| 1. |
- Allam, S., et al.
(författare)
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Investigation of aerodynamic installation effects for an axial fan
- 2013
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Ingår i: 19th AIAA/CEAS Aeroacoustics Conference. - 9781624102134
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Konferensbidrag (refereegranskat)abstract
- The aim of the work reported in this paper is to investigate the aerodynamic installation effects for a generic cooling fan system. This is done by treating the ducted fan as a multi-port source and performing a source characterization, i.e., determining the source strength and source reflection matrix. From the source strength, which represents the sound radiated by the fan in a reflexion free (infinite) duct, the effect of various inflow distortions can be studied. The work is part of the EU-project ECOQUEST where the data will be used to validate the acoustic prediction tools.
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| 2. |
- Bodén, Hans
(författare)
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Acoustic properties of perforates under high level multi-tone excitation
- 2013
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Ingår i: 19th AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624102134
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Konferensbidrag (refereegranskat)abstract
- This paper discusses the effect of high level multi-tone acoustic excitation on the acoustic properties of perforates. It is based on a large experimental study of the nonlinear properties of these types of samples without mean grazing or bias flow. Compared to previously published results the present investigation concentrates on the effect of multiple harmonics. It is known from previous studies that high level acoustic excitation at one frequency will change the acoustic impedance of perforates at other frequencies, thereby changing the boundary condition seen by the acoustic waves. This effect could be used to change the impedance boundary conditions and for instance increase the absorption. It could obviously also pose a problem for the correct modelling of sound transmission through ducts lined with such impedance surfaces. Experimental results are compared to a quasi-stationary model. The effect of the combination of frequency components and phase in the excitation signal is studied.
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| 3. |
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| 4. |
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| 5. |
- Holmberg, Andreas, et al.
(författare)
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A linearized Navier-Stokes method including turbulence damping
- 2013
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Ingår i: 19th AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624102134
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Konferensbidrag (refereegranskat)abstract
- In this paper, a method for including damping of acoustic energy in regions of strong turbulence is derived for a linearized Navier-Stokes method in the frequency domain. The result is applied in a study of the linear interaction of the acoustic and the hydrodynamic field in a 2D T-junction, subject to grazing flow at Mach 0.1. As the acoustic waves travel in regions of strong shear, there is a need to include the interaction between the background turbulence and the acoustic field. For this purpose, the oscillation of the background turbulence Reynold's stress, due to the acoustic field, is modeled using an eddy Newtonian model assumption. The time averaged flow is first solved for using RANS along with a k-ε turbulence model. The spatially varying turbulent viscosity is then added to the spatially invariant kinematic viscosity in the acoustic set of equations. The response of the 2D T-junction to an incident acoustic field is analyzed via a plane wave scattering matrix model, and the result is compared to experimental data for a T-junction of rectangular ducts. A strong improvement in the agreement between calculation and experimental data is found when the modification proposed in this paper is implemented.
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| 6. |
- Holmberg, Andreas, et al.
(författare)
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Aeroacoustics of rectangular T-junctions subject to combined grazing and bias flows - An experimental investigation
- 2013
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Ingår i: 19th AIAA/CEAS Aeroacoustics Conference. - 9781624102134
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Konferensbidrag (refereegranskat)abstract
- Experimentally determined scattering matrices are used to study the low-amplitude interaction between the acoustic and the hydrodynamic fields in a T-junction of rectangular ducts. In particular, combinations of grazing and bias flows are investigated in the study. Common for all flow combinations is that waves incident on the junction at the downstream side are only attenuated, while waves incident at the other branches may be amplified or attenuated, depending on the Strouhal number. When bias in-flow is introduced to a grazing flow, there is first an increase and then a decrease in both amplification and attenuation, as the bias in-flow Mach number is increased. Comparing with T junctions of circular ducts, the interaction is stronger for rectangular duct junctions.
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| 7. |
- Kierkegaard, Axel, et al.
(författare)
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Simulations of a liner cell using a frequency-domain linearized Navier-Stokes methodology
- 2013
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Ingår i: 19th AIAA/CEAS Aeroacoustics Conference. - 9781624102134
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Konferensbidrag (refereegranskat)abstract
- We present numerical simulations of a liner concept, where a thin layer of metallic foam material is located in between the perforated plate and the backing cavity. This liner concept was first presented in Ref 1, where experimental tests showed that the acoustic characteristics of such a liner are nearly independent of both flow and temperature. We use the linearized Navier-Stokes equations in frequency domain to simulate the acoustic waves and the coupling to the hydrodynamic field in the main duct and the liner neck. The effect of the metallic foam is modeled using the Delany-Bazley model. The numerical results are in good agreement with the experimental results presented in Ref 1. In addition, a parameter study on the influence of the flow resistivity of the metallic foam, mean flow speed and mean flow temperature is performed.
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| 8. |
- Krank, Benjamin, et al.
(författare)
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Evaluating stretched grids and introducing black hole layers as alternative non-reflecting buffer zone
- 2013
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Ingår i: 19th AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624102134
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Konferensbidrag (refereegranskat)abstract
- Grid-stretching is often used close to non-reflecting boundaries in order to under-resolve and dissipate outgoing acoustic waves. The resulting buffer zones may be large and computationally expensive, however, since the stretching factor is strongly limited. Two approaches of implementing grid-stretching are considered: Taylor expansion and transformation in curvilinear coordinates. Besides grid-stretching, a new buffer zone technique is introduced, which allows for larger stretching factors and computationally more efficient buffer zones. The speed of sound is varied locally in the buffer zone resulting in spatial under-resolution of outgoing waves similarly to stretched grids. The approach is derived from the concept of acoustic black holes, a method of reducing reflection of bending waves from edges of physical plates, giving the name black hole layer to the new concept. The approach is verified and the performance of stretched grids and black hole layers is evaluated using a linearized Euler equations solver in two space dimensions for several benchmark flows. The most efficient buffer zone is found to be a combination of grid-stretching using Taylor expansion and black hole layer.
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| 9. |
- Miguel, Elena, et al.
(författare)
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Analysis of simultaneous measurement of acoustic pressure in the far-field and density gradient in the near-field in a cold jet
- 2013
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Ingår i: 19th AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624102134
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Konferensbidrag (refereegranskat)abstract
- The purpose of the investigation presented here is to study the degree of correlation between the density gradient in the near field and the aeroacoustic sound generation in the far field of a cold jet. The variations of the density gradient in the flow are visualized and recorded by the Background Oriented Schlieren (BOS) technique. In this method high power LEDs are used as illumination source. Simultaneously the pressure fluctuations in the acoustic far-field are recorded by a set of microphones. Measurements on a round nozzle and a chevron nozzle are conducted and the obtained results are compared. Large structures emitting noise are localized, being in the case of the chevron nozzle further upstream due to the higher level of entrainment of the jet with the surrounded ambient fluid. In addition, it is demonstrated that the noise produced by these structures is radiated mainly downstream, having the highest correlations with the pressure fluctuations recorded with the microphone located at an angle θ = 26° respect to the jet-axis.
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| 10. |
- Nebenführ, Bastian, 1984, et al.
(författare)
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Hybrid RANS/LES Simulations for Aerodynamic and Aeroacoustic Analysis of a Multi-Element Airfoil
- 2013
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Ingår i: 19th AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624102134 ; , s. 64-
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Konferensbidrag (refereegranskat)abstract
- A hybrid RANS/LES modeling approach is used for simulating the turbulent flow around a three-element airfoil in high-lift configuration. A detailed analysis of the flow is made, based on the simulation outcome. A comprehensive aeroacoustic analysis involving all three elements of the airfoil is also presented. To provide input data for acoustic analogies, the results of the simulation are sampled at a permeable stationary surface near the airfoil and at the airfoil itself. The far-field noise signature of the high-lift airfoil is computed with the help of the Kirchhoff integral surface method, the Ffowcs-Williams and Hawkings method for a stationary, permeable surface, and the Curle method. The sound pressure level spectrum exhibits a broad-banded shape with several narrow-banded tonal peaks at low Strouhal numbers. The broad-banded peak at high Strouhal numbers, which is typically associated with vortex shedding behind the blunt slat trailing edge, was also captured. Using Curle's acoustic analogy, the noise emission pattern of the three elements is explored, isolated from each other, revealing that both slat and flap act as dipoles. By refining the used grid, the flow results are significantly improved in terms of slat shear layer instability and resolved turbulent content as compared to our previous work.
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| 11. |
- Sinibaldi, Giorgia, et al.
(författare)
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Analysis of the Characteristic Acoustic Tones of an Impinging Jet
- 2013
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Ingår i: 19th AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624102134
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Konferensbidrag (refereegranskat)abstract
- The acoustic emission of a supersonic jet is an interesting and fundamental issue due to the several technological applications and the diverse physical aspects to be still clarified. An experimental analysis of the characteristic acoustic tones of a supersonic impinging jet has been carried out with a particular attention on the effect of the nozzle pressure ratio and the distance of the impinging surface. The screech tones, typical of an imperfectly expanded supersonic free jet, have been first investigated to have an idea of the deflection from the free jet behavior due to the impinging surface. Then the analysis has been focused on smaller distances of the impinging plate from the jet exit to evaluate the behavior of the impinging tones. The difference between screech and impinging tones, due to the different interactions in the generating feedback mechanism, has been observed.
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| 12. |
- Vilela De Abreu, Rodrigo, et al.
(författare)
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Towards the development of adaptive finite element methods for internal flow aeroacoustics
- 2013
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Ingår i: 19th AIAA/CEAS Aeroacoustics Conference. - 9781624102134
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Konferensbidrag (refereegranskat)abstract
- We report the latest results obtained in the development of an adaptive finite element method for computational aeroacoustics (CAA). The new methodology is based on the General Galerkin (G2) method, which has been successfully used for the computation of incompressible, turbulent flow. Here, we simulate the flow past an in-duct mixer plate and compare the results with available experimental data. The comparisons include mean velocity profiles and frequency content of the turbulent signal. No direct simulation of sound or sound wave propagation has been performed; instead, simple analogy arguments have been used to extract acoustic results from incompressible simulations by assuming a direct correlation between the computed pressure drop signal and the sound at the far field. We were able to reproduce the sound signal from experiments with our incompressible simulation and our results compared well with both the level and the broadband frequency peak of the measured sound. We suggest that the methodology presented here is mainly suitable for the prediction of sound in low Mach number pipe flows.
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| 13. |
- Weng, Chenyang, et al.
(författare)
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Sound-turbulence interaction in low Mach number duct flow
- 2013
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Ingår i: 19th AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624102134
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- When sound waves propagate in ducts with the presence of turbulent flow, turbulent stresses can cause extra attenuation of the wave in addition to that caused by viscothermal effects. Different models for the turbulent stress acting on the sound waves are investigated in this study, and they are applied to compute the wall shear stress impedance of sound waves. Among these models the non-equilibrium model proposed by the present authors in another paper is studied in details. Attempt of including mean flow convection effect in the non-equilibrium model is made. The resulting modified model is then applied to compute the wave attenuation, and the results are compared with experimental data.
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