Noise due to flow through orifice 3

[jonasinc]

We have been testing a piezoelectric transducer connected to a small steel waveguide in close proximity to a flow orifice (~ 1 mm). The flow through the orifice should be subsonic since the pressure ratio is only about 0.4 (24 psig inlet, 14.7 psig outlet). However, we have encountered a phonomenon where the acoustic noise level increases by over 10X when the Reynolds number in the orifice goes above 1000. From 16 to 24 psig the noise level is at 1, then abover 24 psig, the noise increases to 10 or higher. I could understand if there was critical flow that there would be small sonic shockwaves, but that should not occur until approx. 30 psig.

Does anyone know why this occurs? It is just turbulence? We have tested several orifice sizes. While the increase in noise occurs at a different differential pressure for each one, the Reynolds number in the orifice is the same (!1000) at the pressure ratio where the noise increases.

Thanks,
Lee

 

[quark]

I am not knowledgeable about this nor I have experience. But there is a good paper on Fundamentals of Aerodynamic Noise in Control Valves at the link below.

http://www.fisherregulators.com/technicalmonographs/various/

The discussion is very good.

Regards,

 

[monkeydog]

Are you pressures correct?
Do you really mean 24 psig inlet and 14.7 psig outlet?
Can the 14.7 be psia?
If so, then you are going sonic. The 24 psig would be 38.7 psia to the 14.7 psia.

 

[jonasinc]

Actually you are partially correct. The pressures should all be psia. But it is still not critical flow.

 

[monkeydog]

jonasinc
Partially correct?
Hmmm not bad considering the information given.
;-)

 

[vanstoja]

It seems unlikely that transition from laminar to turbulent airflow would produce 10 to 1 increases in airborne (or structureborne ??) noise levels. Also your Reynolds Number appears to be too low for turbulence. The more likely cause of acoustic amplification of 10 to 1 (equals 20dB in sound pressure level) is aeroacoustic resonance at the higher flow Mach number, M. It may be a standing wave resonance in the waveguide chamber upstream of the orifice or within the orifice if its length is sufficient. Ingard,U. & Singhal,1974, "Effect of Flow on the Acoustic Resonances of an Open-Ended Duct", J.Acoust.Soc.Amer. Vol.58,pp.788-792 show 500 Hz spaced resonance peaks in a 0.75in. diameter by 12 in. long duct at M=0.27 that are 10 to 20 dB higher than spectra at M=0.08 and 0.55. Structural amplification of the waveguide walls is another possible contributor if the acoustic transducer is mounted on the waveguide wall rather than being in the airstream.

 

[vanstoja]

Another possible aeroacoustic resonance occurring at Reynolds Numbers above 1000 is associated with thin shear layer lip instability of the jet exiting the orifice.Fig.8.2 in Lucas,etal,1997,"Handbook of Acoustic Characteristics of Turbomachinery Cavities",ASME Press shows shear layer instability starting at Re values of about 1500 with a Strouhal Number given by the equation St=0.0156*Re^0.5. The St value at Re=1500 is 0.604.
The peak frequency at any Re can be determined from the equation f=St*U/d where U is jet velocity and d is orifice diameter. If you get a noise spectrum you can check for jet shear layer instability peaks. Another jet noise source is the so-called "preferred" mode, a downstream columnar resonance present at all Reynolds Numbers with a Strouhal Number between 0.25 and 0.5 which yields lower frequencies than shear layer instability peaks.

 

[vanstoja]

Considering jet flow shear layer instability (SLI) to be the most likely cause of your 10 to 1 noise level increases above Reynolds Numbers of 1000, I ran peak frequency calculations for Re=1500 (the lower limit of SLI) at two air density conditions, ie., Standard(no pressure effects,rho=0.0753lb/ft^3)) and with average 68 degree F air pressure in the orifice for a 24 to 0 psig pressure drop(rho=0.13716lb/ft^3). For jet layer instability Strouhal frequencies I got 93.9Hz and 51.55Hz, respectively. These are stated in Lucas etal to be the upper frequency limits for both jet shear layer instability and preferred columnar modes of the jet. The later would then range between 39-78Hz, and 21-43Hz if also present. Frequencies from 21 to 94 Hz would be below the threshhold of audibility for noise intensity levels(and, nearly, sound pressure levels) from about 70 to 40dB, respectively. Consequently, the jet tones from both shear layer instability and columnar modes may be inaudible at these relatively low airborne noise frequencies and frequency-amplitude spectra are needed to confirm that they are involved.

 

[athomas236]

You could always try "Reduced - Noise Gas Flow - Design Guide" from the NASA Glen Research Centre. It has a section on noise from orifices

athomas236