Aeroacoustics

The modern discipline of aeroacoustics can be said to have originated with the first publication of Lighthill[2][3] in the early 1950s, when noise generation associated with the jet engine was beginning to be placed under scientific scrutiny.

Lighthill[2] rearranged the Navier–Stokes equations, which govern the flow of a compressible viscous fluid, into an inhomogeneous wave equation, thereby making a connection between fluid mechanics and acoustics. This is often called "Lighthill's analogy" because it presents a model for the acoustic field that is not, strictly speaking, based on the physics of flow-induced/generated noise, but rather on the analogy of how they might be represented through the governing equations of a compressible fluid.

The first equation of interest is the conservation of mass equation, which reads

${\displaystyle {\frac {\partial \rho }{\partial t}}+\nabla \cdot \left(\rho \mathbf {v} \right)={\frac {D\rho }{Dt}}+\rho \nabla \cdot \mathbf {v} =0,}$

where ${\displaystyle \rho }$ and ${\displaystyle \mathbf {v} }$ represent the density and velocity of the fluid, which depend on space and time, and ${\displaystyle D/Dt}$ is the substantial derivative.

Next is the conservation of momentum equation, which is given by

${\displaystyle {\rho }{\frac {\partial \mathbf {v} }{\partial t}}+{\rho (\mathbf {v} \cdot \nabla )\mathbf {v} }=-\nabla p+\nabla \cdot \sigma ,}$

where ${\displaystyle p}$ is the thermodynamic pressure, and ${\displaystyle \sigma }$ is the viscous (or traceless) part of the stress tensor from the Navier–Stokes equations.

Now, multiplying the conservation of mass equation by ${\displaystyle \mathbf {v} }$ and adding it to the conservation of momentum equation gives

${\displaystyle {\frac {\partial }{\partial t}}\left(\rho \mathbf {v} \right)+\nabla \cdot (\rho \mathbf {v} \otimes \mathbf {v} )=-\nabla p+\nabla \cdot \sigma .}$

Note that ${\displaystyle \mathbf {v} \otimes \mathbf {v} }$ is a tensor (see also tensor product). Differentiating the conservation of mass equation with respect to time, taking the divergence of the last equation and subtracting the latter from the former, we arrive at

${\displaystyle {\frac {\partial ^{2}\rho }{\partial t^{2}}}-\nabla ^{2}p+\nabla \cdot \nabla \cdot \sigma =\nabla \cdot \nabla \cdot (\rho \mathbf {v} \otimes \mathbf {v} ).}$

Subtracting ${\displaystyle c_{0}^{2}\nabla ^{2}\rho }$, where ${\displaystyle c_{0}}$ is the speed of sound in the medium in its equilibrium (or quiescent) state, from both sides of the last equation and rearranging it results in

${\displaystyle {\frac {\partial ^{2}\rho }{\partial t^{2}}}-c_{0}^{2}\nabla ^{2}\rho =\nabla \cdot \left[\nabla \cdot (\rho \mathbf {v} \otimes \mathbf {v} )-\nabla \cdot \sigma +\nabla p-c_{0}^{2}\nabla \rho \right],}$

which is equivalent to

${\displaystyle {\frac {\partial ^{2}\rho }{\partial t^{2}}}-c_{0}^{2}\nabla ^{2}\rho =(\nabla \otimes \nabla ):\left[\rho \mathbf {v} \otimes \mathbf {v} -\sigma +(p-c_{0}^{2}\rho )\mathbb {I} \right],}$

where ${\displaystyle \mathbb {I} }$ is the identity tensor, and ${\displaystyle$ :} denotes the (double) tensor contraction operator.

The above equation is the celebrated Lighthill equation of aeroacoustics. It is a wave equation with a source term on the right-hand side, i.e. an inhomogeneous wave equation. The argument of the "double-divergence operator" on the right-hand side of last equation, i.e. ${\displaystyle \rho \mathbf {v} \otimes \mathbf {v} -\sigma +(p-c_{0}^{2}\rho )\mathbb {I} }$, is the so-called Lighthill turbulence stress tensor for the acoustic field, and it is commonly denoted by ${\displaystyle T}$.

Using Einstein notation, Lighthill’s equation can be written as

${\displaystyle {\frac {\partial ^{2}\rho }{\partial t^{2}}}-c_{0}^{2}\nabla ^{2}\rho ={\frac {\partial ^{2}T_{ij}}{\partial x_{i}\partial x_{j}}},\quad (*)}$

where

${\displaystyle T_{ij}=\rho v_{i}v_{j}-\sigma _{ij}+(p-c_{0}^{2}\rho )\delta _{ij},}$

and ${\displaystyle \delta _{ij}}$ is the Kronecker delta. Each of the acoustic source terms, i.e. terms in ${\displaystyle T_{ij}}$, may play a significant role in the generation of noise depending upon flow conditions considered. ${\displaystyle \rho v_{i}v_{j}}$ describes unsteady convection of flow (or Reynolds' Stress, developed by Osborne Reynolds), ${\displaystyle \sigma _{ij}}$ describes sound generated by viscosity, and ${\displaystyle (p-c_{0}^{2}\rho )\delta _{ij}}$ describes non-linear acoustic generation processes.

In practice, it is customary to neglect the effects of viscosity on the fluid, i.e. one takes ${\displaystyle \sigma =0}$, because it is generally accepted that the effects of the latter on noise generation, in most situations, are orders of magnitude smaller than those due to the other terms. Lighthill[2] provides an in-depth discussion of this matter.

In aeroacoustic studies, both theoretical and computational efforts are made to solve for the acoustic source terms in Lighthill's equation in order to make statements regarding the relevant aerodynamic noise generation mechanisms present.

Finally, it is important to realize that Lighthill's equation is exact in the sense that no approximations of any kind have been made in its derivation.

In their classical text on fluid mechanics, Landau and Lifshitz[4] derive an aeroacoustic equation analogous to Lighthill's (i.e., an equation for sound generated by "turbulent" fluid motion), but for the incompressible flow of an inviscid fluid. The inhomogeneous wave equation that they obtain is for the pressure ${\displaystyle p}$ rather than for the density ${\displaystyle \rho }$ of the fluid. Furthermore, unlike Lighthill's equation, Landau and Lifshitz's equation is not exact; it is an approximation.

If one is to allow for approximations to be made, a simpler way (without necessarily assuming the fluid is incompressible) to obtain an approximation to Lighthill's equation is to assume that ${\displaystyle p-p_{0}=c_{0}^{2}(\rho -\rho _{0})}$, where ${\displaystyle \rho _{0}}$ and ${\displaystyle p_{0}}$ are the (characteristic) density and pressure of the fluid in its equilibrium state. Then, upon substitution the assumed relation between pressure and density into ${\displaystyle (*)\,}$ we obtain the equation (for an inviscid fluid, σ = 0)

${\displaystyle {\frac {1}{c_{0}^{2}}}{\frac {\partial ^{2}p}{\partial t^{2}}}-\nabla ^{2}p={\frac {\partial ^{2}{\tilde {T}}_{ij}}{\partial x_{i}\partial x_{j}}},\quad {\text{where}}\quad {\tilde {T}}_{ij}=\rho v_{i}v_{j}.}$

And for the case when the fluid is indeed incompressible, i.e. ${\displaystyle \rho =\rho _{0}}$ (for some positive constant ${\displaystyle \rho _{0}}$) everywhere, then we obtain exactly the equation given in Landau and Lifshitz,[4] namely

${\displaystyle {\frac {1}{c_{0}^{2}}}{\frac {\partial ^{2}p}{\partial t^{2}}}-\nabla ^{2}p=\rho _{0}{\frac {\partial ^{2}{\hat {T}}_{ij}}{\partial x_{i}\partial x_{j}}},\quad {\text{where}}\quad {\hat {T}}_{ij}=v_{i}v_{j}.}$

A similar approximation [in the context of equation ${\displaystyle (*)\,}$], namely ${\displaystyle T\approx \rho _{0}{\hat {T}}}$, is suggested by Lighthill[2] [see Eq. (7) in the latter paper].

Of course, one might wonder whether we are justified in assuming that ${\displaystyle p-p_{0}=c_{0}^{2}(\rho -\rho _{0})}$. The answer is affirmative, if the flow satisfies certain basic assumptions. In particular, if ${\displaystyle \rho \ll \rho _{0}}$ and ${\displaystyle p\ll p_{0}}$, then the assumed relation follows directly from the linear theory of sound waves (see, e.g., the linearized Euler equations and the acoustic wave equation). In fact, the approximate relation between ${\displaystyle p}$ and ${\displaystyle \rho }$ that we assumed is just a linear approximation to the generic barotropic equation of state of the fluid.

However, even after the above deliberations, it is still not clear whether one is justified in using an inherently linear relation to simplify a nonlinear wave equation. Nevertheless, it is a very common practice in nonlinear acoustics as the textbooks on the subject show: e.g., Naugolnykh and Ostrovsky[5] and Hamilton and Morfey.[6]

• Acoustic theory
• Aeolian harp
• Computational aeroacoustics

• M. J. Lighthill, "On Sound Generated Aerodynamically. I. General Theory," Proc. R. Soc. Lond. A 211 (1952) pp. 564–587. This article on JSTOR.
• M. J. Lighthill, "On Sound Generated Aerodynamically. II. Turbulence as a Source of Sound," Proc. R. Soc. Lond. A 222 (1954) pp. 1–32. This article on JSTOR.
• L. D. Landau and E. M. Lifshitz, Fluid Mechanics 2ed., Course of Theoretical Physics vol. 6, Butterworth-Heinemann (1987) §75. ISBN 0-7506-2767-0, Preview from Amazon.
• K. Naugolnykh and L. Ostrovsky, Nonlinear Wave Processes in Acoustics, Cambridge Texts in Applied Mathematics vol. 9, Cambridge University Press (1998) chap. 1. ISBN 0-521-39984-X, Preview from Google.
• M. F. Hamilton and C. L. Morfey, "Model Equations," Nonlinear Acoustics, eds. M. F. Hamilton and D. T. Blackstock, Academic Press (1998) chap. 3. ISBN 0-12-321860-8, Preview from Google.
• Aeroacoustics at the University of Mississippi
• Aeroacoustics at the University of Leuven
• International Journal of Aeroacoustics Archived 2005-10-30 at the Wayback Machine
• Examples in Aeroacoustics from NASA Archived 2016-03-04 at the Wayback Machine
• Aeroacoustics.info