laminar flow; videos and deep discussion

Discussion in 'PHYSICS & MATHEMATICS' started by nomendubium, Mar 29, 2013.

  1. nomendubium

    nomendubium scraping by, since '97 Expert Identifier

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    if you have a few spare minutes, lets learn about laminar flow



    I think there are a lot of inferences we can make based on this information. I'll start
    1. the lower the speed (of air for our purposes) equates to a greater affinity for laminar flow
    [​IMG][SUP][5[/SUP]
    [​IMG] is the density of the fluid (kg/m³).
    so v is what I'm talking about, and since v is in the numerator a lower v will always equal a lower equation (Reynolds number) see 5:50 in the first video :)
    2. A smaller diameter vessel will equal a greater affinity for laminar flow. This is L. The linear dimension the air passes over the object. Also in the numerator, so also the smaller the number the smaller the equation (Reynolds number)
    3. (8:15 in the first video) the primary distribution of turbulent flow occurs between +30*-+60* and -30*- -60* relative to the direction of flow. A good place to not work or place objects. It seems (to me) that perpendicular to the flow (90*) would be the place to do your work. (coming in from the side of the work) that's my take on it anyway.
     
    Last edited by a moderator: Nov 29, 2013
  2. rogue

    rogue ♥ Hooked on Mycelium ♥ Moderator Mushroom Doctor Supporter

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    Interesting stuff.

    This has been my take too. Of course we break the laminar flow while we work and need to understand how best to work with it (i.e. how we lift a perti lid and position it while we work)
    We also need the laminar flow to overcome any reverse flows we create through movement and breath.
     
  3. nomendubium

    nomendubium scraping by, since '97 Expert Identifier

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    I'm with you rogue. That's why I didn't attempt to elaborate further, there are so many factors involved in how one uses a hood. For instance a small diameter tool, like an innocultion loop, would create a lot less turbulence than a scalpel, but when the air has been filtered, how much of a difference is that really going to make? I need to find out the mass of mold (spores)and try to equate how much flow it would take to overcome that mass (to keep contaminats from falling right through the flow) A "sneeze shield" is looking like a better and better idea, but then again, why fix what isn't broke?
     
  4. Terry M

    Terry M Member Mushroom Doctor

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    The mass per se doesn't matter. A small particle falls and reaches its terminal velocity according to a simplified equation derived from Stokes' law:

    v = 2r^2·pg/9n
    where r=radius of particle, p=density of particle, g=acceleration of gravity, and n=viscosity of air

    I found that here.

    I couldn't resist filling in some numbers with the following assumptions:
    a spore is maybe 10 microns in diameter, so r ~= 0.5 10^-6 meters
    a spore is approximately the density of water

    From the equation above, I calculated that the terminal velocity of a 10 micron spore is around 3x10^-3 meters/second. That's very slow! It will drop very little distance moving along with laminar flow air at 100 fpm.

    This has interesting implications for a glove box. How long does this spore take to settle to the bottom of the glove box?
    3x10^-3 meters/second = 0.18 meters/minute
    If your glove box is 1/2 meter tall, it would take a little more than 2 1/2 minutes to fall from top to bottom.

    Since v is proportional to r^2, what about a 5 micron particle instead of 10 microns? This would then fall at 1/4 the speed, and would then take around 11 minutes to fall in your glove box. A 2.5 micron particle takes about 44 minutes to settle. This provides a rough guide for how long you should let the air in your glove box settle!

    This is in line with general guidlines for using a glove box. Cool result, huh?
     
    Last edited: Apr 18, 2013
  5. GGTBod

    GGTBod One chance at life and this is it so lap it up Administrator Cannabis Doctor Cacti Doctor

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    As the old guides used to say, put all your kit in your GB spray it inside and out and go and be busy for 10 minutes
     
  6. nomendubium

    nomendubium scraping by, since '97 Expert Identifier

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    indeed, thanks Terry M, for clue-ing us in!
     
  7. GGTBod

    GGTBod One chance at life and this is it so lap it up Administrator Cannabis Doctor Cacti Doctor

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    You guys weren't kidding about deep :hatsoff:
     
  8. Popcorn

    Popcorn New Member

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    This is the information I have been looking for, thank you!