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The present work presents an experimental study in which resonant surface oscillations inside a lateral cavity are reconstructed, often denoted as se- iching, which are excited by a shallow main stream flowing past the horizontal basin. Firstly, the flow configurations that trigger transverse and/or longitu- dinal seiching are studied using pressure measurements in the corners of the cavity, which shows that a transitional Froude number exists, approximately 0.63, at which the dominant seiching mode changes from transverse to longi- tudinal seiching. For both types of resonant conditions, the surface shape is analyzed in detail using a three-dimensional particle tracking velocimetry (3D- PTV) setup. Based upon floating seeding particles, the 3D surface is recon- structed with a superior spatial resolution compared to traditional measurement techniques, which confirms the multimodal aspect of the surface oscillations.

Lateral cavities adjacent to open-channel flows are dead zones located on one side of a main stream. With an approaching flow with a high (subcritical) Froude number, the free-surface of the dead-zone oscillates with high amplitudes and generates a so-called seiche. This configuration is reproduced in a rectangular cavity (with an interface length equal to the main stream channel width) in which the impact of the three dimensionless parameters (Froude number, dimensionless water depth, and geometrical aspect ratio) affecting the seiche is studied experimentally. For all configurations, a natural mode of the cavity is observed, this mode being either longitudinal or transverse, except in the case of a square cavity where bi-directional seiching occurs. Moreover, we show that while the approaching Froude number (0.55 < Fr < 0.7) and dimensionless water depth do not affect the oscillation mode, the selected natural mode is strongly dependent on the geometrical aspect ratio of the cavity. For narrow cavities (small [W + b]/b with W and b the cavity and channel widths, respectively), a longitudinal mode occurs while for wider cavities transverse modes occur, with an increasing number of nodes as the width of the cavity increases. Finally, measuring the time-resolved 2-dimensional field of free-surface deformation in the cavity and the adjacent main stream permits us to identify the vortices shed along the mixing layer at the cavity/main stream interface and thus to analyze the synchronization between the surface oscillation and vortex shedding (at the upstream edge) and impinging (at the downstream edge) processes.

This paper presents a Lagrangian laboratory study of the passive tracer transport in and around a lateral, open-channel (square) cavity. Using 3D-particle tracking velocimetry (PTV), the trajectories of neutrally buoyant seeding particles are measured and analyzed to investigate the processes governing the particle exchanges between the cavity and the adjacent main stream for a selected subcritical flow condition. The tracked particles are classified using a Lagrangian approach based on their start and end positions, i.e., the cavity or the main stream region. Next, the spatial distribution of the particles at the main stream-cavity interface is analyzed to distinguish the typical transport processes of the different particle classes and identify preferential zones of net particle inflow, net particle outflow, and local zigzagging across the interface. Finally, this paper investigates the influence of the zigzag motion of particles on the (net) mass exchange coefficient. Derived from the same 3D-PTV dataset, a comparison between the common Eulerian (velocity-based) and Lagrangian mass exchange coefficients suggests that the transverse velocity method overestimates the net exchange significantly because of the particle zigzag motions.

This work presents the design and application of a Lagrangian measurement and analysis methodology, which is employed to study the flow and passive tracer exchange between a main channel and a lateral cavity in a laboratory experiment. For this purpose, a 3D-PTV system is implemented for which a static and dynamic experimental validation show that the use of a multiplane camera calibration technique and a recent trajectory linking strategy allow one to track the neutrally buoyant particles accurately in time. The resulting 3D particle trajectories are then used to quantify the 3D flow field and study the entrainment mechanisms between the main flow and the cavity using an Eulerian and a Lagrangian methodology, respectively. Analysis of the flow velocities at the geometrical interface between the main flow and the cavity indicates that the inflow is mainly concentrated at the downstream end of the cavity opening, closer to the bottom, while the particles tend to leave the cavity in the upstream part of the interface more uniform over the water depth. A vertical profile of the mass exchange coefficient is quantified based on the transverse velocity components at the interface, which confirms that the exchange intensity varies significantly with depth. More importantly, however, a novel Lagrangian trajectory classification strategy is proposed to study the transport of particles more in detail and overcome problems related to the time-dependent and 3D shape of the hydrodynamic boundary between the main channel and the cavity. Compared to the common Eulerian approach, this enables to refine the definition of mass exchange and exclude those particles that do not add to the net exchange. Subsequently a Lagrangian definition of the (depth-averaged) mass exchange coefficient is proposed, for which the current (preliminary) results indicate its potential to reliably quantify mass exchange.