high–speed observations of detailed free–surface deformation processes to quantify various aspects of the phenomena to determine the air entrainment
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