FLOW AROUND A CYLINDER - istiarto

– 3.22 –
3.3 Turbulence characteristics around the cylinder
3.3.1 Vertical distributions of the turbulence stresses
The results obtained from the turbulence measurements are shown in Fig. 3.9 for some
selected profiles in the planes � = 0°, 45°, 90°, 135°, and 180°. The shear velocity in the
far-field approach flow, u �,� � 0.0265 [m s], is used to normalize the data. The
following is to be observed:
� In the plane � = 0° and outside the scour hole, z > 0, the profiles of the turbulence
intensities and the Reynolds stresses reasonably collapse, showing no significant
change from the profiles in the far field approach flow. Inside the scour hole, z ≤ 0,
there is an increase of turbulence intensities and shear stresses approaching the
cylinder. Near the scour bed, the longitudinal component of the Reynolds stress,
�u �� w ��,
has negative values; this is in agreement with the reversed flow observed in
that region. In the entire region, the longitudinal components of the turbulence
intensities, u �� u ��,
and of the Reynolds stresses, �u �� w ��,
are the dominant ones; this
holds over the entire flow depth.
� In the plane � = 45°, a similar observation as in the plane � = 0° can be made, with
the exception that the transverse component of the turbulence intensities, v �� v ��,
and
of the Reynolds stresses, �v �� w ��,
have values close to the longitudinal ones.
� In the plane � = 90°, the longitudinal turbulence intensities, u �� u ��,
slightly decrease
from the ones in the previous two planes, but the transverse components, v �� v ��,
increase. On the other hand, the longitudinal Reynolds stresses, �u �� w ��,
show
significant activities, notably inside the scour hole; distinguishable are the bulges of
�u �� w �� beneath z = 0, moving downwards with decreasing values as they approach the
cylinder.
� In the downstream planes, � = 135° and 180°, the turbulence is strong and rather
isotropic, notably in the wake region behind the cylinder. It shows that the wake flow,
being known to be 3D one, fluctuates about its mean value without directional
preferences. The Reynolds stresses inside the scour hole in the plane � = 135° are
quite strong, as much as twice or more of those in the upstream planes. The transverse
stresses, �v �� w ��,
are also strong; their profiles display positive and negative values
along the flow depth, indicating a transverse momentum exchange. Behind the
cylinder in the plane � = 180°, the Reynolds stresses are strong but demonstrate a
chaotic picture. The flow itself is also chaotic and thus the interpretation of the
measured data is rather difficult.
Unfortunately —to the best of our knowledge— no research is available for comparison.
Studying a backward-facing step (Etheridge and Kemp, 1978, p. 555; and Nakagawa and
Nezu, 1987, p. 70) showed rather similar trends.