Turbulence areas around Obstacles

[Di0 1]

Hi All,

This discussion was raised during the discussion of a landing accident:
http://www.dropzone.com/cgi-bin/forum/gforum.cgi?post=4632821;sb=post_latest_reply;so=ASC;forum_view=forum_view_collapsed;;page=unread#unread

Although probably unrelated to the specific accident, it is probably interesting to continue on here.

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Somehow related, although it might not be what happened, but maybe good to think about this: also, correct me if I'm wrong, please, most people tend to think as turbulence being "horizontal" from obstacles and don't worry too much about what happens "above" objects. In other word, the wind shadow is not only at the same height of the obstacles but also extends above them, kinda like a "box" no-flight zone.

[...]

My mistake.
Lesson Learned (or rather, reinforced): away from "thick" obstacles means both horizontally and vertically, even if you know you have enough altitude to fly above them and clear them, turbulence might "suck you" when you're on top of them.

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10x downwind from object, 1x upwind and 2x above object is turbulence areas.

Don't fly near objects if you can avoid it.

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Shouldn't there be sides included then?

So here it goes, everybody knows about the turbulence behind obstacles, the "dead air drop" etc., the principle behind the turbulence forming also IN FRONT and ABOVE a thick obstacle on the other hand might not be as known, but it is clear if you think of a drawing, see the first picture here:
http://www.esru.strath.ac.uk/EandE/Web_sites/05-06/wind_resource/wind/wind03.html

I know these are maybe known concepts but maybe it's good to stress them every once and a while.
Anything worth adding to the discussion that hasn't been covered already?

I'm standing on the edge
With a vision in my head
My body screams release me
My dreams they must be fed... You're in flight.

[peek 20]

Di0

http://www.esru.strath.ac.uk/EandE/Web_sites/05-06/wind_resource/wind/wind03.html

I think that graphic could use a lot more swirls with arrows downwind, and actually, making the drawing longer to include area further downwind. The drawing as-is would seem to cover only low winds.

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Anything worth adding to the discussion that hasn't been covered already?

Even smooth ditches that are only a few feet deep can cause turbulence when the winds are moderate to high.

[format 0]

Di0

***10x downwind from object, 1x upwind and 2x above object is turbulence areas.

What's the reason with skipping sides of an object?

What goes around, comes later.

[pchapman 262]

Di0

but it is clear if you think of a drawing, see the first picture here:
http://www.esru.strath.ac.uk/EandE/Web_sites/05-06/wind_resource/wind/wind03.html

Actually the 2nd picture on the page you found is much more useful. Unfortunately it has text so small it is hard to read, and cannot be expanded. (Zooming in your browser helps a little.) --> So I've added a copy of that below.

It looks like it is only a math model, so not guaranteed accurate to reality. It is also 2-D, so it doesn't look at wind around the sides of a building. Nor does it get at the turbulence and variation in wind speed bound to occur.

The obstacle is 20m high and 60m wide, with zero porosity -- rather like a large hangar perhaps. The vertical and horizontal scales are not the same -- stretch it out about 5 times horizontally to be like an actual photographic cross section.

Ignore the yellow bar - that represents a wind turbine the original researchers were looking at, downwind of an obstacle.

Unfortunately the actual wind speed is not shown, and surely the effect will vary somewhat with wind speed, even if dealing in percent changes.

If a number 95 is shown, that's 95% of the original wind speed, or a 5% velocity deficit as is sometimes said.

Lets say for our purposes, a 10% loss of wind speed is "Significant" rather than trivial. Who knows what an appropriate level really is, especially if turbulence and gusting is also included. And 20% loss will be considered "Serious".

Winds stay above 90% of the original value only at 40m up, and 80% at 33 m.

Thus:
Significant changes in wind speed can occur below 2* the obstacle height, and Serious changes below 1.6* the height.

Horizontally, winds stay above 90% only from 365m onwards, and above 80% from 257m onwards.

Thus:
Significant changes in wind speed can occur downwind at up to 18* the obstacle height, and Serious changes downwind at up to 13* the obstacle height. [Edit for typo - numbers were reversed]

At 10 times the obstacle height downwind, one can have wind speeds as low as 74% of the original!

Again, this is just the result of one math model for which we have no background references - use it as an example but don't take numbers as gospel! Because it is 2-D it may be a little pessimistic compared to any real obstacles that aren't too wide. (Like a single building, but not a forest, although a forest is more permeable.)


[inline wind_speeds_behind_obstacle_(zoomed)esru.strath.ac.uk.jpg ]

[Di0 1]

peek

I think that graphic could use a lot more swirls with arrows downwind, and actually, making the drawing longer to include area further downwind. The drawing as-is would seem to cover only low winds.

Well, it's a fairly generic graph, so it really wasn't meant to be applied to a skydiving situation.
More like a quick way to visualize WHY there is turbulence in front of an obstacle.
I agree with you.

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Even smooth ditches that are only a few feet deep can cause turbulence when the winds are moderate to high.

True, another point often overlooked, as ditched are often not thought as "turbulence creating" obstacles, or obstacles at all for what matters.

I'm standing on the edge
With a vision in my head
My body screams release me
My dreams they must be fed... You're in flight.

[Di0 1]

pchapman

...

That's a lot of content, thank you! I'll digest and reply tomorrow.
Really, thank you for taking the time to write it and explain very clearly, it's worth a lot!
For the time being, I'll add that, when talking about turbulence and related phenomena, the most important "scale factor" is certainly the Reynolds number (http://en.wikipedia.org/wiki/Reynolds_number) and that you're totally right, to elaborate, as you can see from the description of the Reynolds Number, it's not just the speed of the wind that determines how "strong" the effect of turbulence is, but more like a combination of speed, size of objects involved, etc. etc.
Now, since all other factors are pretty much fixed when skydiving (even the biggest difference in "size" between different canopies is never so big to really change the ballpark, since we are talking about scaling order of magnitudes), wind speed is probably the most important factor.

I'm standing on the edge
With a vision in my head
My body screams release me
My dreams they must be fed... You're in flight.

[wan2doit 6]

There's another thread currently on wind tunnels regarding flying in a burble. I'm new to this but the subject here seems to be very similar but on a larger scale.

Is my perception correct?

This link describes burble impacts behind aircraft carriers. I don't know how they can even land a plane on a carrier after reading this http://www.dtic.mil/dtic/tr/fulltext/u2/a527798.pdf