Quantitatively characterizing the large-scale structure of buoyancy flows inevitably induced by bubble swarms is essential to deeply understand turbulent bubbly flows. In the present investigation, we discuss how to extract the long-period fluctuations of gas- and liquid-phase hydrodynamics of the flows in a large-diameter pipe, which correspond to the large-scale structure of the turbulent bubbly flows. For this specific purpose, using a newly developed method (combination use of a four-tip optical fiber probe (F-TOP) and a newly developed data processing algorism), we extracted and characterized long-period fluctuations of the turbulent bubbly flows in a large-diameter pipe apparatus of 380 mm in diameter and 2000 mm in height. First, we discuss a method to extract such fluctuations formed in the bubbly flow based on the time-series point-wise void fractions measured by the F-TOP. The fluctuations of the void fractions in time and space are modeled from a point-wise void fraction based on consideration of the relationship between time-average and space-average void fraction. We have found out a simple relationship between them. The space-average void fraction at the larger region shall reflect some larger organized structure of the bubbly flow. As a result, the time-average point-wise void fraction at a given point in appropriately integral time range shall indirectly represent corresponding organized structure. We demonstrate the effectiveness of the proposed method. Analyzing the time-series point-wise void fractions measured via the F-TOP by applying the above method, we have succeeded in extracting the long-period fluctuations. The extracted long-period fluctuations well agree with those obtained from visualization (i.e. large-scale structure of the bubble swarms). Second, the large-scale liquid-phase motion is characterized based on the results obtained by LDA measurements. We discuss the liquid-phase large-scale motion and the coupling between the large-scale motion and the long-period fluctuations of void fraction.

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