FLOW AROUND A CYLINDER - istiarto

Flow in the plane � = 45° and 90°
– 3.11 –
� In the plane � = 45°, a similar observation as in the previous plane (� = 0°) can be
made, notably on the downward flow close to the cylinder and the rotating flow
inside the scour hole, the horseshoe vortex, (see Fig. 3.2b). The velocity contour plot
(see Fig. 3.3b), however, indicates a diminishing flow intensity compared to the one
in the plane � = 0°. Similarly, the downward velocity along the cylinder face is also
less pronounced as has been shown previously (see Fig. 3.1). In the context of the
horseshoe vortex, it can be concluded that this vortex is diminishing.
� In the plane � = 90°, the radial velocity components are very weak (see Fig. 3.2c).
The flow intensity (see Fig. 3.3c), on the other hand, is high and comparable with the
one in the plane � = 45°. This shows that much of the flow is dominated by the
downstream velocity components, u � �� u r (u � � u , u r � �v), as has also been
evidenced in the velocity profiles discussed in the previous section (see Sect. 3.2.1,
Fig. 3.1). Close to the cylinder (Fig. 3.3c), nevertheless, there is a noticeable radial
velocity component, showing a flow direction away from the cylinder in the upper
layer, z > 0, and vice versa in the lower layer, z ≤ 0. The horseshoe vortex is
practically not detected in this plane.
Flow in the plane � = 135° and 180°�
� In the downstream planes (� = 135° and 180°), the flow is directed away from the
cylinder (see Fig. 3.2d,e). The downward flow is weak in the plane � = 135° and is
no longer noticeable in the plane � = 180°. Behind the cylinder, a wake flow,
characterized by a flow reversal towards the surface, takes over. Leaving the scour
hole, the flow is recovering into a unidirectional one; at r = 100 [cm] it has not,
however, entirely reached the (far-field) approach flow condition.
3.2.3 Transverse velocity
The alteration of the flow pattern due to the cylinder and the scour hole can also be
investigated by looking into the directional change (deviation) of the flow with respect to
the far-field approach flow. The vertical flow deviation can be readily illustrated by the
vertical velocity component; this will be discussed in the next section. The horizontal
flow deviation could be manifested by the skewed velocity profiles; this was
experimentally investigated, for example, by Ahmed and Rajaratnam (1997). From the
measured data obtained outside the scour hole, they defined the skewed profiles by
making use of the normal (cross flow) velocity component and the streamwise velocity at
the surface. In the present work, since the surface velocity is not measured, the transverse
velocity component, v, is chosen as a measure of the horizontal deviation of the flow with
respect to the main direction of the far-field approach flow. It has been shown in the
preceding sections that the v component is always less prominent than the u component,
meaning that the flow deviation (in the horizontal direction) is low. The evolution of the
v-velocity component around the cylinder is here investigated, whereas that of the w
component will be discussed in the following section.