Scientists Have Finally Explained Why Helicopters Are So Loud
It goes without saying that helicopters are really loud. What’s trickier is explaining why, and finding a way to make the explanation visible to the naked eye.
But researchers from the German Aerospace Center (DLR) are the first to provide a visual explanation of the cause for all that noise.
more…
Without looking at the link it has something to do with the speed of the tips.
Peak Warming Man said:
Without looking at the link it has something to do with the speed of the tips.
Are you in the right thread?
Silly me thought it was due to the tip of the rotor breaking the sound barrier.
jjjust moi said:
Silly me thought it was due to the tip of the rotor breaking the sound barrier.
Me too.
bob(from black rock) said:
jjjust moi said:
Silly me thought it was due to the tip of the rotor breaking the sound barrier.
Me too.
And specifically of the advanceing rotor tip .
now that they know the cause, someone might discover how to make them quieter
CrazyNeutrino said:
now that they know the cause, someone might discover how to make them quieter
There’s other causes. Think of the sound a UH-1 made/makes. A lot of that is due to that 21-inch wide rotor blade swinging over the wide turbine exhaust
CrazyNeutrino said:
now that they know the cause, someone might discover how to make them quieter
CSIRO colleagues did that with fans for a Warman Pump project. They found that the cause of the sound was vortex shedding from the rear edge of the blades, and by adjusting the chord length and leading edge they could make almost all of the sound go away. I could probably dig out the mathematical equation for sound generation from rotor blades.
mollwollfumble said:
CrazyNeutrino said:
now that they know the cause, someone might discover how to make them quieter
CSIRO colleagues did that with fans for a Warman Pump project. They found that the cause of the sound was vortex shedding from the rear edge of the blades, and by adjusting the chord length and leading edge they could make almost all of the sound go away. I could probably dig out the mathematical equation for sound generation from rotor blades.
I would appreciate that.
Postpocelipse said:
I would appreciate that.
Could probably find a use for it in the slap dancing thread. ;)
Postpocelipse said:
mollwollfumble said:
CrazyNeutrino said:
now that they know the cause, someone might discover how to make them quieter
CSIRO colleagues did that with fans for a Warman Pump project. They found that the cause of the sound was vortex shedding from the rear edge of the blades, and by adjusting the chord length and leading edge they could make almost all of the sound go away. I could probably dig out the mathematical equation for sound generation from rotor blades.
I would appreciate that.
Darn, now I’ll have to find it.
The intensity of the radiated sound in the far field is proportional to the square of the fl uctuating pressure (eq. 7 of http://ctr.stanford.edu/ResBriefs95/lilley)
The original work was done by Hourigan and Thompson and was commercial in confidence. These journal articles relate closely to that work:
Aerodynamic sources of acoustic resonance in a duct with baffles
Self-sustained oscillations in flows around long blunt plates
Flow Induced Acoustic Resonances for a Bluff Body in a Duct: A Numerical Study
Cheers Moll. Would this area be applicable in fluid dynamics?
Postpocelipse said:
Cheers Moll. Would this area be applicable in fluid dynamics?
Yes.
mollwollfumble said:
Postpocelipse said:
Cheers Moll. Would this area be applicable in fluid dynamics?
Yes.
Helps not to assume at the beginning. :)
While in hospital a few years ago, post surgery I occupied a room in the ICU
(Intensive Care Unit) situated immediately above the Emergency Room Entrance
and facing the hospital’s emergency heliport and quickly learned that there’s such
a thing as a “whisper mode” that was surprisingly effective.
You would hear the ‘copper coming from far off… but as it approached the hospital
the sound would become muffled with the effect of having the sound “Sucked UP”
as opposed to “Blown DOWN”… it’s difficult to put into words.. but it was like having
your “ears ring” when the pressure changes… and the only evidence you’d have
of the medevac chopper landing and taking off again, within 10 meters distance,
was a low hum & leaves & paper swirling passed my window. AWESOME!
Anyway.. this was almost 20 years ago.. so this technology has been available,
if not widely used, for a relatively long time.
As for VISUALIZING blade wash.. I’d think SMOKE would work quite well. (???)