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lehpron
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Exhaust/turbulence Effect Supersonic Area Rule?

Tue Nov 13, 2007 12:39 am

I've hesitated asking these for a while, but wondered anyway.

  • I don't mean to be general either, the thinnest parts that trail miles behind that which are equal to the atmosphere no doubt are part of the atmosphere and don't displace any airflow. But the higher pressure gas just aft of the engines, do they displace any air (before mixing) as if the flow shape was like a solid object?


  • Think of supersonic flow going around a bullet or blunt object. Flow has to 'right itself' from going around -- the tail shock wave doesn't form until much farther behind and away from the object. How does area rule 'see' this? Does it make the object seem bigger by righting the flow?


  • In both cases, I wonder how much of supersonic flow pattern is the original object, and the rest is just distrubed flow (from ambient standstill) going back to normal as the object passes, i.e. wake but still moving above Mach 1.
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    tdscanuck
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    RE: Exhaust/turbulence Effect Supersonic Area Rule?

    Tue Nov 13, 2007 4:37 am



    Quoting Lehpron (Thread starter):
    I don't mean to be general either, the thinnest parts that trail miles behind that which are equal to the atmosphere no doubt are part of the atmosphere and don't displace any airflow. But the higher pressure gas just aft of the engines, do they displace any air (before mixing) as if the flow shape was like a solid object?

    Well, the mixing starts as soon as you get out of the nozzle so there's always some but the exhaust plume will expand a little bit and the streamlines will "see" it like a solid body if the outlet pressure is higher than ambient. It doesn't actually have to be higher than ambient though...you can have it above or below. A properly designed nozzle will exhaust exactly at ambient for maximum thrust.

    Quoting Lehpron (Thread starter):
    Think of supersonic flow going around a bullet or blunt object. Flow has to 'right itself' from going around -- the tail shock wave doesn't form until much farther behind and away from the object. How does area rule 'see' this? Does it make the object seem bigger by righting the flow?

    The tail shock should be forming right at the tail. It's what turns the flow inwards. You can get shocks reflecting off each other for quite a distance behind the tail (hence shock diamonds) but the shocks still start right at the tail.

    Tom.
     
    lehpron
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    RE: Exhaust/turbulence Effect Supersonic Area Rule?

    Wed Nov 21, 2007 7:36 pm

    Thanks for the quick response, I didn't expect that.  Smile

    Quoting Tdscanuck (Reply 1):
    The tail shock should be forming right at the tail. It's what turns the flow inwards. You can get shocks reflecting off each other for quite a distance behind the tail (hence shock diamonds) but the shocks still start right at the tail.

    Okay, so where are the tailshocks in these pictures? If a supersonic object completes a cycle of pressure rise/fall when the bow follows the tail wave, then does this area aft of the bullet count into the area rule?



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    tdscanuck
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    RE: Exhaust/turbulence Effect Supersonic Area Rule?

    Thu Nov 22, 2007 5:30 am



    Quoting Lehpron (Reply 2):
    Okay, so where are the tailshocks in these pictures? If a supersonic object completes a cycle of pressure rise/fall when the bow follows the tail wave, then does this area aft of the bullet count into the area rule?

    I'm assuming these are Schlieren pictures. In the upper picture, the band around the bullet at about the 2/3 point is throwing a strong oblique shock...that appears to be enough to turn the flow back to normal without additional major shocks (although you can see many more very faint oblique shocks behind that one, which are basically forming an expansion fan).

    In the lower picture, the aft-most edge of what's labelled the "Mach wave" is the shock coming from back corner of the bullet.

    As far as I'm aware, the separated area aft of the bullets does not count towards the area rule since you're picking up a lot of drag from that large separated area. If the bullet was tapered at the rear to shrink the separated area (which would be following the area rule) the drag should go down.

    Tom.
     
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    Starlionblue
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    RE: Exhaust/turbulence Effect Supersonic Area Rule?

    Thu Nov 22, 2007 5:49 am



    Quoting Tdscanuck (Reply 3):
    If the bullet was tapered at the rear to shrink the separated area (which would be following the area rule) the drag should go down.

    Indeed. But you need at least some flat surface in order to impart force from the propellant gases without causing wobble issues.
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    tdscanuck
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    RE: Exhaust/turbulence Effect Supersonic Area Rule?

    Thu Nov 22, 2007 7:39 pm



    Quoting Starlionblue (Reply 4):
    Quoting Tdscanuck (Reply 3):
    If the bullet was tapered at the rear to shrink the separated area (which would be following the area rule) the drag should go down.

    Indeed. But you need at least some flat surface in order to impart force from the propellant gases without causing wobble issues.

    Good point, I should have mentioned that. Almost all violations of area ruling are because some other design consideration, which area ruling doesn't take into account, becomes dominant. For a bullet, you need a flatish surface for the propelling gases to push on evenly. For a lot of fighters, you need an engine nozzle at the back. Good design is all about juggling the compromises. Area ruling is a technique to reduce trans/supersonic drag but you can't do it to its fullest without screwing up a bunch of other things.

    Tom.
     
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    Starlionblue
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    RE: Exhaust/turbulence Effect Supersonic Area Rule?

    Sat Nov 24, 2007 1:59 am



    Quoting Tdscanuck (Reply 5):
    For a lot of fighters, you need an engine nozzle at the back.

    "A lot"? I thought engine nozzles were a pretty universal concept.  Wink Unless it's reactionless.

    Anyway just messing with you.
    "There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
     
    tdscanuck
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    RE: Exhaust/turbulence Effect Supersonic Area Rule?

    Tue Nov 27, 2007 1:44 am



    Quoting Starlionblue (Reply 6):
    Quoting Tdscanuck (Reply 5):
    For a lot of fighters, you need an engine nozzle at the back.

    "A lot"? I thought engine nozzles were a pretty universal concept. Wink Unless it's reactionless.

    Anyway just messing with you.

    Fair enough, I walked into that. However, what was going through my head at the time is that not all fighters have their nozzles at the rear-most part of the aircraft, like the very early German stuff. In those cases, the nozzle isn't your limiting factor for area-ruling.

    Tom.
     
    lehpron
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    RE: Exhaust/turbulence Effect Supersonic Area Rule?

    Sun Dec 09, 2007 3:17 am

    Well what about the external combustion senario associated either ram/SCramjet engines? That is if it is assumed that the bow shock from inlet can act as a partial-containment for the combustion process and exhaust expansion.
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