> Rolf, Mark and All,
    >
    > I have to correct my previous post. Mark was right. The total
   pressure
    > in the turbine is the sum of the static and dynamic pressure.
    >
    > This is exactly what Bernoulli's Equation tells us:
    >
    > Along a streamline: static pressure + dynamic pressure = total
   pressure
    >
    > Expressed mathematically:
    >
    > static pressure + (density * Velocity^2 / 2) = total pressure
    >
    > In reference to that turbine chart, the pressure obviously shows
   the sum
    > of hte static and dynamic pressure.
    >
    > The reason the flow does not back up is still explained by
   inertia, but
    > it is true that the total pressure equals the dyanmic and static
    > pressure, as Bernoulli tells us. No doubt about it.
    >
    >
    > Regards,
    >
    > Gordon.
    >
   Gordon,

   (I cut the bottom of message off as we all know it.)

   Good that we have it correct now. At least I agree with it.

   One point, would the 'dynamic' pressure not be the same as is described
   by 'inertia" of the gas.
   I think that is one and the same thing. How do you see it?
   However, if so, the dynamic pressure would have a direction, it would be
   a vector quantity.
   The direction, of course, is toward the exhaust nozzle.
   Any comments on that?

   Regards

   Rolf

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   Rolf,

   I'm glad this thread has come up -- it gives us a chance to brush up
   on some basic physics that are fundamental to both aerodynamics and
   propulsion. I realized just how rusty I was on these basics when I
   started thinking about Doug's question about turbine size.

   You are right, of course, that dynamic pressure, which describes
   air or another fluid that is flowing, is a vector quantity -- it has
   both magnitude and direction.

   The tricky part is that static pressure is a scalar quantity. If a
   gas is static and not flowing, the measured pressure is the same in
   all directions. But if the gas is moving, then the measured pressure
   depends on the direction of motion.

   Now about the inertia part. I believe it is correct to say that the
   moving gas inside that turbine engine does not reverse direction and
   flow back to an area of lower pressure due to its inertia. However,
   this is not a full explanation of what is going on. Remember that
   inertia also applies to objects at rest, which resist being
   disturbed by an outside force. Newton gave us this idea of inertia
   in his First Law of Motion.

   So yes, this tells us that the moving gas in the turbine will resist
   changing direction because of its inertia.

   But I believe Newton's Second Law gives us better insight into the
   question of fluid flow because it deals with the question of
   "force." Specifically, it tells us that force is a change in
   momentum over time, or mass times acceleration.  With this insight
   we can now go further and figure out just how much force the moving
   air is exerting.

   To understand dynamic pressure, we turn to the conservation of
   momentum, a fundamental concept of physics.

   Remember the analogy of sticking your hand out the window of your
   moving car? The faster you go, the greater the force on your hand.
   This is how the pitot tube works to move the needle on our airspeed
   indicator. The greater the dynamic pressure, the more the needle is
   deflected.

   As noted earlier, dynamic pressure is simply half the product of
   density and velocity squared:

   Q = rho * V^2 / 2 = 0.5 * rho * V^2

   If we add to this the coefficient of lift and wing area, we have the
   lift equation:

   L = 0.5 *rho * V^2 *Cl *Sref

   And if we add dynamic pressure to the thrust equation, we can
   analyze propeller thrust. So right off the bat we have lift and
   thrust, which is pretty much everything there is to flight. Yes,
   understanding dynamic pressure is a good thing to understand if we
   aspire to fly...

   There is a good page on the NASA educational website that gives a
   more detailed explanation of dynamic pressure:

   http://www.grc.nasa.gov/WWW/K-12/airplane/dynpress.html


   Btw, I think we should be able to make some progress on Doug's
   question about turbine size. We should be able to work out the
   thermodynamics of the rotary engine using standard equations that we
   use in gas turbines. After all both are internal combustion engines.
   From there, we use the same tools to figure out the right size
   recovery turbine.

   I'm going to keep plugging on this one. I think we should have a
   spreadshet for the exhaust side like the one I put together for
   compressor selection.


   Regards,

   Gordon.

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