Wind gradient

While I was RV’ing I was sitting and watching the ducks on the pond. The cottonwoods where dispersing their fluff and it was slowly floating down on the wind and settling on the water. It occured to me that I should be able to see the effects of the wind gradient by watching the path of the fluff. I thought that if the wind slowed close to the surface that the path of the fluff should get steeper as it gets closer to the water. It didn’t. If anything the path was constantly getting lower until the fluff got to within a foot or so of the surface and then it leveled out for 10’ or so. This seems to indicate that the windspeed was fairly constant and then increasing right at the surface. Was I seeing things? Was I interpreting them wrong?

Most interesting observation Don. I think that I have seen something similar but it did not register in my skull that this might be natures demonstration of gradient phenomena.

On the other hand, I have tinkered at length with sails with streamers attached. The lower streamers always seem to indicate either less wind velocity or a different wind direction than the upper ones. That has been the case in full sized boats as well as models with a tall mast. The two observations appear to be at odds with one another. I expect that there is more to know than this.

There are so many variables out there on the pond that it is difficult to arrive at indisputable fact. For example there are wind events called micro bursts. The wind in those events rush downward almost vertically and spread out in a mushroom like pattern at the surface. One would need to be a meteorologist to understand the cause of such things. One thing that happens in one of thse things is that you can be headed in one side of the mushroom, tack to compensate, only to be headed by the opposite side of the mushroom. Mother nature teases us sometimes.

There is a nifty book on one of my shelves. Wind And Sailing Boats by a brit named Alan Waites. He is a weather man as well as a sailor. He clarifies a lot of the mysteries of local weather phenomena. I would recommend that book for any competitive sailor of full sized boats. Really good stuff in the book. How that all may apply to model boats, I do not know. I do know that weather patterns on lakes and ponds are different from coastal patterns. For me, when sailing models, I just look for cats paws and try to go where they are if they seem persistant. I confess that they have often moved by the time my boat gets there. Part of the fun, I reckon.

Work has been done on the migratory patterns of butterflies whose paths are against the pevailing wind. In general they seem to fly at altitudes of between 1 ft and 10 ft. It is very unclear whether the lower limit is because of some wind effect, because the butterflies do not like flying through vegetation or detection problems.

The person who put me onto this and almost certainly knows far more about it than I do is John Amoroso (Tallastro)


I think this might very likely have something to do with “Ground Effect” where aerodynamic lift is enhanced when close to the ground:
It benefits airplanes when taking off, & landing and is always a factor in the underbodies of race cars.

Another possibility is that evaporative cooling from the pond water created a thermocline just above the water’s surface. Cooler air will be denser, hence enhance lift.

Or maybe it’s a combination of the 2?
Bill K

Interesting. Because of the butterflies erratic flight path, I wonder if they deliberately fly closer than a foot to anything except to land on it.

Since the fluff was just drifting on the wind I am not convinced that it would be producing any lift, drag etc. so I’m not sure that the ground effects would effect it. The thermocline is a possibility although not all the fluff leveled out but I guess varying temperatures could account for that.

Yes, as I thought about it more the thermocline explanation made more sense. And the fluff wouldn’t need to level out entirely (as demonstrated by the fact that it eventually landed on the water), but only that the change in air density slowed the rate of decline.
I’m not entirely sure that lift is the proper term for a “flying” fluff, but if there is not a force akin to lift, the fluff would surely drop straight down. Newton demonstrated that 2 objects of differing masses will fall at the same rate, but this assumes a perfect vacuum or equal surface area. The high surface area/mass ratio of the fluff allows it’s slow decline.

Another thought is that there is a “boundary layer” just above the pond’s surface, and that your observation occurs at that point where the fluff is transitioning from “laminar flow”. Lester, help us out here!!

Maybe I’ll take my Kestrel wind guage & measure the wind speed gradient at incremental elevations. I just checked Frank Bethwaite’s book “High Performance Sailing” & he had some interesting photos of 1 meter strips of spinnaker cloth attached at differing heights on a bare dinghy mast. His opinion was that wind gradient is much more pronounced in light air than a heavier breeze, but none of his comments address speeds down at Footy level.

Hi Bill, lurking on this one, it is difficult to analyse what Don saw without making too many assumptions (smile)!

I wasn’t drinking:zbeer::

There does in fact seem to be considerable knowledge about what happens to things in the boundry layer, but whenever I’ve tried pursuing it on the web I’ve come up against either something that says ‘Monsanto Use Only’ or ‘Pay $$$$’. Presumably this is very hot property in twerms of pollen and seed dissemination from GM plants.

Well then, I want to know what you were smoking, and for that matter, Angus too!!..:wink:

BTW Angus, you were spelling it wrong, that’s why you got led astray…:wink:

Here’s the wiki

I wouldn’t doubt that someone has done a PHD dissertation on the effect. I’ll try to watch for it when it occurs here, maybe with a video. And I’ll promise to be sober…:wink:

Bill - my typing may be bad but there are limits!

The difficulty is that we are looking at a turbulent boundary layer. As the final piece of the Wikipedia article says, this needs a turbulence model - which is mega complex. I might be able to understand the general maths, [beads of sweat stand out on skull] but most of the data to drive such models seems to be commercial property.

The surface roughness coefficuents used in naval architecture, meteorolgy, etc. are f**k factors that work well enough in practice where people are interested - which is generally a few feet up. They are assumed to operate on a log scale in respect of height - lthough I suspect that logs are a readily accessible pre-calculated example of steeply rising curve, rather than because there is any particular theoretical reason why it should be a log.

Mostly this does not matter. Normal windsheer curves are of significant interest up to, sat, 12-13 metres. But we are interested in the bottom 0.5 metres. This makes the shape of the bottom of the curve very important. I once started out giving rather pompous - if well meaning - sries of 'tutorials on rig aspect ratio on USA Footy. Half way through I realised that it was not just matter of not having ALL the answes: I didn’t have ANY of them!

Can anyone help. My searchings in the past have been along the lines that biodiversity and soil erosion people are the most likely to have useful information, but I have yet to tag onto a good non-paying literature chain.

Or else does someone want to build wind tunnel to examine the problem?