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Homework Help: Archived Filling Empty Tank

  1. Jun 28, 2014 #1

    A cylindrical object is reentering at a Mach number of say 25 at an altitude of 80 km. I like to calculate static pressure and static temperature inside the cylinder.

    Yes Obvious, normal shock will form. But the problem is a small orifice (say 20 cm ) is available in the cylinder, the air starts entering the cylinder through this orifice until the total pressure outside is equal to inside pressure.

    I have solved/attempted this problem in the following way

    Step1: I assumed cylinder as a CD nozzle having convergent portion ,orifice (as throat) and divergent portion as my cylindrical volume and also assumed sonic flow in throat. Since I know the critical mass flow rate, I calculated the density inside the cylinder and using the isentropic relation I calculated the Mach number (Initially flow reaches supersonic speed in the cylinder) inside the cylinder and subsequently pressure and temperature also calculated. This process exist until static pressure inside reaches the pressure at throat.
    Step2: Reduction in Mach number from 1 to 0 until it reaches the outside pressure.

    Is this process is correct? Static temperature inside the cylinder also will reach the temperature behind the shock?
  2. jcsd
  3. Mar 17, 2016 #2
    At 80km, is the air dense enough to use continuum mechanics? Otherwise you might have to employ more interesting formulas.
  4. Mar 18, 2016 #3
    We loose about half the atmosphere every 5.5Km of altitude. So by my calculations, at 80km we would be down to 0.03Torr - or about 1/24000 what we have at the surface. That's still plenty dense.

    But getting back to the OP. Is this a closed cylinder? Where is the orifice? If it's on the backside of a closed cylinder, it will draw a vacuum. And if 20cm is a "small orifice", how big is this cylinder?
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