An experimental study of a turbulent boundary layer at
Rθ≈1070 and Rτ≈543
was
conducted. Detailed measurements of the velocity vector and the velocity
gradient
tensor within the near-wall region were performed at various distances
from the
wall, ranging from approximately y+=14 to
y+=89. The measured mean statistical
properties of the fluctuating velocity and vorticity components agree well
with previous
experimental and numerically simulated data. These boundary layer measurements
were used in a joint probability density analysis of the various component
vorticity
and vorticity–velocity gradient products that appear in the instantaneous
vorticity
and enstrophy transport equations. The vorticity filaments that contribute
most to
the vorticity covariance Ω[bar]xΩ [bar]y
in this region were found to be oriented downstream
with angles of inclination to the wall, when projected on the streamwise
(x, y)-plane,
that decrease with distance moving from the buffer to the logarithmic layer.
When
projected on the planview (x, z)- and cross-stream
(y, z)-planes, the vorticity filaments
that most contribute to the vorticity covariances
Ω [bar]xΩ [bar]z and
Ω [bar]yΩ [bar]z have angles of
inclination to the z-ordinate axis that increase with
distance from it. All the elements of the
ΩiΩj
∂Ui/∂xj
term in the enstrophy transport equation, i.e. the term that
describes the rate of increase or decrease of the enstrophy by vorticity
filament
stretching or compression by the strain-rate field, have been examined.
On balance,
the average stretching of the vorticity filaments is greater than compression
at all
y+ locations examined here. However, some
individual velocity gradient components
compress the vorticity filaments, on average, more than they stretch them.