CFD Flow Domain for Ship Hulls from NAPA IGES Files

From time to time we receive NAPA IGES files from our maritime cus­tomers which are basi­cally an assembly of often hundreds of single surface patches. The question is then: How can we quickly create a water­tight, box-like flow domain for these ship hulls that can be readily meshed, for use with tools such as FINE/​Marine, STAR-CCM+ or SHIPFLOW?

An example of such a flow domain is shown above in the header picture of this blog post. Since this is in most cases a sym­met­ric shape, only one half of the ship hull body is considered.

To give you a better idea of what kind of data we are talking about, check out the fol­low­ing picture. It shows a typical NAPA geometry, imported into CAESES®:

The Chal­lenge

Creating a flow domain from a ship hull body that is per­fectly closed (e.g. a solid body or at least a set of exactly adjacent surfaces) is not a big deal — you can simply use a set of Boolean oper­a­tions. However, in our sit­u­a­tion this is not the case: If we look at the tes­sel­la­tion of the surface patches, we can observe that often they do not match exactly at the bound­aries. There can be larger gaps between the single surfaces. For many meshing and grid gen­er­a­tion systems on the market, these gaps are critical and possibly an auto­mated meshing cannot be realized with this.

The problem: Gaps between the imported surface patches and over­lap­ping surfaces.

Even if we try to apply a fast joining pro­ce­dure based on our trimesh func­tion­al­ity, the result still shows that some gaps are simply too large.

If we further increase the snapping tol­er­ance for joining the surfaces auto­mat­i­cally, other tiny surface patches are acci­den­tally joined — so there is some limit to this approach, and it is only applic­a­ble if you cope with STL data for meshing (i.e. the surface topology remains the same, only the tes­sel­la­tion is fixed).

We can also zoom into one surface patch­work at the pro­peller region where the flat of bottom dis­ap­pears, which is shown in the next picture. From the distance every­thing looks fine, but using our mouse wheel to zoom in, it suddenly reveals that the surfaces do not cor­rectly meet each other in the corner:

We have also observed surface patches that are self-over­lap­ping in sin­gu­lar­i­ties where the corner vertices should actually meet each other exactly. It can be quite a tedious task to manually repair these surfaces in tra­di­tional CAD systems, just to receive a clean surface mesh that can be used for sim­u­la­tion. So we need some­thing that does the job with just a few mouse-clicks.

An Auto­mated Solution

We have recently spent some time to come up with an auto­mated solution in CAESES®. If the input geometry is heavily cor­rupted in terms of the surface quality, there are also lim­i­ta­tions. However, for most of the geometry data that we have received, this auto­mated process works well and appears to be an elegant and fast way to go.

Driven by the NAPA IGES files but also various demo files from other CAD systems (Rhino etc.), we have devel­oped gen­er­al­ized methods to import typical ship hull geome­tries and apply an intel­li­gent sewing and healing to the unclean BRep topology. This results in a closed surface topology, from which we can generate our target flow domain without any problems.

How It Works

It’s easy to use: You only chooses the IGES file and some color settings for the domain patches, such as the symmetry plane, the hull parts and so forth. This is impor­tant for an optional colored STL export, where you want these parts to be stored sep­a­rately. The fol­low­ing video shows how this process works:

In this video, a feature gets created that does the import and the creation of the flow domain. Option­ally, you can switch off the domain creation process and only import the IGES file. Note that the green edges on surfaces rep­re­sent closed edges in CAESES®, while red edges indicate non-matching or open edges. If you need STL data as a result instead of an IGES or a STEP file, you can addi­tion­ally specify some accuracy settings to fine-tune the tes­sel­la­tion. The deck and the stern part are addi­tion­ally sep­a­rated by using a split-by-angle command.

Download

Download this demo STL file and import it into your meshing system to take a look at it. Thanks to the team from NUMECA Germany for checking our gen­er­ated files!

More Infor­ma­tion

Any feedback to this func­tion­al­ity is highly welcome, we are still testing and improv­ing it. If you have geometry data for testing from any CAD/CAE system includ­ing NAPA — perfect, just let us know. We’ll take a look at it, and send you the result­ing flow domain in this current testing period. Any other ques­tions? Please don’t hesitate to get in touch with us, also if you are inter­ested in using this func­tion­al­ity or if you have a similar task to solve.

We are still testing this! Send us your example geometry and we create a flow domain for you.

Finally, see the marine section on our website for an overview about what you can do with CAESES® in the context of ship design and hull shape optimization:

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