Mary,

It may probably be best if you named the individual parts you find yourself struggling to understand. Otherwise there will be to much to write.

vortices :

really something common. That funnel that you see when you empty your washbasin is a vortex. A tornado is a very large vertical vortex. Vortexes on the tip of wing (or any object that has a high and low pressure side) are just invisible horizontal spinning (around centreline) volumes of fuidum (air) and are called Tip vortices. However similar spinning volumes can be found on leading and leech edges as well depending on the design of the object. They can also be found behing chimnees and buildings. A vortex is a special type of turbulences. In general it is turbulence with a highly predictable and uniform shape. General turbulence moves all over the place while vortices can be very clearly defined. Dependent on their rotational speed these vortices can sap alot of energy from the pressure difference that is causing then. Of course energy is the final baseline of all processes. The sail develops power by milking energy from a energy rich medium.


spanwise flow : Just what is, says flow along the span of a wing.


Mary, you said that your father indicated that sails are pushed along.

How strange it may seem but many tests have shown that a wing section experiences much greater low pressures on the leeward side than high pressures on the windward one. So there is indeed a push component on the sails but a far bigger of the portion is the sailpower is developped by an underpressure (low pressure) on the leeward side.

Mary, the hard part is that fuidum mechanics and fluidum dynamics is a very strange world and often it is just a little different form what you would expect. Our dear God had a really jolly mood when he worked on this area.

One example is that a flow through a pipe at the part where the pipe narrow and then widens again will first drop in pressure and than rise back to normal pressure instead of the other way around. This is called the Venturi effect. Normal people will just expect the very opposite.

Anyway, what else do you want explained ?

Wouter