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The Virtual Tank Utility in Europe

Final Report Summary - VIRTUE (The Virtual Tank Utility in Europe)

VIRTUE was an EU initiative of leading marine CFD Centres to create a virtual basin by improving available CFD tools and integrating them in a comprehensive simulation environment of ship behaviour at sea. It aimed to complement model testing in real basins in the provision of hydrodynamic services.

The combination of leading edge CFD tools, formally integrated based on successfully proven approaches, and world leading test tanks will allow the EU marine industry to address hydrodynamic problems cost-effectively and open a route to full multi criteria hydrodynamic optimisation of ships.

The project was structured into the following work packages (WPs):

WP1 - The virtual towing tank
At the start of VIRTUE, the subject of WP1, Resistance and flow computations, was the one for which most CFD development in ship hydrodynamics already had been done and the largest maturity had been reached. Nevertheless, in the proposal preparation phase of VIRTUE, participants clearly identified the principal topics on which further work was needed to enhance the benefit of CFD in actual ship design work for the industry:
1. While there had been much focus on more qualitative results such as wake fields, the quantitative accuracy of the ship resistance prediction at model and full scale required improvement. Variable results and a large spread between predictions from different computations were observed prior to VIRTUE. Predicted scale effects raised questions.
2. This was even more so for propeller/hull interaction computations. Different approaches had been proposed and some results shown, but there was no clarity on the best formulation, the detailed modelling and the achievable accuracy.
3. The numerical accuracy and efficiency of the methods still limited the applicability and would need to be improved.
4. While development on optimisation for minimum resistance was being done elsewhere, the practical usefulness of the result hinges upon the accuracy of the resistance computations and was thus limited. After improving that accuracy, CFD-based optimisation clearly was a subject to pursue.
The proposal for WP1 has thus been made so as to make progress on all these points.

WP 2 - The virtual seakeeping tank
At the end of WP2, a lot of numerical results (based on benchmarks) became available for a wide range of seakeeping applications and for very different numerical models (free surface, waves etc), in both commercial/industrial codes and in-house / R&D codes.

The three deliverables D234, D245 and D256 appear as the key issues of WP2. They contain both the description of the algorithms and codes and all the results of the benchmarks for each method and codes.

The main results and conclusions of these results have been introduced in the WP2 section of the best practice guidelines. As seakeeping analysis includes a various physical problems, pragmatic methodologies are proposed for each specific problem.

WP3 - The virtual manoeuvring tank
The overall objective of WP 3 was the development of the approach based on prediction of manoeuvrability performance of ships using a series of numerical flow simulations for prescribed simple modes of motion as in captive model tests to obtain hydrodynamic derivatives of forces and moments with respect to the individual degrees of freedom and subsequent analysis of the resulting time histories of the hydrodynamic forces. The derivatives are then used in manoeuvrability simulators to evaluate ship performance in real manoeuvres for regulatory, design and operational purposes. Once the set of hydrodynamic coefficients has been calculated, all desired manoeuvres could be simulated with little computational effort. This approach follows the current practice of towing tanks, but replaces the experimental and potential methods-based techniques for derivatives calculation with numerical RANSE simulations. The derivative approaches are enhanced by direct numerical simulation of ship manoeuvrability through coupling RANSE with ship motion programs.

WP 4 - The virtual cavitation tank
The overall objective of the virtual cavitation tank was to develop and validate a limited number of promising codes that are able to predict the flow about both non-cavitating (necessary for cavitation inception) and cavitating propulsors. In particular, should these codes be able to compute the adverse effect of cavitation on the propulsion characteristics, such as radiated pressure fluctuations that lead to vibrations, and cavitation erosion.

The proposed project on the numerical cavitation tank (WP 4) was built up from three major tasks, forming the main building blocks:
- Task 2: development of an improved RANS code for local flow prediction on propulsors.
- Task 3: prediction of hull pressure fluctuations.
- Task 4: prediction of the risk of cavitation erosion.

Initial studies were conducted under Task 1 aiming at reviews of state of the art multiphase CFD codes and suitable datasets for benchmarking and validation. A third subtask here was the benchmarking of existing codes such as Fluent (at BEC and SSPA), COMET (at HSVA) and EOLE (Principia R&D). A final demonstration task consisted of a propeller design exercise, demonstrating the use of optimisers coupled to a propeller panel code.

WP 5 - The integration platform
In WP5, work by the partners was carried out across a range of activities in software specification, design and integration of data structures, and development of the main software platform prototype. WP5 was subject to major restructuring after the second year review that dramatically altered the mode of delivery of the work from one of pre-planned deliverables to user driven delivery. As a summary, the consequence was that:
- all major pre-planned objectives were met apart from agreed withdrawal of data mining work (original task 5.6)
- thirty-six user requirements resulted from the second year review, growing during the remaining years to seventy-five of which forty of the highest priorities were fully and seven partially completed,
- ten test cases were integrated and demonstrated rather than the originally planned two, and
- three additional partners were integrated into the package.

Based on the developments in all CFD related work packages in VIRTUE, a large variety of CFD tools can be reliably applied to a large range of hydrodynamic analysis tasks arranging from practical problems and considerations in the maritime industry. This is especially important in view of the ever increasing need for improved fuel savings and accompanying reductions of ship emissions which form a major issue in the public discussion today.

New and improved CFD tools together with a comprehensive Best Practice Guideline document which has been produced from contributions from all work packages covering areas such as resistance, propulsion, seakeeping, manoeuvring and propeller / cavitation predictions provide a significant boost to numerical analysis of maritime flow problems.

Together with the software and process integration platform developed in work package 5, VIRTUE tools today offer an unprecedented opportunity to analyse and optimise new ship designs from a hydrodynamic perspective and hence contribute substantially to solving the pressing problems of the maritime industry.