1,720,963 research outputs found
Prediction methods for propagation in bypass ducts and comparison with measured data
No full textA hybrid numerical procedure is presented for predicting the propagation and radiation of fan noise from aero-engine bypass ducts. In this approach, an accurate in-duct finite element model which captures well the physics of multimode propagation within the duct, is coupled to a more approximate radiation model in the external domain. The objective is to develop a relatively simple scheme which can predict the effects of bypass duct geometry and acoustic treatments on the radiated sound at modest computational cost. Predictions
of the attenuation and the far field directivity for no-flow prototype model bypass ducts are validated against test data
Reduction of aerodynamic noise from square bars by introducing spanwise waviness
No full textThis paper presents an investigation, using both numerical and experimental methods, of the application of spanwise waviness to reduce aerodynamic noise from square bars. The numerical simulations are performed using the Delayed Detached-Eddy Simulation approach to obtain the near-field unsteady flow properties, which are then used to calculate the equivalent source terms in the Ffowcs Williams-Hawkings equation for far-field noise prediction. For a straight square bar in cross-flow, which produces strong tonal noise associated with the vortex shedding, a benchmark study showed good agreement between numerical simulations and measurements in terms of far-field noise spectra. Waviness is then introduced along the bar span and the influence of the amplitude and wavelength of the waviness is studied. When the wave amplitude is nearly half the bar width, a large noise reduction of as much as 30 dB is found from both numerical simulations and measurements, including a 10 dB reduction in the broadband level. The influence of the wavelength is much smaller. Analysis of the flow features showed that, with increased wave amplitudes, the spanwise flow becomes significant and strong crossflow vortices develop in the near wake which effectively suppress the primary vortex shedding. This reduces the noise level significantly, especially the tonal noise associated with the vortex shedding
Comparison between measurements and predictions of fan broadband noise on a low speed fan rig
No full textA comparison between measured and predicted fan broadband noise due to rotor-stator interaction
No full textThis paper presents a comparison between predictions and measurements of the broad-band noise generated in a low-speed fan rig. It is assumed here that the dominant generation mechanism is due to rotor-stator interaction. In the broadband noise model the stator is modelled as a cascade of flat plate airfoils on which turbulent wakes due to the rotor impinge. The turbulence transverse length scale and intensity of the rotor wake are determined by data from measurement and RANS calculations. The theoretical expression for the acoustic power spectra is shown to be in the form of the product of an aerodynamic response function and the spectrum of the velocity fluctuations normal to the stator blade direction. The response function is obtained by solving numerically an aerodynamic integral equation, while the turbulence velocity spectrum is expressed analytically in a form similar to an isotropic homogeneous turbulence model. The turbulence is also modelled by random velocity fluctuations superimposed on a statistic distribution of the rms velocity. The statistical distribution is composed of a constant term, modelling the background turbulence and a periodic Gaussian-profile to represent the rotor-wake turbulence. Extensive hot wire measurements of both the steady and unsteady wake velocity was measured. Comparisons between these measurements and RANS predictions are also presented
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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