Aeroacoustics

Aeroacoustics investigates the aerodynamic generation of sound.

Modern aeroacoustics science was initiated in the 1950’s by Sir James Lighthill who derived a theory for the estimation of the intensity of sound that radiates from a turbulent flow. Before this, flow-generated noise studies were focused on the relation between the frequency of the fluid fluctuations and the emitted sound.

The frequency of the aeolian tone was mathematically related to flow parameters by Strouhal who, as a result of quantitative observations, introduced the non-dimensional frequency - the Strouhal number. Lord Rayleigh related the Strouhal number to the flow Reynolds number and further recognized that the direction of most intense sound emission coincides with the direction of the fluctuating force on the wire.

In 1952, Lighthill established the theoretical background that is generally referred to when investigating aerodynamic noise [69]. Lighthill first introduced the concept of aeroacoustic analogy which consists of replacing the actual flow field responsible for generating noise with an equivalent system of noise sources. The noise sources act on a uniform stagnant fluid that is governed using standard acoustic propagation equations. The aerodynamic characterization of the sources then becomes the main issue in noise prediction.

In 1955, Curle [51] extended the concepts that Lighthill developed to include the effect of flow-body interaction on sound generation. In 1969, Ffowcs Williams and Hawkings [58] extended the Lighthill analogy to account for arbitrary surface motion. This formulation was used for noise prediction in rotor blades aerodynamics, such as in the helicopter and turbine industries [46] and [54].

The main mechanisms of sound generation in the presence of solid structures can be classified as [83]:

Vortex shedding noise
Vorticity that is released from a bluff body in a flow. The time varying circulation on the body due to vortex shedding induces a fluctuating force on the body itself which is transferred to the fluid and propagates as sound.
Turbulence-structure interaction noise
Vortical structures impinging on a solid surface generate local pressure fluctuations on the body surface which feed the acoustic far field.
Trailing edge noise
This type of noise is important for all rotating blade technologies, due to the interaction of the boundary layers instabilities with the surface edges.

In current engineering practice, computations of sheared and free shear flows using the unsteady compressible Navier-Stokes equations are performed routinely. It remains challenging, however, to resolve the acoustic field of those simulations whose acoustic energy is many orders of magnitude smaller than the energy of the hydrodynamic field. There are four sources of difficulty in directly resolving the acoustic waves using Navier-Stokes computations (from [38]):

The acoustic-field is larger than the flow-field.
The acoustic-field has a smaller energy content than the flow field.
The numerical discretization can act as a more significant source of sound than the simulated flow-field.
The difficulty of imposing free space boundary conditions appropriate for acoustics in the far-field, at an artificial computational boundary that is positioned at a finite distance away from the source region.

Artificial boundaries must be appropriate to ensure non-reflection of acoustic waves, but also in certain regions to provide inflow and outflow of the aerodynamic field.

Simcenter STAR-CCM+ offers a choice of models for simulating acoustics in liquids and gases.

To date, the primary focus has been on aeroacoustics, the study of noise from flows of gases as they interact with surfaces. However, Simcenter STAR-CCM+ also allows you to model hydroacoustics, the study of noise transmission in water.

The following general types of acoustics models are available in Simcenter STAR-CCM+:

Note	Use the double precision version of Simcenter STAR-CCM+ for all aeroacoustic analyses. For more information, refer to the aeroacoustics guidelines section.