| 1. |
- Bodén, Hans
(författare)
-
Determination of the acoustic properties of liners under high level multi-tone excitation
- 2016
-
Ingår i: 22nd AIAA/CEAS Aeroacoustics Conference, 2016. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624103865
-
Konferensbidrag (refereegranskat)abstract
- This paper discusses the effect of high level multi-tone acoustic excitation on the acoustic properties of acoustic liners and perforates. It is based on an experimental study of the nonlinear properties of these types of samples without mean grazing or bias flow. Compared to previous studies results from normal incidence impedance tube measurements are compared to liners placed in a grazing incidence configuration. 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 also pose a problem for the correct modelling of sound transmission through ducts lined with such impedance surfaces. The effect of the combination of frequency components is also studied.
|
|
| 2. |
- Gojon, Romain, et al.
(författare)
-
Azimuthal organisation of turbulent structures in underexpanded impinging round jets
- 2016
-
Ingår i: 22nd AIAA/CEAS Aeroacoustics Conference, 2016. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624103865
-
Konferensbidrag (refereegranskat)abstract
- The azimuthal organisation of the turbulent structures in underexpanded round jets impinging on a flat plate have been investigated using compressible large eddy simulation. The jet shear-layer region, as well as the region of the wall jets created on the plate after the impact are considered. The jets are characterized by a Nozzle Pressure Ratio of 4.03, a fully expanded Mach number of 1.56, and a Reynolds number of 6 × 104. The distance between the nozzle and the plate varies from 4.16r0 to 9.32r0. The jets generate acoustic tones due to a feedback mechanism. In this paper, the near pressure and density fields of the jets are analysed using Fast Fourier Transform on the nozzle exit plane, the plate, and an azimuthal plane. The amplitude and the phase fields on these sections at the tone frequencies are represented. Similar organisations of the turbulent structures are found in the jet shear layers and the wall jets. Thus, axisymmetric and helical arrangements of the structures in the shear layers lead to spiral and concentric distributions of the structures on the plate, respectively. In particular, for one of the jets, a spiral shape and concentric rings, associated with two tone frequencies generated simultaneously, are observed on the flat plate in the pressure and density phase fields. Moreover, the convection velocity of the turbulent structures on the plate is evaluated from the phase fields. Several diameters away from the jet axis, the velocities found for the present jets compare well with those found experimentally in the phase-averaged distributions of fluctuating pressure for impinging ideally expanded jets using fast-response Pressure-Sensitive Paint. Near the jet axis, given that the present jets are underexpanded, differences are observed, due to the presence of shock cell structures in the jets. Finally, the convection velocity of the turbulent structures on the wall are estimated from cross-correlations of radial velocity. The values obtained compare well with those determined from the phase fields.
|
|
| 3. |
- Hammar, Johan, et al.
(författare)
-
Simulation of aerodynamically generated noise propagation using the wave expansion method
- 2016
-
Ingår i: 22nd AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624103865
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A numerical method to simulate aerodynamically generated sound and its propagation is presented in this paper. The flow is solved using both analytical expressions and numer- ical methods. The aeroacoustic source terms are then defined by using the aeroacoustic analogies of Lighthill, Ffowcs-Williams and Hawkings, Powell and Howe, and are evaluated from the flow solutions. The acoustic propagation with these sources is then performed using the wave expansion method. This is a discretization method suitable for solving wave propagation through inhomogeneous potential flows. Two different cases are considered in the study, a co-rotating vortex pair and the flow around a 2D cylinder at Re=150. The fo- cus of the work presented is to show a robust introduction of aeroacoustic sources in a wave expansion acoustic propagation solution procedure. The numerical results are compared to experimental and numerical results from other studies of the same configurations.
|
|
| 4. |
- Na, Wei, et al.
(författare)
-
Acoustic characterization of a hybrid liner consisting of porous material by using a unified linearized navier-stokes approach
- 2016
-
Ingår i: 22nd AIAA/CEAS Aeroacoustics Conference, 2016. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624103865
-
Konferensbidrag (refereegranskat)abstract
- In this paper, the acoustic properties of a hybrid liner placed at the end of an impedance tube are investigated using numerical simulations. The hybrid liner constitutes of three components, a perforated plate, a porous layer and a rectangular back cavity. The presence of the porous layer is to enhance the absorptive performance of a liner. The main objective of the paper is to verify the proposed numerical methodology - a unified linearized Navier-Stokes Equations (LNSE) approach. In the unified LNSE approach, the combination of the Helmholtz Equation, LNSE as well as the equivalent fluid model are solved in different regions of the impedance tube. To achieve this, the continuity of the coupling condition between the LNSE and the Helmholtz equation is examined. Another objective is to analyze the effectiveness of the porous material to the acoustic performance of the liner. Acoustic liner simulations with and without porous material, porous material with different flow resistivity are carried out. A good agreement is found between the numerical results and the measurements previously performed at KTH MWL.1 Compared to previous work234, several improvements have been made in the numerical methodology, such as that the energy equation has been added in order to include the damping due to viscous dissipation as well as the thermal dissipation in the vicinity of the perforated plate.
|
|
| 5. |
- Peerlings, Luck, 1988-, et al.
(författare)
-
The acoustic equivalence of a mass and heat source
- 2016
-
Ingår i: 22nd AIAA/CEAS Aeroacoustics Conference, 2016. - Reston, Virginia : American Institute of Aeronautics and Astronautics Inc, AIAA. - 9781624103865
-
Konferensbidrag (refereegranskat)abstract
- In combustion systems, unsteady heat release acts as a source term to the acoustic field within the combustor and under the right conditions the energy of the acoustic field can exponentially grow, leading to a combustion instability. An acoustic driver such as a loudspeaker or horn also acts as a source term to the acoustic field and is often modelled as a fluctuating mass source. Considering the similarity of the flame and the acoustic driver to acts as a source to the acoustic field, the question arises if these two types of sources can be interchanged. This contribution investigates that question by considering a 1-D system with mean flow. In the governing equations a mass source term is included which is linearly related to velocity fluctuations. The results are compared with that of a system with a compact heat source. It is found that the two systems are equivalent when there is no mean flow. In the presence of flow, the response of both systems can approximately be the same when special conditions are met.
|
|
| 6. |
- Pignier, Nicolas, et al.
(författare)
-
Aeroacoustic study of a submerged air inlet using an IDDES/FW-H approach and sound source modelling through direct numerical beamforming
- 2016
-
Ingår i: 22nd AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624103865
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The flow-generated sound from an air inlet at a low Mach number is studied using detached eddy simulation and direct numerical beamforming. The flow is solved through RANS and compressible detached eddy simulation for various velocity ratios, defined as the ratio between the flow speed at the duct entrance and in the free stream. Results for the flow show very good agreement with experimental data from the National Advisory Committee for Aeronautics (NACA). Pressure data is extracted at the position of a virtual microphone array inside the simulation domain, outside of the unsteady flow region. The array data is beamformed on a source test grid over the inlet using standard beamforming and Linear Programming deconvolution approaches. As beamforming is performed from simulated data directly in the acoustically resolved domain, the method is referred to as Direct Numerical Beamforming (DNB). The results give information about the location, amplitude and frequency content of the sound sources around the inlet. The resulting incoherent sound sources can be used as a model for the sound radiated from the inlet. Using a simple Green's function, the corresponding far-field spectrum is computed for two velocity ratios, showing a very good agreement with the spectra obtained from a Ffowcs Williams and Hawkings integral.
|
|
| 7. |
- Sack, Stefan, et al.
(författare)
-
Full multi-port characterization of a circular orifice plate
- 2016
-
Ingår i: 22nd AIAA/CEAS Aeroacoustics Conference, 2016. - Reston, Virginia : American Institute of Aeronautics and Astronautics Inc, AIAA. - 9781624103865
-
Konferensbidrag (refereegranskat)abstract
- A multi-port method is used to extract complex mode amplitudes from experimental data of in-duct orifice plates for frequencies with cut-ons of several higher order modes. The two-port method is therefore extended to resolve higher order modes, i.e. spinning modes and radial modes. The work presented in Sack et all.1 is continued and a new test- rig design is presented which enables more stable measurements and cleaner data. The scattering and source data for higher order modes of a circular sharp-edged orifice plate with low Mach-number mean flow is presented and used for advanced post-processing. The gained data is used to predict the scattering of a tandem orifice configuration and the prediction is compared to measurement results.
|
|
| 8. |
- Schickhofer, Lukas, et al.
(författare)
-
Aeroacoustics of an elastic element in unsteady flow of low Reynolds numbers
- 2016
-
Ingår i: AIAA Technical Paper 2016-2700, 22nd AIAA/CEAS Aeroacoustics Conference, American Institute of Aeronautics and Astronautics, 2016. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624103865
-
Konferensbidrag (refereegranskat)abstract
- Vibrations of elastic structures are a common occurrence in numerous fields of engineering such as aeronautics, aerodynamics, civil engineering, and biomechanics. Particular e ort is dedicated to aeroacoustics of elements that are excited to oscillatory behaviour due to fluid instabilities. The current study is concerned with the numerical investigation of the flow-induced vibrations of a flexible, beam-like element in crossflow at low Reynolds numbers of Re = 100 − 1000 by means of fluid-structure interaction simulations. The aeroa-coustics in the near field are assessed with direct computation of the compressible airflow. Additionally, an acoustic analogy is applied, characterising the acoustic sources and the corresponding sound propagation. At low Reynolds numbers and high elastic moduli the dipole source produces the highest pressure perturbation in the near field. At higher Reynolds numbers and low elastic moduli, however, the monopole source due to structural vibrations becomes the important sound generating mechanism.
|
|
| 9. |
- Weng, C., et al.
(författare)
-
Experimental investigation of sound field decomposition with
- 2016
-
Ingår i: 22nd AIAA/CEAS Aeroacoustics Conference, 2016. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624103865
-
Konferensbidrag (refereegranskat)abstract
- The DUct aCoustic Test rig with Rectangular cross section (DUCT-R) is designed at the German Aerospace Center (DLR), Engine Acoustics Department, to test commercial aircraft engine nacelle liner samples. An air flow with speed up to Mach 0.3 can be achieved in the liner section. An overall number of 106 microphone positions are spread over the rig, making it capable of investigating the scattering of the fundamental and higher order acoustic modes above the liner samples. In this paper, the DUCT-R facility is described. The measurement setups currently being used at DLR are introduced. In addition, a methodology for decomposing measured sound-field data with fundamental and higher order modes is described. The methodology is applied to data sets obtained from a “zero measurement” (a measurement without liner samples) with and without mean flows for validation purposes. The validation results are reported and discussed.
|
|
