The interaction of time-dependent rotating and stratified flow with isolated topography

Abstract

Laboratory experiments concerned with a time-varying, spatially homogeneous, free stream current past truncated right circular cylinders in the presence of a linear stratification and uniform background rotation are discussed. The current is composed of a sinusoidal component superimposed on a uniform flow. It is shown that such a flow can be investigated by the suitable translation of an obstacle through a fluid otherwise at rest with respect to a rotating observer. The principal dynamical parameters of the system are the Burger, Ekman, Rossby and temporal Rossby numbers and the velocity ratio u 1 u 0 where u 1 is the amplitude of the oscillating component and u 0 is the magnitude of the uniform flow. The ranges of dynamical parameters considered are of relevance to certain oceanographic current systems. A series of characteristic flow regimes are described for u 1 u 0 = 1.0 and fixed Burger and Ekman numbers; these characteristic flows are depicted on a Rossby against temporal Rossby number flow regime diagram. Emphasis is given to those flow features that are dependent on the time-varying nature of the free stream. For example, for the velocity ratio u 1 u 0 = 1.0 , the free stream comes to rest at a given time in each flow cycle. At this instant, for sufficiently small Rossby and temporal Rossby numbers, a residual anticyclonic motion around the obstacle is present. In another region of the flow regime diagram, the wake flow at this phase consists of a jet-like motion toward the topography. Quantitative measures are made of such observable features as the maximum bubble extent for non eddy-shedding flow regimes and eddy speeds for cases of vortex shedding. Some qualitative observations are also made for cases in which there is no background rotation.