sail aerodynamics

Vertical wind gradient is included, yes, as well as the crew sitting on the rail, although it doesn t show. The plot is from one frame with motions in the waves, so forces vary a lot from one instance to another - I don't have the total forces here, but you could integrate them from the curve. The plot is from this simulation

 

The caption in the movie says the turbulence is due to separation on the windward side. But maybe there is more is going on than just that.

The windward separation occurs when the angle of attack is reduced by the head moving forward as the boat pitches down. But the turbulence on the lee side is shed when the head is moving aft as the boat pitches up. I'm not sure the time difference is due to the convection of the turbulence from the windward side of the jib to the leeward side of the main.

It looks to me like there's a starting vortex being shed from the leech of the jib as the lift on the jib changes due to the boat's pitching.

 

I noticed the same sequence as Tom.

Regardless of the exact mechanism of the turbulence, a sail with lots of pitching motion does not have seem to have much chance for maximum efficiency.

For a long time, I have felt that there can be a big overall impact of pitching motion. It looks like optimum trim would be a trim that is not optimum for the median condition, but is instead geared more toward maximize the percentage of time in the "good region" of performance.

One of the significant advantages when cats & tris go for "foil assist" is probably the reduction in pitching motion.

 

For clarification Tom, what are you using as your definition for 'head' in this posting?

 

Top of the mainsail. The corner opposite the tack and clew.

 

Thanks. I wanted to make sure it was not the 'leading edge' of a sail, as some folks refer to the head of the sail, nor the headsail, etc.

(PS:I should have known you would be using the correct terminology. Perhaps it was just the manner in which I read it at first that I a little confused)

 

In other words the most productive portion of the 'rig' as a whole is where to two sails work together ?

With that in mind I would ask Mikko if the top portion of the mainsail could be improved by having a mastheaded jib?

 

Interesting. When you look at the colour graph the low velocity blue indicating separation occurs up high on the windward side, mostly on the mainsail, when it is pitching forward. The pitching forward creates an apparent wind shift forward so you lose pressure on the windward side and separation occurs, but it does seem to reattach as the boat pitches back again. However when you look at the smoke simulation on the leeward side that is where you see trouble on the leeward side, as it pitches back. The apparent wind shift back again and, perhaps because of the immediately preceeding loss of pressure on the windward side as fell, some turbulence or vortex shedding occurs on the leeward side, but only on the mainsail not the headsail.

This raises some interesting questions. Assuming you maintain optimal heel at least on average, how does it affect boatspeed and pointing ability compared to if it was flat water with less pitching. Also, the mainsail leech is fairly tight, perhaps like it should be if the boat doesn't have to depower. How would things be different if the rig was able to work a little bit more with the waves? Would it be better or worse?

Here is a simulation of a Star, in windier conditions, and with a more fractional and considerably more bendy rig...
https://www.youtube.com/watch?v=zCpVfFDYfWE

It's not a fair comparison and I don't know which is better. The star is designed as a one design class with a HUGE amount of sail area and a slender rig that has a very wide range of ability to power up and depower, at the expense of no headsail downwind. The more conventional racing boat has a less fractional and less slender and depowerable rig, so less range of draft in the mainsail, but the ability to carry spinnakers and such downwind. Both boats appear to be handing the waves pretty good in my opinion, The Star rig is really working, but it has lots of extra sail area to work with and so it san probably have the leech open and close quite effectively in these conditions without worrying about losing too much power. The more conventional racing sloop is sailing with a tighter leech and trying to maintain more power and pointing ability through the waves without losing too much, because it has less to work with relative to its ability to carry sail. So that is my take anyway. Interesting simulation to see what is going on, and where it is going on. If I sailed either of these boats it would make more sense though. Love these simulations. Would like to see one of a Laser sailed upwind with and without vang sheeting, and with body kinetics. There are some Finn simulations but it is quite a different boat I think. Great stuff.

 

So the vertical wind gradient can be accounted for with modern CDF methods?
The reason I ask is I went back to one of Arvel Gentry's documents and read this in the last paragraph.

 

CFD. Computational Fluid Dynamics.
Gentry was a wizard. CFD is getting better and better though as processing power improves, now with large array parallel processing. There will always be limits though, with things like turbulence, and when dealing with significant things happening on boat a large scale and a small scale, and how nothing is ever steady state in sailing. It's impressive how far we have come though. It does provide a lot of insight.

I would like to see some simulations of sailing laser, finn, optimist etc. by-the-lee. Lots of three dimensional flow I think, big vortex over the top as well as around the sides, and lots of dynamics, but I think it could be handled by CFD. Still thinking about trying a smoke bomb some day, or sailing in a snow storm.

 

I see now more clearly what you were saying.

And that image arose on its own,...as a consequence of flow, not as a directed input.

This CDF stuff is quite amazing. I just don't know much about it from a technical standpoint.

 

In CFD they break up the volume of fluid into a very large but finite number of small elements. They don't have to all be the same size and shape now. You can squeeze more elements in where there is more going on and you need more accuracy. Then for each of these elements, for all of these elements at once, you solve the Navier-Stokes Equations for fluid flow. That includes things like conservation of mass, conservation of energy, viscosity, pressure, density if not constant, temperature if not constant, etc. To solve these equations required a number of iterations to converge on a solution of pre-determined accuracy, and that is just for one finite segment of time. From there you walk forward in time and iterate all over again, although it is a little quicker now that you already have the previous time segment, which is pretty close. Turbulence complicates things immensely. There are different ways of dealing with that depending on the type of flow and what you are after. There is a bunch of other stuff I am forgetting, like boundary layer theory and so on, but basically it is brute force iteration, and you end up with a pretty decent approximation of pressure distribution on your sails and mast and so forth, and the hull also of you are solving that at the same time. If there are waves then you have gravity waves and buoyant forces in addition to the hydrodynamic and aerodynamic forces, and if the hull and rig is dynamic then you have all those inertial forces as well. But that is basically it. I don't know what simplifications and assumptions they make but they always have to make some. They usually calibrate the solution using results from the real world or wind tunnel tests. That's all I got. Hope it helps.

 

Slot Effect again

Sorry if this 'slot' subject seems repetitious, but I've battled naysayers for a long time. As I was looking back thru some of Mikko's archived articles I ran across a pretty good clear explanation he had,....and interestingly he makes a distinction between 2 different 'slots':

http://cdn2.hubspot.net/hub/209338/news/Ad_aerodynamics/index2.htm

 

On that same page of his I found this:

http://cdn2.hubspot.net/hub/209338/news/Ad_aerodynamics/index2.htm

Naturally I would say, why not masthead in lieu of 9/10 ?

 

Yes, that's been explicitly noted in trim and tune articles by racing sailors for decades. For example, it's a factor that one looks at when determining headstay tension in many classes.