Let's go back to the front of the sail again. Before any circulation was created you had a circumstance as shown in Figure 15.14. We now need to label some parts of this diagram, and I have done so using the terminology created by the scientists. The lines of wind flowing across the sail are known as streamlines. Between the flowing streamlines are two stagnation streamlines, i.e., streamlines that end or begin abruptly. These are created by the vacuum that arises when the flowing streamlines separate and attempt to go to either side of the surface or when flowing streamlines are joined at the leeward side of a surface. The surface breaks the flow of the wind, one set of streamlines heads for the windward side of the surface, and the other heads for the leeward side. An inevitable neutral vacuum is created, and this area becomes known as the stagnation streamline. One of each kind occurs at opposite ends of a sail. Figure 15.14 shows the stagnation points quite close to the edges of the sail. Note that this is before any circulation is created and that, as was the case with the first schematic, we are dealing with an incomplete picture. Now that the illustration is labelled, let's see what happens
BEFORE ANY CIRCULATION IS CREATED
Stagnation Streamline (S)
Before any circulation is created.
Shaded areas are where a vacuum exists
COMBINING REGULAR FLOW WITH CIRCULATION FLOW
On the leeward side of the sail regular flow + circulation flow = more speed = loi« pressure.
Regular flow across the sail
Circulation flow around the sail
With circulation even more air is flowing across the leeward side, since the velocity of the regular streamlines has been added to the circulation flow.
On the windward side of the sail regular flow - circulation flow = less speed = high pressure.
when some other the forces created are added to the equation. This is important because the new equation has an effect on the stagnation streamlines.
Without circulation, air was flowing across the leeward side of the surface. With circulation even more air is flowing across the leeward side, since the velocity of the regular streamlines has been added to the circulation flow (Figure 15.15). On the windward side the opposite happens since as the regular streamlines come into contact with the circulatory flow, they start to cancel each other out.
All of a sudden two things are happening. Slow-moving (read high-pressure because of Bernoulli's principle) air is on the windward side of the sail and fast-moving (low-pressure) air is on the leeward side. This creates an accentuated pressure gradient, and by extension, lift. The old theories thought this was happening, i.e., that there was a pressure gradient, but they didn't take into account the effect of the circulation and the fact that it adds velocity to one side and reduces it on the other, which makes a dramatic contribution to overall lift . And that's only the half of it, since the meeting of these two air streams, the external flow and the circulating air, also affect the location of the stagnation point, and because of it, a phenomenon called upwash occurs.
Upwash occurs at the front or leading edge of the foil. Look at Figure 15.14 again. Note how the leading stagnation streamline comes into the surface quite close to the front edge of the surface (marked with an X) and that no circulation has been factored in at this point. Aft of the stagnation streamline (a distance marked with
With Circulation — Note that the stagnation point has moved quite a bit because the regular flow and the circulation flow have canceled each other out. Also the Kutta Condition has been met.
Upwash occurs as the air in front of the stagnation streamline heads for the leeward side of the sail.
a Y on the diagram) the windward streamline is quite far from the airfoil surface. Because it is far away we can expect it to be moving fairly slowly. (Since the closer the streamlines are, the more speed they have, just like isobars on a weather map). Therefore, circulation flow, or what there is of it at this point, will be about the same speed as the windward streamline, but moving in the opposite direction so the two forces cancel each other out and the point at which the stagnation streamline comes into the airfoil shifts aft, as seen in Figure 15.16. The external air flow is moving so slowly at this point that the circulation flow is not only able to cancel it out, but even move it in the opposite direction, i.e., make the flow change direction and go around the front edge of the airfoil. Therefore, air that was once destined to flow past the windward side of the foil is now being diverted around the leeward side, further increasing the pressure gradient. This flow around the front edge is called upwash. Once those air particles get past the deepest part of the sail and begin flowing toward the leech of the sail, the flow is called downwash.
This dynamic can be summed up by saying that an aerofoil actually "feels" the foil long before it gets there, and it starts to separate. If you have driven in a snow storm and watched the flakes coming at you being diverted over the hood of the car long before they get to the windshield, or watched water flow around boulders in a river, you will have witnessed this phenomenon. This phenomenon helps create more lift since the upwash at the leading edge of the sail accelerates as it goes around the front edge of the foil while the air diverted to the windward side is a bit lazy, i.e., it does not want to move up into the concave area created by the foil (toward Y in Figure 15.14) and so it sort of hangs around waiting to hook up again with the air flowing past the leeward side, creating a region of high pressure. Of course, the fast-moving air on the lee side of the sail creates a low pressure area, especially near the front of the sail, which adds even more to the pressure gradient, creating lift.
With the boundary layer creating vortices, the vortices creating circulation, the circulation affecting the speed of the flow and the speed of the flow affecting the pressure on either side of the foil, you end up with a surface that creates much more lift than in the original sail theory model. If all of this seems a bit much to come to grips with, let me try to put it all into plain English. Have you ever played golf? If you are like me you have probably sliced the ball a few times (in my case many times). This slice is nothing more than the ball spinning because of the way you hit it, and the spinning creates circulation, which in turn bends the flight of the ball, or "lifts" it to one side. Baseball is another example. If the pitcher wants to throw a fastball he throws a ball with little or no spin to it. That way the ball flies straight toward home plate. If, on the other hand, he wants to throw a curve ball, he throws it with a lot of spin on it. The spinning creates circulation, and this circulation, when added to the normal air flow on the ball, creates two different pressure areas on either side of the ball. On one side is high speed and low pressure, and on the other side is low speed and high pressure. The ball will curve in toward the area of low pressure.
Let's now think about a boat coming out of a tack. As the bow of the boat moves through the eye of the wind the sails are just flapping. Obviously there is no flow, no attached air and none of those little vortices needed to start the circulation. As the helmsman bears away he presents an angle of attack to the wind, and as soon as the air starts to flow over the sail small vortices begin forming along the trailing edge. The vortices gradually begin to create circulation, but as you can imagine it takes time and effort. The vortices are only small cogs, and the circulation is a big wheel. In light air this takes even longer because the vortices are small and weak. Conversely, in stronger winds, the circulation begins sooner, and the boat is able to get up to speed and pointing much quicker. It's important to understand this because if you are the sail trimmer or helmsman you need to know that each time you make a move that causes the circulation to be disrupted, it takes time before it gets back to a steady state and provides the maximum amount of lift. This is why you will see the biggest difference between a good helmsmen and good sail trimmer and a bad helmsmen and bad sail trimmer when sailing in light winds. It takes that much longer to crank up the old circulation/lift machine. It is one of the reasons why sailing is such a fascinating and challenging sport.
One more variable can be tossed into the mix. The aspect ratio of the sail, or the ratio between the height and width of the sail, will also have an effect on how quickly circulation can be established. A high-aspect sail will generate circulation more quickly than a low aspect sail, and will achieve the same percentage of lift in much less time. This is basically because the wind does not have as far to travel around a high aspect sail, and therefore it can start to get the circulation going
"This slice is nothing more than the ball spinning because of the way you hit it, and the spinning creates circulation, which in turn bends the flight of the ball, or "lifts" it to one side."
Long, narrow keels are quick to generate lift, and the result can be felt in the way the boat sails.
High aspect sails and appendages are quick to generate lift. Low aspect sails and appendages take longer.
that much sooner. This is why dinghies are much more responsive than keel boats. It is also why modern rigs are tending more toward high-aspect sails rather than the squat sails found on older boats.
If this is true for sails it must also be true for the appendages below the surface of the water. We know that the high-aspect keel on a modern racing boat like an Open 60 or a Volvo 60 is infinitely more responsive than a short, wide keel found on an old IOR boat. These long, narrow keels are quick to generate lift, and the result can be felt in the way the boat sails (see Figure 15.17). Compare a full-keel cruising boat with a fin-keel racing boat, and the fin keel always wins. Most sailors do not give any thought to that part of the boat dragging through the water when they look up at the sails flying through the air, but the two go hand in hand. The yacht designer spent many hours planning it that way, so bear this in mind and don't fight it.
You now have a fairly clear picture of how a surface creates lift. The "surface" we have been referring to is actually the jib (or the main on a boat with no headsail) and things get to be even more interesting when you add a more complicated sailplan. For example, we all know that most boats also carry a mainsail, which creates its own circulation. And we also know that adding a mainsail to a sailplan increases the overall efficiency of a boat. But until now the reason for this increased efficiency was wrongly thought to be because of the "slot effect," with the "slot" being that area between the leech of the head-sail and the lee of the mainsail (see Figure 15.18). Old theories claimed that
THE SLOT EFFECT
Two circulations work against each other to dramatically reduce the flow.
The Slot (shaded area)—The headsail and mainsail circulations cancel each other out in the slot.
Apparent Wind the air funneling into this slot increased in speed and revitalised the flow along the backside of the mainsail, thereby making the mainsail more efficient. Unfortunately, that theory was wrong. The good news, however, is that we now know what really happens, and it's all good stuff. If you are interested in how the two sails work together stick with me, grab another cold drink, and let's move on to Phase Two.
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