FLOW AROUND A CYLINDER - istiarto

Velocity fields around the cylinder
– 5.22 –
Further evaluation of the simulation results can be made by presenting the computed
velocity fields in a vector form projected on vertical planes around the cylinder. Fig. 5.11
shows these velocity vector plots, V��u r,w�,
in the planes � = 0°, 90°, and 180°. Note
that for clarity purpose, only selected computed velocity vectors are drawn. Shown also
in the same figure are the ones obtained from the experimental data. The radial velocity
component, u r , can be obtained from the u and v components according to the
geometrical relation given in Fig. 5.9.
The comparison shown in Fig. 5.11 has demonstrated the good reproduction of the flow
field by the model. A good agreement of the computed and measured flow patterns, at
least qualitatively, is clearly ascertained. A good quantitative agreement, to some extent,
is also achieved. This is demonstrated in the following figure (Fig. 5.12). The figure
shows the contour plot of the flow intensity in the same planes, � = 0°, 90°, and 180°, as
in the preceding figure (Fig. 5.11). The flow intensity is expressed by
V U � � u 2 � v 2 � w 2 U � , where U � � 0.68 [m s] is the approach flow velocity. The
resemblance of the computed and the measured flow intensities is observed, except at
close to the surface in the plane � = 90°. The discrepancy at this region, however, is
likely due to the scattered measured data, as seen also in the vector plots (Fig. 5.11). In
the plane � = 180°, where the flow is typically complex, a good agreement between the
computed and measured flow intensity plots can be achieved.�
The vector and isoline contour plots of the flow pattern given in Fig. 5.11 and Fig. 5.12
allow one to observe the flow behavior as it interacts with the cylinder. This is described
as follows:�
� In the plane � = 0°, the unidirectional flow coming towards the cylinder becomes
increasingly two-directional (Fig. 5.11a,b) and its intensity decreases (Fig. 5.12a,b).
Close to the cylinder, a downward velocity develops whose intensity increases with
the depth and reaches a maximum value of 0.3U ∞ at z ≈ 2 [cm]. At the bed, this
downward velocity is deflected toward upstream (Fig. 5.11a,b), forming a reversed
flow with an intensity of 0.2U ∞ (Fig. 5.12a,b). This reversed flow encounters the
incoming flow and a separation takes place. A visual observation to Fig. 5.11a,b
indicate that the separation starts at r ≈ 17 [cm] according to the model and at
r = 19.7 [cm] according to the measurements (see Graf and Yulistiyanto, 1998).
� In the plane � = 90°, the velocity vector, V� u r,w
� �, is directed away from the
cylinder, particularly at the surface (Fig. 5.11c,d), with a decreasing intensity as one
moves away from the cylinder (Fig. 5.12c,d). The radiating flow represents the
skewness of the approaching flow when it passes the cylinder. The flow intensity
close to the cylinder is 1.4U ∞ from the simulation and is slightly higher, 1.6U ∞ , from
the measurement. On the other hand, at the upper corner between the cylinder and the
surface, the simulation indicates a slightly higher intensity, being 1.4U ∞ compared to