Ennes Sarradj

Three-Dimensional Acoustic Source Mapping with Different Beamforming Steering Vector Formulations

Acoustic source mapping techniques using acoustic sensor arrays and delay-and-sum beamforming techniques suffer from bad spatial resolution at low-aperture-based Helmholtz numbers. This is especially a problem for three-dimensional map grids, when the sensor array is not arranged around the region spanned by the grid but on only one side of it. Then, the spatial resolution of the result map in the direction pointing away from the array is much worse than in the other lateral directions. Consequently, deconvolution techniques need to be applied. Some of the most efficient deconvolution techniques rely on the properties of the spatial beamformer filters used. As these properties are governed by the steering vectors, four different steering vector formulations from the literature are examined, and their theoretical background is discussed. It is found that none of the formulations provide both the correct location and source strength. As a practical example the CLEAN-SC deconvolution methodology is applied to simulated data for a three-source scenario. It is shown that the different steering vector formulations are not equally well suited for three-dimensional application. The two preferred formulations enable the correct estimation of the source location at the cost of a negligible error in the estimated source strength.

Silent Owl Flight: Bird Flyover Noise Measurements

Mostgeneraofowls(Strigiformes)havetheabilityto flysilently.Themechanismsofthesilent flightoftheowlhave been the subject of scientific interest for many decades. The results from studies in the past are discussed in detail in thispaperandtherationaleforthepresentresearchisgiven,whichincluded flyovernoisemeasurementsondifferent species of birds. Successful acoustic measurements were made on a Common Kestrel, a Harris Hawk, and a Barn Owl. Measurements on three other birds did not lead to reliable results. The setup and procedure used for the outdoor measurements are discussed. These include the estimation of the trajectory from dual video camera recordings and microphone-array measurements with a moving-focus beamforming technique. The main result from the 50 successful flyovers is that the owl flight produces aerodynamic noise that is indeed a few decibels below thatofotherbirds,evenif flyingatthesamespeed.Thisnoisereductionissignificantatfrequenciesabove1.6kHz.At frequencies above 6.3 kHz the noise from the owl remains too quiet to be measured.

Application of a Beamforming Technique to the Measurement of Airfoil Leading Edge Noise

The present paper describes the use of microphone array technology and beamforming algorithms for the measurement and analysis of noise generated by the interaction of a turbulent flow with the leading edge of an airfoil. Experiments were performed using a setup in an aeroacoustic wind tunnel, where the turbulent inflow is provided by different grids. In order to exactly localize the aeroacoustic noise sources and, moreover, to separate airfoil leading edge noise from grid-generated noise, the selected deconvolution beamforming algorithm is extended to be used on a fully three-dimensional source region. The result of this extended beamforming are three-dimensional mappings of noise source locations. Besides acoustic measurements, the investigation of airfoil leading edge noise requires the measurement of parameters describing the incident turbulence, such as the intensity and a characteristic length scale or time scale. The method used for the determination of these parameters in the present study is explained in detail. To demonstrate the applicability of the extended beamforming algorithm and the experimental setup as a whole, the noise generated at the leading edge of airfoils made of porous materials was measured and compared to that generated at the leading edge of a common nonporous airfoil.

Porous airfoils: noise reduction and boundary layer effects

The present paper describes acoustic and hot–wire measurements that were done in the aeroacoustic wind tunnel at the Brandenburg University of Technology Cottbus on various SD7003–type airfoils made of different porous (flow permeable) materials. The objective of the research is the analysis of the turbulent boundary layer properties of porous airfoils and, subsequently, of the noise generated at the trailing edge. The influence of the porous materials, characterized by their air flow resistivity, is discussed. The acoustic measurements were performed using a planar 56–channel microphone array and the boundary layer properties were measured using constant temperature anemometry. The recorded acoustic data underwent further processing by application of advanced beamforming algorithms. A noticeable reduction of the emitted trailing edge noise was measured for the porous airfoils over a large range of frequencies. At high frequencies, some of the porous airfoils were found to generate more noise than the reference airfoil which might be due to the surface roughness noise contribution. It is found that the turbulent boundary layer thickness and the boundary layer displacement thickness of the airfoils increase with decreasing flow resistivities for both suction and pressure side. Both boundary layer thickness and displacement thickness of the porous airfoils are greater than those of a non-porous reference airfoil

Microphone array method for the characterization of rotating sound sources in axial fans

Methods based on microphone array measurements provide a powerful tool for determining the location and magnitude of acoustic sources. For stationary sources, sophisticated algorithms working in the frequency domain can be applied. By using circularly arranged arrays and interpolating between microphone signals it is possible to treat rotating sources, as are present in fans, as being non-moving. Measurements conducted with a four-bladed fan and analyzed with the “virtual rotating array” method show that it is not only possible to identify the main noise contributors, but also to determine a full spectrum for any rotating component of interest.

Noise Generation by Porous Airfoils

The noise generated by the flow around airfoils continues to be one of the important aeroacoustic noise sources. Innovative techniques for airfoil noise reduction are, therefore, of great importance. The present experimental study deals with the use of porous material for the construction of airfoils and the subsequent noise reduction. The use of such material controls the flow around the airfoil and has an influence on the sound generation. The analysis focuses on the characterization of the influence of the porous material parameters, especially the flow resistivity and the porosity, on the noise generation. Results of aeroacoustic wind tunnel tests on several porous and non-porous SD 7003 model scale airfoils are compared. Using microphone array measurements, it was found that not only the overall sound pressure level, but also the spectral characteristics depend on the parameters of the porous material. While the overall sound pressure level decreases in the order of a few decibel, at lower frequencies the reduction is considerable larger and extends 10 dB in some cases. Additionally, the influence of the material parameters on drag and lift is discussed. The aerodynamic performance of the porous airfoils is in general inferior to that of non-porous airfoils. However, there appear to be sets of material parameters that provide a considerable decrease in sound generation and only a minor degradation of aerodynamic eciency.

Trailing edge noise of partially porous airfoils

The use of porous trailing edges is one possible approach to reduce airfoil trailing edge noise. Past experiments on fully porous airfoil models showed that a noticeable noise reduction can be achieved. However, this reduction is accompanied by a loss in aerodynamic performance. To combine the acoustic advantages of the porous trailing edge with the aerodynamic advantages of a non-porous airfoil, the generation of trailing edge noise of airfoil models that only have a porous trailing edge is investigated. To this end, initial experiments were performed on a set of airfoils with porous trailing edges of varying chordwise extent in an open jet wind tunnel, using microphone array measurement technique and a deconvolution beamforming algorithm. The lift forces and drag forces were measured simultaneously to the acoustic measurements. Additionally, hot-wire measurements were performed to allow conclusions on the underlying mechanisms that enable the noise reduction. It could be demonstrated that, depending on the porous material, airfoils that are non-porous except for their trailing edge can still lead to a noticeable trailing edge noise reduction, while providing a better aerodynamic performance.

Silent Owl Flight: Acoustic Wind Tunnel Measurements on Prepared Wings

wings are characterized in the study as technical airfoils in terms of their acoustic and aerodynamic performance. The experiments took place in an aeroacoustic open jet wind tunnel using microphone array measurement technique and deconvolution beamforming algorithms. Simultaneously to the acoustic measurements, the lift and drag forces of the wings were captured using a six-component-balance. This study, which is a complementary study to the approach of performing yover measurements on ying birds, further conrms experimentally that the silent owl ight is a consequence of the special wing and plumage adaptations of the owls and not a consequence of their lower ight speed only.

An Approach to Estimate the Reliability of Microphone Array Methods

Establishing microphone array methods as tool for precise acoustic measurements necessitates assessing their reliability depending on given boundary conditions. In this contribution, an approach to evaluate the performance of microphone array methods is proposed. Instead of simply testing an algorithm on distinct scenarios, Monte Carlo simulations are performed to allow for a statistical analysis. In a simulation scenario, number of sources, source positions, and source levels are varied. The objective is set to correctly reconstruct the level and position of the sound sources, and a rating criterion is dened. The microphone array methods DAMAS, CLEAN-SC, Orthogonal Beamforming and Covariance Matrix Fitting are tested for their performance on 12 600 generated data sets. The results are then statistically evaluated regarding the precision of the reconstruction abilities depending on the varied parameters. It is shown that this approach helps deducing information about the characteristic performance of the methods that could not easily have been revealed otherwise.

Passive Control of the Vortex Shedding Noise of a Cylinder at Low Reynolds Numbers Using Flexible Flaps

A recent experimental study on the vortex shedding noise of a cylinder equipped with thin, flexible flaps showed that the presence of the flaps leads to a sudden shift of the aeolian tones when above a certain Reynolds number, which resulted in a jump in the corresponding Reynolds-Strouhal number diagram. In the present work, this effect is further investigated by performing acoustic measurements on modified versions of the original flap cylinder, where, subsequently, flaps were cut off to study their individual contribution to the vortex shedding. In addition to the acoustic measurements, the movement of the flaps was captured using a high-speed camera. The eigenmodes of the flaps were calculated numerically. The results confirm that the jump of the Strouhal number is caused by a lock-in of the vortex shedding cycle with the oscillation of the outer flaps at the next higher eigenfrequency. Reducing the number of flaps does not affect the jump, but a shortening of the flaps (and hence a modification of the flap eigenmodes) lead to the fact that the Strouhal number jumped to a lower value than before.