Trans RINA, Vol 152, Part A2, Intl J Maritime Eng, Apr-Jun 2010
The symmetric shape labelled ‘Hull shape’ in Figure 3(b) is representative of a possible aerodynamic hull shape, constructed purely to reduce aerodynamic drag on the hulls. The highly cambered hull labelled ‘Diff hull’ in Figure 3(c) is designed to create a diffuser shape though the hulls which should increase the pressure. The Clark Y shape was specifically chosen for its flat under side, which gives neutral ducting for comparing with the convergent and divergent ducting created by the other foils.
A CFD model was run in Fluent using the k-ε turbulence model. This
turbulence model would seem the most
appropriate choice due to its wide acceptance in industry and its ability to handle a variety of turbulent flows. In particular the rapid length scale changes associated with backwards facing steps, such as the transom of a ship's hull. For specific cases it may be better to use more computationally expensive methods such as Reynolds Stress Model (RSM) or a simpler method such as Spalart-Allmaras, but in order to lend consistency to the results, it is desirable that only one model, for which the limitations are well
understood, should be used.
Accepting that the drag is likely to be slightly over predicted due to the k-ε model's inability to account for transitions from laminar to turbulent flow, there will at least be a uniform over-prediction for all cases, allowing for a fair comparison. That is to say, accuracy over precision may be chosen. The accuracy of the k-ε model has proved to be more than adequate throughout much of industry. The k-ε model is considered to be robust, stable, versatile and accurate, and as such, is used for all of the computations in this analysis.
Validation of the CFD was done in prior work and is presented in detail in [21] with mesh independence found over 100,000 nodes. The model hull is 50m long and has a beam of 25m with the leading edge at a height of 5m and at zero angle of attack. This gives an effective cross deck clearance of 3m for the Clark Y wing. The model was run at 36m/s which corresponds to about 70knots.
3.2 RESULTS
The results from the CFD model are shown in Figures 4 to 6. The lift and drag coefficients for each configuration are shown as a bar chart to give a comparison of their relative values. The final configuration, marked Diffuser hull, is a combination of the diff hulls with the same diff hull cross deck. That is, all parts were constructed from the cambered profile shown in Figure 3(c) instead of having the Clark Y cross deck. This configuration provides a complete divergent convergent duct, with the exception of the free surface which remains flat.
3.3 DISCUSSION OF RESULTS
The results for the different hulls in conjunction with the Clark Y wing clearly show that the shape of the hull has a dramatic effect on the pressure distribution between the
Figure 6: Lift-to-drag configurations. A - 44 ©2010: The Royal Institution of Naval Architects ratio for the four hull
Figure 5: Coefficient of drag configurations.
for the four hull hulls. The results for coefficients of lift and drag
demonstrate the change in performance clearly. Figures 4 and 5 show the lift and drag for the various hulls in combination with the Clark Y as well as the complete diffuser shape outlined above. From this it can be seen that although the drag remains quite constant for all of the configurations, the lift varies dramatically from 0.31 up to 1.15. The lift-to-drag ratio shown in Figure 6 is also very dependent on the hull shape with considerably better results for the diffuser hull, which has about four times more lift than the hull shape for approximately the same level of drag. Figure 7 shows the effect of the various hulls as a percent change from a theoretical two dimensional wing.
Figure 4:
Coefficient of configurations.
lift for the four hull
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