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| Titel |
Laboratorial study of continuously fed low-submergence gravity currents over smooth and rough beds |
| VerfasserIn |
Andreia F. Lopes, Helena I. S. Nogueira, Rui M. L. Ferreira, Mário J. Franca |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250079970
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| Zusammenfassung |
| Density or gravity currents are geophysical flows driven by density differences between two
fluids which may be caused by temperature differences, dissolved substances or particles in
suspension, among others. Examples of gravity currents include: in the atmosphere, sea
breeze fronts driven by differences in temperature, avalanches of airborne snow, plumes of
pyroclasts from volcanic eruptions and sand storms driven by suspended particles; in the
water, oceanic fronts, resulting from differences in temperature and salinity, and turbidity
currents caused by suspended particles. The release of pollutants into rivers and oceans
and desalination plant outflows are examples of anthropogenic gravity currents
frequently with negative environmental impacts. Closures for governing equations
of gravity currents, mass and momentum conservation equations, are needed in
what concerns the interaction between these and the lower fixed-bed and the upper
permeable boundary. Herein experimental data obtained under laboratory controlled
conditions is used to investigate the interaction between density currents and the fixed
bed and the surrounding fluid. Instantaneous velocities are analyzed and discussed
after application of double-averaging methods (D-AM), in time and space, for data
processing. Experiments were performed at the Laboratory of Hydraulics and Water
Resources, Department of Civil Engineering and Architecture at Instituto Superior
Técnico, in a 12.5Â m long, 40.9Â cm wide and 50Â cm deep recirculating, glass-walled
channel with variable slope. Saline currents with two different initial densities of
1010 and 1020Â kg/m3 were simulated over two types of horizontal beds (smooth
and rough constituted by one layer of cobbles with 3.5 mm mean diameter) into
the channel filled with fresh water (density of approximately 1000Â kg/m3). The
saline water was injected continuously, by means of a submersible pump (flow
rate of 0.0339Â l/s), at the upstream section of the channel and let flow through
the lower layer of the channel. The water depth in the channel was between 11
and 13.8Â cm and a downstream controlled outlet allowed keeping the water level
constant during the experiments. Measurement of the instantaneous velocity fields
of the flow were made with PIV instrumentation. This allows the photographic
record of the position of tracing particles (both added in the saline solution and
water at room temperature) within the measurement zone illuminated by means of a
laser sheet, using a CCD camera. Two-dimensional instantaneous velocities are
inferred, based on the determination of the displacement of the tracer particles in the
time interval between two flow images acquired consecutively, by application of an
adaptive-correlation algorithm. A return flow is observed with velocities of the same order of
magnitude of the main density current. The application of D-AM to the velocity
measurements allows the interpretation of turbulent mechanisms at the interface
between the bed and the current and between the surrounding fluid and the current by
analysis of velocity profiles, Reynolds stresses and dispersive stresses. The latter ensue
from the intermittency in time and space of both lower and upper boundaries of the
current.
Research supported by Portuguese Foundation for Science and Technology: research
project PTDC/ECM/099752/2008. |
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