Re: turbofan noise

From:         crossley@imap2.asu.edu
Organization: Arizona State University
Date:         02 Dec 94 02:48:59 
References:   1
Followups:    1
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James B Nichols (jbn@apple.com) wrote:
: I was wondering, what is the source of that pronounced, droning/growling
: sound one hears when a widebody is climbing?  It seems to start about 30
: seconds after liftoff and lasts for about 10 minutes.  It is particularly
: noticable on the DC-10s...

High-bypass turbofans emit noise from two different sources: jet noise and
fan loading noise.

The core of the engine exhausts a high-velocity jet that produces "jet 
noise"; this noise is created in the same manner as turbojet on 
low-bypass turbofan engines on older aircraft (eg Boeing 727 and
737, or DC-9) and has a "roaring", low frequency sound.  This sound
is created as the high-velocity flow exchanges momentum with the 
surrounding atmosphere.  Newer, high-bypass turbofan engines mix this 
high-velocity core exhaust with the surrounding slower moving fan exhaust.
The net effect is that the jet noise component of these high-bypass 
turbofan engines is reduced, so that the "fan loading noise" dominates.

The fan of a high-bypass turbofan engine has a relatively high loading;
it produces a significant amount of thrust per unit area of the fan. 
Because of this, each blade in the fan (which has an airfoil shape) is
operating at a high lift coefficient.  Under these conditions, pressure on
the front surface of the fan blade (upper surface of the airfoil) is much
lower than the pressure on the rear of the blade.  This difference in 
pressure must be resolved at the trailing edge of the blade.  Resolving this
difference results in a pressure disturbance; this pressure disturbance is
the source of the fan noise.  This fan noise is often described as having
a "siren" or "droning" sound.  Because these high-bypass turbofans have a
high fan loading and a reduced jet noise, the sound heard by an observer is
dominated by the fan noise.

Here at ASU, we are almost directly under the take-off and landing paths
for Phoenix Sky Harbor airport.  I have noticed that the engines on the
B-757, B-767, DC-10, MD-11 and Airbus 320 aircraft all have this droning
sound.  It is easily contrasted to the sound of B-727, B-737 and DC-9 
aircraft.

I hope this wordy description helps.

--
- William A. Crossley, graduate student
  Department of Mechanical and Aerospace Engineering                       
  Arizona State University, Tempe, Arizona 85287-6016