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Séminaire E. Negretti

16 janvier

Spatially developing dense plumes

Séminaire Eletta Negretti - (Chargé de recherche au LEGI)

Jeudi 25 janvier à 14 h 00 - Amphithéâtre Nougaro

Abstract :

Gravity currents often occur on complex topographies and are therefore subject to spatial development. I will present results from laboratory experiments on continuously supplied gravity currents moving from a horizontal to a sloping boundary, that is either concave or straight. The change in boundary slope and consequent acceleration gives rise to a transition from a stable subcritical current with large Richardson number, to a Kelvin-Helmholtz (KH) unstable current. It is shown here that depending on the overall acceleration parameter Ta, expressing the rate of velocity increase, the currents can adjust gradually to the slope conditions (small Ta) or go through acceleration-deceleration cycles (large Ta). In the latter case, the KH billows at the interface have a strong effect on the flow dynamics, and are observed to cause boundary layer separation.
Comparison of currents on concave and straight slopes reveals that the downhill deceleration on concave slopes has no qualitative influence, i.e, the dynamics is entirely dominated by the initial acceleration and ensuing KH billows.
Following the similarity theory of Turner (1976) a general equation for the depth-integrated velocity is derived that exhibits all driving and retarding forces. The numerical solution of this equation predicts well the experimental velocity data. The comparison shows that when Ta is large, bottom friction and entrainment are large in the region of appearance of KH billows. The large bottom friction is confirmed by the measured high Reynolds stresses in these regions. The head velocity does not exhibit the same behaviour as the layer velocity. It approaches gradually an equilibrium state even when the acceleration parameter of the layer is large.
Preliminary results from experiments of saline gravity currents flowing over a horizontal sediment bed will also be presented.