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The authors present a series of new analyses of the problem of stratified flow over a localized two-dimensional obstacle, focusing upon the detailed dynamical characteristics of the flows that develop when the Froude number is such that the forced internal waves ''break'' above their topographic source. Results demonstrate that when the flow is restricted to evolve in two space dimensions, then the intensity of the Kelvin-Helmholtz-like (K-H) perturbations that form in the downstream shear layer that separates the accelerated low-level jet in the lee of the obstacle and the overlying region of decelerated flow increases dramatically with the governing parameter NU/g (U and N are, respectively, the velocity and buoyancy frequency characteristic of the upstream incident flow, while g is the gravitational acceleration). This nondimensional parameter represents the ratio of the acceleration that a fluid particle feels in the wave to the gravitational acceleration and measures the importance of non-Boussinesq effects. A marked change in the global characteristics of the flow is shown to occur with increasing NU/g, characteristics that include the speed of downstream propagation of the so-called chinook front, the drag exerted by the flow on the obstacle, and the intensity of the K-H instability induced pulsations of the surface velocity field. When the flow is allowed to access the third spatial dimension, the authors demonstrate that it develops intense three-dimensional motions in the regions where overturning of the isentropes in the otherwise stably stratified fluid takes place. An instability of convective type first appears in the form of streamwise-oriented vortices of alternating sign. This instability erodes the downstream propagating K-H billows, eventually leading to the complete arrest of their continued propagation as they ''dissolve'' into fully developed turbulent flow.

Evolution of a two-dimensional flow induced by a jet ejected from a nozzle of finite size is studied experimentally. Vortex dipole forms at the front of the developing flow while a trailing jet establishes behind the dipole. The dynamics of the flow is discussed on the basis of detailed measurements of vorticity and velocity fields which are obtained using particle image velocimetry. It is found that dipoles do not separate ͑pinch-off͒ from the trailing jet for values of the stroke ratio up to 15, which fact can be contrasted with the behavior of vortex rings reported previously by other authors. A characteristic time scale that is defined differently from the formation time of vortex rings can be introduced. This time scale ͑startup time͒ indicates the moment when the dipole starts translating after an initial period when it mainly grows absorbing the jet from the nozzle. A simple model that considers the competing effects of expansion and translation is developed to obtain an estimate of the dimensionless startup time. The dynamics of a dipole after the formation is characterized by a reduced flux of vorticity from the jet. The dipole moves forward with constant speed such that a value of the ratio of the speed of propagation of the dipole to the mean velocity of the jet is found to be 0.5. A universality of this ratio is explained in the framework of a model based on conservation of mass and momentum for the moving dipole.

This paper describes a new series of numerical simulations of stratified flow over localized topography which has been designed to address issues arising from a recently published sequence of detailed observations from a coastal oceanographic setting. Results demonstrate that the numerically simulated flow is very similar to that which develops in Knight Inlet, British Columbia, a fjord which is subject to periodic tidal forcing, and that the detailed dynamical characteristics of this flow are also strikingly similar to those of severe downslope windstorms that often occur in the atmosphere. A typical sequence of events observed in such flows includes the 'breaking' of a forced stationary internal wave induced by the topography, which results in irreversible mixing and the formation, through wave-mean flow interaction, of a decelerated mixed layer that extends downstream from the level of breaking. The formation of this mixed layer is a necessary precondition for transition of the flow into a supercritical hydraulic regime in which a low-level high-velocity jet develops in the lee of the topographic maximum. Simulations with both fixed inflow velocity and harmonically varying inflow velocity are performed and intercomparison of the results clearly demonstrates that flow evolution in the unsteady forcing case can be described, to reasonable approximation, by the results of the corresponding quasi-steady simulations, at least during the accelerating stage when inflow velocity is slowly increasing. At later times of flow evolution, however, the well-mixed fluid accumulates and the flow enters a statistically steady hydraulic-like regime which is characterized by a constant mean drag exerted by the topography on the flow even while the inflow velocity slowly decreases.

When a horizontal force is applied locally to some volume of a viscous densitystratified fluid, flows with high concentration of vertically oriented vorticity (vortex dipoles) are generated. The processes of generation and evolution with time of these unsteady flows in a stratified fluid are studied. A convenient way to produce and study these flows in the laboratory is to use a submerged horizontal jet as a ‘point’ source of momentum. The main governing parameter (the ‘force’) is easily controlled in this case. Two regimes were studied: starting jets with dipolar vortex fronts (the force acts continuously) and impulsive vortex dipoles (the force acts for a short period of time). A conductivity microprobe, aluminium powder, shadowgraph, thymol-blue and other techniques have been used to measure the velocity and density distributions in the flows. It is found that in both regimes the flows are self-similar: the lengthscale of the flows increases with time as t½ for starting jets and as t1/3 for vortex dipoles. Detailed information about the generation mechanism, kinematics and dynamics of the flows is obtained. On the basis of similarity principles a theoretical explanation of the experimental results is given. The theory is in good agreement with the results obtained.