Feature 2 |GERMANY
is no single means to predict operational ship resistance as a total. Although modern state-of-the-art RANS codes offer the potential to compute the total resistance of a ship, this is typically limited to clearly defined, standard conditions, that is a new vessel during trial conditions. Further factors imposed during operation over the life cycle of a vessel including hull fouling, added resistance in a seaway, etc., still need to be superimposed on the basis of empirical methods. TARGETS develops a number of
new tools to provide better input to the overall prediction of hydrodynamic forces for a ship in operation. These cover both form – pressure - related components as well as viscosity-related contributions with the aim to shorten complex CFD optimisations. While hull form optimisation is a key element of ship design which largely influences energy consumption, a new path is chosen to overcome the shortcomings of today’s panel-code-based optimisation methods. As much as these methods benefit
modern ship design, they suffer from the absence of viscosity in the simulation and hence do not offer the full potential CFD can provide. In contrast, RANS methods allow capturing all relevant flow phenomena at a largely increased accuracy, but due to much larger computational times they do not lend themselves to fully automatic optimisations easily. A promising way forward is a novel
technique using adjoint equations which compute the sensitivity to disturbances of a target variable or function inside the RANS code FreSCo+
parallel to the solution
of the momentum equations, (Stück et. al. 2011). Tis approach is used in TARGETS to perform hullform optimisation for a complete ship including propulsion. Te following figure shows examples for computed sensitivities for two different vessels; the upper figure indicates the results for a composite objective function combining ship resistance and wake quality applied to a bulk carrier with active propulsion while the lower figure shows results for the influence on resistance at four different speeds on the aſt body of a car carrier. Te adjoint RANS solver is applied to
explore the design space for a new ship design. It helps to find optimal hullforms
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for a new vessel and, thus provides straightforward input to the DEM at the design stage of a ship. Another important problem arising
in both, design and operation is to find optimal hullforms and floating conditions to minimise wave resistance. As the quality of standard potential
flow predictions
is limited for blunt ships and off-design conditions that is ballast or significantly lower speeds, attempts are made to replace them with RANS based Volume of Fluid (VoF) predictions. Tese however require substantially more computational effort so that speed improvements are necessary. The in-house code FreSCo+
has been
amended with free surface initialisation and changes to the internal algorithms to accept higher Courant numbers. Tis lead to an improved computational performance that reduces the wall clock time by a factor of four to seven depending on grid size and Froude number. Results shown below in the section on energy audits and simulaion for operational scenarios have been computed using this approach. Besides form related resistance effects,
the project performed fundamental investigations related to frictional drag. One of the major outcomes of the first phase was a systematic characterisation of surface roughness. TARGETS has researched viscous
resistance in a
combination of experimental and numerical approaches which lead to a characterisation and correlation database for surface coatings and their effect on energy loss. Te main parameter affecting viscous resistance is surface roughness. Having performed a large number of tests with different coatings in a dedicated flow cell, roughness effects are now modelled into CFD to capture the effects of a deteriorating surface over time. Tis novel capability of CFD codes also allows it to perform comprehensive investigations of life-cycle effects of ships over a broad range of different operational conditions. Besides standard surface conditions,
further advanced treatments and surface structures were investigated. In a comprehensive report effects of patterned surfaces (figure 4) have been investigated and their potential has been assessed. In a second step, the potential of different air lubrication techniques will be assessed.
Finally, added resistance caused by
the natural seaway and the wind will be considered. Whilst
in a large number
of practical cases only rough empirical estimates for the added resistance are used to determine the maximum continuous rating of the main engine for a new design, more accurate methods are available today. Potential flow based strip methods or
panel codes are available to predict the effects of a natural seaway over a large range of conditions (sea state). For more extreme cases RANS-based methods may be applied to assess the effect of either more severe wave conditions or fuller hullforms (tankers, bulk carriers), which exceed the limits of the linearised potential flow methods. Partners in TARGETS investigate the use of both types of methods to predict added resistance
Propulsion Ship propulsion equally contributes to hydrodynamic efficiency and hence determines the energy efficiency of operations. Increasing propulsive efficiency consequently is high up on the TARGETS development agenda. Research includes improvement of propeller efficiency as well as propeller- hull interaction using conventional and unconventional means of Propulsion Improvement Devices (PIDs). This development will deliver practical design tools for the improvement of the propulsive efficiency for conventional propellers as well as more advanced unconventional propulsion devices. As a first step, TARGETS developed a focused, high performance standard
The Naval Architect September 2012
Figure 4. Examples of patterned surfaces: Riblets, Shark skins and Dimples
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