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dang911

Laminar VS Turbulent flow in a Wind Tunnel

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I am in the early design stages of building a small vertical wind tunnel for laboratory research of falling animal dynamics at terminal velocity. Although I have gone skydiving a few times, I am no expert. For all of you who are enthusiasts and I'm sure the couple of scientists out there too, I propose the following questions?

What type of flow would best simulate a fall at terminal velocity, Laminar or Turbulent, why?

Has anyone experienced time in a tunnel which has either extreme flow type or even experienced this in true free fall and noticed a difference in fall characteristics?

Will either way make much of an impact on fall dynamics in the scheme of a 65 MPH terminal velocity?

Feel free to get technical with fluid mechanics or however you may feel comfortable explaining for the non scientists, I appreciate all opinions.

Greg

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I am pretty sure the flow is going to be fully turbulent.
The velocity is fairly high.
The characteristic length is relatively small.
Reynolds number for freefall will be on the order of 300,000. In a wind tunnel it is well over 1e6.

The flow in freefall is turbulent. If it were not, there were be little or no burble above you!

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I am in the early design stages of building a small vertical wind tunnel for laboratory research of falling animal dynamics at terminal velocity.



I can't offer a thing about the scientific aspects of airflow. But I sure am curious why you're going to be testing falling animals - can you tell us the nature of the study?

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Thank you very much for the info, that was what I was after. I do want to ask more of an opinion based question as a follow up.

You mention the burble (low pressure pocket), would that not be caused also with lamnar flow as well, I'm pretty sure it would?

I think we agree that once the air impacts the falling body, it becomes turbulent, but what I am after is if the air below the free falling body is laminar or turbulent will it change the fall characteristics and ultimately stability?


The study is still being focused on specific research criterion but topics to be explored are relationships of density and animals coefficient of (drag) friction. Animals of both classifications, gliders and non gliders will be studied. At this time more detail cannot be provided.

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I'm no wind tunnel expert (though I once operated a mach 2 wind tunnel), but I doubt any research wind tunnels are ever made to purposely have turbulent air. Turbulence is just variation in wind velocity. Wind tunnels are designed to minimize this. Purposely creating turbulent air will just make it harder to make accurate measurements.

On the other hand, some wind tunnels will use vortex generators on the walls to create a turbulent boundary layer, preventing flow separation where the tunnel expands (behind the test section). But just before the test section, flow straightening devices and netting are used, along with a sudden contracting section, to minimize turbulence in the flow inside the test section.

Real freefall happens through "steady" air. Sure, there's turbulence, but the variation in wind velocity is very small compared to the speed of the object falling through the air. To simulate that in a wind tunnel that does 65 mph, you'd need very tiny variation in wind velocity. So in other words, I think you'd best simulate freefall by smoothing the flow as much as possible.

Dave

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Real freefall happens through "steady" air. Sure, there's turbulence, but the variation in wind velocity is very small compared to the speed of the object falling through the air. To simulate that in a wind tunnel that does 65 mph, you'd need very tiny variation in wind velocity. So in other words, I think you'd best simulate freefall by smoothing the flow as much as possible.



And so the debate inside my head is fully revealed. Yes, I have had this exact argument going on for some time. And I think you enthusiasts can help narrow my decision.

Laminar air is harder to achieve, especially with the type of rig I plan on building, my next question is open to anyone is whether you can feel a difference in the tunnels turbulent flow to a nice blue sky dive? Any form of explanation of the difference in flight characteristics would be useful.

Last point, some WT suck others blow (insert jokes here), one would tend to be more laminar the other more turbulent respectively, anybody fly in both and could compare those experiences if they differed any?

Thanks

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Consult the Pope?
Alan Pope's classic tunnel testing textbooks, that is.
(Not that I've looked at one in ages.)

Like others, I'd guess that the usual issue is to get turbulence levels down in a tunnel. Only for very smooth shapes like airfoils would I think it would matter much, what the fine tuning of the turbulence level would be. There must be ways of characterizing turbulence levels.

As for skydiver wind tunnels, I think only the really early ones (or simple amusement park style ones) blew from below. Other non-circulating ones have the fans above, and some sort of smoothed bell or cone intakes below. A modern tunnel that is recirculating has vanes to get the airflow around the corners, but no flow straightening grid or whatever you'd call it, as found in aeronautical tunnels to reduce turbulence. Others with more tunnel experience than me would know more.

So I'd guess: Just get some sort of flow straightener in there, avoid too much blockage by having the tunnel large enough, and worry more about the issues of getting measurements for non "hard" objects. It is one thing to get the drag of a specific airfoil, another to get the drag of some complex biological object that can be oriented in many different ways.

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I think you need to be very clear in your definition of laminar and trubulent, Organized, uni-directional flow is not necessarily laminar. It can be highly turbulent.
Flow in a wind tunnel is wall-bounded. Even though it is organized and conditioned to be straight, it is still highly turbulent. It may be small scale turbulence, not visible to the naked eye, but still turbulent. Reynolds numbers for this flow are very large.
Flow over a skydiver in the clear blue sky will also be turbulent.
True laminar flow is actually very difficult to produce, even in a lab setting.
The burble above a skydiver is the region where the flow has separated. If you ever see a video of a bag lock malfunction, you will see the bag getting buffetted around abover the jumper. The bag is being pushed around by the vortices in the vortex street that is formed when the flow separates around the sides of the jumper.
It is also the reason why most ratty student jumpsuits are torn at the sides of the arms and legs. They are in the region where the flow separates and they whipped around in the vortices!

PM me if you want to discuss this further!

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YES, the Pope has been consulted haha, he a frequent friend of mine for turbulent flow.

If we want to keep a solid definition of Laminar, its Re<2300 but for the sake of this experiment i'll include the transient phase as well and say Re<4000. Also for this project I am ignoring all small scale turbulence's and using 6" as the reference length.

I am actually getting pretty close to doing a conceptual test run using an existing large WT the AAC, which has a bottom blowing fan which I'm sure is going to produce near fully developed turbulent flow.

So the main less scientific question for you guys remains. I would like to know if there is a difference in flight characteristics between blue sky flying, blowing WT and sucking WT.

If there answer is no significant differences than the condition of the flow will be of less importance to my test.

Greg

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the answer to your question is yes... there are many differences in the characteristics to each style of flight.

* in skydiving, you are falling regardless of the wind conditions. the air is smoother and feels thinner/ weaker which requires more body input, as opposed to a strong wt, in order to cause a reaction (i.e. turning or moving fwd/bkwd)

*a "sucking" wt creates a smooth strong air stream with a constant even pressure throughout the diameter of the tunnel (with the exception of some tunnels that "choke" if you get too close to the edge/ perimeter). [however, many tunnels vary in this aspect as far as the strength of the wind at different altitudes, i.e. iFLY Orlando's wind gets noticeably weaker the higher up you go, due to the widening of the tunnel's diameter past the glass (~ 1st 12ft). whereas, Paraclete XP's wind has a larger area's worth of constant even pressure.]
keep in mind: a wind tunnel is designed to replicate freefall, therefore, it must create strong winds in order to keep you afloat, as opposed to naturally falling at terminal velocity. these strong generated winds require less body input than skydiving in order to cause a reaction, which makes the wind feel stronger.

* a "blowing" wt, usually creates a vortex/ cone in the center of the wind stream in which the air is stronger, leaving a weaker air stream around it. the moment you leave that cone it feels as if you were getting spat out. AAC in my opinion is the one exception to the cone (as far as the "blowing" tunnels i have flown in so far go). the wind stream there is constant so long as you stay within the red circle on the net which gets trickier the higher you go...

*in my opinion AAC or a "sucking" wind tunnel would be your best bets... good luck with the tests at the L1 at AAC. you will find that not only is it a great place, but also the staff (including the owner) is great as well. I am sure you will feel welcome and have no problem with any help you may need (so long as it is all ok and regarded as safe by the staff) as John and Keith are 2 of the nicest/ chill pple I have met in the sport.

anyone feel free to correct me on any mistakes or wrong info i may have posted, I hope this helps.

*Itchy
Life is all about experiences...
Luck, is when opportunity and preparation meet...

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I am pretty sure the flow is going to be fully turbulent.
The velocity is fairly high.
The characteristic length is relatively small.
Reynolds number for freefall will be on the order of 300,000. In a wind tunnel it is well over 1e6.

The flow in freefall is turbulent. If it were not, there were be little or no burble above you!



??? The turbulence you speak of is created by the skydiver falling through the air. If you're talking about building a wind tunnel to simulate freefall, you want laminar flow in the tunnel. An ideal freefall simulator would create no turbulence in the flight chamber until someone got in there to fly. In the real world, you get some turbulence near the door at the very least, but most modern designs do their utmost to minimize turbulence in the flight chamber due to the machine itself.

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most modern designs do their utmost to minimize turbulence in the flight chamber due to the machine itself.



certainly would cut down the electric bill

...
Driving is a one dimensional activity - a monkey can do it - being proud of your driving abilities is like being proud of being able to put on pants

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I think you need to be very clear in your definition of laminar and trubulent, Organized, uni-directional flow is not necessarily laminar. It can be highly turbulent.
Flow in a wind tunnel is wall-bounded. Even though it is organized and conditioned to be straight, it is still highly turbulent. It may be small scale turbulence, not visible to the naked eye, but still turbulent. Reynolds numbers for this flow are very large.
Flow over a skydiver in the clear blue sky will also be turbulent.
True laminar flow is actually very difficult to produce, even in a lab setting.
The burble above a skydiver is the region where the flow has separated. If you ever see a video of a bag lock malfunction, you will see the bag getting buffetted around abover the jumper. The bag is being pushed around by the vortices in the vortex street that is formed when the flow separates around the sides of the jumper.
It is also the reason why most ratty student jumpsuits are torn at the sides of the arms and legs. They are in the region where the flow separates and they whipped around in the vortices!
!



I am always in awe when realizing that aerodynamics is such a widespread and enjoyable hobby

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The flow in a wind tunnel needs to be as laminar as possible.

You can treat a skydiver in free fall like stationary object placed in a laminar flowing fluid. THE VELOCITY OF THE FLOWING FLUID IS CONSTANT. Fnet=Fdrag+Fgrav=ma=0 

The flow over the skydiver will form a boundary layer. Inside of this boundary layer is where you will see all of your turbulence (burble). Ik this is a 10yr old thread but im guessing you went with a laminar flow.! 

 

 

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