Design of a common table fan A few questions about the design of a common table fan, such as below.


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*Does the round disc (silver in the image) in front of the blades serve any functional purpose?

*Does the metal casing around the blades serve any functional purpose except protecting the blades?

*What exactly are the sources of noise in a well-designed fan? Does the metal casing in a well-designed fan have an impact on the noise level? 


http://img193.imageshack.us/img193/8081/60933751.gif
 A: I won't address the details of the fluid dynamics that cause the cage to produce noise because it is well out of my area of expertise. But I have performed an experiment in my office. The small desk fan I bought was too noisy. Since it lives on the top of a tall bookcase and well out of reach, I removed the cage completely. It is a lot quieter now.
My guess is that turbulence around the bars of the cage causes the noise. With the cage removed, the air stream throws further. There is still some noise, so I'm guessing that the fan's blades are not optimally shaped.
A: A related question. Compare a table fan's blades versus a wind turbines blades. It is obvious that the former covers more than half the area of the disk prescribed by its circumference, but the wind turbines swept area is mostly empty air. I've heard the power of a wind turbine depends upon the swept area, which implies that the blades already cover enough area, so I would surmise the table fan is probably seriously inoptimal. Probablt the blades are as wide as they are because of pschological marketting, rather than aerodynamics. Does anyone know if this is in fact the case?
A: To the last comment - I disagree.  Wind turbines operate in a fairly different flow regime, and compare the table fan to the prop of a ship, they are about the same shape.  The key difference is moving fluid volume.  The larger blades push against the fluid more strongly, which is desirable in the fan case and in the ship case.  Wind turbines are different as the builder cares nothing about the exhaust wind.
Anyway...
1. I don't think it's 'functional' in any sense other than the fact that the motor is behind it.
2. I don't believe so and I agree with other commentators that the fan would be more simple and less noisy without them.  I'll withhold comment on the obvious safety considerations.
3. Everything dealing with pumping or pushing a fluid makes noise, and it is quite considerable.  You could remove the blades and listen to the electric motor itself and you would find it constitutes only a small fraction of the noise.  Noise minimization for fans is a major engineering topic and it's more or less impossible to reduce it.  Obviously, the faster you get the bigger of a problem it is.  I believe we're talking about turbulent flow in general, I don't think many pumps are actually laminar.  The metal bars will have an influence, but I could not say how much.
