Aeroacoustics

Aeroacoustics

1. Aircraft Noise

   • Propulsive
     • Take-off & Landing
   • Airframe
     • Landing

2. Phenomenon of Sound

   • Definition
     • Longitudinal wave motion in an elastic medium
   • Analysis
     • Assumptions
       • Viscosity is unimportant.
         • Pressure represents a far bigger stress field than that induced by the viscosity at frequencies of practical interest.
       • Pressure fluctuations are much smaller than atmospheric pressure.
         • Linearised Flow Equations
           • Mass/Continuity
             • Particle velocity
               • +x direction
               • -x direction
           • Momentum
       • Wave propagation speed is so high that the motion is isentropic.
     • Wave Equation
   • Measuring
     • Sound Pressure Level (SPL)
     • A-Weighting

3. 1D Sound Waves (Plane-Waves)

   • Disturbances which propagate to the right and left respectively, with constant speed c0 and constant shape.
   • Pipes Acoustics
     • Dominated by 1D low frequency acoustics.
     • Pipe Ends/Area Changes
       • Apply linearised mass + pressure continuity.
     • Convecting Flow

4. Acoustic Resonances & Instabilities

   • Acoustic Resonance Devices:
     • Organ Pipe Modes
       • Frequencies at which BC's sustain standing waves.
     • Helmholtz Resonator
       • Air in volume compresses like a spring, air in neck acts like an oscillating mass.
       • Large inflow/outflow for small incident acoustic wave.
   • Acoustic Instabilities
     • Induced by 2-way interaction between acoustic waves and another phenomenon.
       • Thermoacoustic Instabilities:
         • Acoustics + Unsteady Heat Release
       • Cavity Resonances:
         • Acoustics + Shear Layer Disturbances

5. 3-D Sound Waves & Sources of Sound

   • 3-D Sound Waves
     • Spherically Symmetric
       • Rule out incoming wave.
   • Sound Sources
     • Non-zero term on RHS of the Wave Equation
       • Sound Field
         • q is zero
       • Source Region
         • q is non-zero
     • Solving directly for pressure fluctuation
       • Green's Function
         • Multi-dimensional impulse response.
         • Solution of the PDE with the source concentrated at a point, both in space & time.
     • Monopole Source
       • Spherically symmetric sound field.
     • Dipole Source
       • Angular dependence
         • Cancellation - Dies out more rapidly than monopole.

6. Lighthill's Theory & Jet Noise

   • Lighthill's Theory
     • Lighthill Stress Tensor
   • Jet Noise
     • Lighthill's Eighth Power Law
       • Sound power scales as the jet velocity to the power 8 (explains drive to reduce aero-engine jet exit velocities via ever-increasing bypass ratios).

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