 |
| Titel |
Periodic cycle of stretching and breaking of the head of gravity currents |
| VerfasserIn |
H. I. S. Nogueira, C. Adduce, E. Alves, M. J. Franca |
| Konferenz |
EGU General Assembly 2012
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250061458
|
|
|
|
|
|
| Zusammenfassung |
| Gravity currents, which are geophysical flows driven by density differences within a fluid, are
herein investigated under unsteady conditions by means of lock-exchange releases of
saline water into a fresh water tank. Generally, gravity or density currents are caused
by temperature differences or the presence of dissolved substances or particles
in suspension. Examples of gravity currents include avalanches of airborne snow
and plumes of pyroclasts from volcanic eruptions, in the atmosphere, releases of
pollutants and turbidity currents, in rivers, lakes and reservoirs, and oil spillage and
oceanic fronts in the ocean. A controlled and convenient fashion to investigate in
detail hydrodynamics of unsteady gravity currents is by means of lock-exchange
experiments.
The propagation of unsteady density currents, produced by lock exchange experiments,
present three distinct phases, a first so-called slumping phase when buoyancy and inertial
effects are balanced and front celerity is constant, a second (self-similar) phase when the
reflected bore from the upper layer ambient fluid upstream drive, caused by continuity within
the limited length tank, reaches the current front and causes the front celerity to decrease and
provokes a diminution of the current head and, finally, a third viscous phase when viscosity
plays a role and its effects overcome inertial effects. On the first and second phase, the current
propagation is ruled by buoyancy effects counterbalanced by inertia, Reynolds stresses on the
upper mixing layer and bed shear. Buoyancy is reduced due to entrainment and
consequently the front velocity, leading to lower Reynolds number flows allowing
thus viscosity effects to play a role. As for its anatomy, the current presents two
distinct regions, the head and the remaining body or tail. On the very first instants of
the release, the flow is bulky driven by the whole current mass while the head is
not yet well defined. Later, this detaches from the main body and its particular
buoyancy drives the advance of the current, with a different celerity from the tail. The
head is highly concentrated being the main engine of convection of the released
mass, being subjected to entrainment at the interface with the ambient fluid. The
aim of the present work is to experimentally investigate the dynamics of the head,
including continuous entrainment and cycles of stretching and breaking observed in the
laboratory.
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. Four
lock-exchange release tests were performed varying the density of the saline water. For
smooth bed and for a fixed value of water depth, h = 0.20 m, the following four different
initial densities of the salt-water mixture were analysed: 1015, 1030, 1045 and
1060 kg/m3. 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 25 Hz CCD camera under controlled light conditions. The
resulting video frames are thus converted into grey 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.
The experiments allowed the observation of the dynamics of the head of unsteady density
currents in detail, including a cyclic increase in dimension and mass due to entrainment
followed by a division in two distinct patches. A frontal one continues the drive
downstream whereas a subsequent one is left behind and incorporated in the tail, thus
indicating that the loss of saline mass in the head is not only due to continuous
entrainment at the interface layer. Entrainment follows a decaying trend along the
current development whereas periodic division of the head seems to be kept. The
division of the head is related to mass ejections directing upstream with a clear
signature in the current-depth time and spatial evolution maps. Initial density of the
released saline current seems to be related to the period of the cyclic division of the
head and the amplitude of the mentioned mass ejections; averaged periods of the
occurrence of the divisions are 3.40, 1.63, 1.07 and 0.91 s respectively for initial
densities of the salt-water mixture corresponding to 1015, 1030, 1045 and 1060
kg/m3.
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. |
|
|
|
|
|
|