Cyclic flow oscillations in a system of repeatedly branching channels

The unsteady nature of the flow within a cascade of bifurcating branches was investigated for Reynolds numbers of 900 and 2700 in the mother branch. The cascade consists of three generations of planar symmetric bifurcations with a fixed branching angle and ratio of branch length to width. Chronological flow field measurements were carried out using digital-particle-image-velocimetry (DPIV) to determine the time-dependent velocity distribution within the branches. Though the flow input was steady, low-frequency flow oscillations were found within the branches which were coupled with the dynamics of the separation regions and the cross-talk between the communicating daughter branches. The toggling flip-flop character of the flow was explained by means of a feedback loop in which the cyclic growth, wash-out and re-generation of the separation regions is assumed to be the inner motor for the periodic continuation. These local excitation mechanisms were suggested to be similar to those causing the generation of low-frequency oscillations in shear-layers impinging on edges like in a rectangular cavity. The results demonstrate that unsteady effects and excited oscillations play an additional role in uneven flow partitioning in flow bifurcation cascades, however, they do not favor any flow partitioning in a preferential pathway constant in time