Local flushing timescales in the flow separation zone of a T-shaped open-channel confluence

To acquire knowledge of the surface storage effects in the flow separation zone of a T-shaped open-channel confluence, independent Large Eddy Simulations are performed of the flushing with fresh water of a downstream branch which is initially uniformly contaminated by a passive scalar. Based on the ensemble averaged concentration, the spatial distributions of the local flushing lag and the local flushing time are determined. The flushing lag is much smaller than the flushing time, except in the maximum velocity zone, where both timescales are of the same small magnitude. For the local flushing time, the separation zone shear layer forms a transition between the high values in the flow separation zone and the low values in the maximum velocity zone. Inside the separation zone, the highest flushing times occur in a small zone near the downstream junction corner. Delineations in the time-averaged velocity field of the separation zone or its proxies, the recirculation zone and the reverse flow zone, are also assessed as pragmatic means to find the zone with the highest local flushing times. Finally, a regional flushing time and a residence time distribution were also determined on the basis of performed simulations.