No abstract

Measurements of the spreading rates of gravity-driven currents at both the surface and the bottom of a fluid layer of different density are reported. For the case of a constant inflow rate the spreading relations are derived by estimating the order of magnitude of the forces involved. After an initial balance between gravity and inertia forces the final spreading phase is governed by the balance between gravity and viscous forces. For the latter flow regime, measurements in plane and axisymmetric flow geometries agree well with the spreading relations for gravity currents with a no-slip boundary. The proportionality factor, which is not predicted from this model, is then determined from the measurements and a good agreement is found with the theoretical value derived in the accompanying paper by Huppert (1982).

A vortex-induced unsteady separation was investigated experimentally in the laminar boundary layer produced by an axisymmetric jet impinging normally onto a flat plate. By forcing the air jet, primary ring vortices were periodically generated in the jet shear layer. Phase-locked flow visualization showed that the wall-jet boundary layer separated periodically and evolved into a secondary vortex counter rotating with respect to the primary vortex. The unsteady separation is induced by the primary vortex and moves downstream in the radial mean-flow direction. Phase-averaged hot-wire measurements using a parallel-wire sensor in the vicinity of the unsteady separation provided data for locating the onset of separation in space and time. The data revealed that the unsteady separation originated from a local shear layer which was initiated by the unsteady adverse pressure gradient produced by the primary vortex.

Mesoscale fluctuations in the western tropical Atlantic are analyzed in Geos � t al � imetry sea smfac � height (SSH) and geostrophic velocity anomalies to inves _ tigate t � e rol � of eddies m the North Br � z1l Current (NBC) retroflection zone. The detachment of anl!cyclonic eddies from the NBC retroflect1 ? n is observed during November through January, when the NBC retroflection into the North Equatonal Countercurrent (NECC) weakens and finally breaks down. These eddies are traced over more th 1:� 2 months between 50° and 60°W on their way toward the Caribbean, at average speeds of 15 cm s . In one case an apparent merger of two anticyclonic eddies occurs, one detached from the retroflect10n zone and one detached from the NECC. Cyclonic eddies are also observed but are generally less persistent. Mesoscale SSH variance just west of the retroflection increases by � factor of 2 f r om early summer to winter, mainly because of the anticyclonic eddies. Interhem1sphenc water mass transfer associated with the eddy flux out of the NBC retroflection may amount to an average transport of 3 Sv.