FLOW AROUND A CYLINDER - istiarto

– 5.25 –
1.2U ∞ of the measurement. Note that the vector plot of the measured velocity at this
corner indicates some scattered data as has also been mentioned in the preceding
section.
� In the plane � = 180°, both the simulation and the measurement exhibit a counter
rotating flow towards the surface (Fig. 5.11e,f). Except close to the bed, the
intensities of the rotating flow obtained by the simulation and measurement are quite
comparable (Fig. 5.12e,f). Immediately behind the cylinder, the agreement is quite
satisfactory. Leaving the cylinder, the rotating flow diminishes and the flow returns
back to the unidirectional flow condition.
Vorticity fields in the bottom corner around the cylinder
Another way of observing the flow around a cylinder is to use the vorticity fields whose
component in a vertical plane � is defined by (Graf and Yulistiyanto, 1998):�
�� � �w
�r � �ur �z
(5.3)
in which the velocity gradients are obtained by using finite volume techniques (see
Chapter 4) for the computed data and by using central finite difference for the measured
data. It must be emphasized, however, that the vorticity is highly sensitive to the quality
and quantity of the measured data points. The measured vorticity fields thus have to be
interpreted with caution.
Shown in Fig. 5.13 are the contour plots of the vorticity fields in the planes � = 0°, 90°,
and 180° obtained from the simulation and the measurement. Only the bottom corner
region, where the vorticity is of importance, is presented. The following is to be
remarked:
� Upstream of the cylinder, in the plane � = 0°, a qualitative agreement is observed
between the computed and measured vorticity fields, notably the negative vorticities
formed by the downward flow close to the cylinder. A qualitative, bot not
quantitative, agreement is also observed by the positive vorticity fields in the bottom
corner of the cylinder formed by the downward flow and the reversed flow. The
strong concentrated vorticity shown by the measurements, however, cannot be
captured.�
� On the side of the cylinder, in the plane � = 90°, the agreement is less evident, apart
from the negative vorticity region close to the bed. This negative vorticity is formed
by the radiating flow away from the cylinder as has been mentioned in the preceding
section. There is also a negative vorticity region along the cylinder, produced by the
downward flow. The strong positive vorticity at z = 2 [cm], which is shown by the
measured fields, cannot be reproduced by the simulation.�