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The rise velocities of in-chain bubbles continually released from a single orifice in still water with and without crossflow are investigated in a series of laboratory experiments for wobbling ellipsoidal bubbles with moderate Reynolds number. For the limiting case in still water, that is, crossflow velocity = 0, the theoretical turbulent wake model correctly predicts the in-chain bubble rise velocity. In this case, the bubble rise velocities VB are enhanced compared to the terminal velocities of the isolated bubbles V0 due to wake drafting and are scaled with flow rate Q and bubble diameter D. Here, we also derive an updated wake model with consideration of the superposition of multiple upstream bubble wakes, which removes the nonlinear behavior of the non-distant (i.e., local) wake model. For the cases with crossflow, the enhancement of the in-chain bubble rise velocity can be significantly reduced, and imaging of the experiments shows very organized paring and grouping trajectories of rising bubbles no...

Turbulencecharacteristics within bubble plumes are quantified in the laboratory using particle imagevelocimetry(PIV) data and postprocessing techniques to identify continuous phase vortices and their properties. Experiments are conducted using a single, high-speed Phantom camera (frame rate 250 Hz); images are preprocessed to eliminate signatures from the bubbles, resulting in high temporal and spatial resolution PIV data for the continuous phase only. The local swirl strength is used to identify vortices in the flow field. By combining the velocity fields and identified vortex maps, the vortex size probability distribution and the relationship of the vortex size, circulation and enstrophy with respect to the position in the plume are quantified. Results demonstrate that these quantities are self-similar when nondimensionalized by the plume radius and center line velocity. A direct relationship is also identified between vortex size and the corresponding average circulation and enstrophy. Modulation of the turbulent energy spectrum compared to a single-phase plume is noted and is attributed to the presence of the bubbles in the flow.Vortex size and energy density are greatest in the shear layers at the edge of the round plume, indicating that shear instability, and not bubble wakes, is the dominant process of large-scale turbulent and coherent structure generation in bubble plumes.

International audience; Two-layer stratified flows are commonly observed in geophysical and environmental contexts. At the interface between the two layers, both velocity shear and buoyancy interplay, resulting in various modes of instability. Results from a temporal linear stability analysis of a two-layer stratified exchange flow under the action of a mean advection are presented, investigating the effect of a mild bottom slope on the stability of the interface. The spatial acceleration is directly included in the governing stability equations. The results demonstrate that increasing the bottom slope has a similar effect on the stability of the flow as does increasing the ratio R of the thickness of the velocity mixing layer dv to that of the density layer dp as it causes the flow to be more unstable to the Kelvin-Helmholtz instabilities. The transition from Kelvin-Helmholtz modes to stable flow occurs at lower Richardson numbers and wavenumbers compared to the horizontal two-layer flow. Kelvin-Helmholtz modes are decreasingly amplified for 1 < R < v2. When 2 < R < v2, Kelvin-Helmholtz modes are first amplified and then damped as the Richardson number increases. This suggests that the behavior of the Richardson number alone is not sufficient to predict the stability tendency of the interface. © 2008 American Institute of Physics.

Shallow flows are ubiquitous in nature and are prone to instabilities that result in the formation of large-scale, two-dimensional coherent structures that are expected to significantly enhance eddy diffusivities compared to the stable turbulent base flow. We present the results of an experimental study in free-surface flow to determine the mixing coefficient for a passive tracer plume in two-dimensional grid turbulence. The grid consists of a row of cylinders 2.5 times the water depth and spaced to achieve a porosity of 50%. Depth-averaged dye concentrations are measured using a light absorption planar concentration analysis method; turbulence statistics are calculated from measurements of horizontal flow velocity using a two-component laser Doppler velocimeter. A base case without grid turbulence (having only bottom-generated turbulence) and three cases with grid turbulence are presented. For experiments with grid turbulence, injections were made at 40 water depths downstream of the grid, at the cylinde...