Proudman [ 293], using Lighthill's acoustic analogy, derived a formula for acoustic power generated by isotropic turbulence without mean flow. More recently, Lilley [ 210] rederived the formula by accounting for the retarded time difference which was neglected in Proudman's original derivation. Both derivations yield acoustic power due to unit volume of isotropic turbulence (in W/m ) as
where and are the turbulence velocity and length scales, respectively, and is the speed of sound. in Equation 21.2-10 is a model constant. In terms of and , Equation 21.2-10 can be rewritten as
FLUENT can also report the acoustic power in dB, which is computed from
where is the reference acoustic power ( by default).
The Proudman's formula gives an approximate measure of the local contribution to total acoustic power per unit volume in a given turbulence field. Proper caution, however, should be taken when interpreting the results in view of the assumptions made in the derivation, such as high Reynolds number, small Mach number, isotropy of turbulence, and zero mean motion.
The Jet Noise Source Model
This source model for axisymmetric jets is based on the works of Goldstein [ 125] who modified the model originally proposed by Ribner [ 305] to better account for anisotropy of turbulence in axisymmetric turbulent jets.
In Goldstein's model, the total acoustic power emitted by the unit volume of a turbulent jet is computed from
are the radial and angular coordinates of the receiver location, and
is the directional acoustic intensity per unit volume of a jet defined by
21.2-15 is the modified convection factor defined by
The remaining parameters are defined as
FLUENT reports the acoustic power both in the dimensional units ( ) and in dB computed from
The Boundary Layer Noise Source Model
Far-field sound generated by turbulent boundary layer flow over a solid body at low Mach numbers is often of practical interest. The Curle's integral [ 71] based on acoustic analogy can be used to approximate the local contribution from the body surface to the total acoustic power. To that end, one can start with the Curle's integral
Using this, the sound intensity in the far field can then be approximated by
The total acoustic power emitted from the entire body surface can be computed from
FLUENT reports the acoustic surface power defined by Equation 21.2-31 both in physical ( ) and dB units.
Source Terms in the Linearized Euler Equations
The linearized Euler equations (LEE) can be derived from the Navier-Stokes equations starting from decompositions of the flow variables into mean, turbulent, and acoustic components, and by assuming that the acoustic components are much smaller than the mean and turbulent components. The resulting linearized Euler equations for the acoustic velocity components can be written as
The right side of Equation 21.2-32 can be considered as effective source terms responsible for sound generation. Among them, the first three terms involving turbulence are the main contributors. The first two terms denoted by are often referred to as "shear-noise" source terms, since they involve the mean shear. The third term denoted by is often called the "self-noise" source term, for it involves turbulent velocity components only.
The turbulent velocity field needed to compute the LEE source terms is obtained using the method of stochastic noise generation and radiation (SNGR) [ 30]. In this method, the turbulent velocity field and its derivatives are computed from a sum of Fourier modes.
Note that the source terms in the LEE are vector quantities, having two or three components depending on the dimension of the problem at hand.
Source Terms in Lilley's Equation
Lilley's equation is a third-order wave equation that can be derived by combining the conservation of mass and momentum of compressible fluids. When the viscous terms are omitted, it can be written in the following form:
Lilley's equation can be linearized about the underlying steady flow as
21.2-35 into the source term of Equation
21.2-34, we have
The resulting source terms in Equation 21.2-36 are evaluated using the mean velocity field and the turbulent (fluctuating) velocity components synthesized by the SNGR method. As with the LEE source terms, the source terms in Equation 21.2-36 are grouped depending on whether the mean velocity gradients are involved ( shear noise or self noise), and reported separately in FLUENT.