sonicAZ 0 #1 November 13, 2006 Are there any scientists, engineers, or serious drunks on this forum that can explain how a wind tunnel works in depth? Sorry if this is a super top secret discussion because I have looked at howstuffworks.com etc... no results. Quote Share this post Link to post Share on other sites
Peej 0 #2 November 13, 2006 Well i'm no scientist or engineer, but i have been seriously drunk once or twice. First off, you might have better luck posting this in the tunnel forum, maybe a mod can move it for you... Ok, have you ever been in the tunnel? What surprised me the first time i climbed in there was that the fans that create the air are at the top of the tunnel and not the bottom. (In the Skyventure Tunnels anyway) So, imagine a vertical tube with a series of fans at the top and a nothing but a wire mesh net on the bottom - that's the tunnel, basically of course. The fans at the top suck wind up through the "tube" and this simulates the relative wind that we skydive on/ in. There's a room that looks into the tunnel where a controller sits and he/ she controls the fan speed and flow of wind through the tunnel. This was how i understood it anyways. What else would you like to know specifically? edited for clarity and to add: Like Packerboy said, there are also tunnels with the fams in the bottom, ie the blowing ones, but i don't know where they are situated. There are also outdoor "skydiving simulators" with the fans underneath - like the one in the movie "Dropzone" Advertisio Rodriguez / Sky Quote Share this post Link to post Share on other sites
packerboy 3 #3 November 13, 2006 Well... there are two kinds of wind tunnels. One sucks One blows HARD!! -------------------------------------------------- In matters of style, swim with the current; in matters of principle, stand like a rock. ~ Thomas Jefferson Quote Share this post Link to post Share on other sites
stallboy 0 #4 November 13, 2006 There's a pretty neat diagram on the Bodyflight Bedford site... http://www.bodyflight.co.uk/facilities/ Quote Share this post Link to post Share on other sites
skycat 0 #5 November 13, 2006 QuoteWell... there are two kinds of wind tunnels. One sucks One blows HARD!! Actually their are 3 kinds, you forgot the ones that suck and blow. Fly it like you stole it! Quote Share this post Link to post Share on other sites
tunnelfly 0 #6 November 13, 2006 Quote Actually their are 3 kinds, you forgot the ones that suck and blow. ...and they all swallow... ... lots of money! No.1 reason NOT to be an astronaut: ...You can't drink beer at zero gravity... Quote Share this post Link to post Share on other sites
brettski74 0 #7 November 13, 2006 Quotethere are also tunnels with the fams in the bottom, ie the blowing ones, but i don't know where they are situated. Aerodium in Latvia is a blower. Check out Bodyflight.net for details of lots of tunnels all around the world. In general, the SkyVenture tunnels all suck whereas most of the others blow... air that is. :) (I just couldn't resist ). The Skyventure design is probably the most modern. My understanding is that they place the fans above the tunnel in order to avoid putting turbulent air into the tunnel. Basically, when an object moves through the air, it leaves turbulent air in it's wake. This includes the blades of a fan, but since the air being drawn into the tunnel by a SkyVenture design has not yet encountered the fan blades, they do not contribute any turbulence to the air stream. Instead, the turbulent air is travelling out the top, above the fans and the working area. From their website and photos, it also appears that they use multiple fans in order to help avoid dead spots either in the centre or near the walls. The design is supposedly better, but I haven't tried one, yet, so you'll have to wait until January when I can compare my experiences. You can read more about the design at SkyVenture's website. Blowers usually have some system of fixed vanes in front of the fan in order to try to smooth out the air flow and fill in the dead spot in the middle where the shaft of the fan is. Check Aero Interactive's web page, although the last time I looked, some of the more useful information that used to be there seems to have been removed. Note that although the name is similar, this is not the same mob that runs the Aerodium tunnel in Latvia, although I believe that they may have been involved in the initial construction. Quote Share this post Link to post Share on other sites
pilotdave 0 #8 November 13, 2006 There are 3 basic types of wind tunnels used for freefall simulation (there are many more types of wind tunnels in general). Blower tunnels have the fan on the bottom, in-draft tunnels (skyventure style) have the fans at the top, and recirculating tunnels have the fans typically wherever the diameter of the tunnel is highest (at the top). Most tunnels have some similarities... Just below the flight chamber, the diameter of the tunnel contracts very suddenly (and smoothly). The flight chamber is the smallest part of the tunnel. In subsonic flow, the contraction of the cross sectional area causes the airflow to speed up, since subsonic air is basically incompressible. Putting the fans where the diameter is biggest makes the tunnel most efficient (ie before the contraction section or as high above the flight chamber as possible). In in-draft, recirculating, and some blower tunnels, the diameter of the tunnel slowly increases above the flight chamber. The more it grows, the slower the air will be going at the top and the more efficient the tunnel will be. But if the diameter increases too quickly, the air will separate from the walls and cause a lot of turbulence, which makes the tunnel less efficient. Lots of research tunnels use vortex generators (little fins) along the expanding section to help prevent flow separation. In in-draft or recirculating tunnels, turbulence caused by flow separation can kill fan efficiency too. For research (horizontal) blower tunnels, they usually have a big expanding-contracting section just before the test section. No room to do that on a vertical tunnel, but it helps to reduce the turbulence from the fan. They also use nets or meshes to help reduce the turbulence and flow straighteners to...umm... straighten the flow. Dave Quote Share this post Link to post Share on other sites
Paulipod 0 #9 November 13, 2006 QuoteFrom their website and photos, it also appears that they use multiple fans in order to help avoid dead spots either in the centre or near the walls. The design is supposedly better Mulitple or single fans above the airflow dont make any difference to the laminar flow of flight chamber.... but does remove the turbulence from blades etc as you say. This design has been used in scientific facilities since the late 40's as it is the best way to achieve a good test flow. 'Dead spots' against the wall would be caused by frictional losses on the walls and openings like doorways. Bodyflight Bedford www.bodyflight.co.uk Quote Share this post Link to post Share on other sites
brettski74 0 #10 November 13, 2006 QuoteMulitple or single fans above the airflow dont make any difference to the laminar flow of flight chamber.... but does remove the turbulence from blades etc as you say. Any ideas why the multiple fans instead of a larger single fan? Is it perhaps more efficient or something? I would imagine that multiple fans would complicate the control systems, so there must be some other pay-off if it's not improving the quality of the flow. Quote Share this post Link to post Share on other sites
Paulipod 0 #11 November 13, 2006 QuoteAny ideas why the multiple fans instead of a larger single fan? There are a few pro's and con's for either multiple or single.... Considerations for multiple :- 1. More 'off the shelf' availability 2. Offers a little resilience - ie if one of 5 break you may still be able to run. ... although in most SV designs losing a fan will seriously affect air quality 3. VFD technology for speed control is ALOT cheaper for 250HP or less... and more reliable 4. It is harder to ensure the same frequency etc of multiple Considerations for Single :- 1. Power range of prop technology is greater (ie.. our prop ranges to over 4000HP) 2. Less moving parts... larger bearings etc... 3. Drive is less affected by loading / ie flyers in chamber etc 4. Variable pitch more feasible on larger units Just a few - the decision can be made on many other factors too Bodyflight Bedford www.bodyflight.co.uk Quote Share this post Link to post Share on other sites
Eule 0 #12 November 14, 2006 Besides all the sucking and blowing, there is some electrical fun too. Historically, the speed of an electric motor was determined by the local powerline frequency (60 Hz in North America; 50 Hz in Europe, etc) and how you built the motor. In the US, speeds of just under 1800 rpm and just under 3600 rpm were/are fairly common. It wasn't easy to change the speed of the motor once you built it; some motors were built with multiple windings so you could get two or three discrete speeds, but there wasn't a way to smoothly vary the speed up and down. Most speed changes were done with gearing or belts and pulleys external to the motor. Once computers and really huge transistors* got cheap enough, it became possible to build a relatively cheap "variable frequency drive" or "VFD". (Prior to this, there were some VFDs built with manual control and tubes, particularly for electric subway cars. But computers and transistors made it more practical.) Basically this lets you select whatever frequency you want to run the motor. It takes the incoming AC power and rectifies it to DC. Then the huge transistors, under control of a computer, chop the DC back into AC to run the motor. The frequency can be smoothly varied in very small steps, so you can get pretty fine control over the speed of the motor. You also get to replace several moving parts (gears or belts+pullies) with some non-moving parts (the transistors), saving maintenance and weight. When I was at SkyVenture Arizona, the VFDs were in a room off to the side of the flight chamber. The door was open and I looked in there; it was at least two or three refrigerator-sized cabinets with "Toshiba" and (if I remember right) "SkyVenture Vector Drive" logos. The basic units are probably off-the-shelf items at your friendly local industrial electric supply, and the software that comes with them probably works just fine for running one motor in a factory. If you want to coordinate four motors in a wind tunnel, you get to modify the software a little, which is probably why they had the "SkyVenture" badges on them. There are also any number of sensors scattered around the wind tunnel. Some of them are plugged into the VFD so it can shut itself down if one of the motors overspeeds or overheats; others are plugged into the operator's display so he or she can look at the temperature, pressure, airspeed, vibration in the motors, etc. Eule * OK, probably thyristors in some cases, but if you know that then you probably already know how a VFD works.PLF does not stand for Please Land on Face. Quote Share this post Link to post Share on other sites
mdrejhon 8 #13 November 15, 2006 I created the Wikipedia entry for Vertical Wind Tunnels: http://en.wikipedia.org/wiki/Vertical_wind_tunnel Quote Share this post Link to post Share on other sites
