FLOW AROUND A CYLINDER - istiarto

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5.5 Simulation of flow around a cylinder on a flat channel bed
5.5.1 Computational domain
The computational domain covers a longitudinal section of 4 [m] with the cylinder at its
center. The grid was generated in two steps. Firstly, the Joukowski conformal mapping
was used to generate an orthogonal horizontal (xy plane) grid. Finer grids are
concentrated around the cylinder. Secondly, the horizontal grid was repeated in the z
direction according to a pre-determined interval to cover the entire flow depth. This is
known as a “sigma stretching” method. The number of cells in the x, y and z directions is
99 � 49 � 22 (see Fig. 5.8). The cylinder is represented as a 31 �1� 22 -cell out-blocked
region. There are thus 106,040 computational nodes, 16,834 boundary nodes, and 750
dummy nodes. The z distribution of the grid is identical to that of Test A (uniform flow
simulation) to enable the use of the computed velocity and turbulence profiles of that test
run as the inflow boundary conditions of this simulation.
In presenting the results of the simulation and the comparison to the experimental data,
cylindrical and Cartesian coordinate systems are frequently used together. The origin of
both coordinates is defined at the center of the cylinder, at the original bed level (see Fig.
5.9).
5.5.2 Boundary and initial conditions
The boundary conditions are as follows:
� Upstream inflow: Q = 0.250 [m 3 /s] and prescribed distributions of u,v,w, k, � which
are taken from the results of Test A.
� Downstream outflow: h ∞ = 18.3 [cm].
� Left and right channel (glass) walls: wall boundary with k s = 0 [mm] (hydraulically
smooth).
� Channel (metal) bed and cylinder (PVC) boundaries: wall boundary with
k s = 0.22 [mm] (u �k s � �6.6, hydraulically smooth solid boundaries).
� Top boundary: water surface.
The initial conditions over the computational domain are uniform flow depth
corresponding to the known downstream depth, h∞ = 18.3 [cm], hydrostatic pressure,
p � ��gz �zsurface � z�
[Pa], velocity and k-� according to the known distributions at the
inlet section. At early time steps, the water surface was kept constant until the flow field
was more or less stable. The simulation was then restarted by letting the water surface
move according to the surface boundary treatment. A similar method of surface updating
as of Test C in the preceding test run was used. There are two different criteria for the
pressure computation, namely the one for terminating the pressure computation in the �-
c
c
iteration ( pmax � 10 [Pa], which is equivalent to pmax � ≤ 1 [mm]) and the other for
c
c
updating the surface boundary ( pmax � 0.01 [Pa] or pmax � ≤ 0.001 [mm]).