Rod H. Self

Interaction between a flat plate and a circular subsonic jet

This paper reports an extensive nearand farfield analysis of the noise generated by an isothermal, subsonic, circular jet in the presence of a solid, flat plate shield. Farfield polar and azimuthal acoustic images are presented initially to characterize the interaction noise source. Nearfield streamwise microphone phase anal ysis along the plate trailing edge reveals a deeper understanding of the link between the jet hydrodynamic field (both linear and nonlinear regions) and the mechanisms behind i nteraction noise generation. Nearfield point spectrum data have also been used successfully to validate Amiets farfield trailing edge dipole prediction code for lowspeed jet acous tic Mach numbers. Nomenclature a0 = ambient speed of sound c = 2b = plate chord length D = jet nozzle diameter r = far(field propagation distance from jet (geometric) centre(line ρ = fluid density 0 r = distance of the centre of an eddy from the plate trailing edge δ = eddy radius R = separation distance between the source point and the field point I = far(field sound intensity generated from a single eddy

Deformation of nematic liquid crystals in an electric field

The behaviour of liquid crystal materials used in display devices is discussed. The underlying continuum theory developed by Frank, Ericksen and Leslie for describing this behaviour is reviewed. Particular attention is paid to the approximations and extensions relevant to existing device technology areas where mathematical analysis would aid device development. To illustrate some of the special behaviour of liquid crystals and in order to demonstrate the techniques employed, the specific case of a nematic liquid crystal held between two parallel electrical conductors is considered. It has long been known that there is a critical voltage below which the internal elastic strength of the liquid crystal exceeds the electric forces and hence the system remains undeformed from its base state. This bifurcation behaviour is called the Freedericksz transition. Conventional analytic analysis of this problem normally considers a magnetic, rather than electric, field or a near-transition voltage since in these cases the electromagnetic field structure decouples from the rest of the problem. Here we consider more practical situations where the electromagnetic field interacts with the liquid crystal deformation. Assuming strong anchoring at surfaces and a one dimensional deformation, three nondimensional parameters are identified. These relate to the applied voltage, the anisotropy of the electrical permittivity of the liquid crystal, and to the anisotropy of the elastic stiffness of the liquid crystal. The analysis uses asymptotic methods to determine the solution in a numerous of different regimes defined by physically relevant limiting cases of the parameters. In particular, results are presented showing the delicate balance between an anisotropic material trying to push the electric field away from regions of large deformation and the deformation trying to be maximum in regions of high electric field.

Effect of Centerbody Scattering on Advanced Open-Rotor Noise

Formulas for calculating the effect of centerbody scattering on the sound radiated from an advanced open rotor are presented. The effects of blade sweep and distributed blade loading are considered.Mach number effects are also implicitly included in the model. The work extends a previously published method and applies it to a practical situation in which scattering by the centerbody has a significant effect on the radiated sound field.

A CFD Coupled Acoustics Approach for Coaxial Jet Noise

Whilst the most general methodology to predict jet noise utilises Direct Numerical Simulation or Large Eddy Simulation to model the full unsteady ∞owfleld, such an approach is unfeasible in an engineering context. A method is proposed to couple a standard Reynolds Averaged Navier-Stokes CFD method with a Lighthill based noise model for coaxial jets. This has relatively low computational resource requirements, whilst possessing the physical mechanisms to re∞ect how changes in nozzle geometry modify the noise spectra. The three parameters in the noise model were calibrated using single stream jet noise data and then applied to coaxial jet ∞ows. Coplanar coaxial jet problems for difiering area and velocity ratios showed reasonable agreement with measured noise spectra. However, a three-quarter cowl nozzle conflguration showed poor agreement with experiment. Serrations added to the nozzle produced only small changes in the turbulence intensity and length scale predicted by the RANS CFD model and consequently only minor changes were observed in the spectra - a reduction in low frequency noise coupled with an increase at higher frequencies.

Counter-rotation propeller tip vortex interaction noise

Counter-rotation propeller tip vortex interaction noise occurs when tip vortices, shed from each of the upstream propeller blades, interact with the blades on the downstream propeller. This paper describes two separate models for calculating the noise produced by this interaction. The first model approximates each tip vortex as a helical vortex tube of infinite extent while the second is based on a previously published study and uses a twodimensional approximation to model the tip vortex interaction and represents the velocity field induced by the tip vortices as a piecewise function for which the flow incident on the downstream blade row must be calculated numerically. The unsteady loading on the downstream propeller blades is determined from the incident flow predicted using either of the models and is used to calculate the radiated sound field using an analytic propeller noise formula. Nomenclature B = Number of blades on the propeller i C = Constant with value between 1 and 3 L C = Lift coefficient vtx K = Constant with value between 0.5 and 2 L = Streamwise length from upstream blade mid-chord to the downstream blade leading edge x

A model for the rapid assessment of the impact of aviation noise near airports

This paper introduces a simplified model [Rapid Aviation Noise Evaluator (RANE)] for the calculation of aviation noise within the context of multi-disciplinary strategic environmental assessment where input data are both limited and constrained by compatibility requirements against other disciplines. RANE relies upon the concept of noise cylinders around defined flight-tracks with the Noise Radius determined from publicly available Noise-Power-Distance curves rather than the computationally intensive multiple point-to-point grid calculation with subsequent ISO-contour interpolation methods adopted in the FAAs Integrated Noise Model (INM) and similar models. Preliminary results indicate that for simple single runway scenarios, changes in airport noise contour areas can be estimated with minimal uncertainty compared against grid-point calculation methods such as INM. In situations where such outputs are all that is required for preliminary strategic environmental assessment, there are considerable benefits in reduced input data and computation requirements. Further development of the noise-cylinder-based model (such as the incorporation of lateral attenuation, engine-installation-effects or horizontal track dispersion via the assumption of more complex noise surfaces formed around the flight-track) will allow for more complex assessment to be carried out. RANE is intended to be incorporated into technology evaluators for the noise impact assessment of novel aircraft concepts.

Effects of multi-scattering on the performance of a single-beam acoustic manipulation device

The effects of multiple scattering on acoustic manipulation of spherical particles using helicoidal Bessel-beams are discussed. A closed-form analytical solution is developed to calculate the acoustic radiation force resulting from a Besselbeam on an acoustically reflective sphere, in the presence of an adjacent spherical particle, immersed in an unbounded fluid medium. The solution is based on the standard Fourier decomposition method and the effect of multi-scattering is taken into account using the addition theorem for spherical coordinates. Of particular interest here is the investigation of the effects of multiple scattering on the emergence of negative axial forces. To investigate the effects, the radiation force applied on the target particle resulting from a helicoidal Bessel-beam of different azimuthal indexes (m = 1 to 4), at different conical angles, is computed. Results are presented for soft and rigid spheres of various sizes, separated by a finite distance. Results have shown that the emergence of negative force regions is very sensitive to the level of cross-scattering between the particles. It has also been shown that in multiple scattering media, the negative axial force may occur at much smaller conical angles than previously reported for single particles, and that acoustic manipulation of soft spheres in such media may also become possible.

Utilization of turbulent energy transfer rate time-scale in aeroacoustics with application to heated jets

This paper presents a study of the effects of the form of the time-scale used in aerodynamic noise prediction methodologies with application to the prediction of noise from hot jets. It was motivated by the need to improve the spectral shape of predictions obtained using Lighthill Acoustic Analogy based schemes such as the MGBK method in conjunction with a RANS CFD flow simulation. Acoustic Analogy formulations require knowledge of the unsteady characteristics of the turbulence whereas using a RANS calculation as the starting point provides only steady characteristics of the flow and it is then necessary to model the unsteady behaviour in some way. A critical issue is the proper definition of the acoustic time scale which empirical studies have shown to be frequency dependent. The authors discuss how this dependence arises from the underlying physics of the flow by considering a timescale based on the rate of energy transfer through the turbulent cascade. The technique is applied to noise prediction for both isothermal and heated jets using the MGBK method. It is shown that using the new definition of timescale results in good agreement with experimental measurements in both cases. It is suggested that such a definition will prove robust and find application in other areas of aeroacoustic noise prediction.

Improved jet noise modeling using a new time-scale

To calculate the noise emanating from a turbulent flow using an acoustic analogy knowledge concerning the unsteady characteristics of the turbulence is required. Specifically, the form of the turbulent correlation tensor together with various time and length-scales are needed. However, if a Reynolds Averaged Navier-Stores calculation is used as the starting point then one can only obtain steady characteristics of the flow and it is necessary to model the unsteady behavior in some way. While there has been considerable attention given to the correct way to model the form of the correlation tensor less attention has been given to the underlying physics that dictate the proper choice of time-scale. In this paper the authors recognize that there are several time dependent processes occurring within a turbulent flow and propose a new way of obtaining the time-scale. Isothermal single-stream flow jets with Mach numbers 0.75 and 0.90 have been chosen for the present study. The Mani-Gliebe-Balsa-Khavaran method has been used for prediction of noise at different angles, and there is good agreement between the noise predictions and observations. Furthermore, the new time-scale has an inherent frequency dependency that arises naturally from the underlying physics, thus avoiding supplementary mathematical enhancements needed in previous modeling.

Jet noise prediction using different turbulent scales

The turbulent energy dissipation rate time-scale and length-scale has been routinely used for the prediction of noise from turbulent flows, particularly jet streams. However, this is not the only possible choice. In general, scales evolving in a turbulent medium are threefold. First, those associated with the mean flow; second, those attributed to the turbulence and the mean flow interactions; and third, scales related to the turbulence-turbulence interactions. In this paper, special attention will be paid to further study of the underlying physics of aerodynamic noise by examining various time-scales. To do so, three time scales, namely, dissipation, production, and strain rate time scales, are defined and used in the source modelling to emphasis the effect of the turbulence structures at different jet regions on the jet noise production mechanism. The required mean value and turbulence parameters are obtained using a modified k − ∈ turbulence model, and Lighthill’s Acoustic Analogy is used for the prediction of the emanated noise.