New R&D on added resistance in waves
Today, route simulations play an increasingly important role in connection with ship design and retrofitting where it is essential to evaluate the performance of the ship sailing on a given route to judge the quality of the new design.
The performance evaluation is typically focused on determining the average roundtrip duration at a given probability, but also on calculation of the fuel consumption to estimate for instance the fuel cost or the Ship Energy Efficiency Operational Indicator (EEOI) proposed by IMO. One of the key elements in a reliable route simulation is a good prediction of the added resistance in waves. Therefore, FORCE Technology has just initiated a new 3-year R&D project together with IIHR Hydroscience and Engineering at University of Iowa on prediction of added resistance in waves.
RANS CFDThe new project on added resistance in waves is partly sponsored by Office of Naval Research (ONR) and was launched in the summer of 2012. The goal is to investigate the possibility of using RANS CFD for prediction of added resistance in head and oblique waves. Traditionally, added resistance has been predicted by means of empirical and/or potential theory-based methods. However, these methods have limitations which for instance means that they cannot handle flow separation, they do not include viscous effects, and they apply linearization. Further, when it comes to following sea conditions and short waves, the potential flow methods are experiencing difficulties which means that empirical methods are most often used for these conditions. However, in spite of this,
the combination of empirical and potential theory methods is widely used since the computational effort is limited and many conditions can be swept quickly.
Possibilities of RANS
The question is how accurate RANS is for ships sailing in waves, and if it is possible to extract information about the added resistance at different wave headings which can, for instance, be used for route simulations. RANS offers the possibility of calculating the flow field, the forces and the motions with one tool without many of the limitations of the potential theory tools, and it seems a promising tool in connection with data generation in the simpler head sea case. When it comes to generation of large data sets, RANS is not as fast to use as the simpler methods, but it offers the possibility of studying the flow in detail while the ship is moving in the waves and can hereby contribute with information that is useful for the hull designer in the early design phase.
Further, it may be possible to still use the potential theory methods and then supplement the results with RANS computations in the cases where the potential theory is weak or breaks down. In theory, RANS should be able to generate the required added resistance information, but there are also limitations to this approach. Therefore, it is necessary to validate the computations against measured data. The present project will cover a numerical part where a number of wave cases will be computed, an experimental part where the same cases will be tested in the towing tank and finally a validation part where the comparison between computation and experiment will be made to ensure that the numerical method performs satisfactorily.After the project, the goal is to have a more complete toolbox for generation of added resistance data for route simulators in order to improve the quality of the simulations and give our clients a more accurate picture of the ship’s performance on the route during the initial ship design phase.
FORCE Technology and IIHR at University of Iowa have successfully been working together over the last decade. The collaboration between FORCE Technology and IIHR has partly been sponsored by US Office of Naval Research (ONR). The latest project ending in the summer 2012 was focused on the CFD-based PMM test in order to predict IMO manoeuvres without experimental data.
As part of this work, FORCE Technology has participated in a number of research activities related to the NATO Research and Technology Organization (RTO) which promotes and conducts cooperative research and information exchange, develops and maintains a long-term NATO research and technology strategy and provides advice to all elements of NATO on research and technology issues. One of the RTO research groups which FORCE Technology was part of has focus on stability and control of air and sea vehicles, and it has just been announced that this group is selected to get the RTO Scientific Achievement Award 2012. This award is the highest research group award within NATO.