flow related vibrations 1

[Mpino]

I need some information about vibrations generated by flows in a duct. The aim is to identify diferents spectra for diferent flows, like water, gas, etc.
Thanks

 

[dgallagher]

Most flow turbulance spectra will show random low frequency vibration with raised noise floor, typically below 40 Hz.

 

[Rob45]

Can't point you toward a particular reference, but I can give you a couple of general principles:
You'll have two things to look at;
First, there's the flow noise/vibration "driver". What are its' characteristics? How many blades does it have, if it's a fan, how fast does it turn? This gives you the blade passing frequency.
If you have pump, how many cylinders does it have? Or vanes, if it's vane-type, and how fast does it turn?
In short, how many pulses are generated per second?
Next, you need to know the duct response characteristics:

How long are the duct runs between impedance mismatches - things like plenums (plena?) outlets or inlets, diffusers, etc. ? Then knowing the speed of sound in the medium you're dealing with - and correcting it for any temperature or pressue variation - you can calculate the system acoustic response according to half-wave principles. Calculating the first several acoustic modes should give you an indication of whether or not any of these lines up with the "driver" frequency input to your system by the fan or pump. If this seems likely, you'd want to modify the system characteristics in some way, like changing the pump or fan speed.
A last thing to look at regarding duct characteristics is the wall of the duct: if it has flat surfaces like for example an HVAC duct, youwant to avoid driving those surfaces at resonance (any of the lower-freqquency modes) by the fan input frequency. If either wall resonance or piping acoustic resonance is a problem, this can be addressed by using a piping or duct silencer or muffler to reduce the input energy. For specific problem frequencies, a Helmholtz tuner can be incorporated into the ducting or piping to cancel that specific frequency.
I realize this is just a starting point; hope it helps.
- R

 

[rstates]

Fans naturally surge at hi pressure dp - the fundamental is blade pass. The duct will organ piipe at its fundamental and harmonics. And a flexible duct will oil can. If any two of these are accidentally at the same frq, look out!

Best thing is FFT the noise, and pick out the big offenders, look for a frequency matched mechanism, and you're done. HVAC ducts are notorious for oil canning.

 

[electricpete]

what does oil can mean?

 

[GregLocock]

Generally it is the poor vibration response of a large simply supported flat sheet of steel. Specifically it is the non-linear behaviour seen when a point load is applied to such a sheet. I think the guy talking about ducts is more concerned by (1) than (2). (2) is an issue on the floors of cars for instance, and flat sided oil cans. Cheers

Greg Locock

 

[electricpete]

So it's when the flat part of the can "pops"?

 

[GregLocock]

Yup Cheers

Greg Locock

 

[Rob45]

"What does 'oil-can' mean?"
LOL!
"Once upon a time, engine oil came in metal cans..."