Page 5 of 5 FirstFirst 12345
Results 41 to 46 of 46

Thread: DIY Water screen

  1. #41
    Join Date
    Mar 2006
    Posts
    2,478

    Default

    Some of you keep missing the point. You want HIGH pressure in very fine jets. Think about it. The high speed of the laminar flow is very important, a fast flow causes low pressure to train the higher pressure of surrounding air (plus any fog or mist) into the low pressure field so it will stay there for a greater distance. (It's the same reason aeroplanes fly.) You'll have to experiment with best even-ness for diffusing the light beam but no amount of AC fans or computer fans is going to do anything but piss you off. All that would do is give you a second-rate hazer. What you're after is a little local jetstream.

  2. #42
    Join Date
    Jan 2005
    Location
    Norway
    Posts
    1,303

    Default

    Doctor: I dont want to make discussion about high power air, and low power air.
    As I have not yet tested out high power jet air with mist

    Maybe you are correct....?

    But the fact that I know the laminar water jets DONT like high pressure water.
    My guess is that a mist "floating free" in the air will not stay laminar when it jets down in high speed.

    The light weight particles that fall down will not stay on course when they are pumped out in extremely high speed.


    I base this on the PROfessional fig screens that only use small amount of wind to push the mist out\down from the machine.


    But again, I have not tried anything with "jet" speed, so maybe it can work...

  3. #43
    Join Date
    Mar 2006
    Posts
    2,478

    Default

    I mentioned air jets, not water jets. Water jets are different, you don't want too great a change in pressure or you end up with an aerosol spray.

    Back to air jets.... by 'jet' I don't mean screaming supersonics. I just mean jet, literally, a very narrow fast flow. To start with the jet has to be fast enough to propel mass firmly in one direction, and to create a low pressure region in the flow once the initial expansion is done. The speed will drop as air from outside is 'trained' alongside the main flow, as kinetic energy is exchanged between the two regions. Don't take my word for it, people use this a lot in industry, it was news to me once but easy to see as true just by thinking about it. There is no 'maybe' about this. Mist or fog wouldn't be propelled at high speed, you just have to produce it close to the start of the flow, and it will be drawn alongside it before it merges into it. It might drift fairly slowly if it isn't directed right into the flowing region, that's another aspect open to experiment, the effects on angle of intercept and speed of fog production. What matters is that the fast flow of one fluid in another of similar density will create a region that is clearly defined because the faster flow has lower pressure, so its content is maintained in a narrow stream by the higher pressure alongside it.


    Regarding 'professional', that doesn't always mean best method. You only have to see the way PRO is slapped on seriously dodgy cheap audio stage amps to know how tawdry and hidebound some PRO practises really are. PRO very often means 'as cheap and basic as possible so no-one cares if the club punters smash it up'. Hardly a guide to innovation. Until the late 80's a 'professional' microphone meant a moving coil type barely fit for broadband speech. Anything else was considered a 'studio' mic, and too fancy for 'real pros'.

    Another obvious merit to the jet flow idea is the source can be a very lightweight narrow pipe fitted with jet nozzles. In a large installation it would be a very discreet system, finer than a curtain rail. After the start, the mass of fog and gas will be moving downward in a stream that gravity will help with. If you can chill the fog before it meets the laminar flow that will help too. it might even be possible to get the chilling done by mixing something with the air fed to the jets, but I don't know a cheap or environmentally kosher way to do that. Best to chill the fog as that's where most of the mass is anyway. Cold fog alone tends to drop like a slow ghost of a stone.

    Edit: Actually the bulky pipe needed to duct fog to the top of the curtain would make the thin pressure rail pointless. I mean why hide one if you can't hide the other? But then again, in showbusiness, part of the magic is in illusion, and things that CAN be hidden always offer more scope for good illusions.
    Last edited by The_Doctor; 04-07-2010 at 08:35.

  4. #44
    Join Date
    May 2007
    Location
    1 hr from everything in SoCal
    Posts
    2,753

    Default

    I think a large 1/2 hp squirrel cage blower would provide enough pressure to feed into the laminar tube system that I need to build. Should be fairly high pressure. Those fans have enough guts to push a/c through a two story home. Should work here. I am thinking of trying drinking straws for the jets. Maybe making the bank about 20 layers thick of straws. That is going to take a LOT of straws. I am also wondering... would it be best to use a thin felt or window screen to stablize the flowing air? I want to elimiate the buffeting that you would feel from a conventional fan. That way, it is just smooth and constant pressure. I think it would also equalize the flow over all the jets so that they are at the same pressure. Or, now that I think about it, incorporating your idea of a pipe with many holes in it. That would spread the air flow out so there is no 1 inlet but rather several all the way across.

    The_Doctor, I apologize for not knowing your name, but what do you think? Very high pressure, across several outlet holes that feed into the chamber before exiting out of the laminar tubes?

    The fog, be it atmoized mist or glycol, i think that will be the last thing I worry about. The laminar flow, high pressure creating a low pressure zone is my first obstacle.
    Last edited by absolom7691; 04-07-2010 at 13:20. Reason: spelling
    If you're the smartest person in the room, then you're in the wrong room.

  5. #45
    Join Date
    Mar 2006
    Posts
    2,478

    Default

    No need to apologise, I rarely tell people it because it probably doesn't sound any more believable than this one. It's a sort of open secret, it's Crow. Alias Lostgallifreyan in several bits of net including eBay. I think a few people here knew all that but not many. I like using this netname though so I'll stay with it. Old habit.

    Anyway.... I saw a YouTube page:
    http://www.youtube.com/watch?v=OV-IazRk0sU
    Didn't see the vid, I rarely install flash in any session, but I read the comments, people doing the same thing as in this thread. I also looked into 'air knives' and the Coanda effect.

    Apparently compressed air isn't as efficient, nor are separate nozzles. Ideally a 'plenum' space formed by a pipe having a teardrop (or apple pip) shaped cross section, with the slit at the sharp end, is most efficient, and is best fed by an industrial blower, perhaps the kind used for inline pumping of air in 100 mm ducting might be enough. High pressure CAN also be efficient but only if the coanda effect's main business is confined to a ring nozzle a bit like that inside a 'vortex tube' as used for heating and cooling stuff. In this case it would circulate to form a small ring surrounding a region of low pressure (not by accident are vortex tubes sometimes called tornado tubes). This low pressure draws in high volumes of ambient air and a bias to exit the ring plane in one direction pushes all that air in high volume and high speed in the same direction. Not saying that this is any good for fog curtains, but it's interesting and might have some use in laser shows, for bulk diffusion of fog perhaps, as the fog won't get reduced by impacts with fan blades.

    Basically it seems you can use an AC blower, if it's an inline duct thing, but it still has to feed the flow generating slot thing, a kind of big gentle air knife. Someone mentioned a 95% efficiency for the method but I don't understand the context for the claim so I won't do any more than mention it too. Separate jets drop the efficiency to 60% as claimed in that same forgotten reference, but it is easier to build that way. I think a lot of trial and error might be needed to get the slot or jet aperture size (and separation) right, and the flow rates, etc, as well incident angle and speed at which fog is passed to the main flow.

  6. #46
    Join Date
    May 2007
    Location
    1 hr from everything in SoCal
    Posts
    2,753

    Default

    Thank you very much for all the info. I will check all this out. I just bought 1000 drinking straws ($3). Cheap enough to try out. I am going to try and knock together a small one and see how it goes. It looks like a have a bit of trial and error in front of me, but that is the best way I learn. i will try this out and see the results. I am going to also look into your suggestions as well. I have several months before halloween so, I have some time!

    Well, seeing as how this thread was titled Water Screens.... I think, once I begin construction of my test design, I will get another thread going.

    Thanks again for the info, Crow. My name is Brian, BTW.
    If you're the smartest person in the room, then you're in the wrong room.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •