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Measurement of the noise generation at the trailing edge of porous airfoils   总被引:2,自引:0,他引:2  
Owls are commonly known for their quiet flight, enabled by three adaptions of their wings and plumage: leading edge serrations, trailing edge fringes and a soft and elastic downy upper surface of the feathers. In order to gain a better understanding of the aeroacoustic effects of the third property that is equivalent to an increased permeability of the plumage to air, an experimental survey on a set of airfoils made of different porous materials was carried out. Several airfoils with the same shape and size but made of different porous materials characterized by their flow resistivities and one non-porous reference airfoil were subject to the flow in an aeroacoustic open jet wind tunnel. The flow speed has been varied between approximately 25 and 50 m/s. The geometric angle of attack ranged from −16° to 20° in 4°-steps. The results of the aeroacoustic measurements, made with a 56-microphone array positioned out of flow, and of the measurements of lift and drag are given and discussed.  相似文献   
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Phased microphone arrays are used in a variety of applications for the estimation of acoustic source location and spectra. The popular conventional delay-and-sum beamforming methods used with such arrays suffer from inaccurate estimations of absolute source levels and in some cases also from low resolution. Deconvolution approaches such as DAMAS have better performance, but require high computational effort. A fast beamforming method is proposed that can be used in conjunction with a phased microphone array in applications with focus on the correct quantitative estimation of acoustic source spectra. This method bases on an eigenvalue decomposition of the cross spectral matrix of microphone signals and uses the eigenvalues from the signal subspace to estimate absolute source levels. The theoretical basis of the method is discussed together with an assessment of the quality of the estimation. Experimental tests using a loudspeaker setup and an airfoil trailing edge noise setup in an aeroacoustic wind tunnel show that the proposed method is robust and leads to reliable quantitative results.  相似文献   
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The reserve polymeric inulin from dahlia tuber (>12% or >60% yield, wet or dry basis, respectively) follows as an attractive source for both free fructose or difructofuranose anhydride (DFA III). Although DFA biological activity is not completely understood, there is interest in characterizing other DFA III-producers besidesArthrobacter ureafaciens. The inulinolytic bacterial isolate named “YLW,” owing to the yellow hue in agar slants, is such a producer. Its biochemical characterization showed the presence of galactosylated and mannosylated glycolipids associated with the bacterial cells. Immobilization of fungal inulinase I and bacterial inulinase II, the respective enzymatic catalysts for the production of fructose and DFA III by inulin hydrolysis, was attempted using controlled-pore silica (CPS). The effects of pH, temperature, and incubation time was analyzed and compared for both enzymes in the free and immobilized forms.

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Based on data sets from previous experimental studies, the tool of symbolic regression is applied to find empirical models that describe the noise generation at porous airfoils. Both the self noise from the interaction of a turbulent boundary layer with the trailing edge of an porous airfoil and the noise generated at the leading edge due to turbulent inflow are considered. Following a dimensional analysis, models are built for trailing edge noise and leading edge noise in terms of four and six dimensionless quantities, respectively. Models of different accuracy and complexity are proposed and discussed. For the trailing edge noise case, a general dependency of the sound power on the fifth power of the flow velocity was found and the frequency spectrum is controlled by the flow resistivity of the porous material. Leading edge noise power is proportional to the square of the turbulence intensity and shows a dependency on the fifth to sixth power of the flow velocity, while the spectrum is governed by the flow resistivity and the integral length scale of the incoming turbulence.  相似文献   
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The aeroacoustic wind tunnel at Brandenburg University of Technology at Cottbus is a newly commissioned research facility for the experimental study of sound generation from bodies immersed in a fluid flow. The paper discusses the design criteria for the open jet wind tunnel that provides a maximum wind speed of 72 m/s at continuous operation and may be operated with nozzles of different dimension between 35 cm diameter (circular nozzle) and 12 cm by 14.7 cm (rectangular nozzle). Experiments may be performed either in a reverberant or in an anechoic environment. Both the aerodynamic and the acoustic design of the wind tunnel components are discussed in detail. Background noise measurements in the completed facility revealed very low levels comparable to other wind tunnels. The results of aerodynamic wind tunnel calibration confirmed a uniform flow quality in the jet and a very low axial turbulence intensity which is less than 0.2% for the 35 cm nozzle and less than 0.1% for the other nozzles. A final benchmark is provided by results of successful trailing edge noise measurements on an SD7003 airfoil that are presented and compared to results from the literature.  相似文献   
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