Feature 1 | CFD AND HYDRODYNAMICS
to be fitted. Part of the object in placing a ‘soſt’ tunnel on the naked hull form prior to fitting the actual intake duct was to provide a beneficial environment for the waterjet to operate in, the object being to focus some of the boundary layer towards the waterjet intake and to provide a local static pressure rise. Te naked AWJ hull form body plan is given in Figure 2 (see page 33). Based on an initial assessment of the
requirements for the waterjet installation a design of waterjet and inlet was produced as shown in Figure 3 (see page 33) and Figure 4. Te waterjet was mounted horizontally with the front part of the waterjet ahead of the impeller housing shaped to match and ease the intake duct ahead of the waterjet. Te waterjet was partially faired into the hull to reduce nacelle drag. CFD calculations where undertaken for
Figure 4: Oblique View of AWJ Second Iteration. Figure 5: General View of Naked and Propelled Hull.
both the resistance and the self propelled case. Te resistance results were used to give some indication of what the actual resistance for this vessel would be and the propulsion to give some insight into the propeller hull interaction factors. Te high degree of integration of the waterjet and the hull coupled with the fact that the waterjet also protrudes into the flow means that a resistance calculation with the non-driving waterjet unit fitted is not so useful. Te drag of the waterjet installation would be very high when it is not in operation. For this reason it was decided to regard the waterjet and the recessed intake duct as part of the propulsion arrangement and to run a separate CFD model without these items fitted in order to get a resistance estimate for the ‘naked’ hull. Te ‘naked’ and the ‘propelled’ models are shown in Figure 5. BMT Fluid Mechanics were contracted
Figure 6: Predicted wave Pattern at 30knots.
more risk of ventilation and emergence/ slamming in a seaway. Te forward position was discounted as this was believed to be less efficient due to the higher inflow speed and implications for vessel draught and location of the propulsive machinery. Tis
34
leſt the conventional position. The AWJ hullform was developed
based on the conventional hullform. Modifications included the slight tunnelling of the stern aſt and widening of the hull to provide a suitable location for the waterjet
to carry out the CFD computer runs with the detailed interpretation conducted by BMT Defence Services. Te calculations were performed using the multi-purpose CFD soſtware, CFX. Free surface effects have been incorporated. Te computational mesh used for the simulations comprised approximately 4.5 million tetrahedral and prismatic cells. Te model was allowed to sink and trim so that buoyancy equilibrium was maintained using a method described in Reference 2). An example of the wave pattern results is shown in Figure 6 (see page 34) which gives
The Naval Architect July/August 2010
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