torsdag 23 juli 2009

Interview with WB-Sails: Quality from Design

Interview with WB-Sails.

CJ: On your web site you state "In the last few years, advances in CFD (Computer Fluid Dynamics) and FSI (Fluid Structure Interaction) have changed the way we perceive sail aerodynamics. Old beliefs are proven wrong and new features found."
What old beliefs are wrong and what new features are found, in short?

WB-Sails/Mikko Brummer: These are listed on WB-Sails website. In short
  • The mast is not just a drag-device: it can contribute significantly to the  driving force of the sails.
  • On the windward side of the sails, flow separation occurs rather  through luff separation vortices than bubbles.
  • The turbulence always present in natural wind may lead to different performance on different days & weather.
  • The presence of the hull effects the flow around sails, so it needs to be included in the simulation.
  • On leeward side of the main, the leech vortex from the head of the jib "rips" the flow loose from the mainsail surface, causing a conical separation area on the main at the hounds level.
  • On the leeward side of the jib, separation propagates upwards from the tack, as the foot vortex bursts from the deck.
CJ: Thanks for interesting information. Would you say that you now can realistically simulate the action of sails + mast + hull computationally? If not, what computational simulations would be of interest to you?

MB: All computational simulations around a sailboat would be very much of interest to me ;-). Hard to say how realistic or accurate our simulations are... there are plenty of issues in collecting experimental data from sailing, too, both in the windtunnel & in the real world, so we hardly know exactly what we are comparing against. While the inviscid methods (vortex lattice/panel methods coupled with BL theory) could provide very similar forces to those in windtunnel tests (for upwind cases), the recent N-S solvers certainly have brought qualitative flow prediction to a new level of realism, when compared to windtunnel tests or real world observations (with tell tails etc). On the quantitative (forces) level, N-S results may differ from windtunnel, but there are issues with windtunnels too: turbulence level, inflow twist & shear, R-number, model geometry etc.

CJ: Interesting!

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