single surface sail with round mast

[walt12345]

I would appreciate any references which have at least some coverage of single surface sails using round mast (such as a windsurfing sail). Im specifically looking for references which deal with the seperated flow in the area behind the mast.

Thanks in advance,
Wally Hall

 

[walt12345]

Maybe I need to be more specific with my question... Im looking for ways to possibly reduce the seperated flow behind the mast on a windsurfing sail and reading a previous post in this forum, vortex generators on the mast sleeve might be interesting at least for an experiment? Typically there is a fairly large seperation bubble on the windward side of the sail just behind the mast (maybe 1 to 2 foot in lenght) and likely a smaller one on the leeward side but its small enough that I cant detect it with my tell tails. What I was thinking is simply using "accessory chord" (ie, polyester rope) and sewing it onto the mast sleeve in a zig - zag fashion. This zig zag rope might be sewn at maybe just downwind of the widest part of the mast on both sides and would extend over maybe the lower 10 foot of the sail where all the shape is. The rope idea is nice because I can try it myself and also still roll the sail up for storage. Some specific questions are:

1. In the typicall operating range (described below), would this both result in less seperated flow and hence lower drag without affecting lift much (ie, an improvement in L/D?). Is the rope sewn in a zig zag manner going to create the desired effect or just add more overall drag?

2. How large a structure is needed (rope diameter, distance from bend to bend) and where would it optimally be placed?

Some specifics about the application: Sail width is about 2 meter, height is about 5 meter. Round Mast diameter is approx 4 to 5 cm and the mast is inside of a sleeve which is part of the sail. The sleeve width is typcially about 10 to 15 cm - sometimes even more. Speeds are typcially 20 to 35 mph for a windsurfer and up to 60 to 70 mph when used on an ice or dirt boat.

Thanks,

Wally Hall

 

[GregLocock]

An amateur replies: I've been messing about with Microtunnel (google to find it) for sails. I don't think you've really got a separation bubble on the windward side, I think you have a stagnated, or even backwinded, area (sorry, don't know the proper term for this). This is actually good, in that velocities are low so pressures are high. Low velocities in the windward side of the sail towards the front help push you along. It is also unlikely, but possible, that you have separation just behind the mast on the leeward side. You camber inducers should fill out that part of the shape nicely to prevent that, or else your sail should rotate around the mast to do so in a simple sail, so that your wing profile resembles a comma.

Roughening the surface of the wing in the nose area helps prevent separation further back at high AoA, I don't think (I'm guessing) it really has much effect where you still have laminar flow.

Cheers

Greg Locock