Computational fluid dynamics modelling of wind conditions
In order to assess whether breeding distributions are affected by
airflow characteristics, wind conditions around the cliffs of Skomer
were simulated using the computational fluid dynamics (CFD) model
OpenFoam (openfoam.org version 5.x). OpenFoam is widely used for
modelling atmospheric boundary layer flows (e.g. in the wind energy
industry) and has been extensively validated over a similarly steep
island51.
The initial coarse model domain was 5300 x 5000 x 1000 m, with a
horizontal resolution of 20 m and a vertical resolution of 10 m. The
bottom boundary represented the surface of the island which was taken
from a DEM of Skomer with 2 m resolution (Lle Geo-Portal
http://lle.gov.wales). A simulation was also run using the DEM with 1 m
resolution, however, the outputs were not significantly different. After
establishing the initial mesh, the tool snappyHexMesh in OpenFoam was
used to incorporate the DEM into the modelling process, refining initial
mesh cells close to the surface up to 3 times. This resulted in a finer
resolution close to surface of 2.5 m in the horizontal and 1.25 m in the
vertical. Simulations were completed when convergence was achieved using
a steady-state incompressible solver with a k-ε turbulence closure
scheme.
Wind simulations were run for NW, NE, SE and SW wind directions to cover
a representative sample of wind directions, including the prevailing SW
direction (see Results). This allowed us to test whether birds selected
sites that were predicted by a specific set of wind parameters across
wind directions. The initial wind speed was set to 10 m
s-1 at 20 m height. The following airflow
characteristics were extracted from the model output at 2 m normal to
the ground surface (this height was selected in order to estimate the
airflow conditions that birds would be exposed to close to their
breeding cliffs): The two horizontal and vertical wind vectors (U_0,
U_1 and U_2 respectively), mean wind speed (MeanU), turbulent kinetic
energy (TKE), pressure (P), kinematic viscosity (Nut) of the air medium
and turbulence dissipation rate (ε). These outputs were further used to
estimate horizontal wind speed; gustiness and turbulence intensity (TI)
following31.