FLOW AROUND A CYLINDER - istiarto

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are moved to define the surface position. These procedures clearly imply
computational simplification and approximation. Another method of surface
positioning, which is based on the application of kinematic boundary condition along
the surface, is discussed in Sect. 5.8.
� Downstream of the cylinder, an over-estimation of the computed water surface is
observed. Here also, a rather steep surface profile prevails, in which the high velocity
(low flow-depth) in the plane � = 90° is readapting to the lower velocity (higher
flow-depth) in the wake of the cylinder. Again, the method of surface positioning has
certainly a low accuracy. In the scoured-bed case (see the next section), a similar
over-prediction occurs; another interpretation of this problem, which attempts to
relate it to the production of the turbulent kinetic-energy, will be discussed (see Sect.
5.6.3). One expects that the disagreement of the water surface should disappear at a
distance further downstream away from the cylinder. Unfortunately, the limited
measured data do not allow such control. In the scoured channel bed case, it will be
seen that the measured water surface further downstream returns back to the one of
the approach flow.
5.5.4 Conclusions
The numerical model (see Chapter 4), conclusively, has demonstrated its applicability
and ability to simulate flow around a cylinder on a flat channel bed. Detailed comparison
of the computed velocity fields to the measured ones (see Fig. 5.10) shows satisfactory
agreements. The downward and reversed flows upstream of the cylinder and the rotating
flow downstream of the cylinder can be reproduced by the model (see Fig. 5.11 and Fig.
5.12). A qualitative agreement is also seen from the vorticity fields (see Fig. 5.13). The
model, however, shows a rather disagreement of the water-surface profile, notably along
the cylinder circumference and behind the cylinder. A good agreement, nevertheless, is
seen along the upstream water surface, including the bow wave at the cylinder upstream
face (see Fig. 5.14). The method of surface boundary positioning, in fact, becomes less
accurate in regions of a strong pressure gradient.
5.6 Simulation of flow around a cylinder in a scoured channel bed
5.6.1 Computational domain
The computational domain covers a 5-[m] channel-reach with the cylinder at 2.5 [m]
from the upstream boundary (see Fig. 5.15). The origin of the coordinate system is
defined at the center of the cylinder, at the original uneroded channel bed. The flow is
assumed to be symmetrical about the channel centerline and, therefore, the computational
domain represents half of the channel with a symmetry plane on the left (north)
boundary, y = 0. The grid is formed by 109 � 43 � 22 cells in the x, y and z directions,
respectively. There are 103,114 computational nodes, 16,062 boundary nodes, and 704
dummy nodes. The grid was generated by the same method as in the flat-bed case, i.e. by