Geometry preparation is a very important step in the CFD process. In order to have a successful computational grid generation, the geometry needs to be watertight, without manifolds, and it needs to accurately represent the real vessel. For us, Rhino has always been and continues to be the main tool for adjusting the input geometry we receive from our clients. This is mainly due to the quality of the STL files that it produces, which is the input format for our simulations. In this post you can check out how we prepared the geometry for one of our recent CFD studies on the influence of rudders on overall propulsion power.
Separating hull parts
Once we have a clean, watertight geometry, we can start separating the hull into different parts in order to make the meshing process more efficient. The requirements on the computational mesh are quite different on different parts of the vessel. In general, we tend to separate the hull into following parts:
- Appendages (rudders, struts, skegs, spray-rails, shafts and similar),
- The bare hull.
Below you can see how we separated the rudder and the transom from the bare hull for the KRISO Container Ship, used in the recent study on the influence of rudder on propulsion power.
Once the different hull parts are separated, we need to export them into a format which can be used to create the computational mesh, the Stereolitography file, or STL. In order to do that we use the Export command, and then use the detailed export control to make sure our STL has the appropriate quality. This is achieved by specifying the largest angle tolerance between the actual geometry and the local triangle in the STL, as well as the largest linear distance between the two (see below).
The result is a high quality STL without manifolds, as shown below, which can then be used to produce the computational Finite Volume grid, needed for the CFD simulation.
STL surface representation of the bare hull
Computational grid ready for the CFD simulation
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