# Tag Info

2

Flutter is only possible if you have similar structural and aerodynamic frequencies. One without the other would produce much lower amplitudes. Look at a mass-spring system suspended on an eccentric tappet which sits on the edge of a small rotating wheel. When the wheel turns, it raises and lowers the top of the spring, and the mass on the bottom will ...

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In every boundary layer (except for exotic hypersonic cases), the speed at the wall is zero. At the trailing edge, the upper and lower layers meet, and if you imagine a plane which extends from the trailing edge backwards and follows the streamlines, the speed at the trailing edge is equally zero. The more you now move away from the trailing edge along this ...

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when you look at an airplane propeller, you will se that parts of the propeller blades are slanted. These slants, when rotating, push the air back, creating a low pressure zone, sucking more pressure in, while at the same time pushing more air back.

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This is a most excellent and astute question. Ultimately it comes down to experiment: the model below works pretty well for many fluids. What this must mean therefore is that the loss is small enough that each particle of fluid, in flowing past the region of disturbance, loses a fraction of its energy that is small enough that it doesn't upset the energy ...

-1

To add one more factor into the answer, IC engine efficiency is optimized for a certain RPM range. Going higher or lower reduces it's efficiency at converting fuel into motion.

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because higher speed pulls more oxygen to burn into the engine and less fuel is needed.

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1) The $sin$ term appears to be resolving the velocity of the water relative to the face of the disk. $U$ is the just the speed, it's necessary to determine what part of that velocity will produce force on the object. It might help to visualize extreme cases, setting $\alpha$ and $\beta$ to 0, $\pi /2$ etc. For $\alpha = \beta =0$ you have a perfectly level ...

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The power required to push the car forward increases with the cube of speed. Your car's engine has been designed to deliver a certain power, and asking less from it will make it run at a lower efficiency. You still need to move all those bearings and turn the coolant pump and the generator, so a certain base load is always there. Driving in a lower gear, ...

1

How can it require less petrol to run the engine at 2,500rpm pushing a heavy car at 55mph than the exact same engine doing the same rpm only pushing at 20mph (far less air resistance)? It doesn't require less petrol, it requires more. However, if it requires less than $\frac{55}{20}$ times more petrol at 55mph, then the car is more efficient ...

1

I disagree with the answers for the case with flow parallel to the axis of symmetry. The drag should be higher for the solid cylinder. Yes, the friction drag will be higher for the hollow cylinder case due to there being a higher surface area - however, there are generally two main sources of drag - friction drag (caused by viscous effects at the wall) and ...

2

This is all about drag. The current answer by xpda is the correct idea, but I thought it might be worth going into a little more detail, to explain exactly why there is "no suction to speak of pulling back on it" (in fact, I wouldn't say there is "no suction"...rather, "less suction" would be more apt). Drag results from a difference in pressure between ...

2

Yes, the reference area is commonly the projection of the exposed wing area on the wing's X-Y plane, the plane spanned by the wing chord and the wing span. However, at the low Reynolds number you should find other airfoils more suitable. The NACA 6-digit range is more commonly used at Reynolds numbers > 10$^6$.

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I found a NASA technical memo for very accurate "Real-Time Aerodynamic Heating and Surface Temperature Calculations for Hypersonic Flight Simulation." The authors also discuss a bit how they obtained their expression. I have to say, though, Dave's dissertation and tgp2114's answer are wholly sufficient and more straightforward.

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For viscous hypersonic flows, the heating takes a form: $$q_w = \rho_\infty^N V_\infty^M C$$ where the parameters $N$, $M$, and $C$ depend on the configuration and $q_w$ is the heating in $W/cm^2$ (this is all from Hypersonic and High Temperature Gas Dynamics and I highly recommend this book). For the stagnation point (like the leading edge of a body): ...

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The total energy doesn't change, only the fraction that is potential energy (equivalent to pressure) and the fraction that is kinetic energy (equivalent to speed squared). So each particle will be somewhere on a continuum between only potential energy (at the stagnation point) and only kinetic energy (at high speed at a point of vanishing pressure). The sum ...

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It's very simple, as explained clearly here. The way a parcel of fluid accelerates from one velocity to another is by falling through a pressure difference.

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