FLOW AROUND A CYLINDER - istiarto

– 2.28 –
identify the uneroded area beyond the outer line of the scour hole and the one inside the
scour-hole perimeter, respectively (see Fig. 2.11).
2.5.3 Measurements in the plane � = 0°
The scour hole in the plane � = 0° starts at r ≈ 45 [cm] (3D p ) with the maximum scour
depth of ds = 25 [cm] (1.67h � ) at the leading edge of the cylinder; the downward incline
into the scour-hole is thus � = 35°. At 26 stations, vertical distributions of velocities,
Reynolds stresses, and turbulence intensities are obtained from the measurements in the
plane � = 0°, being the plane of symmetry upstream of the cylinder. The data were
obtained in the radial distance of 10 ≤ r [cm] ≤ 80, which cover the far region (the
approach flow) and the scour hole region. For r ≥ 20 [cm], the data cover almost the
entire flow depth (zones A and B), while for r < 20 [cm], the data are only available at
z ≤ 5 [cm] (zone B). The measured distributions are plotted and presented in Fig. 2.12;
here the data at r = 80 [cm], being very much similar to those at r = 70 [cm], are not
shown.
Velocities in the plane � = 0°. The u-component is dominant along the most part of this
plane of symmetry. In the far region, r ≥ 45 [cm], the v- and w-components are
practically negligible. Entering the scour-hole region, r ≤ 45 [cm], the u-component
remains important up to close to the cylinder, notably within the upper layer, z ≥ 0. This
component does not show the effect of the cylinder until as close as r = 20 [cm]. Taking
the u-component at z = 0, one observes that it is almost constant at u ≈ 0.4 [m/s] (0.9U∞)
before decreasing to u ≈ 0.2 [m/s] (0.45U∞) approaching the cylinder. From r = 30 [cm],
at the lower layer and approaching the cylinder, the w-component, being a downward
component, becomes important. It usually reaches a negative maximum value in the
upper layer of the scour hole, w ≈ –0.27 [m/s] (–0.6U∞) and tends towards zero or slightly
positive values at the solid boundary of the scour hole. Close to the solid boundary of the
scour hole and notably at the range of 16 ≤ r [cm] ≤ 32, noticeable is a return flow
formed by a negative u-component and a positive w-component. The v-component shows
a noticeable deviation from zero at the lower layer and approaching the cylinder; it has a
tendency of being biased towards one side. Close to the cylinder, the v-component shows
non zero values (positive and negative ones) with a maximum value of v = –0.045 [m/s]
(0.1U∞) at the lower layer. This suggests that the flow along the plane of symmetry,
notably at the lower layer, is not completely two-dimensional.
Turbulence intensities in the plane � = 0°. The measured turbulence intensities,
presented as the root-mean-square (rms) of the fluctuating components of the velocity,
u �� u ��,
v �� v ��,
and w ��w
��,
are shown in the second column of Fig. 2.12. In the far
region, r ≥ 45 [cm], their vertical distributions are similar to those of the uniform
approach flow. Within the scour-hole region, the distributions show little effect of the
cylinder within the upper layer and be particularly characterized by a sharp turn towards a
peak underneath the z = 0. The peak becomes noticeable at r = 36 [cm] and remains so up
to r = 10 [cm]. The u �� u ��-peak
is more or less constant of about u �� u �� ≈ 0.08 to
0.09 [m/s]. The peaks of the other components are always smaller, being about 0.5 u �� u
��