Application of 3D-PTV to mass exchange in an open-channel flow past a lateral embayment

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.