Thomas Geyer

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

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.

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.

Silent Owl Flight: The Effect of the Leading Edge Comb on the Gliding Flight Noise

The feathers of owls possess three adaptations that are held responsible for their quiet flight. These are a comb-like structure at the leading edge of the wing, fringes at the trailing edge and a soft and porous upper surface of the wing. To investigate the effect of the first adaptation, the leading edge comb, on the aerodynamic performance and the noise generation during gliding flight, wind tunnel measurements were performed on prepared wings of a Barn owl (Tyto alba) with and without the comb. In agreement with existing studies it was found that the leading edge comb causes a small increase in lift. Additionally, at high angles of attack the results from the acoustic measurements indicate that the presence of the comb leads to a reduction in gliding flight noise. Although this reduction is relatively small, it further helps the owl to approach its prey during the final stages of the landing phase.

Flow noise generation of cylinders with soft porous cover

The use of porous materials is one of several approaches to control or minimize the generation of flow noise. As a simple model for struts and other protruding parts (for example components of the landing gear or pantographs from high speed trains), the noise generated by circular cylinders with a soft porous cover was measured in a small aeroacoustic wind tunnel at Reynolds numbers between approximately 16,000 and 103,000. The porous materials were characterized by their air flow resistivity, a parameter describing the permeability of an open-porous material. The aim of this study is to identify those materials that result in the best noise reduction, which refers to both tonal noise and broadband noise. To this end, measurements with single microphones were performed on a set of cylinders whose porous materials cover a large range of air flow resistivities. The results show that materials with low air flow resistivities lead to a noticeable flow noise reduction. Thereby, the main effect of the porous cylinder covers is that the spectral peak due to the aeolian tone is much narrower, but is not suppressed completely. Additionally, a reduction of broadband noise can be observed, especially at higher Reynolds numbers. The noise reduction increases with decreasing air flow resistivity of the porous covers, which means that materials that are highly permeable to air result in the best noise reduction.

Experimental assessment of the noise generated at the leading edge of porous airfoils using microphone array techniques

ow{permeable materials is a known method for the reduction of airfoil aeroacoustic noise. Detailed acoustic measurements on the noise generation at the leading edge of porous airfoil models were performed in an open jet wind tunnel using microphone array measurement techniques and three{dimensional beamforming algorithms. A set of three dierent grids provided the required inow turbulence. Measurement results are presented for the noise generated at the leading edge of porous airfoils, which are characterized by their air ow resistivity, compared to a non{porous reference airfoil. The comparison of the leading edge noise spectra measured for the reference airfoil with theory yields good agreement. The results of the acoustic measurements show that porous airfoils with low air ow resistivities lead to a noticeable noise reduction, which is assumed to be caused by the larger pores of these materials compared to porous airfoils with a higher air ow resistivity.