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In this paper, we use Large Eddy Simulations (LES) in combination with the Ffowcs Williams - Hawkings method to study the influence of chevrons on the flow field as well as the noise produced by a closely installed M = 0.6 jet. The LES simulations are performed with the spectral/hp element framework Nektar++. Nektar++ uses the high-order discontinuous Galerkin method and an implicit scheme based on the matrix-free Newton-GMRES method to discretize the unfiltered Navier-Stokes equations in space and time, respectively. The far-field noise is computed using Antares. Antares solves the Ffowcs Williams - Hawkings equation for a permeable integration surface in the time-domain using a source-time dominant algorithm. The aerodynamic results show good agreement with experimental data obtained in the Doak Laboratory Flight Jet Rig, located at the University of Southampton. Some discrepancies are observed in terms of the far-field noise levels, especially for higher polar observer angles relative to the downstream jet axis. In terms of noise reduction potential, the simulations predict that the chevrons reduce the OASPL by 1dB compared to an installed round nozzle for all observers located on the unshielded side of the wing. This should be compared to the experiments, which predict a 1.5dB noise reduction for the same chevron nozzle.

Large-eddy simulations (LESs) are performed for two isolated unheated jet flows corresponding to a Doak Laboratory experiment performed at the University of Southampton. The jet speeds studied correspond to acoustic Mach numbers of 0.6 and 0.8 as well as Reynolds numbers based on the nozzle exit diameter of about one million. The LES method is based on the compact accurately boundary-adjusting high-resolution technique (CABARET) and is implemented on graphics processing units (GPUs) to obtain 1000–1100 convective time units for statistical averaging with reasonable run times. In comparison with the previous jet LES calculations with the GPU CABARET method, the mean-flow velocity and turbulent intensity profiles are matched with the hot-wire measurements just downstream of the nozzle exit. The far-field noise spectra of the Doak jets are evaluated using two methods: the Ffowcs Williams–Hawkings approach and a reduced-order implementation of the Goldstein generalized acoustic analogy. The flow and noise results are compared with hot-wire and acoustic microphone measurements of the Doak Laboratory and critically analyzed in comparison with the NASA small hot jet acoustic rig database.

Jet-surface interaction represents a significant community noise problem for the installation of modern ultra-high bypass ratio turbofan engines. The use of chevron nozzles is known to reduce low-frequency jet mixing noise by increasing the mixing rate close to the nozzle. It is currently unknown, however, to what extent such a nozzle lip treatment affects the Kelvin-Helmholtz instability, generated in the vicinity of the wing, which will modify the source of jet-surface interaction noise. To clarify the physics of the jet-surface interaction noise source, an extensive experimental investigation was conducted using the Flight Jet Rig in the anechoic chamber of the Doak Laboratory, at the University of Southampton. Various measurements were carried out on a round and a chevron single stream, unheated subsonic jet, both in an isolated configuration and installed beneath a 2D NACA4415 airfoil "wing". The wall-pressure field on the wing surface was investigated using a pair of miniature wall-pressure transducers and a set of ultra-thin precision microphones. These sensors were flush-mounted in both the stream-wise and span-wise directions on the pressure side of the wing and the unsteady wall-pressure data were analysed in the time and frequency domains. The far-field noise results show significant broadband noise reduction by the chevron jet. This is further evidenced by a reduction in the span-wise correlation length along the wing trailing edge over a wide range of frequencies. Significant reduction of the tonal trapped wave energy is also observed.

When a jet is closely installed under a solid surface, turbulent jet plume is modified. Two levels of jet-plate interactions are present. In addition to linear evanescent instability wave scattered by the plate trailing edge, nonlinear jet turbulence strongly interacts with the plate underneath surface. Both of the interactions could lead to far-field noise change but with different mechanisms. Large-Eddy Simulation (LES) has been performed on two installed jets and one baseline isolated jet to investigate the noise generation mechanisms for this closely installed jet. The distance between jet and plate surface is adjusted for the two installed jets so that strong hydrodynamic interaction is only present in one of the configurations. Far-field noise, computed using Ffowcs Williams-Hawkings (FW-H) equations, is compared for the two installed jets to show the effect of close installation. The near-field sound sources are exmined by analysing quadrupole souces of turbulent mixing and dipole sources of unsteady plate surface loading.

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Additional file 3.

International audience; An investigation into the installation noise from a semi-finite flat plate next to a jet was performed. The experiment was carried out in the ISVR's anechoic Doak Laboratory, at the University of Southampton, using wall pressure transducers flush mounted over the plate surface and a far-field microphone array. In order to reproduce a setup representative of a real scaled jet-wing configuration the plate trailing edge was located at three nozzle exit diameters axially downstream of the jet exhaust. The radial position of the plate was varied from 0.76 to 1.50, nozzle exit diameters from the jet axis to the plate surface. The jet Mach number was also varied from 0.3 up to 0.9. The wall pressure transducers were positioned in the streamwise direction parallel to the jet centerline on the plate surface, including one position upstream of the nozzle exhaust. The pressure transducers in the spanwise direction were positioned close to the plate trailing edge. Data analyses were performed using multivariate statistical quantities in the time and frequency domains. Furthermore, application of the wavelet transform has been chosen to understand the relative contribution of turbulent boundary layer and jet shear layer pressure fields on the scattered far-field noise. Wall pressure data will be also correlated with the far field signals illustrating the propagation effects of the detected wall pressure signatures.