The flow configuration studied herein consists in a straight open channel flow adjacent to a lateral cavity, representing harbors or oxbows in the riverine environment. It is known that, in such configuration, as the Froude number of the channel flow exceeds a given value, a resonance of high amplitude generates a strong oscillation of the free-surface in the cavity; this process is named "seiching". In this case, a standing wave is observed in the cavity, with a particularly high amplitude.

The aim of the present work is to gain information on (i) the parameters governing the direction of this standing wave (parallel or perpendicular to the channel) and the corresponding number of nodes and (ii) the impact of the standing wave on the characteristics of the vortex shedding process taking place at the channel/cavity interface.

The experimental configuration is a simplified geometry with a straight open- channel and a rectangular cavity with vertical walls. The strategy developed here consists in keeping the same flow in the channel, a cavity of fixed length (in the direction parallel to the channel) and in regularly increasing the width of the cavity (in the crosswise direction). The standing wave parameters are measured using ultrasound sensors while the turbulence in the mixing layer is measured using 2D PIV at the cavity/channel interface.

It appears that the variation of the geometrical aspect ratio of the cavity then impacts the characteristics of both the free-surface oscillation and the vortex shedding process:

(1) the direction and number of nodes of the standing wave varies as the aspect ratio of the cavity increases, passing from one resonance mode to another. For narrow cavities, the axis of the standing wave is parallel to the main channel while for wider cavities, it becomes perpendicular and the number of nodes increases as the width of the cavity continues increasing.

(2) while the frequency of vortex shedding at the channel/cavity interface remains always equal to the frequency of the standing wave, the trajectory of the vortices and the general characteristics of the mixing layer are, in turn, affected by the resonance mode of the free-surface.

 We will present experimental evidence of this two-way coupling between standing wave and vortex shedding.