Phase Analysis of the Stretching Cycles of the Head of Unsteady Gravity Currents Developing over Smooth and Rough Beds

Gravity currents are buoyancy driven flows occurring spontaneously in nature or resulting from human intervention. Examples of gravity currents in the water are oceanic fronts, resulting from differences in temperature and salinity, and turbidity currents caused by high concentration of suspended particles. The release of pollutant materials into rivers, oil spillage in the ocean and desalination plant outflows are examples of anthropogenic gravity currents in the water, frequently with negative environmental impacts. The present work experimentally investigates the dynamics of unsteady gravity currents produced by lock-release of a saline mixture into a fresh water tank. Seven different experimental runs were performed by varying e density of the saline mixture in the lock and the bed roughness. The experiments were conducted in a 3.0 m long Perspex flume, of horizontal bed and rectangular cross section of 0.20 x 0.30 m2, and recorded with a 25 Hz CCD video camera. An image analysis technique was applied to visualize and characterize the current allowing the detailed analysis of the gravity current dynamics and more specifically of the head dynamics. The temporal evolution of variables assessed at the head of the gravity current i.e. length, surface, volume and mass, per unit width, shows repeated cycles of stretching and break of the head. During the stretching phase, ambient fluid is entrained into the head causing its growth. However this is not unlimited, a limit in which the head becomes unstable and consequently breaks exists. There is a strong similarity in the head aspect ratio, of maximum head height to length, and mass between the cycles and between runs, thus a phase lumped analysis within the periodically well-behaved cycles is presented in terms of head aspect ratio, head mass and mass rate. In the instants of head break, the head aspect ratio shows a consistent limit of 0.2, for all runs, and the mass of the head is of the order of the initial mass in the lock. Regarding the periodicity of the break events, it is seen to increase with bed roughness. Entrainment at the head is evaluated through mass rate and is seen to occur during all the stages of the current development.