FLOW AROUND A CYLINDER - istiarto

– 5.8 –
(� 2 � d 50 ) was considered as the one suitable as the model k s -value after evaluating the
model results using several different k s values. This corresponds to k su � � = 118, which
signifies that the channel bed is hydraulically rough.
The calibrated k s and h ∞ were subsequently used as the model values for the test runs
using uniform flow (Sect. 5.4) and also for the simulation of the flow around a cylinder
on a flat channel bed (Sect. 5.5) and in a scoured channel bed (Sect. 5.6).
5.4 Test run using uniform flow condition
5.4.1 Boundary and initial conditions
Having found the k s and h ∞ values for the model, test runs were carried out in which the
basic performance of the model under uniform flow conditions was examined. The
uniform flow condition according to Yulistiyanto measurement data was selected as the
test case. The same quasi 2D flow as in the calibration runs was considered (see Sect.
5.3.1).
Using the calibrated values of k s = 0.22 [mm] and h ∞ = 18.3 [cm], three test runs using
different initial and boundary conditions were performed:
� Test A: the initial flow depth along the channel varies linearly between the inflow
depth, h inflow = 17.3 [cm], and the outflow depth, h outflow = 18.3 [cm]. The initial
conditions for the other variables were taken from the results of the calibration run,
which were linearly adjusted according to the ratio of the local flow-depth. The same
spatial discretisation as used for the calibration run ( 500 �1� 22 ) as shown in Fig.
5.1 was used. This test run is aimed at validating the method of the surface
computation. Given a variable depth along the channel, the model shall find the
uniform depth when it converges to the steady-state condition.
� Test B: uniformly distributed velocity and turbulence parameters were specified at the
inflow boundary. These values were also used as the initial conditions along the entire
domain. The initial flow depth was h = h ∞ = 18.3 [cm] everywhere. The
computational domain covers the entire channel reach (38 [m]), but the number of
cells was kept the same as that of Test A (500 �1� 22 ). The coordinate system was
oriented such that the z-axis was vertical. This is different from that of Test A where
the z-axis was normal to the channel bed. This test run is aimed at verifying the wall
function implementation. Given a uniformly distributed velocity at the upstream
boundary, the model shall produce the logarithmic velocity distribution after a certain
distance away from the entrance. This can be considered as a developing flow
condition.
� Test C: this is a run identical to Test B, except that the surface boundary is not
necessarily updated at every time step (the n-iteration), but only after several time
steps. The pressure convergence criterion in the time-step iteration was set to 10 [Pa]