The three-dimensional turbulent structure of steady state gravity currents

The structure of gravity currents has been extensively studied using both laboratory and numerical methods. Much of the previous work has focused on lock-exchange type flows that typically result in an exaggerated current head and a distorted turbulence distribution. The work presented herein investigates steady state gravity currents; in most natural flows the body of the flow forms the majority of the current. This study aims to quantify the three-dimensional turbulent structure of steady state gravity currents. A combination of planar particle imaging velocity (PIV), shake-the-box particle tracking (StB) and acoustic measurements were used to investigate the body of pseudo-steady gravity currents, focusing on the turbulence structure and formation of coherent turbulent structures. These structures are of interest due to their ability to control the distribution of mass, momentum and temperature, as well as their potential impact on erosion and deposition in particle laden flows. PIV was used to investigate a range of Reynolds numbers by considering various slopes with a constant influx, as well as a constant slope with varying influx. StB was used to provide 3D characterisation of single Reynolds number flow in the same geometry as the PIV study. Acoustic measurements were used to quantify a number of unconfined gravity currents with a range of topographical controls. The StB data describes experimentally the three-dimensional turbulent structure of the body of pseudo-steady gravity current flow for the first time. The data reveals the complex three-dimensional flow and internal waves present within gravity currents from a simple ducted domain. The results show that cross-stream and vertical flow velocities within these currents are of very similar magnitude. The unconfined study reveals the presence of significant complexity within gravity currents partially bounded by topography providing insights into the formation and spatial distribution of distinctive bedforms, such as hummock-like and sigmoidal bedforms, sediment dispersal pattern, and process controls on onlap termination styles.