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Mikko Brummer

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About Mikko Brummer

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  • Birthday 02/23/1954

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    http://www.wb-sails.fi/en

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    Helsinki, Finland

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  1. I added a simulation in Youtube about the bad air zone/wind shadow behind the AC 75: Upwind, there is not that much difference to the sub-wind speed case, even if the bad air zone is wider and lasts longer behind the boat. Downwind, however, the bad air zone is totally different: When foiling at several times the wind speed, the wind shadow is left in the wake as a persisting wind shift, extending nearly right behind the boat and for a long time. There is no effect to the side or to downwind of the boat, as would be in the case of a traditional yacht, sailing at sub-wind speeds. You cannot
  2. I added a simulation in Youtube about the bad air zone/wind shadow behind the AC 75: Upwind, there is not that much difference to the sub-wind speed case, even if the bad air zone is wider and lasts longer behind the boat. Downwind, however, the bad air zone is totally different: When foiling at several times the wind speed, the wind shadow is left in the wake as a persisting wind shift, extending nearly right behind the boat and for a long time. There is no effect to the side or to downwind of the boat, as would be in the case of a traditional yacht, sailing at sub-wind speeds. You cannot
  3. Oh yes, absolutely, to the deck and also the sea. If you look at the vortices down at the sea surface, the one most "leeward" is from the keel or "bustle". It breaks down and dissipates fairly quickly, while the one from the head of the main, and from the windward foil, persist. The big turbulence behind the hull comes from the trenches, where the winch pedestals are modeled but the sailors are missing - I should add them to see if they make things better or worse (probably better?). Towards the top of the sails, you can see the jib tip vortices rolling around the mainsails upper vorticit
  4. Orthoganal view and in a larger domain for less "side effects". The colors are swapped (sorry), red is a header and blue a lift, and the scale is also a little tighter to show more nuances - red & blue represent now a shift of 3,5 deg.
  5. Some views of the AC 75 trailing vortices in 3D. When it comes to the fog photos of the VOR 60s, I'm not sure they are showing just trailing vorticity... as I recall, it was a very special meteo situation, with a strong low inversion preventing the warm humid air rising any higher. It's water vapor, or clouds that we see, and probably air temperature & humidity, sea water temperature, wind gradient etc. are involved. CFD simulations are usually "isothermal", and don't take these in to consideration - we should consult a meteorologist ;-). They use the same equations to predict weather, but
  6. In a true wind field of 5 kn a crosswind velocity of ±0,5 kn (the red & the blue areas) means a wind shift of nearly 6 degrees. So you want to avoid the blue areas... the dark blue blob right downwind of the boat is the one that no-one ever managed to break through in the pre-starts.
  7. These are 2D simulations, so essentially "flat", at about 11 m height, not showing tip vortices, for instance. I hope to repeat these in 3D, but it takes time - while these run in minutes, each 3D run takes days or even a week.
  8. No pressures shown here, only velocities. I've been using the word pressure here as sailors do in their speech, more "pressure" indicating a stronger wind (velocity).
  9. Vorticity sailing downwind. The starting vortices from the mainsail & the jib remain floating around each other, until the stronger main-vortex swallows up the jib-vortex. 638802722_LunaRossaDnWind2DvortiWB.mp4
  10. Animation of the crosswind velocities. I need to re-run this in a wider domain, now the periodical sides influence too much towards the end. 1846494501_LunaRossaUpwnd2DcrossWindVeloWBS.m4v
  11. Here's a still pic of the crosswind velocities, blue is a header, red is lift for a boat on starboard.
  12. Yes, I already did this, will have to shrink the video down to the 9 MB limit of this forum.
  13. Downwind, things look weird at the first glance: The pressure (or rather wind speed) is less on the leeward side/aft of the boat, and higher on the windward side, the opposite of what happens windward, and when sailing at speeds slower than the wind. But if you remember that we are looking at the true wind field here, from the perspective of someone standing still in the water (in the earth/inertial reference frame), it’s (perhaps) logical: The sails are accelerating the flow in front and on the leeward side of the boat, and decelerating it behind the boat. At the start, there’s a red &am
  14. Probing the wind speed around the boat in the upwind case. On the windward side, the air is slowed down some, and on the leeward side it is accelerated. 686885976_LunaRossaProbingupwndWBS.m4v
  15. A simple take at the AC 75 bad air, upwind and downwind. Upwind, the flow field looks quite normal, there’s the usual backwind/less pressure zone (or wind speed, if you prefer, the blue circle) on the aft hip of the boat, and the increased pressure on the leeward bow (red circle). In the general plan, the wind is little disturbed, the green color is the true wind speed 10 kn in the simulation, blue colors are about 9 kn and red 11 kn. The big disturbance is limited to the narrow wake back almost in the direction of motion, moving laterally over the field as the boat proceeds along diagona
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