# reynolds number for a main acted on by a jib

Discussion in 'Sailboats' started by Paul Scott, May 24, 2007.

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### Paul ScottSenior Member

What wind speed number do you use when figuring the chord Reynolds Number of a mainsail when it is being inflenced by a jib? Freestream? Velocity at the mast? Other?

Paul

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### Tim BSenior Member

Freestream. The difficult question is what's the chord?

Tim B.

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### Paul ScottSenior Member

Tim, I was assuming a rectangular planform, with a wingmast. I'm trying to understand why you'd use freestream velocity. Is this a convention? Seems you'd want to deal with the flow actually around the main, esp. the leading and trailing edge, or am I wrong here? Another way of stating my question, if the jib changes the flow in the vicinity of the main so much, it seems that you'd want to design the main around the modified flow, rather than the freestream? Or do you average the speeds over the entire chord, and that equals freestream? Or is the main so much part of the the overall flow in the jib/main system that you can't separate it from the freestream?

Paul

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### tspeerSenior Member

Freestream is the convention for Reynolds number based on chord length. But there are lots of Reynolds numbers, such as the one based on local velocity and boundary layer momentum thickness for the length that's used to predict transition.

The jib not only affects the main, the main has a huge effect on the jib and its boundary layer development. If you're going to take into account the interaction, the way to do it is to calculate the pressure distribution for both surfaces in combination, then compute the boundary layer development on each one due to the combined pressure distribution.

This paper, http://www.boatdesign.net/forums/attachment.php?attachmentid=12710&d=1177653204, explains what the effects are and how they affect the boundary layer on both surfaces.

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### Paul ScottSenior Member

Tom, now I'll really have to finish reading AMO's lecture. Which is quite elegant. But so far, the Re numbers he's been using are a bit high for a dinghy in lower wind speeds. Given the aero reading I've been doing from the micro unmanned vehicle, UAV, and big R/C model community, things get.... interesting under chord Re 200,000, which got me thinking about what might be going on around a 20" chord main acted on by an overlapping jib under 8K of wind. As far as computations go, what do you think of the flow image on

http:syr.stanford.edu/09.gif

for example??

Paul

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### Tim BSenior Member

pretty standard, no obvious detached flow, this will not be far off optimum Drive/Side.

How much computation are you thinking of doing? 2D is not too hard, 3d is harder to get good results from.

Tim B.

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### Paul ScottSenior Member

I'll let you know when I finish AMO's lecture.

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### tspeerSenior Member

There's no obvious regions of separation, so this may be an inviscid calculation. One would expect to see a separation bubble on the windward side of the main and on the leeward side of the mast, but perhaps these bubbles are smaller than the spacing of the streamlines in the figure.

It shows the mast is in a region of flow that is slower than freestream, due to the reduced velocity on the windward side of the jib. The flow on the lee side of the main is accelerating to about the belly of the main, so any separation off the mast is probably going to reattach.

It doesn't appear that the forward quarter of the mainsail is directly contributing a great deal, but you can get a sense of how the jib benefits from the presence of the main, making the combination more powerful.

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### Paul ScottSenior Member

Thanks Tom (and Tim), it's hard to know what I'm looking at sometimes. It did look kind of theoretical, which is what I think you mean by inviscid (?), although it does give me some idea of the interaction between the two sails. I esp like how what I think is a representation of the kutta condition is really easy to see at the leach of the jib. Do you agree with Gentry that placing the leach of the jib at the point of maximum flow speed at the lee side of the main raises the overall speed of the flow off the leach of the jib, and by doing that reduces the de-acceleration of the flow on the lee side of the jib (and it's consequent turbulance) because the Kutta condition at the lee side of the jib is defined by a higher flow speed on the windward side that it otherwise would see by itself? Doesn't this argue for overlap, and does this jibe with AMO's observation that extensive laminar flow in the forward part of the (i guess jib, in the Stanford image, or in Gentry's presentation) helps to delay separation considerably, because it reduces the reynolds number based on momentum thickness (?). He also points out that if the pressure rise is not great, it may be extremely steep, as far as boundary layer suction forward of the pressure rise. I'm not even sure I really understand this as I write it, but is this even close to the influence the main has on a short chord jib like the one in the Stanford image, since it does have some overlap into the lower pressure area of the lee side of the main?

!!Editorial Comment!!-This is as difficult to understand as the theoretical musicology of Strauss's opera Elektra. Maybe Schoenberg was the breakdown into turbulance of Strauss's classicism. Elektra does push it right to the breaking point. It may have played itself out in Shoenberg's subconcious! Try playing Verklarte Nacht, and then a tone row sometime.

Paul

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