Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Air flow differance.Square versus round tubing

  1. Oct 30, 2011 #1
    Hello.I am trying to make a snorkel for my 4x4 truck.I would like to use 2 1/2 inch square tubing but all snorkel manufactures use round.My dad says round flows more air.I believe round must be better.But I want to no why?I now I can get more mass or volume per square inch in a square tube at 2 1/2 inch versus a round at 2 1/2 inches so why/how doese this change when I start to move the air through the tube?Doese it change if the tube is straight?Any info or recomendation's to help me understand this would be great please keep in mind I am not very educated at all.So if you could try to keep it simple it would be easier for me to understand.Thank you.

    My truck's stock air tube is 2 and 3/4 inches round. The system I am adding will be one straight piece.Will 2 1/2 square get me the same cfm as 2 3/4 round?
  2. jcsd
  3. Oct 30, 2011 #2
  4. Oct 30, 2011 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Pardon a bit of hand waving. In a circular crossection tube the speed of material through the tube is lowest near the sides due to drag. The speed increases as you move away from the edges with highest speeds in the center. This results in a parabolic speed profile.

    Now in a square tube the regions near a corner are influenced by 2 sides so the speed will be even slower there. As you move away from the sides a speed profile much like a circular tube will develope. What you will end up with is that the majority of the material will be flowing in a circular region inside the square. The concludion would be why pay for and carry around the corners, they do not flow nearly as much flow as the center region.

    (opps I am on a different computer then usual, do not have a spell checker installed. Will check this post later. Apologies)
  5. Oct 30, 2011 #4
    Thank's for the help.I do not no how to do that math for Reynold's # and don't even no how the get the info needed to get my sister to calculate it for me.Like friction velocity etc.But I do kinda understand it a little better.I tried to find a illustration of air flow through round and square tube.I have had no luck.But it sound's like the air swirls longways if the air in the center moves faster then the edges.This is making sense now.Awsome.I would really like to use the square tube still for look's and to be differant.Can you guy's give me a rough guess as to how much differance in flow there is.I could live with Around 5% or 10% but if it's 15 or 20% I need to change my plan's.

    P.S. thank you for being nice and offering help to me even though I cannot spell and lack so much knowledge.I have posted on other forum's for other stuff and peapole are rude and just harrase me instead of offering any help.So thank's again and happy Holloween
  6. Oct 30, 2011 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Hydraulic diameter is given by the equation Dh=4A/U
    Where for square tube:
    So hydraulic diameter for a square tube is Dh = L. In other words, a square tube has the same hydraulic diameter as the same sized round pipe so the flow restriction between the two will be the same. Make it out of either square or round. You shouldn't notice the difference.

    Edit: I should also mention that square and round tube is generally specified by specifying the characteristic length as measured from the outside, not the inside. Pipe of a given diameter on the other hand, does not have an OD equal to the characteristic dimension, it is always larger. 21/2" pipe has an OD of 2.875, so the ID will be larger than the 21/2" square tube. It's the internal dimension you are concerned about, not the outside dimension. If the two have the same internal dimension, they will flow the same.
    Last edited: Oct 30, 2011
  7. Oct 30, 2011 #6
    Thank you everyone.Tomarrow or the next day I will begin building it and give the square tube a shot.I look forward to talking to you all again on the next project.If you have any info to add please feel free anytime.
  8. Oct 31, 2011 #7
    The Does it REALLY matter?

    No - if the tube has the same cross sectional area and it's clean inside, and it is only a few meters long.

    Yes - if the tube is smaller in cross sectional area, it has dags and intrusions into the tube and it's many meters long - like 100 or 200 meters...

    The car engine moves low amounts of air with a low pressure differential.

    The ONLY real time that these are issues are with very high pressures, high velocities - in the kinds of things you get in liquid fuel rocket engines etc..

    People use round steel tube etc., because it's cheap, light, easy to weld, all the fittings are common off the shelf gear or easy to make or adapt.

    Get a vacuum cleaner, hook it up and make up a simple manometer to test the difference - if you can get tubes with the same or nearly the same cross sectional area - and then modify the equation to adjust the results.
  9. Oct 31, 2011 #8

    You are making a snorkel, not designing a fighter pilot.
    1) 2.5" Square tubing will sustain more capacity than the round stock because it has a greater cross-sectional area.
    2) If you are making your own snorkel, I imagine you don't have a "performance" truck. You aren't fine tuning anything. The snorkel will have about the same if not greater intake capacity than the stock air intake depending on where it is located.
    3) Why use steel? Plastic works the same and is lighter and easier to fasten to the vehicle. Steel will be rough on the mounts when you are bouncing around in mud and water.
    Last edited: Oct 31, 2011
  10. Nov 4, 2011 #9
    Hey Iron-Stein.You are right.I had to fabricate my square with to piece's at 90's.The round would have ben much easier to get.Great idea with the vacume.Those kind of home made test's are more my speed.Thank you.

    Travis-King I agree doesnt have to be exact.It's not a space shuttle lol.And the square lets me keep cross section smaller then round because the square edges get me more volume.(if it's to big it will impair my field of view)I leaned to the safe side with my calculated size so 2 1/2 should be slightly extra if anything.My truck is sorta fine tuned, highly modified so I don't want to hurt any performance.They do not make a snorkel for my truck because the motor in it is not factory.Otherwise yah I would have used something lab tested.I used metal because I can cut weld grind paint etc..I said its one straight piece for simplicity to my post but really it's more complex and has some water traps I intergrated in it.To seperate water and snow.I will take a pic later and try to post it.So you all can see what your time has went into.Thank's again everyone.
  11. Dec 1, 2011 #10
    Sorry for taking so long.This is what it look's like.It work's great no problems yet.Thank you everyone for the help.I have gotten some criticism and some complements on how it look's.Honestly Im not completley satisfied with the look's.I might change it.Feel free to comment on it good or bad.I like the honest input.Have a great day everyone.

    Attached Files:

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook