Gravity currents

We review various aspects of buoyancy-driven flows arising from the sudden release of a fixed volume of heavy fluid into a larger body of less dense ambient fluid. The driving density contrasts for these flows may be due to temperature or salinity differences in the case of compositionally driven flows or to the presence of suspended material for their particle-driven counterpart. These so-called gravity currents play an important role in a vast array of geophysical and engineering applications and have been the subject of extensive theoretical, numerical, and experimental investigations over the past few decades. We shall first explore some of what we feel arc the pertinent issues related to these flows in the context of their compositionally driven manifestation. In particular, we examine the efficacy of hydraulic theory in this context and explore its limits. We also discuss the notion of hyperbolicity as it relates to the governing hydraulic equations and the role of the stability Froude number in this concept. For the particle-driven flows we demonstrate the role played by the particles in the production of nonhydraulic effects. The roles of settling velocity and volume fraction of suspended particles in the generation of velocity shear will also be explored. The notion that particles give rise to nonhydraulic effects in the form of velocity shear has not been included in any published studies to date that we are aware of. We shall also present a means of accounting for initial turbulent energy of mixing in the release volume. This energy is that which would be injected into the system in the classic lock release experiment involving a well-mixed fixed-volume suspension maintained behind the lock gate prior to release. Results will be related to experimental data where possible. (C) 2001 Elsevier Science B.V. All rights reserved.