Investigation of the exchange processes between an open channel flow and a lateral embayment using 3D-PTV

The open channel flow past a lateral embayment, in this paper simplified to a square cavity, generates a flow field that is dominated by a slow-moving main gyre occupying almost the whole cavity area and a mixing layer at the main channel-cavity interface. Vortices shed at the upstream corner generate periodic alternating transverse velocities along the interface (Mignot et al., 2016) and at the downstream corner, part of the eddies become entrained inside the cavity where they drive the circulating flow. The interface vortices thus influence the flow field in and around the cavity and as such, govern the mass and momentum exchange with the cavity. Both for river and harbour management, the exchange of nutrients, pollutants, sediments... is important for ecological and economical reasons.

In the literature, mass exchange with lateral cavities is most often quantified by means of the dimensionless mass exchange coefficient k (Valentine and Wood, 1979; Weitbrecht et al., 2008; Mignot et al., 2017), a lumped parameter that describes the exchange with the cavity as a first order process. The experimentally determined k can then be used for one-dimensional modelling, useful for practitioners interested in e.g. water quality in rivers with groyne fields. Nevertheless, it remains difficult to explain which processes govern the net mass exchange, and how they are influenced by the flow and cavity parameters. Previous research (Constantinescu et al., 2009; Akutina, 2015) has shown that the flow in the cavity and at the bottom region of the interface is highly three-dimensional. This makes the simplified approach using k insufficient if a better understanding of the exchange processes is envisaged.

Therefore, this paper will investigate the flow in 3D, in which 3D-PTV will be applied in combination with neutrally buoyant particles. The resulting 3D flow field will be investigated to determine the 3D aspect of the interface velocities and flow field inside the cavity. Moreover, this paper will present a new and more representative, Lagrangian analysis of passive scalar exchange based on the statistics of the 3D paths of the tracked particles. Herewith, only particles that traverse the interface and have a sufficiently long residence time will be included.