Tuesday, January 24, 2012

entry level CFD

CFD has been - and probably still is - the realm of people who know what they're doing. However recently Autodesk released a little bit of software called Vasari, which is really a software preview of some of the things that might be available in future versions of Revit. The 'wind tunnel' in Vasari is admittedly a bit crude compared to other things on the market, so you have to take the results with a grain of salt, but it does seem to give a reasonable impression of what goes on.

The thing I have been initially trying to test for ís whether the hull tramps in any particular configuration acts as sail, or otherwise increases the efficiency of the sail. This could probably come about in a couple of ways:

  • The wing tramps act as an end plate, and reduce the size of the vortex coming off / wrapping around the boom
  • The leeward tramp generates lift in a downward and to windward direction when the boat is heeled strongly to windward, acting as not very productive sail
  • A fixed foredeck fairing of whatever size could generate lift as long as the AOA is reduced somehow so it doesn't stall, or it is a very high stall angle section
  • A fixed fairing might fill the gap between the deck and the boom and increase the end plate effect, even if it is stalled.
  • The underwater profile of the hull generates lift

There has been mention on Facebook that there might be some legality issues with fixed aerodynamic fairings attached to the hull. I couldn't immediately see where aerodynamic fairings of this sort were ruled out, but please if you want to comment do so, I want to open up the discussion.

a couple of screen vids showing the airflow over different tramps, all other things remaining equal:

An early hypothesis is that the vortex trailing off the tramp actually seems to counteract the one from the bottom of the sail slightly. When you have a full tramp, that vortex seems to work better than with a small tramp. Just an early theory though.


Anonymous said...

Cool, wish I knew what I was looking at!

Some of the Tassie Moths tried netting tramps recently and figured they're slow, maybe you've helped prove why. G-string tramps don't seem popular either, maybe less isn't more.

I think your mast-base aerofoil is a great idea - probably be called a sail though and be included in the 8 sq. m (or whatever the new sail area is) or be called a second sail.

Limbic Candy said...

Netting Tramps? Si Payne will remember that he sold me his brother's yellow Magnum 8 K3942 complete with garden net tramps in 1990... My experience was of a major advantage: the boat wouldn't drift faster than I could swim. Fast forward 20 years nothing much has changed - either in my swimming or my ability to stay upright.

Andy said...

Hi there,
some of the tassie boats have been trialling the nets. There are no conclusions about performance but there are observations:

The tramps are difficult for boat handling in waves like what was in Mornington because the wave would pass through the tramps rather than bouncing off the underside.

Any kind of lift from the wing tramps means that you are also going to produce drag. This is the downside to adding lifting surfaces and maybe why the nets feel faster than solid tramps. Reducing all forms of drag is the key to increasing the speeds further.

If you are creating lift from the wings, there is less load on the foils. This means that you may need less lift on them at times but it may operate at odds with the wand.

nick flutter said...

Haha I haven't really been looking at those criteria!

I want to know how we currently define 'sail' and 'hull', because I think that's gotta be an important question in the moth class in the future. I have ascertained that the hulls we currently have work as sails to varying degrees, or maybe contribute to the sail or contribute to the overall drag in varying degrees - I wonder how you go about measuring that. All well and good to say 'measure it as sail', but are you sure? Because where do you draw the line between sail and hull?

That's really the discussion in trying to start up.

nick flutter said...

On mesh tramps - have you guys had a look at the drag from all of those little strings? You could probably work it out pretty easily. I wonder if that's less than the drag of a big flat plate that is stalled?

Alan said...

There's no ball of turbulence around the blob that should be sitting on the windward wing!

Interesting stuff Nick, it seems you have too much time on your hands.

Anonymous said...

I fixed mastbase fairing of some description seems a good idea but not sure if you want to add more lift with it or anywhere. If the breeze starts smoking it's hard enough to stop the boat blowing back over itself.
Also you may remember tassie m2 was using a fairing over the foredeck/mastbase at the belmont worlds.

Doug Culnane said...

Sails are not defined in the current rules so the ISAF definition is probably the one to use:


However these define the traditional established sail equipment not new innovative sails.

From the IMCA rules:

Wings or racks - Structures which are used only to support the crew or rig outboard of the buoyant
hull. May include buoyant components which are not normally immersed.

So if a wing or rack is used for any other purpose than to support the crew or rig then it is not a wing. Therefore if there is an end plate effect then the wings are no longer purely wings.

I think we have to discuss this properly and try to come to some sort of definition before someone turns up with a leeward rack that has sail camber in it and batters etc.... Maybe the current boats are unwittingly (or not) using there extra sails a so we have 10 sqm of sail area.

Does anyone have a good definition / criteria of a wing and a sail or as nick indicates the current boats use the combination of sail and tramp to produce an efficient combined sail system.

I think end plate is not an issue as yachts have always put sails down to the deck but using the leeward wing for driving lift is pushing this a bit far.

Great and very interesting debate.

All the best,


Smilicus said...

Looks awesome Nick. How can I get in contact with you? email address? would like to ask you a few questions regarding building a moth.

nick flutter said...

@ smilicus,
email me, flutter dot n at gmail dot com.

I cant guarentee that I have the answers, in fact a couple of people in the comments above know a LOT more than me! I am happy to try and help.

Doug, thanks for commenting. It's an interesting question, really, and a debate that I think is worth having. Should moths look like bat-mobiles, with fixed wings and appendages to improve airflow?

At present my new boat looks like it'll have thick tramps, because the drag reduction from having attached flow behind the leeward tramp looks significant at this stage. Its an easy one to delete if indeed the class doesn't want it, although I think when they see how easy it is to do / retrofit, and how sexy it will look, they might.

It'll be a fun design exercise at least.

Doug Culnane said...

Hi Nick,

I would advise you to get in touch with Adam May who is chief measurer and has already thought about the legalities of this. I do not know what his thoughts are but worth finding them out before you build, I would have thought. You may be on to something...

All the best and build on,


Anonymous said...

does creating one set of turbulent flow and then trying to negate it with an opposing set of turbulent flow actually result in less drag?

you might end up with a nice free stream again directly downwind of your wings but the horse has already bolted, you've got 2 lots of turbulent air being created.

yassine said...

Hi, can I have your email? We need your help.

nick flutter said...

Above ^