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| Titel |
Comparison of field measurements and CFD simulation of spatial offshore windflow patterns across a foredune system |
| VerfasserIn |
Meiring Beyers, Derek Jackson, Irene Delgado-Fernandez, Andrew Cooper, Andreas Baas, Kevin Lynch |
| 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 |
250053540
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| Zusammenfassung |
| Aeolian sediment input into coastal dunes represents a key component of sediment budget
analysis of beach-dune systems. Calculations of sediment input to the foredunes are
traditionally based on onshore winds, often excluding the role played by offshore wind
events. However, recent work has shown that offshore flows play an essential role in
post-storm dune recovery and maintenance. As a perpendicular offshore flow meets
the dune crestline the surface flow layer detaches from the ground and generates
an area characterised by turbulent eddies in the dune lee slope. At some distance
downstream the flow separates into an offshore “re-attached” component and a
reversed component directed toward the dune toe, with the potential to transport
sediment. The existence of flow separation or steering has previously been qualitatively
linked with dune topography and angle of wind approach, but not yet quantitatively
described.
This study was specifically developed to examine dune re-circulation airflow
characteristics under a range of incident wind velocities and offshore directions. Field work
was undertaken in April-May 2010 at Magilligan, Northern Ireland. Wind data was
measured using a grid of ultrasonic anemometers deployed at the beach-dune surface
covering an area of 90 m long-shore by 65 m cross-shore. The field investigation was
supplemented by Computational Fluid Dynamic (CFD) simulations to help prepare the
field test procedure and help site the rake of masts and it’s mounting of ultrasonic
anemometers. CFD simulations were also performed to explore the potential use
and accuracy of CFD simulation in geomorphological investigations, and improve
understanding of wind flow characteristics through 3D wind flow visualisation.
Previous work by the authors has highlighted differences in the characteristics of
leeward re-circulation and re-attachment patterns under scenarios of perpendicular and
oblique offshore wind flows. In the present work, an attempt is made to identify the
origin of unique spatial wind and flow separation and re-attachment characteristics
discovered in the measured wind data. The ultrasonic anemometer measurements
obtained on a grid 0.5 m above the terrain within the leeward recirculation zone
indicated that the flow re-attachment points varies significantly along the fore-dune
length.
The complimentary use of Computational Fluid Dynamics helps the investigation by
allowing visualisation of basic wind flow characteristics. It also enables us to explore in detail
whether the spatial wind flow features found in the leeward dune zone is primarily a function
of subtle dune topography changes along the shoreline or whether terrain roughness at the
windward and leeward zones is the dominant trigger in dictating leeside flow behaviour.
Previous CFD simulation results were supported by our fieldwork measurements which
suggested that the spatial wind distribution in the zone leeward of the fore-dune is in fact
dependent on the changes in the topography along the dune crests with further correlations to
wind direction and even wind speed. The work here attempts to quantify these
influences.
In the present investigation a simplified geometry of the fore-dune surface terrain is
created that closely mimics the 3-D topography but allows the isolation of terrain form,
roughness and wind directional effects. We initially compare CFD results over the simplified
3-D fore-dune system with measured 3-D anemometry results then present simulations
and comparisons for varying wind direction, terrain roughness and fore-dune form
parameters. Results highlight that for offshore winds, relatively small undulations in
foredune form may be the primary driver for the unique spatial distribution of the
re-attachment point, more so than upstream roughness effects. It also confirms that
dune orientation, relative to the offshore winds, is key in influencing alongshore
steering or recirculating leeward flows and associated potential aeolian sand transport. |
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