-0.35 -0.30 -0.25 -0.20 -0.15 -0.10
-0.6 -0.4 Figure 26, Through plane error, magnitude 7. DISCUSSION OF EXPERIMENTAL RESULTS
7.1 THROUGH PLANE VELOCITY COMPONENTS
Table 13 shows a summary of the non-dimensional errors in the through plane velocity components for each of the locations around the tug. In this table, the non- dimensional parameter Erroru (which is the error in the through-plane velocity component) was calculated from Tables 7 to 12 by non-dimensionalizing the values of ErrorVx with the free stream flow speed.
Table 13, Non-dimensional values of Erroru Flow region
Tetrahedral mesh
Upstream, no fin -0.133
Down stream, no fin -0.068 Downstream, with fin -0.124
7.2 IN-PLANE VELOCITY COMPONENTS
Three numerical values were picked to compare the PIV experiments with the tetrahedral and hexahedral meshes. These were the mean value and standard deviation of Error2D and the fraction of the data where the error between the CFD predictions and the experiments (for the in-plane flow components) were within 10% of the free stream speed. The values were non-dimensionalized based on the free stream speed of 0.5 m/s. The results are given in Tables 14 to 16.
Table 14, Non-dimensional mean, Error2D
Hexahedral mesh
-0.138 -0.080 -0.175
From these values it can be seen that the value of Erroru is consistently negative. This means that the flow component from the CFD predictions was consistently higher than the observed values in the experiments. The difference was consistent with the values of the wake from the seeding rake used for these
experiments
(Molyneux et al., 2007a), which was seen to be between 10 and 12 percent of the free stream flow. It was expected that
the wake from the seeding rake was
reducing the flow speed, relative to the case when the rake was not present. It was also shown that the rake had negligible effect on the in-plane flow measurements, so comparison between the CFD simulations and the PIV experiments should be focussed on the in-plane flow patterns.
Upstream, no fin
Flow region Tetrahedral mesh 0.083
Down stream, no fin
Downstream, with fin
0.074 0.097
Hexahedral mesh 0.076
0.078 0.101 Table 15, Non-dimensional standard deviation, Error2D
Upstream, no fin
Flow region Tetrahedral mesh 0.117
Down stream, no fin
Downstream, with fin
0.049 0.064
Hexahedral mesh 0.107
0.055 0.074 -0.2 y, m 0.0 0.2
0.2 0.1 0.0
-0.1 -0.2
©2008: Royal Institution of Naval Architects
B-57
Error Vx
z, m
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