System rotation is known to substantially affect the mean
flow pattern as well as the
turbulence structure in rotating channel flows. In a numerical
study of plane Couette
flow rotating slowly about an axis aligned with the mean vorticity, Bech
& Andersson
(1996a) found that the turbulence level was damped in the
presence of anticyclonic
system rotation, in spite of the occurrence of longitudinal counter-rotating
roll cells.
Moreover, the turbulence anisotropy was practically unaffected by the weak
rotation,
for which the rotation number Ro, defined as the
ratio of twice the imposed angular
vorticity Ω to the shear rate of the corresponding laminar flow,
was
±0.01. The aim
of the present paper is to explore the effects of stronger
anticyclonic system rotation
on directly simulated turbulent plane Couette flow. Turbulence
statistics like energy,
enstrophy and Taylor lengthscales, both componental and directional, were
computed
from the statistically steady flow fields and supplemented by structural
information
obtained by conditional sampling.The designation of the imposed system rotation as ‘high’
was associated with a
reversal of the conventional Reynolds stress anisotropy so that the velocity
fluctuations perpendicular to the wall exceeded those in the streamwise
direction. It was
observed that the anisotropy reversal was accompanied by an appreciable
region of
the mean velocity profile with slope ∼2Ω,
i.e. the absolute mean vorticity tended to
zero. It is particularly noteworthy that these characteristic
features were shared by
two fundamentally different flow regimes. First, the two-dimensional
roll cell pattern
already observed at Ro=0.01 became more regular and energetic
at
Ro=0.10 and
0.20, whereas the turbulence level was reduced by about 50%. Then, when
Ro was
further increased to 0.50, a disordering of the predominant roll cell pattern
set in
during a transient period until the flow field settled at a
new statistically steady state
substantially less affected by the roll cells. This was accompanied by
a substantial
amplification of the streamwise turbulent vorticity and an anomalous variation
of the
mean turbulent kinetic energy which peaked in the middle of the channel
rather than
near the walls. While the predominant flow structures of the non-rotating
flow were
longitudinal streaks, system rotation generated streamwise vortices, either
ordered
secondary flow or quasi-streamwise vortices. Eventually, at
Ro=1.0, the turbulent
fluctuations were completely suppressed and the flow field relaminarized.
DNS of a turbulent Couette flow at constant wall transpiration up to
