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This experimental laboratory study presents a detailed analysis of the surface oscillations inside a square cavity connected laterally to an open channel. For an approaching steady and one-dimensional main stream with a high (subcritical) Froude number, interaction with the dead water volume in the cavity area triggers high-amplitude free-surface oscillations, denoted seiches. In the present study, the length of the square cavity equals half of the combined width of the cavity and the main channel. This particular geometry enables two-dimensional or bidirectional seiching with standing waves being excited in both directions simultaneously. In this work, the (relative) magnitude of the different seiching modes (transverse and longitudinal) is measured and analyzed for a range of flow conditions with varying Froude number. The current experimental results confirm earlier findings indicating that the magnitude and dominance of the longitudinal seiching mode (with respect to the transverse mode) grows with increasing Froude number of the main stream. Additionally, a wavelet analysis in the time and frequency domains reveals that the amplitude of the two modes fluctuates significantly in the course of a single experiment, such that seiching cannot be considered a steady phenomenon. Therefore, two alternative strategies are presented to quantify the amplitude of both modes using a more time-localized approach that is less affected by the intermittent behavior of seiching. Although more research is necessary to fully understand what determines the dominant mode at a given moment, an in-depth analysis of the surface recordings at multiple locations in the cavity suggests that the transition from transverse to longitudinal seiching (and vice versa) is related to the phase difference between the bidirectional seiching waves at the (inner) corners of the cavity.

Experimental study of bidirectional seiching in an open-channel, lateral cavity in the time and frequency domain