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Mixing induced in a dense current flowing down a sloping bottom in a rotating fluid
No full textA density driven current was generated in the laboratory by releasing dense fluid over a sloping bottom in a rotating freshwater system. Over a wide range of parameter values, the following four flow types were found: laminar, wave, turbulent and eddy regime. The amount of mixing between the dense and the ambient fluids was measured and its dependence on the Froude number and on the distance downslope was determined for increasing values of the Reynolds number. Mixing increased significantly when passing from the laminar to the wave regime; i.e. with increasing Froude number. We believe that mixing between the dense salty water and the lighter fresh water was caused by breaking waves. We quantified the amount of mixing observed and estimated the value of the entrainment velocity at the interface between the dense fluid and the fresh overlying fluid. The results have been compared with previous laboratory experiments which presented the classic turbulent entrainment behavior and observational estimates of the Mediterranean and Denmark Strait overflow
Mixing in a density-driven current flowing down a slope in a rotating fluid
Full text linkAuthor Posting. © Cambridge University Press, 2008. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 604 (2008): 369-388, doi:10.1017/S0022112008001237.We discuss laboratory experiments investigating mixing in a density-driven current flowing down a sloping bottom, in a rotating homogenous fluid. A systematic study spanning a wide range of Froude, 0.8 < Fr < 10, and Reynolds, 10 < Re < 1400, numbers was conducted by varying three parameters: the bottom slope; the flow rate; and the density of the dense fluid. Different flow regimes were observed, i.e. waves (non-breaking and breaking) and turbulent regimes, while changing the above parameters. Mixing in the density-driven current has been quantified within the observed regimes, and at different locations on the slope. The dependence of mixing on the relevant non-dimensional numbers, i.e. slope, Fr and Re, is discussed. The entrainment parameter, E, was found to be dependent not only on Fr, as assumed in previous studies, but also on Re. In particular, mixing increased with increasing Fr and Re. For low Fr and Re, the magnitude of the mixing was comparable to mixing in the ocean. For large Fr and Re, mixing was comparable to that observed in previous laboratory experiments that exhibited the classic turbulent entrainment behaviour.Support was given by the National Science Foundation
project number OCE-0350891
Mixing induced in a dense plume flowing down a sloping bottom in a rotating fluid: a new entrainment parameterization?
No full textWe will discuss laboratory experiments investigating mixing in a density driven current flowing down a sloping bottom in a rotating homogenous fluid. A systematic study spanning a wide range of Froude, Fr, and Reynolds, Re, numbers was conducted by varying four parameters: the rotation rate, the bottom slope, the flowrate, and the density of the dense fluid. Different flow regimes, i.e. laminar, wave, turbulent and eddy regimes, were observed either in different experiments, while changing the above parameters, or simultaneously in the same experiment, as the current descended the slope. Mixing in the density driven current was quantified within the observed different flow regimes and at different locations on the slope. The
dependence of mixing on the relevant non-dimensional numbers, i.e. Fr and Re, will be discussed. Mixing increased with increasing Fr. For low Fr the magnitude of the mixing was comparable to mixing in the ocean. For large Fr and Re, mixing was comparable, or slightly lower, than in previous laboratory experiments that presented the classic turbulent entrainment behavior with larger Re. We will suggest a new empirical
parameterization for entrainment in dense currents that presents two novelties when compared to the classical
Ellison and Turner [1959] parameterization. First, it depends both on the Fr and Re of the flow and it accurately predicts both ocean and laboratory estimates of mixing. Second, it takes into account subcritical (Fr<1) mixing. The subcritical mixing observed in the present experiments could be of fundamental importance when determining the final water mass characteristics of a dense overflow current descending the continental slope. A weak but non zero entrainment can substantially change the final density and,
consequently, the location of important water masses, such as the North Atlantic Deep Water, in the open ocean water column. Finally, a comparison of the laboratory results to those of a “stream tube” model will be presented. We will show that the model predictions are consistent with laboratory observations when the new entrainment parameterization is employed
Laboratory experiments on eddy collisions with seamounts of varying geometry
No full textCollisions of oceanic mesoscale eddies with seamounts often result in major modifications of their structure, having significant impacts to the re-distribution of water properties and mixing rates. We investigated the interaction between a self-propagating barotropic cyclonic eddy with an obstacle and determined the conditions for an eddy
to bifurcate into two eddies. A series of idealized laboratory experiments were carried out in a glass tank mounted concentrically on a 1 m diameter rotating turntable.
As in a previous study, after a self-propagating cyclonic eddy came into contact with
the obstacle, fluid peeled off the outer edge of the eddy and a so-called ”streamer” went around the cylinder in a counterclockwise direction. Under certain conditions,
this fluid formed a new cyclonic eddy in the wake of the cylinder, causing bifurcation of the original eddy into two eddies. In the present study we performed three sets of idealized laboratory experiments with the aim of investigating the importance of the slope of the side walls of the obstacle, the influence of the obstacle horizontal cross
sectional area and the importance of the height of the obstacle. The present results suggest that bifurcation occurs only when the obstacle height is more than 0.85 % of the eddy height and that steep sloping walls do not influence the bifurcation mechanism. In addition, experiments performed using an obstacle with an elliptical horizontal
cross section revealed that the length which the ”streamer” has to travel around the obstacle (and not the dimension of the obstacle in the direction orthogonal to the flow) is the relevant parameter governing the occurrence of bifurcation. Observations of a ”Meddy” bifurcating after collision with the Irving Seamount in the Canary Basin show similar behavior suggesting that these idealized laboratory experiments could be used to infer the result of eddy collisions with seamounts of varying geometry in the ocea
A new parameterization for entrainment in overflows
Full text linkAuthor Posting. © American Meteorological Society, 2010. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 40 (2010): 1835–185, doi:10.1175/2010JPO4374.1.Dense overflows entrain surrounding waters at specific locations, for example, sills and constrictions, but also along the descent over the continental slope. The amount of entrainment dictates the final properties of these overflows, and thus is of fundamental importance to the understanding of the formation of deep water masses. Even when resolving the overflows, coarse resolution global circulation and climate models cannot resolve the entrainment processes that are often parameterized. A new empirical parameterization is suggested, obtained using an oceanic and laboratory dataset, which includes two novel aspects. First, the parameterization depends on both the Froude number (Fr) and Reynolds number of the flow. Second, it takes into account subcritical (Fr < 1) entrainment. A weak, but nonzero, entrainment can change the final density and, consequently, the depth and location of important water masses in the open ocean. This is especially true when the dense current follows a long path over the slope in a subcritical regime, as observed in the southern Greenland Deep Western Boundary Current. A streamtube model employing this new parameterization gives results that are more consistent with previous laboratory and oceanographic observations than when a classical parameterization is used. Finally, the new parameterization predictions compare favorably to recent oceanographic measurements of entrainment and turbulent diapycnal mixing rates, using scaling arguments to relate the entrainment ratio to diapycnal diffusivities.Support was given by the National Science Foundation
Project OCE-0350891 and OCE-0726339
Indagine sperimentale sul mescolamento di una corrente densa su un piano inclinato
No full textL’obiettivo del presente lavoro è lo studio, mediante esperimenti di laboratorio, del mescolamento asso-ciato ad una corrente di gravità che si muove lungo un piano inclinato ed in ambiente rotante. Gli espe-rimenti sono stati condotti utilizzando una vasca parallelepipeda montata su un tavolo rotante. La vasca è stata riempita di acqua dolce, fluido ambiente, e all’interno di essa è stato posizionato un piano inclinato. La corrente di gravità è stata generata immettendo sul piano inclinato, mediante un pompa, un fluido a densità superiore al fluido ambiente. Tutti gli esperimenti sono stati filmati dall’alto mediante una telecamera montata solidalmente alla vasca. Durante gli esperimenti la corrente di gravità si è propagata lungo il piano inclinato, producendo un mescolamento dovuto all’ “entrainment” del fluido ambiente. Sono state condotte differenti prove sperimentali variando la pendenza del piano inclinato, la velocità di rotazione, la portata e la densità iniziale della corrente. Al variare delle condizioni sperimentali si sono osservati quattro regimi di moto della corrente: regime laminare, regime delle onde (frangenti e non), regime dei vortici e regime turbolento. Al termine di tutte le prove si è misurata la densità della corrente di gravità e quindi si è valutato il mescolamento al termine del percorso lungo il piano inclinato. All’aumentare della pendenza del fondo si è osservato un aumento del mescolamento associato al regime di onde che frangendosi producevano maggiore “entrainment” di fluido ambiente da parte della corrente di gravità. Per i valori più elevati della pendenza del fondo, la corrente di gravità non è risultata completamente mescolata con il fluido ambiente. Per una serie di prove si è valutato anche il mescolamento in diversi punti posizionati lungo il percorso della corrente, confrontando il risultato con la teoria del mescolamento di un “plume” verticale e turbolento fornita da Turner (1973). I risultati sperimentali risultano in accordo con la teoria di Turner nel caso di una corrente di gravità in regime di moto turbolento. Al contrario, quando il regime di moto della corrente è di tipo laminare-onde, la teoria di Turner non è più adatta a modellare il mescolamento lungo il percorso della corrente stessa
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