Oscillations of the fluid flow and the free surface in a cavity with a submerged bifurcated nozzle

•Self-sustained oscillations in a thin cavity with submerged nozzle were observed.•Three flow regimes are detected depending on nozzle depth and inlet velocity.•The three flow regimes have been summarized in a flow regime map.•PIV measurements are performed to link free surface behavior to the bulk-flow.•We report a close correlation between jet-behavior and free surface dynamics.

The free surface dynamics and sub-surface flow behavior in a thin (height and width much larger than thickness), liquid filled, rectangular cavity with a submerged bifurcated nozzle were investigated using free surface visualization and particle image velocimetry (PIV). Three regimes in the free surface behavior were identified, depending on nozzle depth and inlet velocity. For small nozzle depths, an irregular free surface is observed without clear periodicities. For intermediate nozzle depths and sufficiently high inlet velocities, natural mode oscillations consistent with gravity waves are present, while at large nozzle depths long term self-sustained asymmetric oscillations occur.For the latter case, time-resolved PIV measurements of the flow below the free surface indicated a strong oscillation of the direction with which each of the two jets issue from the nozzle. The frequency of the jet oscillation is identical to the free surface oscillation frequency. The two jets oscillate in anti-phase, causing the asymmetric free surface oscillation. The jets interact through a cross-flow in the gaps between the inlet channel and the front and back walls of the cavity.