FLOW AROUND A CYLINDER - istiarto

2 Flow Measurements
Abstract
– 2.1 –
Measured three-dimensional flow fields around a vertical cylinder, in an established
(equilibrium) scour hole under a clear-water regime, are reported. An acoustic Doppler
velocity profiler (ADVP) was used to measure instantaneously the vertical distributions
of the three velocity components in vertical planes around the cylinder. The flow field is
presented as the vertical distributions of the (time averaged) velocities, of the turbulence
intensities, and of the Reynolds stresses in the planes � = 0°, 45°, 90°, 135° and 180°.
The flow in plane of symmetry upstream of the cylinder, � = 0°, is characterized by a
reversed flow along the scour bed and a strong downward flow close to the cylinder. This
structure remains, but with diminishing strength, in the other planes, � = 45°, 90°, and
135°. In these three planes, the downward flow has a decreasing magnitude. In the plane
� = 180°, there is a flow reversal close to the surface. As the flow moves downstream,
leaving the scour hole, the flow reversal diminishes; the flow is recovering to the
approach flow condition. Measurements in all planes show that the flow in the far region,
outside the scour hole, does not change with the presence of the cylinder. The flow is
altered only in the scour-hole region.
The turbulence intensities get stronger in the downstream planes. The increase is notably
important from the plane � = 90° to � = 135° and further downstream to � = 180°.
Approaching the cylinder, in all planes except in the plane � = 180°, the turbulence
intensifies within the scour hole, z < 0, but remains more or less unchanged outside the
scour hole, z > 0. Downstream of the cylinder, in the plane � = 180°, the turbulence gets
its strongest intensities. It is interesting that the three components of the intensity, u �� u ��,
v �� v ��,
and w ��w
��,
in the plane � = 180° indicate a tendency of being isotropic.�
The vertical distributions of the Reynolds stresses in the planes � = 0° to 135° have an
almost linear distribution in the upper layer, z > 0, while in the lower layer, they exhibit a
strong peak. From the plane � = 0° to � = 135°, the Reynolds stresses inside the scour
hole get increasingly stronger. In all planes, the �u �� w �� is always dominant compared to
the �v �� w ��.
Downstream of the cylinder in the plane � = 180°, however, the vertical
distributions of the Reynolds stresses do not show any conclusive trend.
Presented also are the vertical distributions of the uniform approach flow, i.e. the flow
without the cylinder being installed. The approach flow is uniform, but shows a slight
tendency of being decelerated; the bed is hydraulically (incomplete) rough.