# The ultimate validation?

Discussion in 'Hydrodynamics and Aerodynamics' started by Mikko Brummer, Sep 25, 2012.

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### Mikko BrummerSenior Member

Our sails, designed with the help of computer simulation, won Gold & Bronze medals at the London Olympics - is that the ultimate validation for our simulation codes?

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### Mikko BrummerSenior Member

OK, maybe it's no validation at all, the medals were won by the sailors, not by the sails, and even less by the CFD simulation. Nevertheless, you could turn it vice versa: without the computer simulation, we would not have won these medals. Without simulation, we would have ran out of ideas years ago, and these sails never would have made it to the Olympics ;-).

Star LIC-flow simulation in waves - LIC= line integral convolution. It provides a representation of the flow analogous to the resulting pattern of a tract of wind-blown sand. Here we can see the effect of the motion of the boat on the flow pattern.

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### daiquiriEngineering and Design

Congratulations for these victories Mikko. It is true that sailors race the races, but every victory is the result of a teamwork, and you were an important part of it.

About that video, what lessons did you learn from it and how did you use it in the optimization of the sails?

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### Leo LazauskasSenior Member

I agree. Well done, Mikko!
Remember that the dietitians, coaches, and uniform makers, are all claiming
their part in the victory too. The bus driver who got them to the event on time
is probably still getting free drinks for his effort.

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### Mikko BrummerSenior Member

A complete analysis would take too much time, but simulation of motion in waves was what we added to our tools during this Olympiad. Perhaps surprising (but good) news is that the average forces remain more or less the same, despite the pitch & heave. Yaw (the steering of the boat) can also be simulated, and it has a considerable effect on the forces. The yaw is changing in the simulation shown here, even if does not show well as the angle is from the side.

If you step the video frame by frame, you can read the Cx (Drive force coeff.) and the Cz (Heel) and see how very much it varies depending on the state of the boat along the wave. The top of the main stalls as the bow pitches up, and there's a separation vortex from the tack of the jib at the same time. Lots of drive is lost, but Heeling force & moment are also at minimum. As the bow pitches down, you can see separation on the inside of the main top, and also in the upper jib luff.

You can also see the disturbance of the spreader on the windward side - with different steering it will show on lee side as well - and the ever present disturbance at the tack of the main, air leaking under the boom at the gooseneck.

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### Mikko BrummerSenior Member

The motion of the boat used for the simulation was recorded on Loof's boat while sailing, with an inertial motion unit (IMU). The IMU, by Cosworth, is the same they use for Formula 1 cars to show you the G-forces on TV. Getting the pitch and the yaw from the gyroscopes is straightforward, but heave is a bit more delicate as it needs to be double-integrated from the accelerations, and the integral has a tendency to "creep".

The animation from ahead shows the steering motion by the Olympic champion - the bow is drawing an elliptic path with the longer axis more or less vertical. You can also compare with the bow shots from the Olympics.

The graph shows the huge variation of the driving force as the boat passes over the wave - there is most drive available as the boat starts it's climb up the wave. Each pitch angle is associated with two very different drive numbers, depending whether the bow is on its way up or down.

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Olympics

Congratulations, Mikko !

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### johneckSenior Member

Very interesting. Is the stretch of the sail material accounted for? What about sheet trimming/easing?

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### Mikko BrummerSenior Member

Oh no... that's still way beyond our (or anyone's) computing capacity... maybe in 4 years or so, for the 2016 Rio Olympics ;-). Sheets/sails are not trimmed in a keelboat over every wave, but the sail shape is far from static, so in that respect the simulation still lacks.

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### PetrosSenior Member

This is very interesting. I think anyone that has operated a dingy, and perhaps smaller keel boats, has noticed or felt the different amount of drive or thrust off the sails as the boat pitches forward and aft. So it does not surprise me you can measure it, it is nice that the "feel" is confirmed with instruments. What I would be interested in knowing, what do you design differently on the sails to take advantage of it?

Do you optimize the shape to for the flow conditions where the thrust is maximum? or try to improve the thrust or driving force when it is not as high?

Would you design different shaped sails if you knew it was going to be on realtivly smooth water?

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### Luc VernetSenior N.A.

Fantastically interesting, and thanks for posting.
The difference of the flow between the intrados and the extrados at the top of the sail is spectacularly shown: laminar on the intrados when the boat pitches up and the opposite on the extrados. Also very interesting - but not surprising - to see that the vortexes created by the mast are on the extrados only.
Has the yaw of the boat, like on the view of the bows, been part of this simulation or only the pitching motion?
However: this is only on the surface - or close to. Do you have/ would you share such video showing the whole flow in 3D, or at least some "frames" taken along this same motion?

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### Mikko BrummerSenior Member

Yes, the yaw on the LIC-simulation is as shown in the view of the bows.

Well, how to show the "whole flow" in 3D? Here's one attempt, mapping the vorticity with some filtering to keep the model in sight as well. This is a different case, there is much more separation on the jib extrados here.

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### nicoSenior Member

great job Mikko.
What RANSE code did you use for this?

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### Mikko BrummerSenior Member

We've measured the motion of the boat with instruments, the sail forces are calculated by simulation, not measured values.

From practice it would seem performance is best when the boat is moving at as steady speed & heel as possible. In the simulation it is interesting to see how different steering techniques (different phases in yaw & pitch) affect the timing of the thrust available, in addition to the average thrust. But for the total performance of the boat you naturally have to mind how the hull is passing the wave, too.

Certainly, very different. It's well known that more twisted sails perform better in waves. Also, different vertical profiles in the wind (shear) call for different sail shape, as does the natural wind turbulence. The trick is to design the sails so they would work automatically in unison with the rig, to allow for changing wind and sea conditions.

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### tspeerSenior Member

Thanks so much. I was going to get to bed at a decent hour, but I couldn't stop watching the movie over and over!

That's really amazing.

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