Surface current and recirculating cells generated by bubble curtains and jets

Abstract

The flow structure associated with a line bubble plume in shallow water is investigated. G. I. Taylor has proposed the use of such plumes as wavebreakers. To be effective the surface current generated should be stable for a distance of the order of the wavelength, which in turn could be several times the depth. It appears that the formation of recirculating cells can affect the wavebreaking potential of line bubble plumes. The paper presents observations and measurements of the cell structure associated with line bubble plumes as obtained in a model towing basin of dimensions 1 × 1 × 40 m. A recirculating region was found on both sides of the plume, but the secondary (and higher-order) cell structures proposed by other investigators were not observed. The primary cells were found to be appreciably longer than those reported in the literature for vertical plane jets. The difference can in part be attributed to different definitions of cell length. The definitions used herein are based on observables, both on the water surface and in the interior of the flow, and they lead to consistent measures of length. Bubble-plume parameters (such as entrainment coefficient) are known to depend on gas flow rate, and it was found that the length of the primary cell is a weak function of this variable as well. Additional experiments with a vertical plane jet were conducted for comparison. Longer cells than those previously reported were again observed. The paper contains a complete theory for line bubble plumes, including the effects of compressibility, bubble slip and finite release volume, as well as a simplified similarity analysis useful in estimating plume properties and horizontal-current depth and velocity.

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 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).…”

Sediment Evacuation from Reservoirs through Intakes by Jet-Induced Flow

“…Neither time dependency nor the flow regime (transient or steady state) and sediment particles were investigated. Fanneløp et al (1991) concluded from their laboratory experiments that large-scale applications of bubble plumes in the ocean at depths typical for off-shore fields are likely to produce recirculating cells rather than an unlimited horizontal current. Riess and Fanneløp (1998) state that there is little reason to believe that bubble plumes and vertical jets of comparable mass and momentum fluxes will produce different flow patterns.…”

“…Without the walls, the cells generated by the jets might have a different shape. Even though this has not been tested in the research reported in this paper, research in the field of recirculating cells induced by bubble plumes and jets (Riess and Fanneløp 1998;Fanneløp et al 1991;Jirka and Harleman 1979) leads to the assumption that the jet-induced axial flow field is generated coherently and limited in the semiinfinite space of the real reservoir upstream (and lateral of the jet arrangement), even without being confined by walls or valley slopes. This assumption has to be proven by future investigations.…”