Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 10/15/1997
Publication Date: N/A
Technical Abstract: Density currents such as pyroclastic flows and turbidity currents can endanger life and property, transport vast quantities of sediment, and disperse pollutants, and their sedimentary deposits may become economically important hydrocarbon reservoirs. Yet little information exists on the turbulent flow structure within these complex geophysical flows. To obtain such information, a series of experiments was conducted with fully turbulent, subcritical, brine underflows in a lock-exc tank where laser Doppler anemometry was used to construct a two-dimensional picture of the velocity structure. The velocity maximum occurs at 20% of the flow depth. The shape of the velocity profile is governed by the differing and interfering effects of the lower and upper boundary. The form of the head is also governed by the velocity profile, which influences entrainment of ambient fluid; this entrained fluid contributes to an overall motion of fluid in the head away from the bed. Mean motion within the head consists of a single large vortex. The turbulence spectrum is apparently discontinuous, with two dominant wavenumbers. The larger of these has a length scale similar to the flow thickness, and these eddies contain the majority of the turbulent kinetic energy. Turbulent kinetic energy reaches 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 where shear stress is low. Lower boundary friction is important in the generation of turbulence with larger wavenumbers.