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Accueil > Actualités > Séminaires

Eletta Negretti - LEGI

Spatially developing gravity currents

Vendredi 13 Oct 2017, INSA, Amphi Est bât. Humanités

Spatially developing gravity currents

Gravity currents often occur on complex topographies and are therefore subject to spatial development. Results from laboratory experiments on continuously supplied gravity currents moving from a horizontal to a sloping boundary, that is either concave or straight, will be presented. 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 (largeTa). 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 comparison of this equation with the experimental velocity data 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.

The presence of the curvature may induce centrifugal instability (Görtler instability). We did cross-stream slit lighting and dye injections at the boundary but could not detect any Görtler vortices near the boundary. Up to onset of KH instability the boundary layer is laminar (low CD) and Görtler vortices if present would only increase CD by less than a factor 4 (Swearingen & Blackwelder 1987).


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