FLOW AROUND A CYLINDER - istiarto

5.1 Introduction
– 5.2 –
The numerical model presented in the preceding chapter (Chapter 4) shall be used to
simulate flow around a cylinder. Two cases of flow around a cylinder were considered,
namely flat channel bed and scoured channel bed. The first case was the object of
experimental and numerical investigations of the previous study (Yulistiyanto, 1997),
whereas the second case was experimentally investigated in the present study (see
Chapters 2 and 3). In both studies, the experimental investigations were performed by 3D
instantaneous velocity measurements using an acoustic Doppler profiler. The
experimental data thus obtained allow detailed comparisons of the simulation results to
be performed, not only for the velocity fields but also for the turbulence quantities. In the
previous numerical work (Yulistiyanto, 1997), a 2D model was used thus restricting the
comparison to the depth-averaged velocity fields. Nevertheless, the result of this 2D
model was promising and encouraged the 3D model development.
The model has various constants, such as those in the k-� transport equations (c � , c 1 , c 2 ,
� k , and � � ) and the ones of the logarithmic velocity law (� and k s ). In the present model,
the k-� model constants (see Table 4.1 in Chapter 4) and the Karman constant (� = 0.4)
are invariant and are applied to all flow conditions. The equivalent roughness of the wall,
k s , characterizes the roughness of the physical solid boundaries (the channel wall, the
cylinder, and especially the channel bed). It is the only “physical link” of the model to the
roughness characteristics of solid boundaries. In order to obtain the correct model-value
of k s , a calibration had to be performed. Using a uniform flow condition (with neither the
cylinder nor the scour hole), various k s -values were tested to obtain the best match
between the computed and measured flow fields. In the calibration run, adjustment of the
uniform flow depth (which will be the approaching flow depth) was also accordingly
needed.
Before performing the simulation of flow around a cylinder, which is a highly complex
3D flow, a series of test runs were carried out for the simple uniform flow case. The test
runs were aimed at investigating the basic performance of the model and verifying some
computational techniques adopted in the model. This simple well-known flow condition
was the best tool to serve this purpose. The experimental data for the comparison were
taken from the same preceding work (Yulistiyanto, 1997).
The computer code of the model is written in FORTRAN-77. All simulation runs were
executed on a personal computer. The computational CPU time of the simulation run is
typically 70 to 100 hours.