Planar Motion Mechanism with CFD

Over the years, FORCE Technology has performed captive model tests for shipowners to assess the manoeuvring capabilities of ships in the late design stage.

8 September 2015

The benefits of simulating manoeuvres

By performing numerical captive model tests in the early design stage, we get an understanding of the vessel’s manoeuvrability and are thus able to improve it. This helps shipowners embody the improvements early in the design stage and thereby avoid costly and time-consuming last-minute changes to the design.

The advantage of CFD is a better understanding of the flow around the vessel because the calculations may be paused and the pressures, velocities etc. can be studied thoroughly. This provides the naval architect with a unique opportunity to improve details based on information that cannot be obtained through physical model testing solely focusing on force measurements.

Finally, the numerical setup is not limited by physical boundaries; the manoeuvring can be predicted in both deep and shallow water and in terms of larger dynamic manoeuvres such as turning circles.

Commercial applications

A number of commercial projects have been performed successfully for various clients. The results from the simulated manoeuvring tests were applied as input to the mathematical ship model used in our in-house ship bridge simulator.

The numerical tests are typically used in three levels of increasing accuracy:

for generic mathematical models (based on database values), the actual ship geometry can be used in the numerical setup in order to improve the accuracy of the mathematical model

for mathematical models built from scratch, a full numerical captive test programme can be performed as input to the model. This is a cheaper but less accurate alternative to performing the captive model tests in the towing tank and is typically selected when no physical model is available

the numerical tests can be used as a supplement to deep-water captive model tests performed in the towing tank. The numerical tests assess the effect on the manoeuvrability of e.g. banks or shallow water and for filling holes in the test matrix.

Development of the method

We continuously developed and validated the numerical PMM (Planar Motion Mechanism) setup against towing tank data from both our own and other facilities. The latest addition to the setup is the modelling of the actual propeller geometry in-place on the vessel. This means that the flow around the aft ship and the forces generated by the propeller are simulated more accurately than by means of the previously used momentum source models.

Compared to towing tank tests, the forces and moments found are usually within ~10%, which is acceptable in most applications. Apart from validating against towing tank tests on commercial, and thus confidential, vessels, a larger study has been performed on the MOERI Container Ship (KCS), which is a “public” ship used for research.

Captive model tests

Over the years, FORCE Technology has performed captive model tests for shipowners to assess the manoeuvring capabilities of ships in the late design stage. The result is a solid knowledge database of manoeuvring results for a large range of ship types and sizes. We still carry out captive model tests in our towing tank. As a new service, we provide the tests as numerical CFD tests in the early design stage.