Directivity and sound power radiated by a source under a boundary layer

This paper considers the radiation of sound from compact 2D or 3D sources located in an otherwise rigid wall, bounding a region of fluid flowing parallel to the wall. The sound radiation problem is modeled using a wavenumber decomposition. Numerical results show how the radiated power and directivity depend on the free-stream flow Mach number and the thickness of the boundary layer adjacent to the wall. The numerical model is validated by comparing the sound power and directivity obtained in the limiting case of a thin boundary layer with that obtained from an alternative analytical model in which the flow is uniform and there is slip at the boundary

Modelling of turbulent jets and wall layers: extensions of Lighthill's acoustic analogy with application to computational aeroacoustics

Two extensions to Lighthill’s aeroacoustic analogy are presented. First, equivalent sources due to initial conditions are derived that supplement those due to boundary conditions, as given by Ffowcs Williams & Hawkings. The resulting exact inhomogeneous wave equation is then reformulated with pressure rather than density as the wave variable, and the right-hand side is rearranged using the energy equation with no additional assumptions. Applications to computational aeroacoustics are discussed, and illustrated with examples based on 2D and 3D simulations

The design, implementation and evaluation of mass conferencing

There have been attempts to classify and analyse the approaches and techniques of using videoconferencing for teaching and learning. Most classifications include the use of videoconferencing techniques to support lecture‐style delivery to large audiences, or what might be referred to as ‘mass conferencing’. This is often dismissed by sceptics as another gimmick: the real thing is better, or it may be viewed as simply just another didactic approach with little to commend it either in the form of communication or in pedagogical terms. However, the key element in its use is the context within which the mass conferencing is being applied Whatever videoconferencing approaches are employed, it is our view that their successful implementation implies both a clearly defined structure and an operational template. Thus, this paper underlines some of the processes which we have used in mass conferencing. We then evaluate the outcomes, and identify, some themes to be incorporated in successful mass conferencing, including the key factors involved in successful delivery, namely in the preparation, activity, and evaluation stages. In operational terms, the introduction of an external element, beyond the control of course tutors, has highlighted many organizational, pedagogical and technical questions, some of which we address

Nonlinear mechanisms of sound generation in a perturbed parallel jet flow

An initial value problem with relevance to jet noise is investigated. A plane parallel jet flow is subjected to a spatially localized initial disturbance and is then left to evolve according to the two-dimensional compressible Navier–Stokes equations. The hydrodynamic response is in the form of a convecting vortex packet. The Ffowcs Williams–Hawkings approach is formulated in the time domain and used to extrapolate from the simulated near field to the acoustic far field. The predominant downstream sound radiation comes from an early stage of nonlinear development of the vortex packet. Two simplified models to account for the radiation are introduced, based on nonlinear mode interactions on a prescribed base flow. The first uses two sets of linearized Euler equations, coupled via the inviscid Lilley–Goldstein acoustic analogy. This formulation separates the linear sound field from the sound field driven by nonlinear interactions; qualitative agreement of the latter with the Navier–Stokes computations demonstrates the importance of nonlinear interactions. The second model uses combinations of linear inviscid eigenmodes to drive the sound field, which allows extraction of the dominant mode interactions responsible for the observed radiation pattern. The results indicate that a difference-wavenumber nonlinear interaction mechanism dominates sound radiation from subsonic instability modes in shear flows

On the extrapolation of acoustic waves from flow simulations with vortical out flow

Feri Farassat was one of the pioneers of the use of the Ffowcs Williams & Hawkings formulation of Lighthill's acoustic analogy as a way to extrapolate radiated waves from simulations of unsteady flows. Current computational limits mean that volume source terms are often neglected, causing inaccurate acoustical predictions when entropy fluctuations or vorticity pass across the extrapolation surface. The derivation of the Ffowcs Williams-Hawkings equation is modified to allow the equivalent surface sources to be distributed over a transition layer of finite thickness rather than being confined to a single layer, in order to reduce the effect of vorticity exiting the computational domain

Extensions of Lighthill's acoustic analogy with application to computational aeroacoustics

Lighthill's aeroacoustic analogy is formulated for bounded domains in a general way that allows pressure-based alternatives to the fluid density as wave variable. The advantage relative to the standard version (Ffowcs Williams & Hawkings 1969 Phil. Trans. R. Soc. A 264, 321–342) is that the equivalent surface source terms needed for boundary value problems do not involve the local density. Difficulties encountered in computational aeroacoustics with standard wave extrapolation procedures, due to advection of density inhomogeneities across the control surface, are thereby avoided. Likewise, in initial-value problems, the equivalent volume source terms that represent initial conditions do not involve the density either. The paper ends with an extension to parallel shear flows, in which a modified aeroacoustic analogy due to Goldstein (Goldstein 2001 J. Fluid Mech. 443, 231–236) is formulated for bounded domains using a similar windowed-variable approach. The results provide a basis for acoustic wave extrapolation from jets and boundary layers, where the control surface cuts through a sheared mean flow

Sound radiation from exponentially growing and decaying surface waves

A simplified model problem is used to illustrate some of the parameters controlling the radiation of sound into an ambient medium due to the growth and subsequent decay of subsonic travelling waves, such as may occur via non-linear interactions in turbulent free shear flows. It is shown that substantial sound may be generated by apparently subsonic modes as a result of their growth and decay characteristics. Low frequency modes that saturate over a short timescale are the most effective radiators