Laboratory investigation of the plume-to-plume interaction in a rotating tank

This paper aims to understand the characteristics and dynamical processes of two rotating buoyant fluids (two adjacent river plumes) under ideal geostrophic conditions and further reveal their interaction mechanism. Experimental studies were conducted on a rotating tank, where the discharge (Q(up)) and density anomaly (g '(up)) in the upstream inflow were modified to investigate the resultant dynamics over two plumes. Instantaneous velocity fields were measured using the particle image velocimetry to re-construct the 3D plume structure. Based on high-resolution velocity measurements, the upstream coastal current wraps around the downstream bulge and transports further downstream, in association with the downstream coastal current. An alongshore transport equilibrium theory applicable to the plume-to-plume system was proposed to classify the plume alignment types at the laboratory scale. We discover that the increasing Q(up) or g '(up) promotes the alongshore transport and inhibits the offshore extension of the downstream bulge. The g '(up) adjustment decides the alignment types of two interacting plumes, while the Q(up) adjustment determines the degree of the interaction effect. Coastal currents transform from parallel to vertical alignment as g '(up) decreases until they become weak coastal currents where the alignment cannot be recognized. The alignment types directly modify the mass and momentum dynamic balance and can further affect the mass exchange between plume water and ambient water. The mechanisms of two interacting plumes provide a comprehensive perspective on the multi-source river plume system in nature, such as adjacent mountainous rivers, delta estuaries, and buoyancy-driven coastal current systems.