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# Rarefied Gas Dynamics - Science topic

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The article is just click-bait; if somebody doesn't understand how the lift is created he/she haven't developed the most basic concepts of fluid dynamics. I'm trying to explain how the aerodynamic forces are generated around a body; excerpt from a course I'm working on:
"According to the kinetic molecular theory of gases ... static air exerts a pressure of about 101kPa on any object at sea level, created by the collisions between the object’s surfaces and air molecules ... this pressure changes when the air is not static but it moves around the object ... the random movement of the molecules changes, their random speed vectors gaining a component induced by the macroscopic air movement. ... This change of the random movement of the molecules is causing variations of pressure in the air volume, and acting over the object surfaces are creating forces, termed aerodynamic forces."
The article focuses on inability of common layman "lift theories" to account for the decrease of pressure and speed increase on the upper wing surface. I'm trying to correct this using some common sense.
Let's imagine a flat barn door (our "wing"), flying at an angle, through static air. Let's imagine the air molecules are un-moving grains of sand as a first approximation. In such a case the wing interacts only with the "molecules" that is able to hit, in the direct path of the wing. On the top region, the "wing" is not interacting with the "molecules", a void is created between the "wing" and the "molecules". This model is called "Newtonian sine-squared law of lift" and developed by - Newton in 1687. Interesting fact, this model is not at all accurate for normal flight, but it is accurate at hyper-sonic speeds and/or very low temperatures and pressures, such as space vehicles re-entry, as the molecular speeds are much lower than the aircraft speed.
Well, let's replace now the grains of sand with air molecules that are moving, according to the kinetic molecular theory of gases, at typical air at room conditions, the molecules are moving at about 500 m/s. A wing flying e.g. at 100 knots, about 50 m/s, is much slower. The void between the wing and the free stream is quickly filled by air molecules that are "pushed" in by elastic collisions with other air molecules. Since the air molecules are moving in the void in a global downward general direction, this has a macroscopic effect of increasing the airspeed. Since the total kinetic energy needs to be conserved, less molecular speed is available for collisions and that accounts for decreasing the pressure over the top of the wing. According to Bernoulli this is correct, as Bernoulli law is an energy conservation law, as applied to fluids. Since accelerating air downward is creating an upward lift force , the Newton's second law is also correct. So Bernoulli's and Newton's laws are actually the interpretation of the same physical phenomena.
Hopefully this short introduction dispels some misconceptions of how "no one can explain lift"
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I am trying to create an impact pressure probe to measure spatial distribution of free stream pressure of a jet plume exiting to a rarefied medium. I found out that sankovich pressure probes is best suited for my purpose. Kindly provide the dimensional details of the above mentioned probe.
check Fig 4
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Greetings, I am trying to understand how the information travels in subsonic flows from a microscopic perspective. From computational physics point of view, the flow field can be described as elliptic in nature and there are no real characteristics. I know that every point in the flow field depends on every other point in the flow field implying it eventually depends on the boundaries of the flow field if there exist any.
Assuming a flow field at rest, if a disturbance is introduced into the system the information propagates through pressure waves. But from a microscopic point of view, I can only imagine information propagating by collisions with particles. But is it the complete the picture of it ? Because in Direct Simulation Monte Carlo method, the velocities before and after collisions cannot be correlated and i don't seem to understand the information travel.
Am i failing to understand something deeper on the information travel in subsonic flows ?
P.S. I am trying to understand this for application in micro-fludics using DSMC method.
I think it is better to focus first on the meaning of the sound velocity. It is a thermodynamic-based quantity that gives a statistical lecture of the microscopic collisions that happen at a certain temperature due to the molecular agitation. Similarily the macroscopic flow velocity u is a statistical resultant of the collisions at microscopic level. In terms of the mathematical character of the PDE, an elliptic field would transmit instantaneously (infinite velocity) a disturbance. Actually, this model is an approximation used for incompressible flow motion where acoustic waves propagating at a>>u are decoupled from the convective velocity. From a more complete and real physical description, the sound velocity a can be greater than u but not infinite. That means that waves (both acoustic and convective) travel at a finite speed and following particular directions. This turns out in the hyperbolic/parabolic character of the PDE.
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CD is the ratio of actual flow rate to the theoretical flow rate
So, how can we calculate the theoretical flow rate?
Go to the definition of discharge coefficient in https://en.wikipedia.org/wiki/Discharge_coefficient
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