[Beowulf] Desktop fan reccommendation

[Lux, Jim (337C)]

I'm not sure that the acoustic noise from fans is from the actual aerodynamic noises (e.g. not like a jet engine, or the pressure/shock waves from the blades).  The blade tips are probably operating in a low speed incompressible flow regime.  

For low speed fans typical of this application, noise is much more from incidental flow behavior and mechanical transmission (e.g. the airflow from the blade hitting a stationary object and creating a pulsed flow which then hits the package side and makes it vibrate).  There's also surprisingly high noise in some fans from the DC brushless motor (a cheap controller uses square edge pulses to the windings, so the torque has pulses, which then are mechanically transmitted to the housing.. a nice "whine" source for a little 6000 rpm motor with a lot of poles)

Actually, not all fans are set up to suck out of the box.    Blowing in works better for heat transfer (you're pushing cold dense air, rather than sucking warm undense air)..  Most test equipment uses the "suck in through a filter and pressurize the box" design approach.  I think PCs evolved the other way because the single fan was in the power supply, and you didn't want to blow hot air, preheated by the power supply, through the rest of the system.   So it is set up as an "exhaust from PS box" fan.

And a lot of higher performance PCs (like the Dell sitting on my desk) use centrifugal fans (with variable speed, to boot)

Jim Lux

-----Original Message-----
From: beowulf-bounces at beowulf.org [mailto:beowulf-bounces at beowulf.org] On Behalf Of Daniel Pfenniger
Sent: Wednesday, June 06, 2012 10:33 AM
To: holway at th.physik.uni-frankfurt.de
Cc: Beowulf Mailing List
Subject: Re: [Beowulf] Desktop fan reccommendation

holway at th.physik.uni-frankfurt.de wrote:
>
>> The Dyson bladeless and silent fans are based om a different 
>> principle, a cylindrical thin air layer carries along the inner air 
>> column, the air flow is then laminar (http://www.dyson.com/store/fans.asp).
>
> Which is not good if your trying to cool stuff.....

Well, the fans we are discussing expel air *out* of the box so the heat carried by the air doesn't care about the downstream laminar or turbulent state of the airflow.

However noise generation does depend on the airflow state, since the acoustic power is proportional to the 8th power of the turbulence eddy speed (Lighthill 1952, 1954).  This is why jet planes are noisy, as their turbulence is almost sonic.  The airplane or helicopter propeller tips, or the fan blade ends move closer to the sound speed, so most of the sound is generated there.

The conclusion is that to keep a computer quiet one has advantage to use large fans rotating at low speed.  For the same air/heat output one gets much less noise, especially if the airflow is laminar.


	Dan






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