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Bernoulli's Principle and Static Gas Pressure 
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#55
Mar2405, 01:31 PM

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Every 1st year aerospace engineering student has measured the lift on a flatbottomed wing in a wind tunnel. That's why I'm so incredulous about this whole thing: in the hundred years since the Wright brothers did it, literally millions of people have done wind tunnel tests. I even own an RC plane with a flatbottom wing: the construction manual is very clear in saying that you need to set the nose gear height for a negative angle of attack, otherwise the plane will take off on its own. How can you be so arrogant as to think that every one of those millions of people missed something so basic when you haven't even looked yourself? You yanked something out of the air and you guessed wrong. Deal with it! (learn from it) Here is something you can do yourself: http://hsc.csu.edu.au/engineering_st...el_final1.html 


#56
Mar2405, 02:44 PM

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In other words, when applying the Bernoulli equation with the incompressible assumption, only kinetic and potential energies are exchanged. The incompressible form of Bernoulli's equation only accounts for this mechanical energy. The internal energy does NOT change along any streamline. You could also derive this knowing the density is constant and the enthalpy is constant. Since H = U + PdV, where H = Enthalpy U = Internal Energy PdV = Density change and since H and PdV is constant, then U is also constant. 1) The system is isothermal 2) The system is isentropic 3) The system is isenthalpic 4) The system is adiabatic Note that this version of Bernoulli's equation is very simple in terms of the assumptions it makes. Perhaps that's why it seems so counterintuitive, because it is not very realistic in its assumptions. Check the following references, they all refer to this type of flow as isentropic: http://www.grc.nasa.gov/WWW/K12/airplane/isentrop.html http://astron.berkeley.edu/~jrg/ay202/node87.html http://astron.berkeley.edu/~jrg/ay202/node91.html http://www.fluidmech.net/tutorials/b...bernoulli.htm http://www.fluidmech.net/tutorials/b...ssumptions.htm 


#57
Mar2405, 02:49 PM

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Andrew:
The flow is isenthalpic. The flow undergoes irreversible expansion given by PdV. Note this is for a real fluid, and is not predicted by the Bernoulli equation we're talking about here. If H = U + PdV and if H stays constant and if PdV changes, The internal energy of the fluid must change which results in a cooling (or heating) of the fluid. 


#58
Mar2405, 04:14 PM

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#59
Mar2505, 09:21 AM

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#60
Mar2505, 10:19 AM

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#61
Mar2505, 10:45 AM

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#62
Mar2505, 01:51 PM

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#63
Mar2605, 08:44 AM

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#64
Mar2605, 09:00 AM

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#65
Mar2605, 10:09 AM

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#66
Mar2605, 10:10 AM

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Regarding refrigerators: compressibility is largeley irrelevant as a cause of temperature fluctuations because the working fluid is undergoing a phase change. That's where most of the energy comes from.



#67
Mar2605, 11:12 PM

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If you are talking about air flow around a wing, the air is considered noncompressible because there is nothing to compress it against (like a pipe wall) so the air just moves out of the way and creates a compression wave that propagates away much faster than the wing speed. AM 


#68
Nov407, 01:03 AM

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An airfoil (wing of an airplane) is designed such that the upper surface of the wing is longer than the bottom. (a curved line is longer than a straight line of same horizontal displacement)
Now, think of a bend in a river. The water on the outer edge of the bend travels a longer distance over the same period of time as it takes the water on the innder edge to round the bend. Since velocity = distance/time, the water on the outer ede of the bend must travel at a faster velocity in order to travel a larger distance over the same period of time. Say the water travelling on the outer edge is travelling at velocity v2 and the water at the inner edge is travelling at v1. From this we have v2>v1. Similarly, v2 represents the velocity of the air flowing on top of the wing and v1 is the velocity of air flowing under the wing. Now use Bernoulli's Equation: p1 p2 = [1/2]rho(v2^2  v1^2) where p is pressure, rho is the greek symbol representing density and v is velocity. Density (rho) remains constant in a given system. (Assume the density of air does not change as it passes around the wing). THEN since v2>v1, p1>p2. So the pressure on the bottom of the wing is greater. A resultant force is created due to the pressure different. (keep in mind pressure=force/area). So the force is acting perpendicular to the wing, thus holding the wing up. Another way to derive this force is by considering the difference in velocities. Since v2>v1, then V2 must be ACCELERATED from v1. There must be a net force inbetween the pressures in order to accelerate the wind. 


#69
Nov407, 06:52 AM

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The equal transit time principle is FALSE.
There is no natural law that says particles travelling along the outer bend must arrive at the same time as those travelling around the inner bend. Utilization of the equal transit time principle yields totally false pressure distributions. 


#70
Nov407, 08:28 AM

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No need to open a 2.5 year old thread...



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