Parametric Hull Optimisation
Parametric Hull Optimisation using CFD
When using CFD to optimise the hull, automatization is key. At Cloud Towing Tank, standard CFD simulations such as calm water resistance, self-propulsion and seakeeping, have been automatised to minimise the amount of man-hours required to run them, and with that their cost too. To conduct parametric hull optimisation, another aspect of the whole process needs to be automatised, and that is geometry manipulation and export. Using Rhino/Grasshopper tool, local or global hull characteristics can be parametrised, and the export of a large number of geometries automatised (see an example here). Together with an automatised CFD environment, this provides a powerful tool that allows quick optimisation of various aspects of the hull.
In our experience with parametric hull optimisation so far, we have found that having three parameters in a single optimisation gives a good compromise between cost and performance. With three parameters, the minimum number of geometries that need to be analysed is roughly 27, which is feasible even if the optimisation need to be carried out at multiple speeds and/or drafts.
Another important component if the generalised Lackenby transformation, which allows:
- Global parametrisation of the hull in terms of LCB and parallel midbody length,
- Maintaining a fixed displacement and LCB during local hull modification.
With this approach, any local changes to the geometry (e.g., bulb) are compensated using Lackenby in terms of the change of LCB and displacement volume. This allows us to conduct the optimisation without changing the displacement or LCB. Beside the hull itself, different kinds of appendages can easily be parametrised and optimised, such as struts, propulsion pods and similar.
Common examples of CFD hull optimisation
Very common part of the hull's geometry that is often optimised is the bulbous bow. Tipically this includes varying the thickness, heigth and lenght of the bulb, but can also incldue the variation of the stem shape (vertical stem vs. classic stem). Below is an example of bulb optimisation for a container carrier. You can read more about this study here.
Optimizing the stern shape of the hull is another common goal of CFD hull optimisation, where propulsion performance is directly analysed. Stern frame shapes as well as the stern gondola and bulb can be parametrised to yield higher propulsive efficiency. In this case the propeller needs to be modelled, which is achieved with the actuator disc model. Below is an image of the original (black) and optimised (red) hull form, where 1.6% power reduction is achieved. Read more here.
To achieve the best results, during the parametric hull optimisation projects we collaborate with the hull design team to make sure that the parametrised features of the geometries are relevant to the project. It is our job to propose features of the gometries we suspect to be important and to parametric the original geometry in the best possible way. Depending on the exact parametrisation technique used, we often need help from the design team to propose alternative hull frame shapes that will be used to create a parametric design space.