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| Titel |
Scaling Laws for Lobe and Cleft Patterns at the Front of Particle-Laden Gravity Currents |
| VerfasserIn |
Andrew Jackson, Barbara Turnbull, Richard Munro |
| 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 |
250046535
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| Zusammenfassung |
| There are many examples of gravity currents in geophysics, such as snow avalanches,
pyroclastic flows, thunderstorm outflows and sea-breeze fronts. These currents are driven by
the buoyancy arising from a density difference, either between two homogenous fluids, or a
fluid and particles suspended by turbulence within it. One feature that most turbulent gravity
currents share is a pattern of lobes and clefts at the moving front of the current. Basal
friction causes the foremost part of the current to rise above the surface, creating a
region of unstable stratification at the moving front. It is from this region that the
instability which causes the initial lobe and cleft pattern originates. The wavenumber of
the lobe and cleft pattern has been shown to be related to the Grashoff number,
which is a dimensionless parameter relating buoyancy induced inertia to viscous
forces.
Findings from a previous study of unsuspended granular flows of polystyrene particles
suggest that, although this instability causes initial fluctuations, vortices formed in the
moving front develop these fluctuations into a more stable pattern of lobes. The wavenumber
of these lobes is determined by a velocity boundary layer, the thickness of which is
determined by the size of the particles.
This study is an experimental investigation of the pattern and development of lobes and
clefts found at the moving front of a gravity current of fully suspended expanded
polystyrene particles in air. Experiments are performed in order to ascertain whether
the wavenumber of the pattern that forms at the moving front of these currents is
determined by a velocity boundary layer, as characterised by the Grashoff number of the
flow.
Various length scales are considered when determining the Grashoff number in the
analysis. One such length scale is based on the diameter of the particles used. Initial results
indicate that fluctuations in the moving front of suspended granular flows of polystyrene
particles develop into a more stable pattern of lobes, unlike flows of two homogenous fluids
which produce a constantly shifting pattern. The size of these lobes scales inversely with the
size of the particles, the wavenumber of the pattern decreasing as the particle diameter
increases. This suggests that the nonlinear feedback growth of pairs of vortices in the moving
front is responsible for developing the initial fluctuations into the more stable pattern
observed. The size of these vortices is determined by the thickness of a velocity
boundary layer, the thickness of which is determined by the diameter of the granular
particles. |
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