FLOW AROUND A CYLINDER - istiarto

– 3.12 –
The spatial evolution of the v-velocity component in the planes � = 45°, 90°, and 135°, is
displayed in Fig. 3.4. Shown on the left are the scalar values of the v velocity, normalized
by the velocity in the approach flow, v U� . Shown on the right are the direction (in
degrees) of the local velocity vector, V h�u,v �, with respect to the plane of symmetry,
such as � � arctan�v u�
with –90 ≤ � [°] ≤ 90. The negative values (indicated by the
dashed contour lines) denote flow being deflected away from the plane of symmetry.
� In the plane � = 45°, outside the scour hole the v-velocity component remains
negligible, implying that the flow is unaltered either by the cylinder or the scour hole.
Beginning at r = 35 [cm], it starts to show negative values (the flow deviates away
from the plane of symmetry) and vice versa inside the scour hole. Approaching the
cylinder, the negative deviation grows stronger in magnitude, v U � ≈ 0.35, in
direction, � ≈ –20 [°], and in area, z ≥ –11 [cm] (see Fig. 3.4a,b). Inside the scour
hole, a weak flow, v U � ≈ 0.1 to 0.15, deviates strongly, � ≈ 80 [°], towards the
plane of symmetry.
� In the plane � = 90°, a weak flow deviation, v U � ≈ ±0.1, is observed away from the
cylinder at the upper layer, z > 0, and towards the cylinder inside the scour hole (see
Fig. 3.4c,d). A small area, confined near the bed, of negative deviation is noticed. In
the rest of the scour hole, r > 30 [cm], the flow is not deviated at all. Compared to the
measurements on the flat channel bed (Yulistiyanto, 1997), this flow deviation is
much less pronounced; the scour hole hinders the cylinder-induced deviating flow
from developing.
� In the plane � = 135°, a weak flow deviation is observed; although the maximum
value is v U � ≈ 0.2, but it is largely around v U � ≈ 0.1. The deviation here is very
much similar to that in the plane � = 90°.
From the above observation on the transverse velocity component, it can be concluded
that influence of the cylinder and the scour hole in deviating horizontally the approach
flow is limited inside the scour hole and close to the cylinder. Outside the scour hole, the
horizontal flow-direction remains the same as that of the far-field approach flow. This is
supported further by the vector plots of the horizontal velocity, V h�u,v �, around the
cylinder at different elevations z depicted in Fig. 3.5.
Fig. 3.5 also reveals the horizontal component of a rotating flow inside the scour hole; at
z = –10 [cm] a clockwise rotating flow is noticeable on the side of the cylinder,
0° < � ≤ 105°, with a weaker counter-clockwise one next to it (see Fig. 3.5c). Deeper
inside the scour hole, z = –15 [cm], this structure remains but has become very weak (see
Fig. 3.5d). Downstream, � > 90°, and close to the cylinder, the flow is accelerated and
directed towards the wake behind the cylinder (see Fig. 3.5c,d). Immediately behind the
cylinder, the flow meets the one coming from the other side of the symmetry plane; both
together form an upward flow.�