Articles about Aerodynamics

Advances in sail aerodynamics
Sail aerodynamics - part two
Sail Dynamic Simulation
Streamlines & swirls
WindTunnel Movies
Sails shape & aerodynamics
The Quest for the Perfect Shape
Note on the effect of side bend
Anatomy of a Mini-Transat
Mini boat - Maxi challenge
Mini boat - Maxi challenge
470 Aerodynamics
Lifting bows with foresails
Telling tales ...
The scientific Finn
Wind tunnel images

WindTunnel Movies

These movies
are from a wind tunnel research program we recently carried out at the HUT and VTT wind tunnels. The purpose of the research was primarily to calibrate and verify the MacSail aerodynamic simulation program. The effect of the hull & rigging on the air flow over the sails was studied, and insight was gained on the influence of various trim lines such as sheets, cunningham, outhaul, mast bend etc.

The trailing vortex emanating from the upper leech of the jib can clearly be distinguished above the leeward back corner of the model. The yarns on the windward side lie relatively still compared to the disturbed flow in the jib wake on the leeward side.

Flow separation is monitored with 10 mm long tell tails cut from regular C-cassette tape. The wind speed is 15 m/s (30 kn).

Alternative JPEG movie, should the better quality Quicktime movie not work for you.


A closer view reveals some intermittent separation in the upper leech of the main. The separation is at least partly triggered by the jib wake vortex interfering with the mainsail flow.

Alternative JPEG movie, should the better quality Quicktime movie not work for you.


Even closer, the turbulent nature of separated flow is clear. See also the yellow tuft at the jib leech, and the disturbance of the mast over the tell tail close to the luff. In see through, you can distinguish the windward tell tail streaming straight up.

Alternative JPEG movie, should the better quality Quicktime movie not work for you.

The tests were carried out in wind speeds ranging from 10 to 20 m/s (20 to 40 kn). Wind speed was chosen to generate sufficient pressure to provide realistic mast bend & sail cloth stretch properties. The model is equipped with all normal sail controls including sheets, cunninghams, outhaul, jib lead, traveler & boom vang.

The sail trim was radio controlled from outside the tunnel, while actual sail driving force, heeling force and heeling moment could be read on the computer screen during the test. Depending on the test run, sail drive (light to medium air tests) or the sail drive to heeling moment ratio (heavy air tests) was maximized.

20 different runs were carried out, with tens of measurements in each run at apparent wind angles ranging from 18 to 60 degrees (corresponding to a true wind angle range of approximately 35 to 90 degrees). The sail shape & tell tail behavior was recorded during each run with 3 video cameras and one normal camera. More than 15 hours of video was recorded during the tests, for later analysis


The movies were made using Apple's Quicktime - they should
under Windows, too, at least in newer browsers (4+).