FLOW AROUND A CYLINDER - istiarto

Velocity fields around the cylinder
– 5.33 –
A more detailed observation of the flow pattern around the cylinder can be facilitated by
plots of velocity vector and flow intensity contour, given in Fig. 5.17 and Fig. 5.18. The
plots show the velocity vector, V� u r,w
� �, and the flow intensity, V U �
(V � u 2 � v 2 � w 2 and U � � 0.45 [m s]), in the planes � = 0°, 90°, and 180°. Note that
for clarity purposes, not all but only some selected computed velocity vectors, being
representative ones, are drawn. Shown also in the same figures are the ones obtained
from the experimental data.
Comparing the computed and the measured velocity fields shown in the two plots, it is
evident that the two closely resemble. As in the flat-bed case, the model here also has
proven its applicability to this highly 3D flow. Further remarks can be put forward from
the plots in each of the three planes:
� In the plane � = 0° (Fig. 5.17a,b and Fig. 5.18a,b), the unidirectional incoming flow
is decelerating and deflecting towards the bed upon entering the scour hole. Close to
the cylinder, the down-deflected flow becomes more pronounced and develops into a
downward flow along the cylinder face. The stagnation pressure stimulates the
downward flow, which is strengthening with increasing depth (decreasing z) and
diminishing close to the bed, where the flow turns to the upstream direction along the
bed. This reversed flow diminishes as it moves away from the cylinder and eventually
joins the downstream facing upper flow. These downward and the reversed flows can
be reasonably well captured by the model; the maximum computed downward flow,
0.5U ∞ , is slightly weaker than the measured one, 0.6U ∞ , but the computed reversed
flow is comparable with the measured one, being both indicate an intensity of 0.2U ∞
(Fig. 5.18a,b). Note that the measurements show a strong reversed flow concentrated
in a small region at the bottom corner of the cylinder, which the simulation cannot
capture due to the limited spatial resolution of the grid.
� In the plane � = 90° (Fig. 5.17c,d and Fig. 5.18c,d), both simulation and
measurement show negligible radial and vertical velocity components as indicated by
the vector plot (Fig. 5.17c,d). The contour plot (Fig. 5.18c,d) reveals, on the other
hand, a comparable flow intensity as in the plane � = 0°. This shows that the flow is
mainly directed towards downstream tangentially to the cylinder and that the lateral
skewness of the approaching flow is negligible.
� In the plane � = 180° (Fig. 5.17e,f and Fig. 5.18e,f), close to the cylinder the velocity
is weak, 0.1 to 0.3U ∞ , and is directly upward towards the surface, where it turns
upstream towards to the cylinder. Further downstream away from the cylinder, the
flow is gradually increasing and returning to the approach flow condition. The close
resemblance of the computed flow with the measured one is in fact a rather surprising
since the flow behind the cylinder is a complex one.