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Hey everyone, I have a quick question for those of you who have jumped large smokestacks, or anyone with some detailed knowledge of turbulence.

A group of 5 of us went out to jump a 395' free standing smokestack the other night, and encountered a rather scary situation. For some insight on the object, it is about 20-25' wide at the exit and 40-50' wide at the bottom. The group consisted of four guys intending to go stowed with 42's, and one that wanted a PCA. When we arrived at the exit point, it had become quite windy (1-3mph on the ground vs 18-22mph at exit) but the wind was in a fantastic direction in relation to the intended landing area. We decided to exit facing directly downwind so as to have maximum protection from object strike in the event of off headings. One of the guys packed for freefall went first, took about a 1 second delay, reached and pulled. It was quite dark, so the exact details of the following five seconds are not quite complete, but I'll describe what the rest of us saw. A strong pull, PC about 4-5 feet to the right of the jumper, but instead of inflating and ending up directly behind the jumper as usual, it stayed about 4-5 feet beside him as he continued into freefall. We lost sight of him into the dark at this point, but after about 4.5 seconds he got a canopy, had enough canopy time to avoid a power line and flare. Because everything that he did looked right to us, we all assumed that the extreme hesitation was something to do with the wind, so after a little stalling and stuttering we all decided to PCA, and I went last with a static line (a very good example of why it's a great idea to keep some things like break tape and a pull-up in your rig by the way). Everyone else had great jumps, the first guy was un-injured and all was well.

Has anyone had any similar experience regarding smokestacks? The best theory that we could come up with was that because we decided to exit of the very downwind side of the object, and it's narrow but solid, the wind was "wrapping around" the stack, and where the two flows (from each side of the stack) met back up with each other created some funky kind of turbulence/downdraft that caused the hesitation. I realize that it was a one-off and it could have just been a "shit happens" moment, but the PC pack job was only about two hours old, his pull was solid, and his pin tension was as normal. We did have a fleeting thought before the subsequent jumpers' exits that going from 90 degrees to the left (which would result in a crosswind from the left) would have solved the problem, but to be honest none of us were really in the mood to experience what our friend did.

Does anyone have any technical insight or similar experience to share? Thanks in advance.

generally, going stowed with a 1 second delay isn't a good idea.
i've had, and seen hesitations, from that exact thing.

A friend and I did a 600ft high, 25ft wide at hight stack this past weekend. I suggested we jump down wind, with the wind being right to left across the normal exit direction. He said no way, turbulence downwind of the stack would be suicide. 2sec delay handheld with the cross wind was a much better option. PC blew over the top of me a bit, otherwise uneventful. So, my point is, after hearing that advice, from someone with 450 jumps, I wouldnt go downwind of it. As with skydiving or anything, wind over an object is always going to create turbulence downwind.

Google "Computer Aided Analysis of Smoke Stack Designs", download the pdf.

scroll down to figure 3-3 (under 3.4.2 Vortex Shedding). Figure 3-4 is also awesome.

Searching for stuff like "bluff bodies vortex shedding" can also give you some cool pictures.

You don't exit a smokestack downwind!
If you do, you either didn't care to do your homework on round shape solids, or are just plain stupid
Best place to exit would be between the dense airflow zone and the turbulent zone, with the wind helping your PC. Here's a crude drawing:

With exits marked A through C, from best to worst. You can jump C if the wind is weak enough, but keep in mind that in the dense zone, winds can be up to 4X of the actual speed. Jumping in any other direction is plain darwinism

That's an awesome reference, thanks dude.

Seems to be we just didn't do enough research first. Thanks for all the help everyone.

Just more info in case your gonna jump it on the crosswind. In those winds you might be begging for off heading/180.

Crosswind

" Light to medium wind will normally open into the wind on SD jumps because the wind takes the canopy and puts in a shoulder low scenario to the relative wind on deployment. Stronger winds tend to turn downwind more to the object 180sh due to the canopy taking you.
Take care,
space "

YMMV

the pic of the smokestack is absolutely correct. i thought this was basic knowledge, particularly applicable to buildings.

Also good to know, thanks bud.

While the points about turbulence are valid, turbulence alone is not likely to cause such massive hesitation - for that, a massive downdraft would need to follow the jumper for something like 100ft or more. In other words, a giant vortex with horizontal axis, not vertical as usual for vertical cylinder.

Since unexplainable PC hesitations when going stowed with short delay happen even in perfect conditions, there seems to be no reason to look for other explanations. If you go handheld and have hesi like this, that's another story...

Offtopic: has there ever been a hesitation when going handheld?

I've had and seen the exact same thing, but our jumps also included a major hesitation and stall before full inflation, SD. Exact same thing on every jumper on the load.

Went back with a bit of light one early morning to jump it and we had quite a bit of fog blow through. Pretty much same wind conditions as the funky jump. You could see the fog blowing round the tower, and the wind shadow "cone" (pretty much the same as the diagram shown in the post above) extended a least 20-30ft and we wouldn't clear it with the launch from exit, it would put you in the worst of the shit.

That object diameter was 15ft max, around 15mph at exit.

I think I'd have to disagree with that to some extent. I agree that it's unlikely that the turbulence would hold the PC in one place for that long, but it's more possible that it could blow it into the jumpers' burble.

Not discounting the short delay with stowed PC factor, I believe that was a large contributor.

I was planning on 2 seconds, which I'll probably do next time. Exiting from a different position relative to the wind of course!

You bet
http://www.youtube.com/watch?v=Tv-8ovfThGI
46" hand held, go n' throw

Why is B better than A?

I'd think B is best for a
right-handed jumper
going hand held.

Cool diagram, thanks.

I'm pretty sure he's saying that A is better than B. I agree with that but I'd lean toward saying that C is better than B because regardless of if you're stowed or HH you have the possibility of your PC blowing back into you on a short delay where your fall rate is less than wind speed.

from that diagram, C > A > B

crosswind from your left to your right, helping keep PC away from you (unless you throw leftie)

Who cares if there's a good chance for a 90 left from C?

All it did was turn you into the wind for landing : )

I'm surprised you had a load with so many people and either no-one understood about winds and solid objects or nobody spoke up about it : (

It's not even BASE-specific knowledge, people should leave their skydiving FJC with a solid fear of turbulence downwind of anything even semi-solid (even trees).

As I mentioned initially, we did think to exit with the crosswind from the left but all decided against free falling it until we could collect our thoughts.

Great discussion... I tend to agree with Yuri base's statement, the turbulence created in this situation is a little different than air flowing over an object that lies horizontal in nature and therefore creates vortices on that plane but in a vertical object as per the discussion yes vortices are created but in a vertical plane thus should not effect PC performance as the free fall speed creates the airspeed for pressurization and deployment in which this plane is unaffected in this situation.... What I think is being experienced in this situation is the lower air pressure/density on the lee/(downwind side of the object) similar to jumping at higher altitudes you expect there to be a slightly longer delay due to the reduced air pressure within the atmosphere.

When airflow over a smooth cylindrical object is induced obviously pressure is increase on the upwind side and then there is a decrease in pressure on the lee/downwind side of that object as suggested previously the airflow increase speed as it goes around the object therefore as it reunites with the air on the lee side it can create a reduced or even negative pressure zone. The effect on the pressure on the lee side is dependant on the wind speed and the surface of the object (drag) so if its rough like a golf ball/grid iron ball there will be less drag therefore less turbulence created but if its smooth as this object would be more turbulence is created and oddly enough the turbulence could quite likely appear at 90' from wind direction so by jumping in some of the positions described in this forum you may not be effectively avoiding any vortices at all if that is the concern?

I have attached a pic to show the phenomena.... If wind velocity is low there will be a uniformed laminar flow as in first pic, second pic if air speed increases reduces air pressure/vortices are created just behind the point at 90' to wind direction on the lee side, the third vortices are created constantly from one side to the next creating oscillations in reduced air pressure from one side to the next.

Now the fourth and fifth pics are a representation of difference in turbulence produced of that of a smooth surfaced object as a pose to a rough surface receptively.
This phenomena is actually used within industry to precisely measure mass airflow and the instrument is called a vortex flow meter or insertion tubes (work from similar principles from memory? They consist of a round tube with holes drilled on the upstream and down steam side in relation to direction of flow of air mass to be measured and measure pressure changes on the upstream and downstream side to help deter-manmass flow) I am sure you can find more technical information on this if desired? If you know the diameter of object and wind velocity you could even calculate at what point the vortices would likely begin to form and at what wind speed would be a concern.

As stated I believe the hesitation was due to short delay/low air speed prior to deployment in conjunction with the reduced air pressure created in this zone possibly some effect of slight down draft coming over the top of the stack with the short delay also? The fact that no one pounded in on landing suggests no vortices in the horizontal plane where evident, I believe it to be relatively safe to fly in the zones as air pressure is changing constantly but in the same vertical plane unlike turbulence in a horizontal plane that affects the wind under/over and within the canopy effecting flight/performance.

Note: These are only my own idea's and opinions from my experiences as a base jumper and in relation to my technical field of work.

Play safe
Sharpy

An excellent post sir, thanks very much for your insight.

I meant handheld with more than 1s delay. With go-n-throw sometimes there are small hesitations caused by insufficient force in the first 1-1.5s to open the pin flap or pull the pin. That's kinda "normal".

And with PC held as mushroom, not packed into a tight ZP-all-around package.

So, >1s delay, handheld with appropriately-sized, mushroomed PC - ever a hesitation caused solely by PC failing to produce nominal pull force? I doesn't thinks so.

Stowed PC? Hundreds of occurrences.

So, go handheld. It's cool. Kinda old school. Feel like a pioneer in the dawn of BASE era. Only pussies go stowed - they're afraid of holding the bridle.

 
"So, go handheld. It's cool. Kinda old school. Feel like a pioneer in the dawn of BASE era. Only pussies go stowed - they're afraid of holding the bridle"

I agree with your statement. Seems a "newer" generation of jumpers goes stowed off EVERYTHING. It never made since to me jumping with others who would go stowed on a 300 ft. object and literally have their hand back on the PC before that last toe left the edge. Then the PC is out before they ever have much speed built up. I prefer to go HH on anything sub-300 and take a nice "comfortable" delay.

Not to get off topic on this but it kinda relates to the OP. Different objects will bring different wind/turbulence senerios and sometimes going stowed is just out right dangerous regardless of the height of the object. To the OP, props to you and the rest of your crew for making a decision to switch to PCA/static line after seeing some freaky shit go down with 1st jumper. Decision like that will keep you un-injuried and in some cases alive. Cheers

this happened to me way back in the early 90's, led to an object strike and broken hip. shouldn't have jumped in those winds.

I actually really enjoy going HH, I was just a little antsy to do some stowed freefall because everything we'd been jumping lately was quite low! There was no worry of reaching before I'd left the edge though, I planned to take a delay on that shit!

i know this post is old, haven't been on for a while. had some input to the discussion, however. there is a powerplant called gavin in oh where they built a 3d stack between 2 others. not too long after, they actually took the top off of it because the wind shear almost knocked it over, i watched some of the dismantling. not too sure if this would translate into winds when jumping, but something to think about. they had computer simulations(not positive, but they should have) and engineers who do this for a living and still got it wrong. just a thought to keep in mind...

I agree with some of the statements of 'Not' being involved with a PC hesetation to inflate after the pitch on the Stack .

I have had this happen a couple times from stacks & buildings . the turbulence from airfow going around both sides of the object and creating turbulence/rotor on the Exit-side where your canopy is opening .
Never affecting the PC on Pitch, inflation or Tow .
IT mostly affecting my Canopy by making it presure breath some with it's inflation . Then holding it there & not letting it immediately fly away from the object. While at the same time the canopy was breathing & stagnant on forward movement . It let/forced the canopy sink many feet of altitude loss . Then the canopy would go 100% as normal & you would fly away only to find yourself low & short of your primary LZ and game plan .
.