Model studies of dense water overflows in the Faroese Channels Topical Collection on the 5th International Workshop on Modelling the Ocean (IWMO) in Bergen, Norway 17-20 June 2013

The overflow of dense water from the Nordic Seas through the Faroese Channel system was investigated through combined laboratory experiments and numerical simulations using the Massachusetts Institute of Technology General Circulation Model. In the experimental study, a scaled, topographic representation of the Faroe-Shetland Channel, Wyville-Thomson Basin and Ridge and Faroe Bank Channel seabed bathymetry was constructed and mounted in a rotating tank. A series of parametric experiments was conducted using dye-tracing and drogue-tracking techniques to investigate deep-water overflow pathways and circulation patterns within the modelled region. In addition, the structure of the outflowing dense bottom water was investigated through density profiling along three cross-channel transects located in the Wyville-Thomson Basin and the converging, up-sloping approach to the Faroe Bank Channel. Results from the dye-tracing studies demonstrate a range of parametric conditions under which dense water overflow across the Wyville-Thomson Ridge is shown to occur, as defined by the Burger number, a non-dimensional length ratio and a dimensionless dense water volume flux parameter specified at the Faroe-Shetland Channel inlet boundary. Drogue-tracking measurements reveal the complex nature of flow paths and circulations generated in the modelled topography, particularly the development of a large anti-cyclonic gyre in the Wyville-Thompson Basin and up-sloping approach to the Faroe Bank Channel, which diverts the dense water outflow from the Faroese shelf towards the Wyville-Thomson Ridge, potentially promoting dense water spillage across the ridge itself. The presence of this circulation is also indicated by associated undulations in density isopycnals across the Wyville-Thomson Basin. Numerical simulations of parametric test cases for the main outflow pathways and density structure in a similarly-scaled Faroese Channels model domain indicate excellent qualitative agreement with the experimental observations and measurements. In addition, the comparisons show that strong temporal variability in the predicted outflow pathways and circulations have a strong influence in regulating the Faroe Bank Channel and Wyville-Thomson Ridge overflows, as well as in determining the overall response in the Faroese Channels to changes in the Faroe-Shetland Channel inlet boundary conditions. © 2014 Springer-Verlag Berlin Heidelberg

Het Wave-experiment bij Feijenoord – een terugblik

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Vortices in time-periodic shear flow

Vortices emerging in geophysical turbulence may experience deformations due to the non-uniform ambient flow induced by neighbouring vortices. As a first approximation this ambient flow is modelled by a linear shear flow. It is well known from previous studies that the vortex may be (partially) destructed through removal of weak vorticity at the vortex edge – a process referred to as ‘stripping’. While most previous studies considered a stationary external shear flow, we have examined the behaviour of the vortex embedded in a linear shear flow whose strength changes harmonically in time. Aspects of the vortex dynamics and the (chaotic) transport of tracers have been studied by both laboratory experiments and numerical flow simulations

Modelling of geophysical vortices

The large-scale vortex structures that are commonly observed both in the oceans (e.g. Gulf Stream rings) and in the atmosphere (hurricanes, cyclones, Jupiter’s Great Red Spot) are in good approximation two-dimensional (2D), due to the combined eff ects of geometrical confi nement (essentially, the oceans and atmosphere are thin fl uid shells), density stratifi cation, and planetary rotation. Th eir dynamics is governed by conservation of potential vorticity, which is here defi ned as PV=(f+¿)/H, with f the Coriolis parameter (=2Osin f, with O the planetary station speed, f the geographical latitude), ¿ the local relative vorticity, and H the local layer depth (column height)

Zelforganisatie van twee-dimensionale stromingen

In tegenstelling tot driedimensionale stromingen (3D) worden tweedimensionale turbulente stromingen (2D) gekenmerkt door een zgn. energieflux naar de grotere lengteschalen. Deze eigenschap is direct waarneembaar in de geleidelijke vorming van grootschalige coherente wervelstructuren, een proces dat ook wel wordt aangeduid als ‘zelforganisatie’ van 2D turbulentie. Als gevolg van de aardrotatie maar ook tengevolge van de stabiele dichtheidsopbouw zijn de grootschalige stromingen in de atmosfeer en oceanen in goede benadering tweedimensionaal. De talloze grootschalige coherente wervels, die men via satellietmetingen heeft waargenomen, zijn een manifestatie van bovengenoemd proces. In het laboratorium worden 2D turbulente stromingen – en in het bijzonder daarin optredende wervels – op kleine schaal onder geconditioneerde omstandigheden onderzocht in experimenten in roterende en/of gestratificeerde vloeistoffen. Op deze wijze is veel inzicht verkregen in de stabiliteit van wervels, hun onderlinge wisselwerking en hun transporteigenschappen. Naast directe stromingsvisualisatie (toevoeging van kleurstoffen) worden er ook geavanceerde optische meettechnieken toegepast waarmee de beweging van duizenden tracer-deeltjes simultaan kan worden gevolgd. Daarnaast spelen numerieke stromingssimulaties een steeds belangrijker rol in het onderzoek op dit terrein. Tijdens de lezing zal een overzicht worden gegeven van de algemene eigenschappen van 2D turbulentie en zullen enkele experimenten worden besproken waarin het fascinerende verschijnsel van ‘zelforganisatie’ waarneembaar is

Shallow flows : 2D or not 2D?

It is commonly assumed that shallow flows are in good approximation two-dimensional (2D) or quasi-2D. We will provide evidence that this is not always the case, and that the simple scaling argument based on the continuity equation does not always apply. Laboratory experiments on vortex flows in shallow fluid layers have revealed that locally significant three-dimensional (3D) effects and substantial vertical motions may occur, clearly destroying the assumed 2D character of the flow. For example, in the case of a dipolar vortex structure, an oscillatory vertical motion is observed in the vortex cores, while a spanwise circulation roll is present in front of the travelling dipole. These laboratory observations are confirmed by 3D numerical flow simulations. Attention will be given to a correct scaling analysis, in which both the aspect ratio of the fluid depth and a typical horizontal scale and the Reynolds number play a role

Two-dimensional turbulence on a bounded domain : the role of angular momentum

No abstract