17,207,582 research outputs found
Experimental investigation of aerofoil tonal noise generation
No full textThe present study investigates the mechanisms associated with tonal noise emission from a NACA 0012 aerofoil at moderate incidence (0°, 1°, 2° and 4° angle of attack) and with Reynolds numbers ranging from 100 000 to 270 000. Simultaneous time-resolved particle image velocimetry (PIV) of the aeroacoustic source region near the trailing edge and acoustic measurements in the far field are performed in order to establish the correspondence between the flow structure and acoustic emissions. Results of these experiments are presented and analysed in view of past research for a number of selected cases. Characteristics of the acoustic emission and principal features of the average flow field agree with data presented in previous studies on the topic. Time-resolved analysis shows that downstream convecting vortical structures, resulting from growing shear layer instabilities, coherently pass the trailing edge at a frequency equal to that of the dominant tone. Therefore, the scattering of the vortical structures and their associated wall pressure fluctuations are identified as tone generating mechanisms for the cases investigated here. Moreover, wavelet analysis of the acoustic pressure and velocity signals near the trailing edge show a similar periodic amplitude modulation which is associated with multiple tonal peaks in the acoustic spectrum. Periodic amplitude modulation of the acoustic pressure and velocity fluctuations on the pressure side are also observed when transition is forced on the suction side, showing that pressure-side events alone can be the cause
Investigation of tonal noise generation on an airfoil with time-resolved PIV
No full textTonal noise generated by airfoils at low to moderate Reynolds numbers has been focus of research since decades. Several explanations of the observed phenomena relate the noise spectrum to the instability of the laminar boundary layer, which has lead to the term laminar boundary layer instability noise. The particular features of boundary layer instability noise have been reported in literature, but remain controversial to date. In the past, studies have been performed relying on hot-wire, Laser Doppler Velocimetry (LDV), phase-locked Particle Image Velocimetry (PIV), and acoustic measurements, as well as numerical simulations and theoretical approaches. In recent years, high-speed PIV has become accepted as a tool for the investigation of aeroacoustic problems since it can provide spatio-temporal information on the structure of the source field. In the present study, far-field microphone measurements are conducted to map the noise emissions of a NACA 0012 airfoil over a range of angles of attack and velocities. The principal features observed in the noise maps agree with findings of previous experiments. For the investigation of the ow structure and source field at the trailing edge, high-speed PIV in conjunction with simultaneous microphone measurements has been performed. The information obtained using this recent measurement technique provides detail on the relation between the acoustic spectrum and the dynamic characteristics of the aeroacoustic source field
Effect of serrated trailing edge on boundary layer instability noise
No full textWings operating at low and moderate Reynolds number such as the ones of UAVs or the blades of small wind turbines and of compressor fans, can be the source of an aeroacoustics phenomenon called laminar boundary layer instability noise. This paper presents an investigation with time-resolved PIV and far field microphones of the dynamical flow phenomena and the acoustic emissions of a NACA 0018 airfoil, with and without serrated trailing edges, operating in a transitional regime (Reynolds number ≈ 3.3 – 4.7 × 105). The measurements are done in an open jet wind tunnel facility with low noise and turbulence intensity. The planar PIV measurements are done on the pressure side of the airfoil in two different planes, parallel and orthogonal to the wing span at the trailing edge region, to capture the flow evolution along the airfoil chord as well as its spanwise arrangement. The results presented here show the effectiveness of the serrated geometries to reduce the noise emitted by this type of flows. Recently published literature outcomes concerning the streamwise evolution of the flow with and without serrations are confirmed. Moreover new insights related to the spanwise coherence of the traveling vortical structures are presented leading to a better understanding of the hydrodynamic and acoustic mechanisms involved in this aeroacoustics phenomenon
A comparative study of turbulent boundary layer wall pressure fluctuations obtained from high-speed tomographic PIV and DNS
No full textTrailing-edge noise diagnostics with low-repetition-rate PIV
No full textA novel approach for trailing-edge noise diagnostics with low-repetition rate Particle Image Velocimetry systems is presented. The approach is based on the application of diffraction theory and implements a recently developed single-snapshot pressure reconstruction in boundary layers using the Vortex-in-cell method in combination with a frozen turbulence assumption to estimate the relevant statistical descriptors of the surface pressure field. The present communication describes the required planar and tomographic Particle Image Velocimetry experiments with a high spatial-dynamic-range system and demonstrates the applicability of the approach by comparison of hydrodynamic and acoustic pressure statistics with available data. The frequency spectra of the hydrodynamic pressure near the surface are found to be sensitive to the model employed for the convective velocity in the application of the frozen turbulence assumption for reduced frequencies f δ*/u∞ > 0.5 (2.3kHz). For lower reduced frequencies, excellent agreement with the reference data is observed
On the estimation of wall pressure coherence using time-resolved tomographic PIV
No full textThree-dimensional time-resolved velocity field measurements are obtained using a high-speed tomographic Particle Image Velocimetry (PIV) system on a fully developed flat plate turbulent boundary layer for the estimation of wall pressure fluctuations. The work focuses on the applicability of tomographic PIV to compute the coherence of pressure fluctuations, with attention to the estimation of the stream and spanwise coherence length. The latter is required for estimations of aeroacoustic noise radiation by boundary layers and trailing edge flows, but is also of interest for vibro-structural problems. The pressure field is obtained by solving the Poisson equation for incompressible flows, where the source terms are provided by time-resolved velocity field measurements. Measured 3D velocity data is compared to results obtained from planar PIV, and a Direct Numerical Simulation (DNS) at similar Reynolds number. An improved method for the estimation of the material based on a least squares estimator of the velocity derivative along a particle trajectory is proposed and applied. Computed surface pressure fluctuations are further verified by means of simultaneous measurements by a pinhole microphone and compared to the DNS results and a semi-empirical model available from literature. The correlation coefficient for the reconstructed pressure time series with respect to pinhole microphone measurements attains approximately 0.5 for the band-pass filtered signal over the range of frequencies resolved by the velocity field measurements. Scaled power spectra of the pressure at a single point compare favorably to the DNS results and those available from literature. Finally, the coherence of surface pressure fluctuations and the resulting span- and streamwise coherence lengths are estimated and compared to semi-empirical models and DNS results
Diffusive author(s), cohesive author: Analysis of S/N (1994)
No full textThis study indicates the ways in which various aspects of the author(s) are brought forth in Dumb type’s performance art, the S/N production. Previous research has suggested a non-hierarchical organization of Dumb type and the absence of a “privileged author” in Dumb type’s collaborative work, S/N. However, the results that I have investigated from member’s interviews on the creative process of S/N along with my analysis of the recorded images of S/N, indicate a different aspect of the author(s). First, S/N was created through, so to speak, the collective ideas of the members of Dumb type. Further, S/N has at least nine quotations from previous performances, installations, and printed writings, besides the work-in-progress technique. Explicating one of the “author functions” as given by Michel Foucault, each text has plural subjects of the author. However, it has been revealed from members’ interviews that Teiji Furuhashi had a decision-making role in selecting the members’ ideas within the performance. Since then, S/N has had plural subjects of creation; however, Furuhashi is one of the subjects of creation along with the “privileged author.” S/N has plural authors (diffusive authors) yet at the same time, it has a “privileged author,” Teiji Furuhashi (cohesive author)
Acoustic Emissions of Semi-Permeable Trailing Edge Serrations
No full textThe trailing edge of a NACA 0018 airfoil is modified through the attachment of serrations with different degrees of permeability. Acoustic beamforming is used to inspect the turbulent boundary layer-trailing edge noise emissions from the unmodified and serrated trailing edges. Different freestream velocities and angles of attack are investigated. The serration permeability is prescribed by having slits cut into the solid surface of the serrations in two different configurations. The results indicate that a certain benefit in noise reduction is obtained from a mixed solid/slitted configuration, while a fully slitted configuration loses most of the noise reduction performance.AerodynamicsAircraft Noise and Climate EffectsWind Energ
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