| 1. |
- Agarwal, A., et al.
(författare)
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A ray tracing approach to calculate acoustic shielding by the silent aircraft airframe
- 2006
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Ingår i: Collection of Technical Papers - 12th AIAA/CEAS Aeroacoustics Conference. - 1563478099 - 9781563478093 ; , s. 2799-2818
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Konferensbidrag (refereegranskat)abstract
- The Silent Aircraft is in the form of a flying wing with a large wing planform and a propulsion system that is embedded in the rear of the airframe with intakes on the upper surface of the wing. Thus a large part of the forward-propagating noise from the intake ducts is expected to be shielded from observers on the ground by the wing. Acoustic shielding effects can be calculated by solving an external acoustic scattering problem for a moving aircraft. In this paper, acoustic shielding effects of the Silent Aircraft airframe are quantified by a ray-tracing method. The dominant frequencies from the noise spectrum of the engines are sufficiently high for ray theory to yield accurate results. It is shown that for low-Mach number homentropic flows, a condition satisfied approximately by the Silent Aircraft during take-off and approach, the acoustic rays propagate in straight lines. Thus, from Fermat's principle it is clear that classical Geometrical Optics and Geometrical Theory of Diffraction solutions are applicable to this moving-body problem as well. The total amount of acoustic shielding at an observer located in the shadow region is calculated by adding the contributions from all the diffracted rays (edge-diffracted and creeping rays) and then subtracting the result from the incident field without the airframe. Experiments on a model-scale geometry have been conducted in an anechoic chamber to test the applicability of the ray-tracing technique. The three-dimensional ray-tracing solver is validated by comparing the numerical solution with analytical high-frequency asymptotic solutions for canonical shapes.
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| 2. |
- Bodén, Hans, 1954-, et al.
(författare)
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Experimental investigation of nonlinear acoustic properties for perforates
- 2006
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Ingår i: Collection of Technical Papers. - Cambridge, MA. - 1563478099 - 9781563478093 ; , s. 67-74
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Konferensbidrag (refereegranskat)abstract
- This paper presents the results of a study of non-linear acoustic properties of perforates and micro-perforates. The results are potentially of interest for perforates and other facing sheets used in aircraft engine liners as well as perforate pipes used in automotive mufflers. In the linear limit the perforate acoustic impedance is independent of the sound field but when the sound pressure level is high it will be dependent on the acoustic particle velocity in the holes. For pure tone excitation the impedance will be controlled by the acoustic particle velocity at that frequency. If the acoustic excitation is random or periodic with multiple harmonics the impedance at a certain frequency will depend on the particle velocity at other frequencies. In this paper a study lias been made of harmonic interaction effects by using multiple pure tone excitation and random noise excitation.
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| 3. |
- Efraimsson, Gunilla, et al.
(författare)
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Analysis of Aeroacoustic Wave Propagation Simulations Using a Higher-Order Accurate Method
- 2006
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Ingår i: Collection of Technical Papers - 12th AIAA/CEAS Aeroacoustics Conference. - Reston, Virigina : American Institute of Aeronautics and Astronautics. - 1563478099 ; , s. 922-931
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We present analysis and numerical investigations of the influence of grid stretching together with artificial viscosity on solutions to higher order accurate centered finite difference schemes. Also, we numerically investigate the accuracy of a sixth order accurate scheme for varying Mach number. In the analysis the first-order wave equation together with artificial viscosity terms are considered. In the numerical calculations a simple model problem is solved for which detailed studies of the downstream and upstream acoustic waves is possible.
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| 4. |
- Elnady, Tamer, et al.
(författare)
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Validation of an inverse analytical technique to educe liner impedance with grazing flow
- 2006
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Ingår i: Collection of Technical Papers - 12th AIAA/CEAS Aeroacoustics Conference. - Reston, Virigina : American Institute of Aeronautics and Astronautics. - 1563478099 - 9781563478093 ; , s. 3093-3107
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Konferensbidrag (refereegranskat)abstract
- This paper is based on a previously developed analytical technique to educe the acoustic impedance of a liner sample, placed inside a rectangular duct in grazing incidence. This technique uses the measurement of complex acoustic pressure at four positions inside the duct, upstream and downstream the lined section, and educing the result using the mode matching method. In the mode matching code, the flow was simplified to be uniform, which is not usually the actual case. This technique was opposed by several doubts regarding the mean flow assumption and the possible pressure discontinuity at the hard-soft and soft-hard steps. It was believed that the pressure field away from this discontinuity is representative of the real case, and the code would be capable to educe the correct liner impedance. In this paper, the code is validated against benchmark data from NASA. First the code was tested to be able to reproduce the pressure field for a given impedance. Second, it was tested to educe the correct impedance for a given pressure distribution. The results of the mode matching code were in excellent agreement with the experimental data. The effect of shear flow was investigated and found out that the differences are quite small for the chosen duct size and frequency of interest. On the other hand, the mode matching formulation was re-written based on the pressure instead of the velocity potential.
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| 5. |
- Åberg, Magnus, et al.
(författare)
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Simulation of the acoustics behind a wall due to a moving vehicle
- 2006
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Ingår i: Collection of Technical Papers - 12th AIAA/CEAS Aeroacoustics Conference. - 1563478099 ; 3, s. 1960-1971
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Konferensbidrag (refereegranskat)abstract
- The influence of a barrier on the acoustic field is investigated using an expansion of the compressible Navier-Stokes equations. Large Eddy Simulations (LES) is used to account for the acoustical sources and the equation for the acoustic density fluctuation accounts for the propagation of the acoustics waves. This combined approach enhances numerical efficiency while maintaining good accuracy. The problem under consideration is related to the spreading of noise due to a moving vehicle and the effects of a noise barrier on the Sound Pressure Level (SPL) and spectral composition. These parameters are computed for several configurations of the barrier in terms of height and distance from the vehicle. The main results show that there can be acoustic modes in resonance between the barrier and vehicle and that barrier height has an effect on SPL that is intuitive. Copyright
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| 6. |
- Åbom, Mats, et al.
(författare)
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Aero-acoustics of flow duct singularities at low mach numbers
- 2006
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Ingår i: Collection of Technical Papers - 12th AIAA/CEAS Aeroacoustics Conference. - Reston, Virigina : American Institute of Aeronautics and Astronautics. - 1563478099 - 9781563478093 ; , s. 3708-3717
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Konferensbidrag (refereegranskat)abstract
- This paper describes the application of an acoustic 2-port model to describe flow generated noise in ducts. An experimental procedure that enables determination of both the passive (the scattering matrix) as well as the active (source) 2-port data is described. The method is applied to investigate the aero-acoustics of an orifice plate in a duct. The passive data is compared with a simple quasi-stationary model and the active part is analyzed using a scaling law procedure, based on the assumption of a compact dipole source.
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