[Beowulf] Desktop fan reccommendation

[Vincent Diepeveen]

On Jun 6, 2012, at 7:28 PM, Lux, Jim (337C) wrote:

> 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.

Exhausting for PC's is most effective for what you probably call 'low  
airspeed' fans when i measured some years ago with a dual k7 machine.
It was far more effective than blowing in some air.

The ballgame changes when you blow in at some massive mercilious CFM  
as getting the lower temperature sooner to the cpu is going to make
  a difference then. This is not so interesting for computers though.

I blew in with far over moped sounds using some delta fans.

Yet it's already cooled really well by then so not such an  
interesting difference.

At that huge blow in rate, it was very effective indeed, yet that  
difference i could only measure when total overkilling the machine
with those fans.

Actually the machines thin aluminium started to bend under that huge  
airpressure, but i figured that out only long after the
experiment, but that's for another time to discuss :)

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

When i googled on centrifugal fans, i saw huge prices in the hundreds  
of dollars.

Would mean the centrifugal fans are more expensive than the entire  
cluster which seems a tad odd.

So it's gonna be the cheapskate cardboard solution with some duct  
tape and glue and relative cheap fans.



> 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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