What the hell is he doing in Auckland?
After posting some stories about my weekend activities, I want to give a short overview of why I am in Auckland in the first place!
The University of Auckland has one of the very few research institutes in the world that deal with sail aerodynamics and have proper wind tunnel facilities to test their theoretical work at the same time. The YRU (Yacht Research Unit) owns the Twisted Flow Wind Tunnel, which is used for doing commercial and research testing mostly for America’s Cup (IACC) and VolvoOpen60/70 class yachts.
Now, why “Twisted Flow” and “How do they do it”?
Everyone of us has experiences that the wind speed picks up from the ground level when we climb up (e.g. taking the elevator to the top of a tall building). This effect is caused by the Planetary Boundary Layer (PBL) which can be assumed to have an exponential shape (just like boundary layer flows over surfaces). Now, imagine you are sailing on a fast boat at constant speed. The wind is blowing from a constant (absolut) direction. The height of the mast can be as much as 30m, so there will be a lot more wind at the mast-top than on the deck level. As the boat – and with it, the mast – are moving at a steady absolut speed, the wind direction you experience on the deck level is the (vector)-sum of the the true wind and the boat speed. This apparent wind has an angle to the yacht centreline called the apparent wind angle.
Now imagine you were sitting on top of the mast. You are still moving forward at the same speed as the people on deck (except you just broke the mast 
), but you feel a much stronger wind. And it’s not only the wind strength that has changed, rather, the WIND DIRECTION that you experience has changed as well (the apparent wind).
As the yacht-model is fixed in the wind tunnel, it is then necessary to twist the flow in the wind tunnel to correctly model the oncoming flow as it would appear on the water.
To achieve this, we have several vertical vanes installed in front of the test section of the wind tunnel. We can adjust the twist and the speed of the flow as a function of height (using crossbars to build up a boundary layer) and thus model the effect of the planetary boundary layer and the change of apparent wind speed and direction.
What does that have to do with me and what do I do there?
Yes, I do drink a lot of coffee. However, after long EUROAVIA nights in the office, that’s nothing new
I work on a project called “Inverse Design of optimal sails”. In simple words, we want to theoretically predict an optimal sail shape under certain external constraints rather than guessing a sail shape, doing some tests on it, and then alter the sail shape to iteratively come up with a good design. This topic is not necessarily new as a lot of people have worked on the topic before. However, this sail design technique is still not broadly used; and it is thus worth pushing forward.
Hence, I am trying to work out a code to analyse the performance of an existing sail using a relatively simple aerodynamic approach. However, I will add the sail specific boundary conditions such as heel (leaning to the side, similar but not equivalent to dihedral in airplanes), the influence of the footgap underneath the sail, and the twisted flow to the equations making the theory much more complicated. After comparing my theoretical predictions with measurements on a stiff sail (made out of fibreglass), I will then try to inverse the process and optimize the aerodynamic parameters under given constraints and come up with an optimized sail in terms of low drag and high thrust.
Well, I think that is enough theory to begin with. I yet have to take pictures in the wind tunnel which will help understand what’s really going on. Coming soon…..
Greetings from rainy Auckland
Timm
