Velocity structure, turbulence and fluid stresses in experimental gravity currents

Gravity currents are of considerable environmental and industrial importanc e as hazards and as agents of sediment transport, and the deposits of ancie nt turbidity currents form some significantly large hydrocarbon reservoirs. Prediction of the behavior of these currents and the nature and distributi on of their deposits require an understanding of their turbulent structure. To this end, a series of experiments was conducted with turbulent, subcrit ical, brine underflows in a rectangular lock-exchange tank. Laser-Doppler a nemometry was used to construct a two-dimensional picture of the velocity s tructure. The velocity maximum within the gravity current occurs at y/d app roximate to 0.2. The shape of the velocity profile is governed by the diffe ring and interfering effects of the lower (rigid) and upper (diffuse) bound aries and can be approximated with the law of the wall up to the velocity m aximum and a cumulative Gaussian distribution from the velocity maximum to the ambient interface. Mean motion within the Plead consists of a single la rge vortex and an overall motion of fluid away from the bed, and this large ly undiluted fluid becomes rapidly mixed with ambient fluid in the wake reg ion. The distribution of turbulence within the current is heterogeneous and controlled by the location of large eddies that dominate the turbulent ene rgy spectrum and scale with flow thickness. Turbulent kinetic energy reache s a maximum in the shear layer at the upper boundary of the flow where the large eddies are generated and is at a minimum, near the velocity maximum w here fluid shear is low.