141 / 2024-02-20 04:34:32

Interaction Between Two Counterflowing Estuarine Bottom Currents: Insights from Laboratory Experiments

Gravity Currents,Collision Dynamics,Turbulent Mixing,Maximum Rise Height,Upwelling

Colliding gravity currents occur when fluid masses of contrasting densities meet and collide, resulting in complex interactions. This natural phenomenon significantly influences the physical, chemical, and biological properties of the geophysical environment where it occurs. Although colliding gravity currents have been observed in estuaries and coastal oceans, the collision dynamics are not fully understood due to inaccessible bottom locations and the limitations of in situ measuring equipment. The mechanisms and implications of bottom currents collision and interaction remain an open question. In this study, we designed laboratory experiments to simulate the interaction between counterflowing saline gravity currents in order to understand the circulation pattern, quantify important collision parameters, and provide valuable insights into the implications of such interactions in nature. Advanced flow visualization and particle image velocimetry (PIV) technology were deployed to monitor the flow evolution and velocity fields of the colliding currents during the interaction process. Results indicate that the overall outcome of the gravity currents collision depends on the buoyancy ratio of the colliding currents (i.e., the ratio between the density differences of the two colliding currents with the ambient fluid). Both symmetric and asymmetric collisions, respectively, generated intense mixing and a significant vertical motion of the mixed fluid at the collision zone. The maximum rise height of the mixed collision front exceeded twice the height of the colliding currents, establishing an exchange pathway between the bottom waters and the overlying waters. A strong relationship was observed between the maximum rise height of the collision front and the buoyancy ratio of the colliding currents. Depending on the density difference between the two colliding currents, internal waves could be generated at the interface between the two fluid layers. This study presents important research findings and explains why a confluence of counterflowing estuarine or oceanic currents is energetic and biologically productive.