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Aug 24, 2015, 09:54 AM
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Data

shape of thermals or updrafts


for all that i have seen (in text and real life), thermals develop as bubbles or columns that rise to the condensation level, where become clouds.
but recently in clear air i have detected updrafts in the shape of waves that run across the wind, very much like sea waves. or should i say "sort of currents that keep the plane up there"?
is there any data on this?
please bring only links or positive, polite comments to the point.
regards
Last edited by phil alvirez; Aug 26, 2015 at 03:55 AM.
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Aug 24, 2015, 01:07 PM
An itch?. Scratch build.
eflightray's Avatar
I thought most thermals were bubble shaped.

Here's one link I just found -- http://soartech-aero.com/Thermals.htm --

By the way, a search on 'thermal shapes', mostly comes up with underwear
Aug 24, 2015, 03:34 PM
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richard hanson's Avatar
Quote:
Originally Posted by phil alvirez
for all that i have seen (in text and real life), thermals develop as columns that rise to the condensation level, where become clouds.
but recently i have detected thermals in the shape of waves that run across the wind, very much like sea waves.
is there any data on this?
please bring only links or positive, polite comments to the point.
regards
Shape?
moving shapes and as varied as the terrain-
we have large salt /mud flats which generate some beauties - but the y also form next to the foothills
Aug 24, 2015, 03:56 PM
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Thread OP

shape


i said: i have detected thermals in the shape of waves that run across the wind, very much like sea waves.
and asked for data or comments to the point.
that's all that i need.
please.
Aug 24, 2015, 03:59 PM
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Thread OP

waves


Quote:
Originally Posted by eflightray
I thought most thermals were bubble shaped.

Here's one link I just found -- http://soartech-aero.com/Thermals.htm --
i thought so too. sometimes they do. but others they are like columns.
or waves, as i have experienced recently.
that is why am asking for data or comments regarding it.
thanks
Aug 24, 2015, 04:39 PM
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richard hanson's Avatar
Pardon me -- I will try to capture some with my camera
Aug 24, 2015, 04:46 PM
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Have a look at https://en.wikipedia.org/wiki/Horizo...nvective_rolls and see if that is close to what you're detecting.

Steve
Aug 24, 2015, 05:01 PM
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ShoeDLG's Avatar
Anything like the mountain wave described here:

http://www.ssa.org/GliderLiftSources

Or are you talking about true thermals perpendicular to the wind?
Aug 24, 2015, 05:23 PM
Grad student in aeronautics
Fascinating. The last diagram on that website you linked suggests to me Kelvin-Helmholtz breakdown of a shear layer, as I have annotated in my attachment. The mountain peak could be shedding vorticity at some roughly constant rate and the resulting shear layer coalesces into discrete concentrated vortices
Aug 24, 2015, 07:06 PM
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richard hanson's Avatar
we get rising columns of air you can watch--actually lifting stuff from the ground on open fields on burning hot days- and the stuff next to the mountains\ all different we also get stuf shifting up and down large open canyons
all different -- How in the ----would you label any of it

the graph of air in mountain soaring is --interesting-
Aug 24, 2015, 09:06 PM
B for Bruce
BMatthews's Avatar
Phil, I've occasionally found my self in a wave too. Often it's from something obvious like a line of trees or buildings located upwind of the flying area. And other times it may be due to some other discontinuity in the landscape. Maybe even something such as a really big ditch or small ravine or possibly a transition from a large pond or small lake to the ground?

But I've never seen (or more accurately explored via model) a thermally generated lift source that was anything but columnar in shape. All the way effects I've ridden were due to physical line features on the ground.

The biggest and baddest of these was the cliff that I wave soared from at the Kent-Boeing airfield in south Seattle about 20 years ago. The cliff was the backside of a 400 foot rise located about 2 to 2.5 miles from the flying field. I ran into it at around 300 feet up and it lifted me to around 500 to 600 feet and I stayed in it while flying the "line" of lift over about a 1/4 mile distance back and forth to achieve the longest flight of the day at the Electric flyin that year.

The wave lift I found at mind field from things like tree lines was never really that strong. In one case I could hold altitude along the line that was about 100 yards past the line of trees but the turns produced some sink which was BARELY recovered by the return leg. And it never did get any higher than that. It was very specific and could only hold up my model over that center line and maybe 15 feet to either side. Beyond that if there was lift it was too weak. It was very set too. The line moved a little this way and that but it stayed there for the whole afternoon session while I was flying. It was so light though that while I was able to get a 15 minute flight from it one time it also left me with sweat soaked armpits. The wind speed that day was brisk but still flyable with the Gentle Lady I had in the air at the time.... but barely.

In my case with The Kent Boeing Space Center I would guess that I was riding the second or maybe third wave as indicated in Dpate's diagram. In the case of the tree wave I'd bet it was the first wave. at that distance. And certainly the only thing I found any closer to the trees was bad turbulence.

Any of this give you a possible clue to your own situation? If the line the air was breaking over was tall enough it doesn't need to be anywhere close. Anything significant up to as much as a mile away could be doing this.

The key with wave lift is that it is stationary for any given wind speed. If the effect you found was a traveling source of lift then I would suspect some odd combination of multiple thermals that were interacting in a way that prevented them from joining. I've seen where myself and a buddy were in different thermals (we tried to fly towards each other only to find sink) located along a line normal to the wind direction. As we circled in our own thermals they moved downwind a ways and then something changed and we both side stepped towards each other and the two thermals mixed. For a few moments there was still lift but then it dissipated quickly due to the energy battle in the mixed air. But if we were to fly from one to the other at the right time I can see where it may seem like a long "line thermal".


Then there was the day where I flew over a moise field early in the morning (the old Gentle Lady again) and found that the lift over this one field was "pillow shaped" and conformed to the size of the field exactly. This wasn't a thermal though. I've seen this in my early morning or late Fall free flight flying where the ground moisture lifts up and creates a lift gradient. The model will settle at the height where the lift is just able to match the model weight. If it sinks lower it'll rise again to that balance point. But it'll never rise above it either. I know because I spent a good 20 minutes trying to find the "core" on that pillow of dew rise. There simply wasn't one. It did stay there until the sun came out from behind the haze and the wind started to flow and that cut it off and scattered it. But for two flights where it was still dead calm I was able to sit my model at 60 feet up on that pillow with the trims set for a stable circle and set the Tx on the ground for a good 5 minutes at a time.

Anyway, enough of my reminiscing. What is around the field and what were the conditions during this "wave" lift? If you can describe it a bit more perhaps something more suitable can be offered.
Aug 24, 2015, 11:22 PM
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ShoeDLG's Avatar
Quote:
Originally Posted by DPATE
Fascinating. The last diagram on that website you linked suggests to me Kelvin-Helmholtz breakdown of a shear layer, as I have annotated in my attachment. The mountain peak could be shedding vorticity at some roughly constant rate and the resulting shear layer coalesces into discrete concentrated vortices
There certainly could be breakdown of the shear layer, but I don't think that's an essential ingredient. The main contributor to wave lift (as I understand it) is a geographic feature that vertically displaces passing air such that it is no longer in hydrostatic equilibrium.

If the lapse rate is stable, then the displaced air will tend to return to its original altitude. The wave is caused by the air repeatedly overshooting its original altitude due to inertia. While this process is typically dynamically stable, it may not be well damped, resulting in "secondary" and "tertiary" wave that is less strong than the primary wave.

I would imagine this basic mechanism could exist even if the shear layer remained essentially laminar.
Last edited by ShoeDLG; Aug 24, 2015 at 11:32 PM.
Aug 25, 2015, 12:07 AM
Grad student in aeronautics
Ah, well I'd say that's fascinating too.
Aug 25, 2015, 04:31 AM
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Thread OP

details


well, gentlemen, thanks for your great contributions. all fascinating.
seems that we all are into some sort of spell regarding these intriguing caracteristics of the air.
let me bring more details: once when i got into the wave, it was high, at about 150 meters (500 ft); wide: all over from left to right, as much as i could keep the plane in sight; long lived: i could keep my plane seemingly forever; strong: when i gave up due to my eyes getting tired, i had a hard time bringing the plane down. i have to explain: i have the privilege of having a vario from the guy from new zealand (no longer available) that sends beeps when feels 'up', and even higher pitch too when it is stronger, so i can see exactly where and how strong the lift is. so at that time i gave the plane increasingly down trim until eventually began to descend. i could measure this exactly because the vario can be changed from beeps to telling the height, so i know this exactly. that is the great advantage of this vario, that i didn't have before. it feels as if you are seeing the thermal. about the field, all around is flat like a billiard table. nothing in sight is elevated. zero. the lake is far away, beyond sight. no lines of trees, so don't think the surface can have any influence of the phenomenom. other times i get the wave but is not as strong. also, i have been there in a thermal that started at about same height (150 meters) but as i could explore it, i could see that it was about 200 meters in diameter. then as i climbed, could see at distance some 20 white birds ( i guess gulls) that were flying fast in large circles, and climbing until disappeared far away up there, so the thermal seemed increasing in diameter as it went higher (sort of conical shape). large, very large.
Aug 25, 2015, 04:44 AM
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Thread OP

no sir


Quote:
Originally Posted by ShoeDLG
Anything like the mountain wave described here:

http://www.ssa.org/GliderLiftSources

Or are you talking about true thermals perpendicular to the wind?
no it's nothing like that, sorry.
am talking about waves, horizontal imaginary lines that extend from left to right and stay there for some time with low wind. when i say 'imaginary' is because this happens sometimes in clear air: no clouds at all. i only can 'see' them with the tones of the vario. it feels as if the time stops-and the plane just keeps flying. i have it making lazy 8s to stay on the line of lift. so it is nothing that seems going up as in the classic thermals that may be bubbles or upright cylinders. i must say that i had no idea of this until now that i have the vario that tells me exactly where the lift is.


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