4,068 research outputs found
Extensions of Lighthill's acoustic analogy with application to computational aeroacoustics
No full textLighthill'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
Modelling of turbulent jets and wall layers: extensions of Lighthill's acoustic analogy with application to computational aeroacoustics
No full textTwo 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
On the extrapolation of acoustic waves from flow simulations with vortical out flow
No full textFeri 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
Acoustics of thermoviscous fluids: the Kirchhoff–Helmholtz representation in generalized form
No full textThe Kirchhoff-Helmholtz representation of linear acoustics is generalized to thermoviscous fluids, by deriving separate bounded-region equations for the acoustic, entropy, and vorticity modes in a uniform fluid at rest. For the acoustic and entropy modes we introduce modal variables in terms of pressure and entropy perturbations, and develop asymptotic approximations to the mode equations that are valid to specified orders in two thermoviscous parameters. The introduction of spatial windowing for the mode variables leads to surface source and dipole distributions as a way of representing boundary conditions for each mode. For the acoustic mode the boundary source distribution is expressible in terms of the fluid normal velocity, the normal heat flux, and the vector ω × n ̂ , where ω is the vorticity on the boundary and n ̂ is the unit normal; only the first of these is present in the usual lossless-fluid version of the Kirchhoff-Helmholtz representation. Use of the generalized thermoviscous representation to project exterior sound fields from surface data, where the data may contain contributions from all three linear modes, is shown to be robust to cross-modal contamination. The asymptotic limitations of the thermoviscous modal equations are discussed in an appendix.</p
On the modal distribution for circular and annular ducts
No full textMany important areas of aircraft engine noise concern regimes where frequencies are sufficiently high that corresponding approximations can be made. At such frequencies modal populations are high and approximate methods based on modal averaging can be applied. In this paper the normalised variance of the deviation of the modecount from its average value is examined for circular ducts. Periodic orbit theory, first developed in quantum physics, is used to provide a value for this normalised variance is found and compared with numerical predictions. The implications for concentric and eccentric annular
ducts are examined. It is shown that as eccentricity is varied eigenvalues show ‘avoided crossings’
Marriage record of Coates, John W. and Wright, Millie
No full textMarriage license for John W. Coates and Millie Wright. C.L. Jackson was the Justice of the Peace
FIGURE 8 in A new forest dwelling button spider from South Africa (Araneae, Theridiidae, Latrodectus)
No full textFIGURE 8. Maximum Likelihood phylogram with bootstrap and posterior probability support overlaid. Values in brackets are GenBank accession numbers or BoLD identifiers. Where more than one individual from a species was included only the species name appears on the phylogram. The GenBank accession number/BoLD identifier can be found in Supplementary Material Table S1.Published as part of Wright, B.M.O.G., Wright, C. D., Sole, C.L., Lyle, R., Tippett, R., Sholto-Douglas, C., Verburgt, L. & Engelbrecht, I., 2019, A new forest dwelling button spider from South Africa (Araneae, Theridiidae, Latrodectus), pp. 584-600 in Zootaxa 4700 (4) on page 597, DOI: 10.11646/zootaxa.4700.4.12, http://zenodo.org/record/355755
Green's functions in computational aeroacoustics
No full textThe theory of Green's functions for the wave and Helmholtz equations is examined with particular attention to their use in aeroacoustics for the extrapolation of acoustic wavefields from numerical flow simulations. In a new synthesis that permits straightforward generalization of previously published results, spatial and temporal windowing functions are employed to provide equivalent-source expressions to account for both initial and boundary conditions. Detailed results describe the transformation of both source terms and Green's functions to take account of uniform subsonic mean flow, and expressions are given for free-field Green's functions, both with and without flow, in time, frequency and wavenumber domains. Aworked example illustrates the non-uniqueness of the Green's function for a simple one-dimensional bounded proble
Aerodynamic sound generation in thermoviscous fluids: A canonical problem revisited
No full textAlthough the Lighthill–Curle acoustic analogy theory is formally exact, the presence of linear source terms related to viscous stresses and non-isentropic density changes makes it unsuitable for studying aerodynamic sound generation in low Reynolds number thermoviscous flows. Here we use an extension of the Ffowcs Williams and Hawkings formulation, with thermoviscous effects explicitly included, to find an analytical solution to the canonical problem of sound radiation from a circular cylinder immersed in a viscous heat-conducting fluid and rotating sinusoidally about its axis. Existing published solutions are compared and an earlier null result is explained. The new analysis reveals the dominant source of sound at low Mach numbers to be unsteady viscous dissipation rather than Reynolds-stress quadrupoles, unless the fluid parameter B=αc 2/c p is zero
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