Some details and text on the Computational Fluid Dynamics simulations that mostly Jari has been doing lately. They are done with the open-source software package OpenFOAM, which is of course free to download and use. We run it under Ubuntu, and use fairly affordable 4-core machines with 8 Gb of RAM.

You start with a CAD model of the geometry you want to simulate. Here the hull-shape has been taken from one program, and the sail shape from another sail-design program. They're imported into a third CAD program and joined into the model we want to simulate.

There are (at least!) two issues at this stage: (1) the meshing-tools that follow do not like infinitely thin surfaces, so in order for meshing to run smoothly the sails have to be drawn as having some finite thickness, about 1-2 mm in this case. We can try to reduce this in the future. (2) This is only a CFD simulation, not a fluid-structure interaction simulation. That means the geometry (sails, mast, etc) is not going to move or flex in any way in response to the fluid pressure. So the trim and shape you draw into the sails is going to stay there...

The geometry is exported as STL-files for each surface. Meshing then follows. It's first done with a simple program called blockMesh, and then refined close to any specified surfaces using a program called snappyHexMesh. The resulting mesh looks like this:

And if we zoom in we see the smaller cell-size around the surfaces of the model:

Here's another potential problem with CFD. (3) How do you know what the right mesh-size is? You don't. Decreasing the cell-size obviously leads to a more accurate simulation, but as the size of the mesh goes up the calculation time increases similarly. Some CFD/FEM-packages have adaptive solvers that can increase the mesh-resolution in areas of the simulation where it's most appropriate. I don't know if OpenFOAM has that. One simple approach is to run the same simulation with successively increasing mesh-sizes and see how the end result (e.g. drive force on the model) reaches a plateau, hopefully the correct value.

Then it's time to specify boundary conditions. For a wind-tunnel type of simulation the in-flow velocity on the upwind wall of our simulation-domain is fixed, and the surfaces of the model are specified as no-slip surfaces.

Solving follows. Now you actually have to select the physics model you want to simulate. Well that's easy, in the continuum-limit fluids should behave according to the Navier-Stokes equations. The only problem is that these are nonlinear and difficult to solve. Some people actually do this, but it takes a big big mesh and lot's of computing power. It's called direct numerical simulation. The rest of us with desktop-computers need to solve something simpler than the N-S equations. A lot of different approximations exist, so (4) how do you know which physics model to choose? The Reynolds-averaged N-S equations (RANS) are a popular choice. Our simulations use the simpleFoam solver, which assumes an incompressible fluid, and includes a k-epsilon turbulence model.

OpenFOAM can run the solver on one CPU-core, or the mesh can be split into parts and each part of the mesh run on a separate CPU-core. These simulations usually take around 6-12 hours to complete on a single 2.5 GHz core.

After solving it's time to visualize the results:

OpenFOAM can also integrate the pressure over select parts of the model to obtain the total force on the model. Here we chose a wind-speed and direction for which Lester Gilbert already has some real-world data. The driving and heel-forces seem to roughly agree.

Animations enhance the visual experience:

Add some music and a few camera-angles, and you would think this qualifies as promotional material for any upcoming Americas Cup syndicate...

(To see this in bigger format, click the "YouTube" logo to go their page, and then click "HQ")

That's all for now. If you're a CFD-expert, please be brave and comment below on my open questions (1)-(4) !

An IOM boat+rig in the digital wind-tunnel sailing downwind. The plan is to run a few of these and compare with actual data. Available in HQ on youtube, remember to turn the sound up!

Jari has put quite a bit of time and effort into learning to use the OpenFOAM open-source CFD package. It's maybe not as polished and pretty as Comsol or Ansys or any of the other 10+ keur costing commercial packages, but it seems OpenFOAM has all the same solvers and physics models and is able to handle complex geometries and meshing - if you spend the time and effort to learn how it all works.

Stay tuned for some "digital wind-tunnel" testing of IOM sails and foils... (bring your 4/8-core machine and 8+ Gb of RAM)

There are a lot of CFD software packages out there, some expensive and some hugely expensive. The most well known open-source alternative seems to be OpenFOAM. It's not as easy to use as the fancy commercial ones, but it's well supported by both academic and industrial users, and it's free! Some installation notes, mostly for myself, on how I installed Ubuntu 8.10 and OpenFOAM 1.5.

1. download 64-bit Ubuntu 8.10 iso-file and burn to CD (8.04 did not want to boot on my Q9300 cpu machine, probably a motherboard conflict?)
2. install Ubuntu as normal, run suggested updates (286 of them actually!)
3. for OpenFOAM I roughly followed instructions from here: https://help.ubuntu.com/community/OpenFOAM
4. download all the .gtgz files from here. Rename them to just .tgz so Ubuntu knows how to extract them
5. run the bashrc magic with '. $HOME/OpenFOAM/OpenFOAM-1.5/etc/bashrc' followed by 'source ~/.bashrc'
6. now in OpenFOAM-1.5/bin we can try './foamInstallationTest' which errors out because it doesn't find ssh or rsh (these are only needed for remote installations or cluster-calculations or similar I think)
7. to make foamInstallationTest happy, install ssh: sudo apt-get install ssh
8. now foamInstallationTest reports that all systems are go.
9. paraFoam which is used for pre- and post-processing needs QT. I'm not sure which packages exactly are required, but mine started working after installing 'libqtcore4', 'libqt4-assistant', 'libqtgui4' and all of their dependencies. (I did this with the synaptic package manager)
10. Ready!

Try running the tutorial cases in e.g. ~/OpenFOAM/OpenFOAM-1.5/tutorials/icoFoam/cavity. This tutorial is explained in the docs here. Everything is set up already, so you only have to run blockMesh to generate the mesh, then icoFoam to do the actual calculation, and then visualize with paraFoam.

Looks like the mast bends to leeward at the middle, while the top of the mast stays in place. I don't think that's a good thing. Should try lower shroud attachment point and maybe shorter spreaders.

Remember to turn up the sound so you hear the wind, water, and the RMG winch! Also try these in higher quality by going to youtube and clicking "watch in high quality".

Here's how the camera looks on the foredeck:

Although we've already built a number of finboxes, I've forgotten to publish the plan - so here it is. As the Noux Mk2 is still somewhat of a prototype this finbox has a long 27.5 x 5 mm slot for the mast which will allow optimisation of the best mast position and rake angle during testing. So as not to carry a whole lot of extra water in the boat we made the mast-box narrow, 5mm, which means that the lower end of the mast needs to be an aluminium extrusion of 5mm width... A future Noux Mk3 might again have a more traditional mastbox for a round mast when we have found the optimal position.

I've had a couple of requests for reduced shadows for the Noux by people wanting to home-build the boat using planking or other methods. Here are the lines with 1-6mm offset from the outer hull surface. A DXF file is attached to the pdf, click the small paper-clip next to the text to save the DXF as a separate file

Here's the part I wanted to make. It's an RC-pot recess for the Noux. I'm using a lid and rim from a food container as the only hatch in the boat. The rim attaches to the lower conical part, and the lid is recessed into the deck by the upper conical part.

Our mill is still very much being upgraded with parts here and there, here you can note the new heavy duty vise (below the part), and a new coolant system (blue lines, now powered by a much stronger and higher volume pump than before). We're now using EMC2 to drive the mill which works much better than turbocnc.

Here is the finished part. It's milled in two parts with a central bolt holding the two pieces together. Here I've positioned the disks on the aft-deck mould, approximately where they will be attached eventually.

Before bolting the disks to the aft-deck mould we wanted to make a test part, so in the upper left of this picture you see a moudling with only the two rings. We've used epoxy mixed with micro-fibers (the brown/pink stuff) to round off the 90 degree corner before applying any fiberglass. To the right the rim and lid of the pot.

Removing the moulding from the metal disks was no easy task although we used the same Frekote release agent that works well with other parts. The aft-deck with the pot-recess will probably require some careful moulding technique or otherwise we will end up with a deck stuck to the mould...

Happily, the rim fits nicely inside the ring as it should after pressing it down quite hard. A tight fit is good since this join needs to be watertight - probably silicone sealant + four screws are going to hold down the rim in the actual boat. On the right I've put on the lid and it recesses nicely and will leave just 1-2mm of the lid above deck level. There's an opening for the bit that sticks out of the lid - I think I'm going to cut that off the lids and have a perfectly round hatch in the boat.

I've thought some more about the placement of the radio components and come to the conclusion that the 'flatpack' thing I was thinking about previously is maybe not so good. It would position the battery inbetween the winch and the servo and that would make moulding a strong structure quite hard. Above is my latest idea, which would position the winch and the servo on a moudling which will attach to the inside rim of the hatch. This part will also require a custom mould which I'll probably cnc cut from aluminium. The battery would then require a separate 'pocket' or 'cage' that must be made separately. The Spektrum DX6 Rx is so small and light that it really does not need any special space considerations, it can be velcroed to almost any surface.

Update 2006 Aug 30: A variation on the same theme: a "flat-pack" version.

Here is a side view of what we came up with. The winch, either an RMG or a Graupner Regatta, is located most forward, the battery in a recess in the middle, and the rudder servo is aft. The Rx is small and light so it can be velcroed almost anywhere. (click image on the right for a pdf)

A top view. Almost half of the available lengthwise space needs to be allocated to the winch (at least if using an RMG. The Regatta is a bit smaller, but on the other hand it uses a larger drum). Click image on the right for a pdf.

This is the moulding that would hold the winch, batteries, and servo. Need to come up with a smart way of moulding this piece... anyone have any ideas ?

Another perspective view of how the radio installation could look like. Compare this to Craig Smith's Obsession below.

I remember seeing a nice compact radio installation on the Roar Edge marbleheads, it was called an 'rc-boot' or something similar. If anyone knows if the current Stollery marblehead radio installation is similar, send me a picture !

A 'flat-pack' version

Placing the winch, battery, and servo all inside the 115 mm circular hatch could make things a bit cramped inside the boat. So here is a version with the battery cells all lined up in one row, I'm calling it a flat-pack.
This saves the width of an AA cell, about 15 mm, from the length of the radio moulding. and that allows me to enlarge the winch side of the moulding by about 5 mm and the servo side by almost 10 mm. Now both the winch and servo seem to fit much better.
The downside ofcourse is that the new boat will need new battery packs. Well, it's a good idea to change batteries every two years or so anyway, so I'm not considering this as a major drawback of this design (It will be nice to upgrade to the new 2700 mAh AA cells anyway, added security and no need to change batteries during a whole day of 3-rig racing).

click the picture on the right to download a pdf drawing.