1,721,070 research outputs found
Acoustic measurement of boundary layer flow parameters
No full textThe principle of an ultrasonic method for measuring the parameters of a boundary layer flow is investigated using a mathematical model of the sound field created by a compact piston in a solid boundary, radiating into a parallel shear flow. The model, based on a wavenumber decomposition of the problem, is used to demonstrate that dispersive effects in the streamwise direction are a function of the free-stream flow velocity and the boundary layer displacement thickness, and that a multifrequency measurement of phase velocity between two wall-mounted transducers can in principle be used to determine these parameters. Results from a preliminary experiment to demonstrate the method are presented
Directivity and sound power radiated by a source under a boundary layer
No full textThis paper considers the radiation of sound from compact 2-D or 3-D 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 wave number
decomposition. Numerical results show how the radiated power and directivity depend on the freestream 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
Aerodynamic noise sources on high lift slats and flaps
No full textThe high lift devices used on the wings of civil aircraft to increase lift at low flight speeds are contributors to the overall noise of aircraft at approach. Although there have been a number of recent steps forward in measurement and numerical modelling techniques for these sources there is still more work to be done in understanding the physical and acoustical reasons why they are so dominant. This paper outlines the analysis and interpretation of noise data from an aircraft in flight and the development of a semi-empirical noise model for slats and flaps. Interference patterns observed in the data
indicate that the sound radiation process involves multi-path sound propagation to the observer. A specific outcome of this observation was a notional source model which permitted the separation of noise radiation from the leading and trailing edges of acoustically non-compact flaps, and for slats a separation between two paths radiating from the slat trailing edge either above the slat or through the slat gap. Evidence for the fundamental source mechanisms is combined with standard theory of trailing edge and
leading edge noise on isolated airfoils and acoustic image source techniques to explain why flap noise dominates the forward arc of the aircraft whereas slat noise dominates the rear-arc and hence why these sources are so significant for an observer on the ground
Validation of a prediction model for aerodynamic noise from aircraft landing gear
No full textThe continued development of quiet engines is now giving rise to situations where airframe noise is comparable with engine noise at approach. The landing gear is a major contributor to airframe noise, and this paper outlines development and testing of a semi-empirical noise model for predicting the benefit of noise control fairings. The model is based on nondimensional source spectra derived from a data base of full-scale tests on A320 landing gear installed in the DNW wind tunnel and is used to predict the noise reduction potential of a variety of fairings installed on the main and noise gears from an A340 aircraft . The model is found to give good agreement with data and this gives confidence that it may be used as an engineering tool to optimise fairing design. The paper also discusses the way in which the model can provide a framework for incorporating CFD data in the design process and also number of factors which complicate the comparison of noise data from wind tunnel and flyover tests
Directivity and sound power radiated by a source under a boundary layer
No full textThis 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
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