FLOW AROUND A CYLINDER - istiarto

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2.5.7 Measurements in the plane � = 180°
The scour hole in the downstream of the cylinder (� = 180°) extends far to the
downstream, r = 105 [cm] (7D p ), with the maximum scour depth of ds = 19 [cm]
(1.3h � ); the upward incline of the scour hole is thus � = 11°. Further downstream, up to
r = 300 [cm] (20D p ), the bed continues with a deposition. The ADVP measurements
were performed at 10 ≤ r [cm] ≤ 100, from which 18 vertical distributions of the
velocities, the turbulence intensities, and the Reynolds stresses were obtained. The data
for the region close to the cylinder, r < 22 [cm], are available only at z ≤ 5 [cm], while
those for the rest, r ≥ 22 [cm], cover almost the entire flow depth. The 18 vertical
distributions are presented in Fig. 2.16
The flow in the wake of the cylinder is characterized by the so-called wake vortices that
are formed by the shear-layers as a result of the flow separation detaching from the
cylinder surface. At the surface and notably near the cylinder, the vortex is strong, the
surface considerably fluctuates, and an air entrainment frequently takes place, generating
air bubbles in the flow. These air bubbles, being of important dimensions, made the
measurements very difficult by blocking the acoustic signal. This was particularly so in
the case of measurements in zone A where the air bubbles stick to the mylar film of the
instrument housing. The positioning of the ADVP-instrument was also difficult due to the
fluctuating surface. For that reason, some of the measurement data from the zone A
(z > 5 [cm]) and r ≤ 38 [cm] have to be interpreted with care.
Velocities in the plane � = 180°. Close and behind the cylinder, r ≤ 38 [cm], the u- and
w-components indicate a flow reversal towards the water surface. The u-component is
small close to the cylinder, being u ≈ 0.1 [m/s] (0.22U∞) at r = 10 [cm]. Moving outward
from the scour hole the u-component increases and the flow reversal diminishes. The ucomponent
is quasi-uniformly distributed along the depth with the value of u ≈ 0.3 [m/s]
(0.67U∞) at r = 70 [cm]; further downstream, it is recovering towards a logarithmic
distribution, nevertheless up to r = 100 [cm], the logarithmic distribution has not been
completely attained. The w-component is always in the upward direction, which is on the
contrary to that in the plane � = 0°. Its maximum value is w ≈ 0.16 [m/s] (0.33U∞) at
r = 10 [cm], which constantly diminishes as the flow leaves the scour hole. The vcomponent
is usually small; immediately behind the cylinder, however, it has larger
values of v ≈ 0.04 [m/s] (0.09U∞), which is an indication of three-dimensional flow.
Turbulence intensities in the plane � = 180°. The intensity of the turbulence at the
plane � = 180° is considerably strong. Close to the cylinder, the three components show
magnitudes of approximately u �� u �� ≈ v �� v �� ≈ 0.2 [m/s] and w ��w
�� ≈ 0.15 [m/s], which
are more than twice higher than those at the plane � = 0°. The maximum intensity is
found at r = 18 [cm], which is unlike the case for the other planes where the maximum is
found at the measured vertical closest to the cylinder, r = 10 [cm]. As the flow moves
away from the cylinder, the turbulence intensities decrease. Along the range of the
measurements, the u �� u �� and v �� v �� are comparable with the one occasionally exceeds
the other, where as the w ��w
�� is always the least dominant.