Inverse Energy Cascade Structure of Turbulence in a Bubbly Flow. PIV Measurement and Results

The inverse energy cascade, which is one of the important phenomena to enhance the large-scale flow instability in bubbly flow, is investigated by measuring a local two-phase flow structure driven by buoyant bubbles using particle imaging velocimetry (PIV). In PIV, the flow field of liquid phase is measured by separating an original image to respective phase images using a statistical thresholding method for separating image parameters of bubbbles and particles. The present results obtained in the case where the bubble Reynolds number and average void fraction are less than 30 and 1.5%, respectively, confirm the large energy decay with a slope index steeper than -5/3 in the log-log diagram of energy spectrum in a high wavenumber region. An important relationship between the energy spectrum and the bubble-bubble interval distance is also detected.