Laminar Flow Wind Question...

[Rick D]

...before all you guys slink back to the underworld...

I recall that wind will rise before a landmass when blowing onshore and will likewise return a ways off it on the backside. That's the working assumption I have made for decades as I try to keep away from steep rises of land mass (and falls). This is more important to me as it related to racing.

So, my question is whether the base assumption is largely correct and secondly if there is some kind of rule of thumb (sorry ) that i can use to figure out where the lift begins for any value of the height of the landmass? Would the reverse be true for a falling mass? This assumes nominal wind of 15 knots, temperature of 60 degrees F and steady barometer. I realize this is just a tiny bit of the variable, but it's also all this tiny brain can handle.

 

[MikeH]

John,
From the 30 years that I raced on lakes and rivers with varying land heights on each side, and from my background as an engineer, I'm not aware of any widespread rules of thumb for this. (And, technically, almost no wind flow is truly laminar, except in the lab.)

The shape and height of the bordering land should have the most effect - like the different front ends of vehicles on the highway. A more gently sloped land mass will delay that lift effect, whereas the shape of a cliff will be much more impactful.

Saying all that, the gut feel that I start with when near shore is a look at the heights of land above water - how high it peaks and how far away from shore - then begin with the assumption that the lift effect carries at least that far onto the water. Most of the time that's good enough to keep my sails full of air. I'm interested to see how others respond to this!

 

[Dan Johnson]

Rick...

Where I race on the Chesapeake, we have a shoreline near one of our drop mark turning marks. It is positioned consistantly by GPS coodinates for each race so it's not a variable.

What we've noticed is that the wind does not flow perpendicularly off the shore as we get closer in to it. The shoreline ends with a point of land about a half mile or more toward the southwest from the mark. As we approach the mark on (let's say) a west wind, the wind moves more to the southwest. Those on the starboard layline end up sailing more of an elipse than a straight line.

It gets even more tricky when the wind is under ten knots and shifty in nature. The further north it moves, the more of a dead spot it creates as you near the shore to make the layline.

Somewhere in the back of my memory I remember reading in beginner sailing books about the wind shadows from shore obstacles being something like ten times the height of the tallest shore component (50 feet of tree height equals a 500 foot dead zone).

The only solution we've found is to read the wind on the water and watch our VMG like a hawk when in closer to shore, tacking before it gets glassy when the VMG begins to drop.

Not exactly an engineering answer, but I hope it helps!

 

[sinnettc]

The hill at the north end of the lake we sail (Fern Ridge Reservior) casts a pretty big shadow though I'm not sure it's 5x the height of the hill. What I do know is that the transitional area between the shadow and the marina will kick your butt if you're not on your toes when it's really windy. Seems like the winds there are stronger there than anywhere else for about 100 yards.