Feature 1 | CFD AND HYDRODYNAMICS
for marine CAD systems and Numerical towing Tank Symposium (NuTTS) for marine CFD applications.
Economic assessment In order to determine if the software investment is financially viable, an economic assessment should be made. Te differences in the functionality provided by CAD and simulation soſtware means that their assessment must be treated differently.
Economic benefits of CAD Software For a first estimate of the expected economic benefits due to the implementation of a new CAD system, a simple cost-benefit comparison may suffice. For this simple analysis, costs can be seen as global sums and the benefits considered as reduced man-hours. To be beneficial, the reduced man-hours must, at least, compensate for the additional costs. In addition, the effects of CAD on profitability include:
• Improved product quality for constant capacity
• Improved productivity, i.e. unless staffing levels are reduced, CAD results
• Flexible work hours increase CAD profitability.
in extended capacity; if there is no corresponding increase in demand and staffing levels cannot or must not be reduced, CAD investments will not pay off
Economic benefits of Simulation Software Te simplistic estimate described above considers only the direct costs and benefits due to the implementation of the CAD system but, neglects possible indirect benefits and savings due to reduced errors etc. For simulation soſtware, like CFD, this simple approach does not work and the expected additional income through the ability to offer a wider scope of services must be also considered. Te value of computer technologies can
be classified according to time, quality and cost aspects. Te main benefits of CFD in these respects are, according to Bertram (1993):
• Problems solved more quickly than when using conventional approaches,
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• Improved designs through detailed analysis and/or formal optimisation
• The speed of CFD now allows its applications during preliminary design.
• CFD does not significantly reduce the cost of the actual design process, but
Te use of CFD early in the design phase reduces the potential risk associated with the development of new ships. Tis is especially important when exploring niche markets for unconventional ships where the design cannot be based on previous experience
it improves quality and helps with the early detection of design flaws and this can lead to significant cost savings. Within the first weeks of design, 40% to 60% of the total ship production cost is determined, Johnson (1990). Te costs of design modifications increase by orders of magnitude the further into the project they are made; ideally no fundamental modifications should be made aſter the conceptual design phase. Achieving this goal can be greatly facilitated by the use of CFD. If CFD is employed consistently to determine the final hull form at an earlier stage, numerous decisions that influence the production costs can be made earlier in the design process, thus reducing the risk of expensive design modifications being necessary later in the project. Tis is especially important in the context of modern workflow methodologies (e.g. concurrent engineering and lean production).
When assessing the economic benefits
of CFD, merely considering cost aspects will, therefore, lead to incorrect strategic decisions. We can assess the benefits of CAD by using an analogy with another computer technology: CIM (Computer- Integrated Manufacturing), aptly put by Dietrich (1988); “In other words, when taking CIM
decisions, the question of ‘What will we save in the short term?’ is not the right one to ask. Te real issue is rather; ‘How will
due to better (direct) insight into design aspects. CFD analyses of ship hulls and appendages before final model testing are now standard practice in ship design. CFD can also help with much faster trouble shooting in cases where problems are found
OpenFOAM
Recently, the open-source CFD package OpenFOAM, has attracted a lot of attention in the maritime CFD community, Schmode and Bertram (2009). In the hands of well-trained users, OpenFOAM yields good results for a variety of complex maritime flows (seakeeping with six degrees of freedom; cavitating flows around propellers; sloshing with breaking waves; etc.), see for example, the 2009 Numerical Towing Tank Symposium, www.uni-due. de/imperia/md/content/ ist/nutts_12_2009_cortona. pdf. Zero licensing fees, wide scope of applicability and access to source code for personal modification make OpenFOAM very attractive, particularly in the academic environment. Industry users, on the other hand, should take a more prosaic perspective: savings in license fees may be far less than the added costs of training. Experienced CFD users have reported (in personal communication) that it took them several months to two years to come to terms with OpenFOAM. Nevertheless, Germanischer Lloyd has decided to employ both commercial CFD software (STAR-CCM+ developed by CD-adapco) and OpenFOAM in parallel. The investment of considerable resources in a “free” software package was motivated by co- operative projects with several academic partners. However, it is fair to say that the bulk of professional consulting work continues to be based on the use of commercial software products.
The Naval Architect July/August 2010
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