Typically in an exhaust manifold/downpipe you'll start to hear Mach noise at around 0.6-0.8 M bulk flow rate, as there is enough local acceleration to get you into trouble.
In an HVAC system you can hear flow noise at much slower speeds than that, .05M or less.
Cheers
Greg Locock
I rarely exceed 1.79 x 10^12 furlongs per fortnight
Thanks, Greg. I am talking about air intake system, and turbocharger intake noise. Would you expand a little more why different systems have various Mach noise? Frequency dependent?
Well to be honest that is far too big a subject to cover in a post. or even a book. Basically it comes down to how the noise is generated, and then how much is filtered out or amplified before anyone hears it.
Cheers
Greg Locock
I rarely exceed 1.79 x 10^12 furlongs per fortnight
So you are talking about noise created by turbulence around the compressor blades then, not regular "flow noise" (e.g. that you would get from an exhaust). This is not really connected to flow rate, more likely where on the compressor map you are operating. If you are near or over the stall line, expect whoosh noise.
In that particular case, any turbulence upstream of the blades is amplified by the pressure change process, in a non linear fashion. So in general if noise is your concern, spend more time refining the intake duct than the exhaust duct around a bladed impeller.
Cheers
Greg Locock
I rarely exceed 1.79 x 10^12 furlongs per fortnight
