Unsteady air–water flow measurements in sudden open channel flows

Measurements of air–water flow properties are reasonably simple in steady flows, but not so in unsteady flows. Some studies investigated periodic flows in which instantaneous data were averaged over several cycles. During the present work, new unsteady air–water flow measurements were performed in sudden open channel flow surges. Unsteady air–water flow measurements were performed in the wave front with an array of resistivity probes. The results demonstrated quantitatively strong aeration of the leading edge in terms of void fractions, bubble count rates and specific interface areas. Experimental results highlighted that this strongly aerated region was relatively short: i.e. typically 0.3 to 0.5 m long. Measurements of air and water chord sizes highlighted a wide range of bubble and droplet sizes. Time-variations of air– water flow structure were observed. List of symbols a Air–water specific area (1/m) defined as the air–water interface area per unit volume of air and water amean Depth averaged specific interface area defined in terms of Y90: amean 1⁄4 1 Y90 R Y90