![Hier klicken, um den Treffer aus der Auswahl zu entfernen](images/unchecked.gif) |
Titel |
Analysis of the entrainment on lock-exchange density currents |
VerfasserIn |
Helena I. S. Nogueira, Claudia Adduce, Elsa Alves, Mário J. Franca |
Konferenz |
EGU General Assembly 2011
|
Medientyp |
Artikel
|
Sprache |
Englisch
|
Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 13 (2011) |
Datensatznummer |
250051841
|
|
|
|
Zusammenfassung |
Density or gravity currents are geophysical flows driven by density differences within a fluid
which can be caused by temperature differences or the presence of dissolved substances or
particles in suspension. Oceanic fronts and turbidity currents in lakes and reservoirs are
examples of gravity currents occurring in masses of water. In the governing equations of
density currents, namely in the total mass conservation equation, the entrainment rate of the
ambient fluid through the upper interface becomes an explicit or source term requiring a
closure model. The entrainment rate may be modelled as a function of the buoyancy velocity
and a characteristic density, and it is usually evaluated through empirical relations. The aim
of the present work is to experimentally investigate the entrainment of ambient
fluid into unsteady density currents performed by lock-exchange releases of saline
water into a fresh water tank. The experiments were conducted at the Laboratory of
Hydraulics of University of Rome “Roma Tre” in a 3.0 m long, 0.20 m wide and 0.30 m
deep transparent Perspex flume. Eight lockexchange release tests were performed
varying the density of the saline water, the water depth in the water tank and the bed
roughness. The experiments were performed by varying one parameter at a time. For
smooth bed and for a fixed value of water depth, h = 0.20 m, four different initial
densities of the salt-water mixture were analysed: 1015, 1030, 1045 and 1060Â kg/m3;
for a fixed initial density of 1030 kg/m3, one test was performed by changing the
water depth to 0.25Â m; the effect of bed roughness was investigated for a fixed
water depth and fixed density, 1030Â kg/m3, by placing a thick layer of sediments
on the bed with three different values of D50: 2.9Â mm, 4.6Â mm and 24.6Â mm,
where D50 is the grain size diameter for which 50% of the sediments have smaller
diameters. A controlled quantity of dye is added to the saline water in the lock to
provide flow visualization and to serve as density tracer. The development of the
current is recorded with a 25Â Hz CCD camera under controlled light conditions. The
resulting video frames are thus converted into gray scale matrices and a calibration
procedure establishes a non-linear relation, experimentally determined, between
the gray scale values and the quantity of dye in the water. The quantity of dye is
converted into salt concentration by assuming a linear relation between quantities,
dye and salt, allowing thus the estimation of the 2D instantaneous current density
distribution. With the resulting 2D density maps, temporal and spatial evolution of
the current height and depthaveraged density and temporal evolution of the front
position, front velocity, mass balance and entrainment rate may be assessed and
discussed taking into account the differences in the initial density of the saltwater
mixture, the water depth and bed roughness. Detailed reconstruction of the current 2D
geometry is possible, allowing the identification of such phenomena as instabilities
in the upper mixing layer; these are observed in the plots of current height and
instantaneous density maps, and may be related with the process of water entrainment. They
prove to be quasistationary, or at least varying at a different (lower) time scale than
the remainder current, being slightly advected downstream by the mean current
velocity.
Research supported by Portuguese Foundation for Science and Technology through the
research project PTDC/ECM/099752/2008 and the PhD grant SFRH/BD/48705/2008. |
|
|
|
|
|