A
simplified
car
wing-mirror,
is
mounted
on
a
flat
plate
in
an
open
wind
tunnel.
The
geometry
is
a
half-cylinder
of
diameter
20 cm
with
hemi-spherical
free
end.
The
far-field
flow
speed
is
40 m/s.
All
calculations
were
done
using
DES
methods,
using
Spalart-Allmaras
and
turbulent
energy
transport
equation
(Yoshizawa).
The
mesh
size
was
approx.
2.4
million
cells.
Animation
shows
pressure
on
the
mirror
baseplate,
with
large
scale
shedding,
a
‘horseshoe’
vortex
and
wake
details.
Clickimagetoviewtheanimation
Below are comparisons of velocities from LES simulation (black arrows) with
LDA measurements (coloured arrows, after A.M.K.P Taylor, Imperial College
London). The direction and magnitude of the velocities is predicted well both
upstream and downstream of the mirror. The prediction of reattachment in the
wake is within 10% of the measured results.
The graph below compares static pressure measurements at points on the
mirror surface with calculated values using different LES models and meshes. A
countour plot of the static pressure on the front of the mirror is also shown below.
The final image shows time averaged vortices.
The
adjacent
image
shows
the
RMS
of
the
pressure
fluctuations
on
the
baseplate
around
the
mirror.
The
vortex
shows
strong
vibrational
motion,
with
the
strongest
noise
sources
seen
to
originate
from
the
mirror
trailing
edges
and
reaching
a
maximum
some
distance
downstream
in
the
wake.
High
surface
fluctuations
are
due
to
a
combination
of
strong
shedding
vortices
from
the
trailing
edge
and
turbulence
production
in
the
shear
layer
between
the
seperation
bubble
and
the
free
stream.
Below shows Fourier transforms of pressure trace for a point on the back of the
wing mirror. The transformed trace is analogous to the noise produced at that
point on the surface according the the Lighthill hypothesis. The comparison is
between experimental data (black), Spalart Allmaras (S-A, green) and SGS
turbulence energy transport DES (1eq, blue). Excellent agreement can be seen,
especially for the SA model up to nearly 4kHz.
