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CFD and Submerged Waterjets
Tom Dinham-Peren, chief hydrodynamicist at BMT Defence Services, discusses the issues of using computational fluid dynamics (CFD) to evaluate the performance of fully submerged waterjet units.
P
ropeller driven hull forms are traditionally dominant for low to medium speed vessels and
transom mounted waterjets have proved effective for higher speed vessels. However, development in submerged water jet technology now presents an alternative, which may be an effective and efficient solution for vessels which operate between these two speed regimes. For Naval vessels this technology may
also offer advantages in underwater acoustics, which would be suitable for vessels with anti-submarine warfare roles. In the commercial environment applications could include those vessels which are required to operate in shallow waters (therefore constrained by draught), or environmentally sensitive areas where reduced underwater noise is needed. Rolls-Royce Naval Marine (RRNM)
has developed a fully submerged waterjet (the Advanced Water Jet for the 21st century or AWJ-21) which is a candidate system for both Naval and Commercial applications. While RRNM has carried out considerable development on the design of the propulsor, little research has been undertaken to explore integration of the system into a concept design. BMT Defence Services (BMT) and RRNM carried out a joint project to explore these issues and the results were presented in a joint paper given at INEC 2010 (Reference 1). Te paper investigated the application
of the AWJ-21 to a Naval vessel design and looked at the overall design integration, the whole ship propulsion efficiency, through life costs and space requirements in comparison with a conventionally propelled vessel. As part of this project a CFD study was undertaken to investigate the design issues and powering performance of the AWJ-21 concept. While the paper is based on a warship study, the findings of the CFD study are not warship specific and would
The Naval Architect July/August 2010
also apply to commercial vessels. As the purpose of the study was to
compare an AWJ-21 propelled vessel with an equivalent conventionally propelled vessel, two hull forms where developed. For the AWJ-21 concept consideration was given to the number and layout of the waterjets prior to development of the hull form. Candidate solutions included the use of two or three waterjet units, the degree of integration with the hull and the fore and aſt position. For the triple waterjet option there was also the issue of whether all three units should be inline or mounted in some form of staggered configuration. These options are summarised in Figure 1. The degree of integration was also a
major consideration in the design and there were four factors involved, the nacelle drag, the wake gain effect, the uniformity of the inflow and the induced drag on the hull and flow losses. With higher integration, the nacelle drag
is expected to reduce, the wake gain effect to increase, the inflow to become less uniform and the induced drag on the hull and inlet losses to increase. Whether the overall effect is beneficial will depend on whether the benefits to be gained from lower nacelle drag and a higher wake gain effect are cancelled out by higher induced hull drag and inflow losses. Te design might also be non-viable due to excessive cavitation and vibration due to the non-uniform inflow. On balance, a decision was made to study
a design with a high level of integration as while there was some risk that this arrangement might not work, the potential gains if it could be made to work are large. On the basis of preliminary cost and efficiency calculations a twin installation was selected. Te remaining decision on the fore and
aſt position of the waterjets was made on the basis that a far aſt position would place the waterjet close to the free surface with
Figure 2: AWJ Naked Hullform Lines Plan. Figure 1: Propulsor Integration Options.
Figure 3: Side View of AWJ Second Iteration. 33
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