FLOW AROUND A CYLINDER - istiarto

– 5.9 –
( p � � 1 [mm]), but the surface was updated only when the pressure computation
converges within 0.01 [Pa] ( p � � 0.001 [mm]) maximum limit. Note that the time
step in the model is a pseudo time, which is used as iteration step marking the
progress of the surface computation (see Chapter 4). This test was performed since
the pressure computation in the previous runs, Test A and B, took a large number of
iterations to converge while the other variables had not changed considerably within
these iterations. In such case, it might be beneficial to relax the pressure convergence
criterion (thus reduces the number of pressure iterations) and let the n-iteration
continue, but without changing the surface boundary. The surface update was done
only when the maximum pressure-correction reduces to 0.01 [Pa].
5.4.2 Results of the test runs
Comparison with the experimental data
To assess the model, the test results were compared to the experimental data, which was
done for the water surface profile along the channel and the vertical distributions of the
velocity, eddy viscosity, and shear stress (see Fig. 5.3 and Fig. 5.4). The computed flow
of Test C is quite similar to that of Test B and thus the comparison with the experimental
data is not presented. Further evaluation of the computational performance was made by
inspecting the computational history of some selected variables as the model iterative
computation marches towards the steady-state solution (see Fig. 5.5 to Fig. 5.7).
The computed water surface profile for Test A (see Fig. 5.3a) satisfactorily results in a
uniform flow depth along the channel. For Test B (see Fig. 5.4a), the water surface
profile shows a decreasing flow depth in the first 7.5-meter reach. Further downstream
the flow depth gradually increases towards the specified depth of h = 18.30 [cm] at the
outlet boundary. Within the downstream half channel-reach, x ≥ 20 [m], a nearly uniform
flow-depth is observed, showing less than 1 [mm] difference between the two ends of this
channel reach. This proves that the method of the water surface computation is
applicable.
The model produces the logarithmic velocity distribution as expected for uniform flows
(see Fig. 5.3b and Fig. 5.4b). In Test A the logarithmic velocity distribution prevails
consistently along the computational domain (see Fig. 5.3b). When a uniformly
distributed velocity is maintained at the inlet boundary (Test B), it gradually develops
into the logarithmic distribution with increasing distance from the inlet (see Fig. 5.4b).
After a half reach of the computational domain, x ≥ 19.04 [m], the velocity distribution is
logarithmic.
The above results show the applicability of the model. To further validate the model, the
simulation results are compared to the experimental data. The comparison between the
simulation and the measurement are made for the vertical distributions of the velocity,
eddy viscosity, and the shear stress. The computed profiles at x = 19.8 [m] for Test A and
at x = 19.04 [m] for Test B are compared to the measured ones as shown in Fig. 5.3c and
Fig. 5.4c, respectively. The following is to be observed: