Aeration‐Induced Circulation from Line Sources. I: Channel Flows

Influencing Flow Patterns and Bed Morphology in Open Channels and Rivers by Means of an Air-Bubble Screen

“…These experiments have shown that a longitudinal bubble screen, parallel to the flow direction, can generate a bubble-induced secondary flow perpendicular to the bubble screen with a size of about 4× the water depth. This size is in agreement with the previously mentioned studies under still-water conditions (Wen and Torrest 1987;Fanneløp et al 1991). Blanckaert et al (2008) have shown that the bubble-induced secondary flow causes redistribution of the longitudinal velocity and of the boundary shear stress, which suggests that the bubble screen would also lead to morphological redistribution in configurations with mobile riverbed.…”

“…Experiments were conducted under both still-water and straightflow conditions with three different water depths, as follows: Table 2). For each water depth, experiments were performed with four different base flow velocities U, as follows: (1) (Table 2) These results are in line with previous studies performed under still-water conditions (Wen and Torrest 1987;Fanneløp et al 1991;Riess and Fanneløp 1998) where the bubble-induced secondary flow size was proportional to the water depth and had a size of 2.5H to 7H.…”

“…According to Wen and Torrest (1987), the vertical flow velocities above the porous tube are zero at the bed and the water surface, and reach a maximum value, v z;flow;max , at about middepth (Fig. 11).…”

“…Experiments performed under still-water conditions reported in literature ( Table 1) have shown that the bubble-induced secondary flow size varies from 2.5 to 7× the water depth, independent of the water depth (Riess and Fanneløp 1998), whereby differences between results are mostly related to the different definitions of the bubble-induced secondary flow size or to the different geometries of experimental setups. Wen and Torrest (1987), for example, found a secondary flow cell size of 4H for water depths between 0.25 and 0.9 m. Goossens (1979) performed experiments at small and large scales and found a range of influence of about 4× the water depth, for water depths between 2 and 5 m. These results indicate that similar secondary flow cell sizes (normalized with the water depth) are found independently of the water depth and base flow velocity, and that the results obtained in the here reported laboratory experiments are relevant for natural rivers and open channels with subcritical flow conditions. For the range of bubble sizes used, the rising velocity of an individual air bubble in water is nearly constant at 0.24 m s −1 (Leifer et al 2000), irrespective of the flow depth.…”

“…According to Fanneløp et al (1991), two regions of recirculating flow, called primary cells, are found on both sides of a linesource air-bubble screen in still-water conditions. Their size varies from 2.5 to 7× the water depth independent of depth and air discharge, except at very low air discharge (Wen and Torrest 1987;Riess and Fanneløp 1998). The cores of the two cells, also called rotor cores, are located close to the bubble screen, independently of the air discharge.…”