# Tag Info

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It is friction with the outer layers of the atmosphere and the relatively large velocities of the spacecraft which produce these amounts of heat. Hope this is helpful

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The answer is due to the area-Mach number relation for hydrodynamic shocks. G.B. Whitham has a great book (check out Chapter 8) on all sorts of various waves and has a good discussion of this topic. The idea is that one can define the Mach number as a function of the cross-sectional area of a ray tube. The simple form is:  \frac{ 1 }{ A } \frac{ d A }{ ...

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The rotation is part of the key to the storm itself. Primarily the pressure and temperature differences are what causes these systems to take the shape and forms that they do. Once a tropical depression starts to form you can already see rotation in the moisture around the low pressure zone, even through it typically looks nothing like a hurricane. Not ...

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Like any object moving through a fluid, a high-speed train distorts the air as it moves through it. Broadly speaking, there are three main regions of flow structure around a high-speed train: the upstream distortion, boundary layer and wake. These can be collectively referred to as the slipstream. The effects of the slipstream on a static observer (e.g. a ...

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The front of the train compresses air which can blow you away, while at the back of the train air rushes back in after the train has displaced it. This backdraft is especially troublesome in closed areas such as subways, where a train exits a small tunnel near a platform and the displaced air rushes back into the vacated tunnel. Next time you see a big truck ...

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If the tips of the propeller blades are moving near the speed of sound a shock wave can form. Supersonic flow has very different character than subsonic. A propeller designed to operate at subsonic speeds will be inefficient at supersonic ones due to shock waves. In general, shock waves cause a loss of efficiency. You might have noticed that subsonic ...

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Any kind of "funnel" - an area where tall buildings create an obstacle to the free flow of air - acts as an amplifier to wind. That is, even a little bit of air moving from point A to point B will notice the "obstacle" that is a pair of buildings; it will build up pressure in front of the buildings and result in a faster flow of air through the passage. An ...

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The highest wind speed ever measured outside of a tornado was 113m/s, approx. one third the speed of sound. Needless to say, no wind turbine will be operational at that wind speed, let alone operate with its wing tips close to supersonic mode. Smells like the pseudoscience it is.

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The statement in the first paragraph "In fact, today’s standard turbines are based on the same physical principles as 18th century windmills." is marketing hooey. They are hanging their hat on the fact that the windmills were unducted props and most of today's turbines are the same, which is true. The airfoils used today are not 18th century designs. They ...

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The Kutta condition is completely artificial. The potential equations are completely artificial. The potential equations are a mathematical construct we use because it's much simpler than the full Navier-Stokes set of equations. We know the Kutta condition is never actually upheld in any real flow ever. However, when we perform all of our mathematical ...

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I disagree with the most voted answer, by CAGT. He says "This area is completely different to the one above", but this means nothing. The equation $p = {F \over A}$ mentioned by the author does hold, and there is no contradiction or paradox in it. In fact, the equation $p = {F \over A}$ holds not only here but anywhere else in physics. You may write it in ...

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A fan changes the average velocity of air molecules. This can be seen as a molecular scattering process for very thin gases, whereby every single molecule hits the surface of the angled rotating fan blade in such a way, that an axial velocity component is imparted on the molecule. Since a single rotating fan blade can not do this without also imparting a ...

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Think about the air around the fan at any given time: The amount of air flowing into the fan must equate to the air flowing out of the fan. The amount of air that passes through an area in a given time is related to the velocity of the air i.e. the faster the air is moving, the more air that can flow through a fixed area/hole/slot. The fan blades apply a ...

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Here's a standard fan with some (hard to see) arrows indicating air flow. The fan works by pulling air in and then making it move faster. The air flow behind the fan is slow moving and wide (you can see the arrows behind the fan coming from above and below the fan blades) whereas the air flow in front of the fan is fast moving and narrow (which follows ...

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There is a YouTube video that visualizes the air flow around a propeller for various configurations. I caught a screen shot of a moment that more or less shows what is going on: As you can see, this happens at 2:07 into the clip - this happens to be for a dual rotor configuration (two counter rotating blades) but the principle is the same. Behind the ...

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You have to balance competing issues, so it's an optimization problem. Yes, more blades move more air, but also cost more power. (Like in a small aircraft, each propeller blade can't consume more than about 100 hp.) If the air is moved through a smaller diameter fan, then it has to move at higher velocity (to get the same overall flow), and power goes as ...

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3 blades would be indeed close to optimum for power efficiency and require less material during manufacturing. But typical engineering goal for fans is different: maximum performance / noise. More blades $\implies$ slower rotation for same performance $\implies$ less turbulence $\implies$ less noise. Also, check out my older relevant question: Wind ...

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The direction of the Resultant force $R$ is always dependent on the direction of the $V\infty$ But however, the direction/orientation of the Normal force $N$ is dependent on the orientation of the body itself ($N$ is perpendicular to the body and axial force $A$ is parallel to the body.) In the above case, since there is not much of surface interaction, ...

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