Topics in gravity and turbidity current research.

摘要

Gravity current flow and related phenomena are studied in this dissertation with the help of two-dimensional numerical simulation of full Navier-Stokes equations. First, we consider the effect of large density contrast on the motion of gravity current. We find that for larger density contrasts the dense front dissipates an increasing amount of energy, while the dynamics of the light front can be approximated by the energy conserving solution. Secondly, we examine the effects of a slope on the gravity current in classical lock-exchange flow. Simulations of full lock releases show that the flow goes through an initial, quasisteady phase that is characterized by a constant front velocity. This quasisteady front velocity persists up to a dimensionless time on the order of 10. The flow subsequently undergoes a transition to a second phase with a larger, unsteady front velocity. Conceptually simple models are proposed and provide good agreement with numerical simulations. Thirdly detailed numerical simulations were conducted of gravity currents released from a lock and propagating at the bottom of a linearly stratified ambient. The objective is to test the predictions of the recent theoretical analysis by Ungarish (2006)[1]. The functional dependence of the front velocity on S is found to agree with the theoretical results for weak stratification. Simulations for deeply submerged currents (small a) in strongly stratified ambients (S > 0.5) show that the front velocities deviate from the fastest predicted theoretical solution, but fall within the range of the slower solutions found by Ungarish[1]. Finally we discuss the formation of submarine channel levees by turbidity currents. Submarine channel levee shapes have been found to have both exponential and power law shapes. A simple model is suggested for describing levee shapes. Entrainment of ambient fluid in the turbidity current plays an important role in determining the shape of the levees. Two-dimensional full Navier-Stokes simulations are performed and found to be in good agreement with the model results.