Gravity currents past thin two-dimensional obstacles

The modification of a gravity current past a thin two-dimensional barrier is studied experimentally, focusing on propagation characteristics as well as turbulence and mixing at the gravity-current head near the obstacle. The broader aim is to develop an eddy-diffusivity parametrisation based on local governing variables to represent gravity-current/obstacle interactions in numerical weather prediction models. A gravity current is produced in a rectangular tank by releasing a salt solution via a lock-exchange mechanism into an aqueous ethanol solution with matched refractive index, and it is allowed to interact with the barrier. A combined particle image velocimetry and planar laser-induced fluorescence system is used to obtain instantaneous velocity and density fields. The experiments span two Reynolds numbers and four obstacle heights, with each case replicated ten times for conducting phase-aligned ensemble averaging. Four evolutionary stages of the front are identified: approach, vertical deflection, collapse and reattachment. Particular focus is placed on the vertical deflection and collapse stages (dubbed collision phase), which includes flow (hydraulic) adjustment, flow modulation over the obstacle, instabilities, turbulence and mixing, and relaxation to a gravity current downstream. The time scales for various flow stages were identified. The results demonstrate that the normalised eddy diffusivity changes significantly throughout these stages and with the dimensionless height of the obstacle.