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Reynold's Number Calculation

  1. Sep 12, 2011 #1
    A passenger jet cruises at 525 mph and an altitude of 39,000 ft. The wing chord is 3 m. If the local density is 6.14 x 10-4sl/ft3 and the dynamic viscosity is 2.97 x 10-7lbf-s/ft2, what is the cruise Re number based on the chord? Make sure your units are consistent in your calculations!

    2. Equation for Re, which I believe is Re=(pvL)/u where p=density, v=velocity, L=length, u=dynamic viscosity. We haven't been taught the equation, supposed to find it online, and I believe this is it.

    3. My issue is I am confused as to what units I should keep everything in and also I have no idea what lb-s/ft2 is.
  2. jcsd
  3. Sep 12, 2011 #2


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    Staff: Mentor

    Why don't you look up (or write down) the units of viscosity that you are familiar with, and maybe you can work backwards from that (swapping one at a time a metric-type unit into its imperial version) to allow you to figure out these imperial ones.

    lbf is probably pounds or pounds force, but I'm guessing.
  4. Sep 12, 2011 #3

    Just tried to solve it myself and I think it worked. I turned 6.14x10^-4 slug/ft^3 to .3164 kg/m^3, 525mph to 234.696 m/s, and 2.97x10^-7 lb-s/ft^2 to 1.422x10^-5 N-s/m^2

    I think those were the correct unit required to keep this dimensionally homogeneous since Re number is unitless. I am pretty sure my conversions are correct, if anyone notices one is off please tell me. After typing these into the equation (I didn't round on my calculator for these conversions like I showed here) I ended up with Re=15667833.16

    I am not sure if that is it or if it's even a reasonable number (never used Reynold's equation before) so please tell me if this looks strange.
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