Reattachment zone characterisation under offshore winds with flow separation on the lee side of coastal dunes

Irene Delgado-Fernandez, Derek Jackson, Andrew Cooper, Andreas Baas, Kevin Lynch, Meiring Beyers

Research output: Contribution to conferencePoster

Abstract

Airflow separation, lee-side eddies and secondary flows play an essential role on the formation and maintenance of sand dunes. Downstream from dune crests the flow surface layer detaches from the ground and generates an area characterised by turbulent eddies in the dune lee slope (the wake). At some distance downstream from the dune crest, flow separates into a reversed component directed toward the dune toe and an offshore “re-attached” component. This reattachment zone (RZ) has been documented in fluvial and desert environments, wind tunnel experiments and numerical simulations, but not yet characterised in coastal dunes. This study examines the extent and temporal evolution of the RZ and its implications for beach-dune interaction at Magilligan, Northern Ireland. Wind parameters were measured over a profile extending from an 11 m height dune crest towards the beach, covering a total distance of 65 m cross-shore. Data was collected using an array of nine ultrasonic anemometers (UAs) deployed in April-May 2010, as part of a larger experiment to capture airflow data under a range of incident wind velocities and offshore directions. UAs were located along the profile (5 m tower spacing) over the beach, which allowed a detailed examination of the RZ with empirical data. Numerical modelling using Computational Fluid Dynamics (CFD) software was also conducted with input data from anemometer field measurements, running over a surface mesh generated from LiDAR and DGPS surveys. Results demonstrate that there is a wind threshold of approximately 5-6 ms-1 under which no flow separation exists with offshore winds. As wind speed increases over the threshold, a flow reversal area is quickly formed, with the maximum extent of the RZ at approximately 3.5 dune heights (h). The maximum extent of the RZ increases up to 4.5h with stronger wind speeds of 8-10 ms-1 and remains relatively constant as wind speed further increases. This suggests that the spatial extent of the RZ is largely independent of incident wind speed and is located between 4-5h. The magnitude of the maximum extent of the RZ is similar to that simulated using CFD and is consistent with previous studies conducted in desert dunes and wind tunnel simulations for offshore winds blowing over tall and sharp-crested dunes. Ongoing analyses are being conducted to evaluate the effect of changing wind direction, dune height and shape.
Original languageEnglish
Publication statusPublished - 2010
EventAmerican Geophysical Union (AGU) Fall Meeting - San Francisco, United States
Duration: 13 Dec 201017 Dec 2010

Conference

ConferenceAmerican Geophysical Union (AGU) Fall Meeting
CountryUnited States
CitySan Francisco
Period13/12/1017/12/10

Fingerprint

dune
wind velocity
anemometer
beach
computational fluid dynamics
wind tunnel
airflow
eddy
desert
DGPS
secondary flow
temporal evolution
wind direction
simulation
surface layer
spacing
experiment
software

Cite this

Delgado-Fernandez, I., Jackson, D., Cooper, A., Baas, A., Lynch, K., & Beyers, M. (2010). Reattachment zone characterisation under offshore winds with flow separation on the lee side of coastal dunes. Poster session presented at American Geophysical Union (AGU) Fall Meeting, San Francisco, United States.
Delgado-Fernandez, Irene ; Jackson, Derek ; Cooper, Andrew ; Baas, Andreas ; Lynch, Kevin ; Beyers, Meiring. / Reattachment zone characterisation under offshore winds with flow separation on the lee side of coastal dunes. Poster session presented at American Geophysical Union (AGU) Fall Meeting, San Francisco, United States.
@conference{de0388d731854105b46308cc4df1752a,
title = "Reattachment zone characterisation under offshore winds with flow separation on the lee side of coastal dunes",
abstract = "Airflow separation, lee-side eddies and secondary flows play an essential role on the formation and maintenance of sand dunes. Downstream from dune crests the flow surface layer detaches from the ground and generates an area characterised by turbulent eddies in the dune lee slope (the wake). At some distance downstream from the dune crest, flow separates into a reversed component directed toward the dune toe and an offshore “re-attached” component. This reattachment zone (RZ) has been documented in fluvial and desert environments, wind tunnel experiments and numerical simulations, but not yet characterised in coastal dunes. This study examines the extent and temporal evolution of the RZ and its implications for beach-dune interaction at Magilligan, Northern Ireland. Wind parameters were measured over a profile extending from an 11 m height dune crest towards the beach, covering a total distance of 65 m cross-shore. Data was collected using an array of nine ultrasonic anemometers (UAs) deployed in April-May 2010, as part of a larger experiment to capture airflow data under a range of incident wind velocities and offshore directions. UAs were located along the profile (5 m tower spacing) over the beach, which allowed a detailed examination of the RZ with empirical data. Numerical modelling using Computational Fluid Dynamics (CFD) software was also conducted with input data from anemometer field measurements, running over a surface mesh generated from LiDAR and DGPS surveys. Results demonstrate that there is a wind threshold of approximately 5-6 ms-1 under which no flow separation exists with offshore winds. As wind speed increases over the threshold, a flow reversal area is quickly formed, with the maximum extent of the RZ at approximately 3.5 dune heights (h). The maximum extent of the RZ increases up to 4.5h with stronger wind speeds of 8-10 ms-1 and remains relatively constant as wind speed further increases. This suggests that the spatial extent of the RZ is largely independent of incident wind speed and is located between 4-5h. The magnitude of the maximum extent of the RZ is similar to that simulated using CFD and is consistent with previous studies conducted in desert dunes and wind tunnel simulations for offshore winds blowing over tall and sharp-crested dunes. Ongoing analyses are being conducted to evaluate the effect of changing wind direction, dune height and shape.",
author = "Irene Delgado-Fernandez and Derek Jackson and Andrew Cooper and Andreas Baas and Kevin Lynch and Meiring Beyers",
year = "2010",
language = "English",
note = "American Geophysical Union (AGU) Fall Meeting ; Conference date: 13-12-2010 Through 17-12-2010",

}

Delgado-Fernandez, I, Jackson, D, Cooper, A, Baas, A, Lynch, K & Beyers, M 2010, 'Reattachment zone characterisation under offshore winds with flow separation on the lee side of coastal dunes' American Geophysical Union (AGU) Fall Meeting, San Francisco, United States, 13/12/10 - 17/12/10, .

Reattachment zone characterisation under offshore winds with flow separation on the lee side of coastal dunes. / Delgado-Fernandez, Irene; Jackson, Derek; Cooper, Andrew; Baas, Andreas; Lynch, Kevin; Beyers, Meiring.

2010. Poster session presented at American Geophysical Union (AGU) Fall Meeting, San Francisco, United States.

Research output: Contribution to conferencePoster

TY - CONF

T1 - Reattachment zone characterisation under offshore winds with flow separation on the lee side of coastal dunes

AU - Delgado-Fernandez, Irene

AU - Jackson, Derek

AU - Cooper, Andrew

AU - Baas, Andreas

AU - Lynch, Kevin

AU - Beyers, Meiring

PY - 2010

Y1 - 2010

N2 - Airflow separation, lee-side eddies and secondary flows play an essential role on the formation and maintenance of sand dunes. Downstream from dune crests the flow surface layer detaches from the ground and generates an area characterised by turbulent eddies in the dune lee slope (the wake). At some distance downstream from the dune crest, flow separates into a reversed component directed toward the dune toe and an offshore “re-attached” component. This reattachment zone (RZ) has been documented in fluvial and desert environments, wind tunnel experiments and numerical simulations, but not yet characterised in coastal dunes. This study examines the extent and temporal evolution of the RZ and its implications for beach-dune interaction at Magilligan, Northern Ireland. Wind parameters were measured over a profile extending from an 11 m height dune crest towards the beach, covering a total distance of 65 m cross-shore. Data was collected using an array of nine ultrasonic anemometers (UAs) deployed in April-May 2010, as part of a larger experiment to capture airflow data under a range of incident wind velocities and offshore directions. UAs were located along the profile (5 m tower spacing) over the beach, which allowed a detailed examination of the RZ with empirical data. Numerical modelling using Computational Fluid Dynamics (CFD) software was also conducted with input data from anemometer field measurements, running over a surface mesh generated from LiDAR and DGPS surveys. Results demonstrate that there is a wind threshold of approximately 5-6 ms-1 under which no flow separation exists with offshore winds. As wind speed increases over the threshold, a flow reversal area is quickly formed, with the maximum extent of the RZ at approximately 3.5 dune heights (h). The maximum extent of the RZ increases up to 4.5h with stronger wind speeds of 8-10 ms-1 and remains relatively constant as wind speed further increases. This suggests that the spatial extent of the RZ is largely independent of incident wind speed and is located between 4-5h. The magnitude of the maximum extent of the RZ is similar to that simulated using CFD and is consistent with previous studies conducted in desert dunes and wind tunnel simulations for offshore winds blowing over tall and sharp-crested dunes. Ongoing analyses are being conducted to evaluate the effect of changing wind direction, dune height and shape.

AB - Airflow separation, lee-side eddies and secondary flows play an essential role on the formation and maintenance of sand dunes. Downstream from dune crests the flow surface layer detaches from the ground and generates an area characterised by turbulent eddies in the dune lee slope (the wake). At some distance downstream from the dune crest, flow separates into a reversed component directed toward the dune toe and an offshore “re-attached” component. This reattachment zone (RZ) has been documented in fluvial and desert environments, wind tunnel experiments and numerical simulations, but not yet characterised in coastal dunes. This study examines the extent and temporal evolution of the RZ and its implications for beach-dune interaction at Magilligan, Northern Ireland. Wind parameters were measured over a profile extending from an 11 m height dune crest towards the beach, covering a total distance of 65 m cross-shore. Data was collected using an array of nine ultrasonic anemometers (UAs) deployed in April-May 2010, as part of a larger experiment to capture airflow data under a range of incident wind velocities and offshore directions. UAs were located along the profile (5 m tower spacing) over the beach, which allowed a detailed examination of the RZ with empirical data. Numerical modelling using Computational Fluid Dynamics (CFD) software was also conducted with input data from anemometer field measurements, running over a surface mesh generated from LiDAR and DGPS surveys. Results demonstrate that there is a wind threshold of approximately 5-6 ms-1 under which no flow separation exists with offshore winds. As wind speed increases over the threshold, a flow reversal area is quickly formed, with the maximum extent of the RZ at approximately 3.5 dune heights (h). The maximum extent of the RZ increases up to 4.5h with stronger wind speeds of 8-10 ms-1 and remains relatively constant as wind speed further increases. This suggests that the spatial extent of the RZ is largely independent of incident wind speed and is located between 4-5h. The magnitude of the maximum extent of the RZ is similar to that simulated using CFD and is consistent with previous studies conducted in desert dunes and wind tunnel simulations for offshore winds blowing over tall and sharp-crested dunes. Ongoing analyses are being conducted to evaluate the effect of changing wind direction, dune height and shape.

UR - http://www.agu.org/meetings/fm10/

M3 - Poster

ER -

Delgado-Fernandez I, Jackson D, Cooper A, Baas A, Lynch K, Beyers M. Reattachment zone characterisation under offshore winds with flow separation on the lee side of coastal dunes. 2010. Poster session presented at American Geophysical Union (AGU) Fall Meeting, San Francisco, United States.