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| Titel |
Gravity currents moving on smooth and rough beds |
| VerfasserIn |
Valentina Lombardi, Claudia Adduce, Giampiero Sciortino, Michele La Rocca |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250047235
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| Zusammenfassung |
| The aim of this work is the analysis of gravity currents moving on both smooth and rough
beds by laboratory experiments. Gravity currents occur when two fluids with different
densities come in contact. The lighter fluid overlays the heavier one, causing a relative motion
between the two fluids. Full depth lock-exchange release experiments were performed to
produce gravity currents in the laboratory and the instantaneous velocity field was
measured by Particle Image Velocimetry (PIV). The experiments were performed in a
Perspex tank of rectangular cross-section 3 m long, 0.3 m deep and 0.2 m wide.
The tank was divided in two portions separated by a vertical sliding gate placed
at a distance x0 from the left end wall of the tank. The lock was filled with salty
water with density Ï01 and the other part of the tank was filled with tap water with
density Ï2 (Ï01 >Ï2). The desired density was obtained dissolving a quantity of salt
into the fresh water and measurements of the initial density were performed by
a pycnometer. Both in the right and in the left part of the tank the total depth of
the fluid was h0. A quantity of dye was dissolved into the salt water to allow the
visualization of the gravity current after the gate’s removal. At the beginning of each
experiment the gate was suddenly removed, and the heavier fluid moved from the left
portion of the tank forming a gravity current. The experiment was completed when
the gravity current reached the right wall of the tank. All the experiments were
recorded by a CCD camera and an image analysis technique, based on a threshold
method, was applied to measure the space-time evolution of the gravity currents’
profiles.
The experiments were performed with two different values of bed’s roughness É = 0 mm
and 4.5 mm and keeping constant the initial density of the salty mixture Ï01 = 1015 Kg/m3,
the density of the lighter fluid Ï2 = 1000 Kg/m3, the position of the gatex0 = 0.1 m and
the depth of the two fluidsh0 = 0.2 m. The desired roughness É was obtained by
gluing sand with a defined mean diameter on the bottom of the tank. In agreement
with previous studies, three different phases in the gravity current’s dynamics were
observed: a first slumping phase, in which the front position varies linearly with
time and the front speed is constant; a second self-similar non-viscous phase, in
which the front speed decreased as t-1-3; a third self-similar viscous phase, which
occurs when viscous effects become predominant and in which the front velocity
decreased as t-4-5. The space-time evolution of gravity currents’ profiles, measured for
different bed’ roughness, shows that as the bed’s roughness increases the front speed
decreases. Preliminary velocity measurements, performed by PIV, show the presence of
interfacial instabilities at the boundary between the lighter and the heavier fluid. |
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